Was the whole Universe close to Big Bang very small, or just very dense?

Was the whole Universe close to Big Bang very small, or just very dense?

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I am sorry this question is probably silly for professional astronomers, of which I am not one.

I often hear at lectures that immediately after the Big Bang, the universe was small, say, the size of grapefruit or something like that. But because of inflation which stretched space at super-light (but finite), speed, the observable universe now, may not be the whole universe, and the whole universe may be infinite. If it is infinite, then it would seem that it had to be infinite in infancy as well, just very dense. In fact, it seems "in the limit", it had to be infinite even at Big Bang.

So, questions:

Was early universe small, or only dense but still infinite?

How about at Big Bang, was it infinite?

When astronomers say early universe was small, do they simply mean "the part of the universe which corresponds to our observable universe, was small"?

I am sorry this question is probably silly for professional astronomers, of which I am not one.

This question is by no means silly. Your question is a common one about cosmology (the study of where the universe came from, how it is evolving and what its fate will be). The media often butchers these concepts horribly, resulting in a lot of confusion (out of all scientific information, they seem to have the hardest time reporting cosmology accurately). Your inquisition is definitely a good thing.

When astronomers say early universe was small, do they simply mean "the part of the universe which corresponds to our observable universe, was small"?

Well, they're usually referring to the entire Universe. In my final paragraph, I explain what this implies for the observable universe.

If it is infinite, then it would seem that it had to be infinite in infancy as well, just very dense. In fact, it seems "in the limit", it had to be infinite even at Big Bang.

You're closer to the truth. When we talk about the expansion of the Universe, we're really saying that space is being created between all matter.

As you mentioned, the Universe may be infinite. It is not like a ball, but rather like a flat grid, and its "expansion" just means that the distances between objects on the grid are getting larger. In essence, more space is being created between the objects. That's what we mean by expansion - that objects are moving away from each other, since more space is being created between them. Below is a gif I've made to demonstrate this:

A more useful way to describe this is to say the grid is expanding - that space itself, as a coordinate system, is growing. As an analogy, imagine are walking your dog. Suddenly, the ground begins expanding between you. You and your dog will separated and continue receding away from each other.

So the same thing is happening with our universe. The grid is in fact growing, and objects are being swept away with it.

OK, now that we've gotten the core concepts down, I'll introduce one more bit of terminology. The "scale factor of the Universe" refers to how much the Universe has expanded, compared to now. For example, if in a billion years the scale factor is 3, that means that every object in the Universe is 3 times farther from each other compared to now. If the scale factor 700 million years ago was 0.8, then everything was closer by a factor of 0.8 at that time. By definition, the scale factor is 1 right now.

So, if the Universe is expanding now, we'd expect it to be smaller as we look further back in time - i.e. the scale factor would be less. General relativity predicts the scale factor to be zero at 13.8 billion years ago. This would mean that every object would be zero times its current distance from us - in other words, there would be no space.

If you think a Universe without space is impossible, you're correct. We apparently have a contradiction. In GR, you can't have a spacetime with zero space.

Our modern physical theories work fine up a few fractions of a second after the moment of contradiction, and our observations do agree with the idea of an extremely dense early universe. However, our theories break down as we try to model the Universe at earlier and earlier times, until they no longer prove accurate, preventing us from explaining the most interesting moment.

This is why the moment of the Big Bang is one of the biggest mysteries in cosmology. Theories like quantum gravity have arisen to try to explain the conditions near the Big Bang, but none are sufficient as of now.

I often hear at lectures that immediately after the Big Bang, the universe was small, say, the size of grapefruit or something like that.

Indeed, the problem stems from the ambiguity when one says "universe". In this case, they're referring to the observable universe, which is actually spherical. The observable universe was indeed much smaller near the time of the Big Bang, compared to its radius now.

This is because its radius actually depends on our Universe's scale factor*, which means that at the moment GR predicts the scale factor to be zero, it also predicts the size of the observable universe to be zero.

Obviously that can't be the case, since as we've explained above, it shouldn't be possible for the scale factor to be zero. However, we can say with reasonable confidence that the observable universe was likely the size of a grapefruit at one point, if not smaller (although "grapefruit" seems an arbitrary choice for comparison. I can't actually find the paper that first uses this analogy, so what they originally meant is a bit unclear).

*Measuring distances is actually a bit tricky in cosmology; in some cases, we want to talk about distances or motion of objects while neglecting the Universe's expansion. To save you the need to learn a lot of terminology, I'm right now taking into account the expansion of the Universe when talking about the observable universe's size. The observable universe also grows due to factors besides the Universe's expansion, i.e. light from further and further galaxies reaching us.

Learn about Georges Lemaitre's view on the origin of the universe and the big-bang theory in relation to space expansion

Physicists used to think that the universe had existed forever, unchangingly, because that's what their observations of the night sky suggested. Needless to say, this view clashed with the origin or creation stories of most major religions, which hold that the universe had a beginning.

So it's not surprising that it was a Catholic priest, Georges Lemaitre, who was one of the first major proponents of a new scientific viewpoint that the universe did have a beginning. Lemaitre, of course, was also an excellent mathematician and scientist and based this conviction not just on his religious beliefs but upon new experimental evidence from Edwin Hubble that showed the universe was expanding. This evidence, combined with the mathematics of general relativity, allowed Lemaitre to rewind cosmic history and calculate that the farther back in time you go, the smaller the universe had to be.

The natural conclusion is that everything we can currently see in the universe was at one point in time more or less at one point in space. Lemaitre called this idea the primeval atom, but of course, today we know it as the big bang theory, except big bang is a horrible name. It would be much more accurate to call it the everywhere stretch, because one of the most common misconceptions about the big bang is that it implies that the entire universe was compressed into a single point, from which it then somehow expanded into the surrounding nothingness.

It is true that the observable universe-- that is, the part of the whole universe we can see from Earth-- was indeed shrunk down to a very, very small bit of space, but that bit of space was not a single point, nor was the rest of the universe also in that same bit of space.

The explanation for this is the magical power of infinity. The whole universe is really big. Current data show it's at least 20 times bigger than the observable universe, but that's just a lower bound. It might be infinite. And if you have an infinite amount of space, you can scale space down, shrink everything to minuscule proportions, and still have an infinite amount of space, kind of like how you can zoom out as much as you want from a number line, but it'll still be an infinite number line.

Essentially, space doesn't need anywhere to expand into because it can expand into itself and still have plenty of room. In fact, this is possible even if space turns out not to be infinite in size, though the reasons are complicated and have to do with the infinite differentiability of the metric of spacetime.

But anyway, the event unfortunately known as the big bang was basically a time long ago when space was much more squeezed together, and the observable universe, which is everything we can see from Earth, was crammed into a very, very small piece of that space. Because the entire early universe was dense and hot everywhere, spacetime was curved everywhere, and this curvature manifested itself as a rapid expansion of space throughout the universe.

And although people call this the big bang, it wasn't just big. It was everywhere, and it wasn't really an explosion. It was space stretching out. It's actually quite unfortunate that the everywhere stretch isn't nearly as catchy as the big bang, which brings us to the big bang singularity, which is an even horribler name because every single word is misleading. Singularity seems to imply something that happened at a single point, which isn't at all what it's referring to. It should be called the part of the everywhere stretch where we don't know what we're talking about.

Basically, our current physical models for the universe are unable to properly explain and predict what was happening at the very, very beginning, when the universe was super, super scaled down. But rather than call it the time when we don't have a clue what was happening anywhere, for some reason, we call it a singularity.

This ignorance, however, does conveniently answer the question what happened before the big bang, because it tells us the question isn't well defined. Back when space was so incredibly compressed and everything was ridiculously hot and dense, our mathematical models of the universe break down so much that time doesn't even make sense.

It's like how at the North Pole, the concept of north breaks down. What's north of the North Pole? The only thing you can say is that everywhere on Earth is south of the North Pole, or similarly, every when in the universe is after the beginning. But once time began, whenever that was, space expanded incredibly quickly all throughout the universe for a little while. Then expansion slowed. The universe cooled. Stuff happened, and after a few billion years, here we are.

One thing we still don't know is why this everywhere stretching happened. That is, why did the universe start off in such a funny compressed state, and why did it follow the seemingly arbitrary laws of physics that have governed its expansion and development ever since?

For Georges Lemaitre, this might be where God finally comes into the picture, to explain the thing science can't, except that experimental evidence doesn't actually rule out the possibility that there may indeed be a time before the beginning, a previous age of the universe that ended when space collapsed in on itself, getting quite compressed and dense and hot, but not enough to mangle up our ideas of what time is. It would have then bounced back out, stretching in a fashion similar to what we call the big bang, but without the we don't know what we're talking about singularity part.

So physics may actually be nudging us back to the view that the universe is eternal and didn't begin after all, in which case, Professor Lemaitre might have to rethink his interpretation of the words "in the beginning."

Was the whole Universe close to Big Bang very small, or just very dense? - Astronomy

My question is "what is at the end of the universe if it really does end?" because if the big bang theory is correct then then when the whole universe was one atom or very small what did it expand into because if there was nothing there it couldnt have expanded. for example if you have a room and you say that the walls are the end of the universe then you build on to the room to make it bigger (the universe expanding) there has to be room on the other side of the wall for you to build into.

There is nothing called the end of the Universe. There are three possibilities of the shape of the Universe.

First, the Universe might have what we call positive curvature like a sphere. In this case, the Universe is called "closed" and it has a finite size but without a boundary, just like a baloon. In a closed Universe, you could, in principle, fly a spaceship far enough in one direction and get back to where you started from.

The second possibility is that the Universe is flat. This kind of Universe can be imagined by cutting out a piece of a baloon material and stretching it with your hands. The surface of the material is flat and not curved. You can expand and contract it by tugging on either end. Flat Universes are infinite in extent and have no boundaries.

Finally, the Universe might be "open" or have negative curvature. Such Universes are also infinite in spatial extent and have no boundaries.

Thus whatever be the shape of the Universe, there is nothing called a boundary and hence nothing called the edge or end of the Universe.

Regarding the second question of expansion, remember that space exists only IN the Universe and there is no meaning to the term "outside the Universe". What happens in expansion is that the space itself is expanding. With respect to your room analogy, it is not that the walls of your room are pushing against something but that the space in the room is expanding there is nothing to push against. Thus, when we talk of galaxies receding from us due to the expanion, it is not that the galaxies are moving, but the space in between us and the galaxies is expanding.

Edit by Michael Lam on February 10, 2016: Recent observations suggest that the Universe is very close to flat. The WMAP mission determined that it is flat to within a 0.4% margin of error.

About the Author

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

New Evidence

Observations of distant ultra-bright galaxies in the 1950s suggested the universe was changing, and measurements of the helium content in the universe didn’t match the steady state model’s predictions. In 1964, the monumental discovery of the cosmic microwave background radiation — direct evidence of a young, hot universe — would deal the final deathblow to the steady state model.

“It really seems to suggest … the universe had very different conditions in early times than today,” Kaiser says. “And that was just not what the steady state model suggests.”

In an ironic twist, Hoyle used the term “Big Bang” in an attempt to dismiss the theory in a BBC interview. Though his own theory would be largely lost to history, the irreverent name would stick.

To his death, Hoyle would never submit to the Big Bang theory. A small subset of cosmologists still work on resurrecting a steady state model but, on the whole, the community overwhelmingly supports the Big Bang theory.

“There are a couple of other puzzles, so cosmologists don't think we're done, but they’re now kind of patching or filling in some holes to the original Big Bang models — certainly not replacing it,” Kaiser says.

A Bouncing Universe

If our Universe originated in a bounce, that means there was another universe before us. That universe went through its life, perhaps expanding, and eventually contracting once again. As all of the matter and spacetime of this universe came together, it ended in a spectacular fireball. Then, in a giant “bounce”, our Universe was born, like a phoenix rising from the ashes of the old universe.

This isn’t entirely a new idea. Physicists have tossed around the idea of the Big Bounce for several decades. Even farther back, cyclic time is present in Hindu cosmology.

The Universe is currently expanding, like an inflating balloon. Contracting, it is like a deflating . [+] balloon. The bouncing universe model says that once the previous universe is "deflated", a new one would "inflate" once again.


The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements, the CMB, large-scale structure, and Hubble's law. [10] The theory depends on two major assumptions: the universality of physical laws and the cosmological principle. The universality of physical laws is one of the underlying principles of the theory of relativity. The cosmological principle states that on large scales the universe is homogeneous and isotropic – appearing the same in all directions regardless of location. [11]

These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10 −5 . [12] Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars. [13] [14] [notes 1]

The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10 −5 via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995. [15]

Expansion of space

The expansion of the Universe was inferred from early twentieth century astronomical observations and is an essential ingredient of the Big Bang theory. Mathematically, general relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, are specified using a coordinate chart or "grid" that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This metric contains a scale factor, which describes how the size of the universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system, the grid expands along with the universe, and objects that are moving only because of the expansion of the universe, remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such co-moving points expands proportionally with the scale factor of the universe. [16]

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distances between comoving points. In other words, the Big Bang is not an explosion in space, but rather an expansion of space. [4] Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local concentrations of matter such as our galaxy do not necessarily expand with the same speed as the whole Universe. [17]


An important feature of the Big Bang spacetime is the presence of particle horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. [18]

Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well. [18]


Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium. Others were fast enough to reach thermalisation. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter. The larger the ratio, the more time particles had to thermalise before they were too far away from each other. [19]

According to the Big Bang theory, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling down.


Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. [20] This irregular behavior, known as the gravitational singularity, indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone can not extrapolate toward the singularity—before the end of the so-called Planck epoch. [5]

This primordial singularity is itself sometimes called "the Big Bang", [21] but the term can also refer to a more generic early hot, dense phase [22] [notes 2] of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics) work. Based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the "age of the universe"—is 13.799 ± 0.021 billion years. [23]

Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible density gradient. [24]

Inflation and baryogenesis

The earliest phases of the Big Bang are subject to much speculation, since astronomical data about them are not available. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures, and was very rapidly expanding and cooling. The period from 0 to 10 −43 seconds into the expansion, the Planck epoch, was a phase in which the four fundamental forces — the electromagnetic force, the strong nuclear force, the weak nuclear force, and the gravitational force, were unified as one. [25] In this stage, the characteristic scale length of the universe was the Planck length, 1.6 × 10 −35 m , and consequently had a temperature of approximately 10 32 degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of quantum gravity. [26] [27] The Planck epoch was succeeded by the grand unification epoch beginning at 10 −43 seconds, where gravitation separated from the other forces as the universe's temperature fell. [25]

At approximately 10 −37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially, unconstrained by the light speed invariance, and temperatures dropped by a factor of 100,000. Microscopic quantum fluctuations that occurred because of Heisenberg's uncertainty principle were amplified into the seeds that would later form the large-scale structure of the universe. [28] At a time around 10 −36 seconds, the electroweak epoch begins when the strong nuclear force separates from the other forces, with only the electromagnetic force and weak nuclear force remaining unified. [29]

Inflation stopped at around the 10 −33 to 10 −32 seconds mark, with the universe's volume having increased by a factor of at least 10 78 . Reheating occurred until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles. [30] [31] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. [4] At some point, an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe. [32]


The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10 −12 seconds. [29] [33] After about 10 −11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators. At about 10 −6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 10 8 of the original matter particles and none of their antiparticles. [34] A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN). [35] Most protons remained uncombined as hydrogen nuclei. [36]

As the universe cooled, the rest energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years, the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background. [36]

Structure formation

Over a long period of time, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. [4] The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter, warm dark matter, hot dark matter, and baryonic matter. The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold (warm dark matter is ruled out by early reionization), [38] and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. [39] In an "extended model" which includes hot dark matter in the form of neutrinos, [40] then if the "physical baryon density" Ω b h 2 >h^<2>> is estimated at about 0.023 (this is different from the 'baryon density' Ω b >> expressed as a fraction of the total matter/energy density, which is about 0.046), and the corresponding cold dark matter density Ω c h 2 >h^<2>> is about 0.11, the corresponding neutrino density Ω v h 2 >h^<2>> is estimated to be less than 0.0062. [39]

Cosmic acceleration

Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 73% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity predominated, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the declining density of matter relative to the density of dark energy caused the expansion of the universe to slowly begin to accelerate. [7]

Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theoretically. [7]

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10 −15 seconds. [41] Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics.


English astronomer Fred Hoyle is credited with coining the term "Big Bang" during a talk for a March 1949 BBC Radio broadcast, [42] saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past." [43] [44]

It is popularly reported that Hoyle, who favored an alternative "steady-state" cosmological model, intended this to be pejorative, [45] but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models. [46] [47]


The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912, Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way. [49] [50] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time. [51]

In 1924, American astronomer Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity—now known as Hubble's law. [52] [53] By that time, Lemaître had already shown that this was expected, given the cosmological principle. [7]

Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the universe. [54] In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence. [55]

In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics this objection was later repeated by supporters of the steady-state theory. [56] This perception was enhanced by the fact that the originator of the Big Bang theory, Lemaître, was a Roman Catholic priest. [57] Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was "repugnant" to him. [58] [59] Lemaître, however, disagreed:

If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time. [60]

During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, [61] the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman) [62] and Fritz Zwicky's tired light hypothesis. [63]

After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time. [64] The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced BBN [65] and whose associates, Ralph Alpher and Robert Herman, predicted the CMB. [66] Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949. [47] [44] [notes 3] For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. [67] Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory. [ citation needed ]

In 1968 and 1970, Roger Penrose, Stephen Hawking, and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of the Big Bang. [68] [69] Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang theory with the introduction of an epoch of rapid expansion in the early universe he called "inflation". [70] Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant [71] and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe). [72]

In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. [73] While there still remain some questions as to how accurately the ages of the clusters are measured, globular clusters are of interest to cosmology as some of the oldest objects in the universe. [ citation needed ]

Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE), [74] the Hubble Space Telescope and WMAP. [75] Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating. [76] [77]

The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis (BBN). More recent evidence includes observations of galaxy formation and evolution, and the distribution of large-scale cosmic structures, [79] These are sometimes called the "four pillars" of the Big Bang theory. [80]

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations. [81] Remaining issues include the cuspy halo problem [82] and the dwarf galaxy problem [83] of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. [84] Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.

Hubble's law and the expansion of space

Observations of distant galaxies and quasars show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed: [52] v = H 0 D D> where

Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable under the assumption of the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Friedmann in 1922 [51] and Lemaître in 1927, [54] well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker.

That space is undergoing metric expansion is shown by direct observational evidence of the cosmological principle and the Copernican principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogeneous, [52] supporting the cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position. [86] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

Cosmic microwave background radiation

In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. [67] Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 Nobel Prize in Physics.

The surface of last scattering corresponding to emission of the CMB occurs shortly after recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly scattered from free charged particles. Peaking at around 372 ± 14 kyr , [38] the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.

In 1989, NASA launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 10 4 , and measured a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.7255 K) then in 1992, further COBE measurements discovered tiny fluctuations (anisotropies) in the CMB temperature across the sky, at a level of about one part in 10 5 . [74] John C. Mather and George Smoot were awarded the 2006 Nobel Prize in Physics for their leadership in these results.

During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the shape of the universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies. [91] [92] [93]

In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general. [75] The Planck space probe was launched in May 2009. Other ground and balloon-based cosmic microwave background experiments are ongoing.

Abundance of primordial elements

Using the Big Bang model, it is possible to calculate the concentration of helium-4, helium-3, deuterium, and lithium-7 in the universe as ratios to the amount of ordinary hydrogen. [35] The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for He 4 / H >> , about 10 −3 for H 2 / H >> , about 10 −4 for He 3 / H >> and about 10 −9 for Li 7 / H >> . [35]

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for He 4 >> , and off by a factor of two for Li 7 >> (this anomaly is known as the cosmological lithium problem) in the latter two cases, there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. [94] Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than He 3 >> , and in constant ratios, too. [95] : 182–185

Galactic evolution and distribution

Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang theory. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then, larger structures have been forming, such as galaxy clusters and superclusters. [96]

Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently, appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures, agree well with Big Bang simulations of the formation of structure in the universe, and are helping to complete details of the theory. [96] [97]

Primordial gas clouds

In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. These two clouds of gas contain no elements heavier than hydrogen and deuterium. [102] [103] Since the clouds of gas have no heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.

Other lines of evidence

The age of the universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars. [104] It is also in good agreement with age estimates based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background. [23] The agreement of independent measurements of this age supports the Lambda-CDM (ΛCDM) model, since the model is used to relate some of the measurements to an age estimate, and all estimates turn out to agree. Still, some observations of objects from the relatively early universe (in particular quasar APM 08279+5255) raise concern as to whether these objects had enough time to form so early in the ΛCDM model. [105] [106]

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift. [107] This prediction also implies that the amplitude of the Sunyaev–Zel'dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult. [108] [109]

Future observations

Future gravitational-wave observatories might be able to detect primordial gravitational waves, relics of the early universe, up to less than a second after the Big Bang. [110] [111]

As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang theory. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the horizon problem, the magnetic monopole problem, and the flatness problem are most commonly resolved with inflationary theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven. [112] [113] [114] [115] What follows are a list of the mysterious aspects of the Big Bang theory still under intense investigation by cosmologists and astrophysicists.

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. [32] It is generally assumed that when the universe was young and very hot it was in statistical equilibrium and contained equal numbers of baryons and antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium. [116] All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.

Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy". [7]

Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. [7] Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, [117] and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos. [39] According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of quintessence or other modified gravity schemes. [118] A cosmological constant problem, sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from Planck units. [119]

Dark matter

During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters. [120]

Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway. [121]

Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem [83] and the cuspy halo problem. [82] Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations. [122]

Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact. [123] The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature. [95] : 191–202

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation. [28] : 180–186

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe. [95] : 207 Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB. [75] : sec 6

If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon. [28] : 180–186

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before electroweak symmetry breaking occurs, so inflation should not be able to prevent large-scale discontinuities in the electroweak vacuum since distant parts of the observable universe were causally separate when the electroweak epoch ended. [124]

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand Unified theories (GUTs) predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness. [123]

Flatness problem

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW. [123] The universe may have positive, negative, or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density positive if greater and zero at the critical density, in which case space is said to be flat. Observations indicate the universe is consistent with being flat. [125] [126]

The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat. [notes 4] Given that a natural timescale for departure from flatness might be the Planck time, 10 −43 seconds, [4] the fact that the universe has reached neither a heat death nor a Big Crunch after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10 14 of its critical value, or it would not exist as it does today. [127]

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch. [18]

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy stars would burn out, leaving white dwarfs, neutron stars, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach absolute zero—a Big Freeze. [128] Moreover, if protons are unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death. [129]

Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip. [130]

One of the common misconceptions about the Big Bang model is that it fully explains the origin of the universe. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state. [131] It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe. [17]

Hubble's law predicts that galaxies that are beyond Hubble distance recede faster than the speed of light. However, special relativity does not apply beyond motion through space. Hubble's law describes velocity that results from expansion of space, rather than through space. [17]

Astronomers often refer to the cosmological redshift as a Doppler shift which can lead to a misconception. [17] Although similar, the cosmological redshift is not identical to the classically derived Doppler redshift because most elementary derivations of the Doppler redshift do not accommodate the expansion of space. Accurate derivation of the cosmological redshift requires the use of general relativity, and while a treatment using simpler Doppler effect arguments gives nearly identical results for nearby galaxies, interpreting the redshift of more distant galaxies as due to the simplest Doppler redshift treatments can cause confusion. [17]

The Big Bang explains the evolution of the universe from a starting density and temperature that is well beyond humanity's capability to replicate, so extrapolations to the most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the "primeval atom" while Gamow called the material "ylem". How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, specific laws of nature most likely came to existence in a random way, but as inflation models show, some combinations of these are far more probable. [132] A topologically flat universe implies a balance between gravitational potential energy and other energy forms, requiring no additional energy to be created. [125] [126]

The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and correcting this will require the development of a correct treatment of quantum gravity. [20] Certain quantum gravity treatments, such as the Wheeler–DeWitt equation, imply that time itself could be an emergent property. [133] As such, physics may conclude that time did not exist before the Big Bang. [134] [135]

While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds on the subject of "cosmogony".

Some speculative proposals in this regard, each of which entails untested hypotheses, are:

  • The simplest models, in which the Big Bang was caused by quantum fluctuations. That scenario had very little chance of happening, but, according to the totalitarian principle, even the most improbable event will eventually happen. It took place instantly, in our perspective, due to the absence of perceived time before the Big Bang. [136][137][138][139]
  • Models including the Hartle–Hawking no-boundary condition, in which the whole of spacetime is finite the Big Bang does represent the limit of time but without any singularity. [140] In such case, the universe is self-sufficient. [141] models, in which inflation is due to the movement of branes in string theory the pre-Big Bang model the ekpyrotic model, in which the Big Bang is the result of a collision between branes and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the universe cycles from one process to the other. [142][143][144][145] , in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe, expanding from its own big bang. [146][147]

Proposals in the last two categories see the Big Bang as an event in either a much larger and older universe or in a multiverse.

As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy. [148] [149] As a result, it has become one of the liveliest areas in the discourse between science and religion. [150] Some believe the Big Bang implies a creator, [151] [152] while others argue that Big Bang cosmology makes the notion of a creator superfluous. [149] [153]

How could an explosive Big Bang be the birth of our universe?

Pretend you’re a perfectly flat chess piece in a game of chess on a perfectly flat and humongous chessboard. One day you look around and ask: How did I get here? How did the chessboard get here? How did it all start? You pull out your telescope and begin to explore your universe, the chessboard….

What do you find? Your universe, the chessboard, is getting bigger. And over more time, even bigger! The board is expanding in all directions that you can see. There’s nothing that seems to be causing this expansion as far as you can tell – it just seems to be the nature of the chessboard.

But wait a minute. If it’s getting bigger, and has been getting bigger and bigger, then that means in the past, it must have been smaller and smaller and smaller. At some time, long, long ago, at the very beginning, it must have been so small that it was infinitely small.

Let’s work forward from what happened then. At the beginning of your universe, the chessboard was infinitely tiny and then expanded, growing bigger and bigger until the day that you decided to make some observations about the nature of your chess universe. All the stuff in the universe – the little particles that make up you and everything else – started very close together and then spread farther apart as time went on.

Our universe works exactly the same way. When astronomers like me make observations of distant galaxies, we see that they are all moving apart. It seems our universe started very small and has been expanding ever since. In fact, scientists now know that not only is the universe expanding, but the speed at which it’s expanding is increasing. This mysterious effect is caused by something physicists call dark energy, though we know very little else about it.

Astronomers also observe something called the Cosmic Microwave Background Radiation. It’s a very low level of energy that exists all throughout space. We know from those measurements that our universe is 13.8 billion years old – way, way older than people, and about three times older than the Earth.

If astronomers look back all the way to the event that started our universe, we call that the Big Bang.

Many people hear the name “Big Bang” and think about a giant explosion of stuff, like a bomb going off. But the Big Bang wasn’t an explosion that destroyed things. It was the beginning of our universe, the start of both space and time. Rather than an explosion, it was a very rapid expansion, the event that started the universe growing bigger and bigger.

This expansion is different than an explosion, which can be caused by things like chemical reactions or large impacts. Explosions result in energy going from one place to another, and usually a lot of it. Instead, during the Big Bang, energy moved along with space as it expanded, moving around wildly but becoming more spread out over time since space was growing over time.

Back in the chessboard universe, the “Big Bang” would be like the beginning of everything. It’s the start of the board getting bigger.

It’s important to realize that “before” the Big Bang, there was no space and there was no time. Coming back to the chessboard analogy, you can count the amount of time on the game clock after the start but there is no game time before the start – the clock wasn’t running. And, before the game had started, the chessboard universe hadn’t existed and there was no chessboard space either. You have to be careful when you say “before” in this context because time didn’t even exist until the Big Bang.

You also have wrap your mind around the idea that the universe isn’t expanding “into” anything, since as far as we know the Big Bang was the start of both space and time. Confusing, I know!

Astronomers aren’t sure what caused the Big Bang. We just look at observations and see that’s how the universe did start. We know it was extremely small and got bigger, and we know that kicked off 13.8 billion years ago.

What started our own game of chess? That’s one of the deepest questions anyone can ask.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts.

Michael Lam does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

The known knowns

Let's start with the general framework. 13.77 billion years ago, our universe was incredibly hot (a temperature of over a quadrillion degrees) and incredibly small (about the size of a peach). Astronomers suspect that, when our cosmos was less than a second old, it went through a period of incredibly rapid expansion, known as inflation.

This inflation event was perhaps the most transformative epoch ever to occur in the history of our universe. In less than a blink, our universe became incredibly larger (enlarging by a factor of at least 10^52). When this rapid expansion phase wound down, whatever caused inflation in the first place (we're not sure what) decayed, flooding the universe with matter and radiation (we're not sure how).

A few minutes later (literally), the first elements emerged. Prior to this time, the universe was too hot and too dense for anything stable to form &mdash it was just a giant mix of quarks (the fundamental building blocks of atomic nuclei) and gluons (the carriers of the strong nuclear force). But once the universe was a healthy dozen minutes old, it had expanded and cooled enough that the quarks could bind themselves together, forming the first protons and neutrons. Those protons and neutrons made the first hydrogen and helium (and a little bit of lithium), which went on hundreds of millions of years later to build the first stars and galaxies.

From the formation of the first elements, the universe just expanded and cooled, eventually becoming a plasma, and then a neutral gas.

While we know that this broad-brush story is correct, we also know that we're missing a lot of details, especially in the time before the formation of the first elements. Some funky physics may have been in operation when the universe was only a few seconds old, and it's currently beyond our theoretical understanding &mdash but that doesn't stop us from trying.

Discovery of the Cosmic Background Radiation

If the model of the universe described in the previous section is correct, then—as we look far outward in the universe and thus far back in time—the first “afterglow” of the hot, early universe should still be detectable. Observations of it would be very strong evidence that our theoretical calculations about how the universe evolved are correct. As we shall see, we have indeed detected the radiation emitted at this photon decoupling time, when radiation began to stream freely through the universe without interacting with matter (Figure 1).

Figure 1. Cosmic Microwave Background and Clouds Compared: (a) Early in the universe, photons (electromagnetic energy) were scattering off the crowded, hot, charged particles and could not get very far without colliding with another particle. But after electrons and photons settled into neutral atoms, there was far less scattering, and photons could travel over vast distances. The universe became transparent. As we look out in space and back in time, we can’t see back beyond this time. (b) This is similar to what happens when we see clouds in Earth’s atmosphere. Water droplets in a cloud scatter light very efficiently, but clear air lets light travel over long distances. So as we look up into the atmosphere, our vision is blocked by the cloud layers and we can’t see beyond them. (credit: modification of work by NASA)

The detection of this afterglow was initially an accident. In the late 1940s, Ralph Alpher and Robert Herman, working with George Gamow, realized that just before the universe became transparent, it must have been radiating like a blackbody at a temperature of about 3000 K—the temperature at which hydrogen atoms could begin to form. If we could have seen that radiation just after neutral atoms formed, it would have resembled radiation from a reddish star. It was as if a giant fireball filled the whole universe.

But that was nearly 14 billion years ago, and, in the meantime, the scale of the universe has increased a thousand fold. This expansion has increased the wavelength of the radiation by a factor of 1000 (see the section A Model of the Universe). According to Wien’s law, which relates wavelength and temperature, the expansion has correspondingly lowered the temperature by a factor of 1000 (see the chapter on Radiation and Spectra).

Alpher and Herman predicted that the glow from the fireball should now be at radio wavelengths and should resemble the radiation from a blackbody at a temperature only a few degrees above absolute zero. Since the fireball was everywhere throughout the universe, the radiation left over from it should also be everywhere. If our eyes were sensitive to radio wavelengths, the whole sky would appear to glow very faintly. However, our eyes can’t see at these wavelengths, and at the time Alpher and Herman made their prediction, there were no instruments that could detect the glow. Over the years, their prediction was forgotten.

In the mid-1960s, in Holmdel, New Jersey, Arno Penzias and Robert Wilson of AT&T’s Bell Laboratories had built a delicate microwave antenna (Figure 2) to measure astronomical sources, including supernova remnants like Cassiopeia A (see the chapter on The Death of Stars). They were plagued with some unexpected background noise, just like faint static on a radio, which they could not get rid of. The puzzling thing about this radiation was that it seemed to be coming from all directions at once. This is very unusual in astronomy: after all, most radiation has a specific direction where it is strongest—the direction of the Sun, or a supernova remnant, or the disk of the Milky Way, for example.

Figure 2. Robert Wilson (left) and Arno Penzias (right): These two scientists are standing in front of the horn-shaped antenna with which they discovered the cosmic background radiation. The photo was taken in 1978, just after they received the Nobel Prize in physics.

Penzias and Wilson at first thought that any radiation appearing to come from all directions must originate from inside their telescope, so they took everything apart to look for the source of the noise. They even found that some pigeons had roosted inside the big horn-shaped antenna and had left (as Penzias delicately put it) “a layer of white, sticky, dielectric substance coating the inside of the antenna.” However, nothing the scientists did could reduce the background radiation to zero, and they reluctantly came to accept that it must be real, and it must be coming from space.

Penzias and Wilson found the distribution of intensity at different radio wavelengths to correspond to a temperature of 3.5 K. This is very cold—closer to absolute zero than most other astronomical measurements—and a testament to how much space (and the waves within it) has stretched. Their measurements have been repeated with better instruments, which give us a reading of 2.73 K. So Penzias and Wilson came very close. Rounding this value, scientists often refer to “the 3-degree microwave background.”

Many other experiments on Earth and in space soon confirmed the discovery by Penzias and Wilson: The radiation was indeed coming from all directions (it was isotropic) and matched the predictions of the Big Bang theory with remarkable precision. Penzias and Wilson had inadvertently observed the glow from the primeval fireball. They received the Nobel Prize for their work in 1978. And just before his death in 1966, Lemaître learned that his “vanished brilliance” had been discovered and confirmed.

Did We Just Find The Largest Rotating ‘Thing’ In The Universe?

Filaments, hundreds of millions of light-years long, were just caught spinning.

In our own cosmic backyard, everything we see spins, rotates, and revolves in some fashion or other. Our planet (and everything on it) spins about its axis, just like every planet and moon in the Solar System. The moons (including our own) revolve around their parent planet, while the planet-moon systems all revolve around the Sun. The Sun, in turn, like all of the hundreds of billions of stars in the galaxy, orbit around the galactic center, while the entire galaxy itself spins about the central bulge.

On the largest of cosmic scales, however, there’s no observed global rotation. The Universe, for whatever reason, doesn’t appear to have an overall spin or rotation to it, and doesn’t appear to be revolving around anything else. Similarly, the largest observed cosmic structures don’t appear to be spinning, rotating, or revolving around any other structures. But recently, a new study appears to be challenging that, claiming that enormous cosmic filaments — the strands of the cosmic web — appear to be rotating about the filamentary axis itself. This is weird, for sure, but can we explain it? Let’s find out.

46 billion light-years in radius today. (NASA / CXC / M.WEISS)

In order to make a prediction, we first have to set up the scenario that we expect, then put in the laws of physics, and evolve the system forward in time to see what we anticipate. We can go all the way back, theoretically, to the earliest stages of the Universe. At the start of the hot Big Bang, immediately following the end of cosmic inflation, the Universe is:

  • filled with matter, antimatter, dark matter, and radiation,
  • uniform and the same in all directions,
  • with the exception of slight density imperfections on the scale of 1-part-in-30,000,
  • and with additional tiny imperfections in the directionality of these fluctuations, the linear and rotational motions of these overdense and underdense regions, and similar imperfections in gravitational wave background that the Universe is born with.

As the Universe expands, cools, and gravitates, a number of important steps occur, particularly on large cosmic scales.

In particular, some things grow with time, other things decay with time, and still other things remain the same with time.

The density imperfections, for example, grow in a particular fashion: proportional to the ratio of the matter density to the radiation density. As the Universe expands and cools, both matter and radiation — made up of individual quanta — get less dense the number of particles remains the same while the volume increases, causing the density of both to drop. They don’t drop equally, however the amount of mass in every matter particle remains the same, but the amount of energy in every quantum of radiation drops. As the Universe expands, the wavelength of the light traveling through space stretches, bringing it to lower and lower energies.

As the radiation gets less energetic, the matter density rises relative to the radiation density, causing these density imperfections to grow. Over time, the initially overdense regions preferentially attract the surrounding matter, drawing it in, while the initially underdense regions preferentially give up their matter to the denser regions nearby. Over long enough timescales, this leads to the formation of molecular gas clouds, stars, galaxies, and even the entire cosmic web.

Similarly, you can track the evolution of any initial rotational modes in a Universe that’s initially isotropic and homogeneous. Unlike the density imperfections, which grow, any initial spin or rotation will decay away as the Universe expands. Specifically, it decays as the scale of the Universe grows: the more the Universe expands, the less important angular momentum becomes. It should make sense, therefore, to anticipate that there won’t be any angular momentum — and hence, any spinning or rotation — on the largest cosmic scales.

At least, that’s true, but only up until a certain point. As long as your Universe, and the structures in it, continue to expand, these rotational or spinning modes will decay away. But there’s a rule that’s even more fundamental: the law of conservation of angular momentum. Just like a spinning figure skater can increase their rate of rotation by bringing their arms and legs in (or can decrease it by moving their arms and legs out), the rotation of large-scale structures will diminish so long as the structures expand, but once they get pulled in under their own gravity, that rotation speeds up again.

Angular momentum, you see, is a combination of two different factors multiplied together.

  1. Moment of inertia, which you can think about as how your mass is distributed: close to the rotation axis is a small moment of inertia far away from the rotation axis is a large moment of inertia.
  2. Angular velocity, which you can think of as how quickly you make a complete revolution something like revolutions-per-minute is a measure of angular velocity.

Even in a Universe where your density imperfections are born only with a very slight amount of angular momentum, gravitational growth won’t be able to get rid of it, while gravitational collapse, which causes your mass distribution to get concentrated towards the center, ensures that your moment of inertia will eventually decrease dramatically. If your angular momentum stays the same while your moment of inertia goes down, your angular velocity must rise in response. As a result, the greater the amount of gravitational collapse a structure has undergone, the greater the amount we expect to see it spinning, rotating, or otherwise manifesting its angular momentum.

But even that is only half of the story. Sure, we fully expect that the Universe is born with some angular momentum, and when these density imperfections grow, attract matter, and finally collapse under their own gravity, we expect to see them rotating — perhaps even quite substantially — in the end. However, even if the Universe were born with no angular momentum anywhere at all, it’s an inevitability that the structures that form on all cosmic scales (except, perhaps, the extreme largest ones of all) will start spinning, rotating, and even revolving around one another.

The reason for this is a physical phenomenon we’re all familiar with, but in a different context: tides. The reason planet Earth experiences tides is because the objects near it, like the Sun and the Moon, gravitationally attract the Earth. Specifically, however, they attract every point on the Earth, and they do so unequally. The points on the Earth that are closer to the Moon, for instance, get attracted a little bit more than the points that are farther away. Similarly, the points that are “north” or “south” of the imaginary line that connects Earth’s center to the Moon’s center will be attracted “downward” or “upward” correspondingly.

Despite how easy this is to visualize for a round body like the Earth, the same process takes place between every two masses in the Universe that occupy any volume more substantial than a single point. These tidal forces, as objects move through space relative to one another, exerts what’s known as a torque: a force that causes objects to experience a greater acceleration on one part of it than other parts of it. In all but the most perfectly aligned cases — where all the torques cancel out, a tremendous and coincidental rarity — these tidal torques will cause an angular acceleration, leading to an increase in angular momentum.

“Hang on,” I can hear you objecting. “I thought you said that angular momentum was always conserved? So how can you create an angular acceleration, which increases your angular momentum, if angular momentum is something that can never be created or destroyed?”

It’s a good objection. What you have to remember, however, is that torques are just like forces in the sense that they obey their own versions of Newton’s laws. In particular, just like forces have directions, so do torques: they can cause something to rotate clockwise or counterclockwise about each of the three-dimensional axes that exist in our Universe. And just like every action has an equal an opposite reaction, whenever one object pulls on another to create a torque, that equal and opposite force will create a torque on that first object as well.

It’s not something you think of very often, but this plays out all the time in our reality. When you accelerate your automobile from a standstill as soon as the light turns green, your tires start to spin and push against the road. The road, therefore, exerts a force on the bottom of your tires, which causes your spinning tires to grip the road, accelerate, and push the car forward. Because the force isn’t directly on the center of the wheels — where the axels are — but rather off-center, your tires spin, gripping the road, and creating a torque.

But there’s an equal-and-opposite reaction here, too. The road and the tires have to push on one another with equal and opposite forces. If the force of the road on the tires causes your automobile to accelerate and then move, say, clockwise with respect to the center of planet Earth, then the force of the tires on the road will cause planet Earth to accelerate and rotate, ever so slightly, a little bit extra in the counterclockwise direction with respect to how it was moving before. Even though:

  • the car now has more angular momentum than it did before,
  • and the Earth now has more angular momentum than it did before,

the sum of the car+Earth system has the same amount of angular momentum as it did initially. Angular momentum, like force, is a vector: with magnitude and direction.

So what happens, then, when the large-scale structure in the Universe forms?

As long as you’re not too large for gravitational collapse to occur — where matter in the Universe can contract all the way down in one or more dimension to a scale where things will go “splat” due to collisions — these tidal torques will cause clumps of matter to pull on one another, inducing a rotation. This means that planets, stars, solar systems, galaxies, and even, in theory, entire cosmic filaments from the cosmic web should, at least sometimes, experience rotational motions. On larger scales, however, there should be no overall rotation, as there are no larger bound structures in the Universe.

This is precisely what the latest study sought to measure, and precisely what they found. For individual filaments, they couldn’t see anything, but when they took thousands of filaments together, the rotational effects clearly showed up.

“By stacking thousands of filaments together and examining the velocity of galaxies perpendicular to the filament’s axis (via their redshift and blueshift), we find that these objects too display vortical motion consistent with rotation, making them the largest objects known to have angular momentum. The strength of the rotation signal is directly dependent on the viewing angle and the dynamical state of the filament. Filament rotation is more clearly detected when viewed edge-on.”

We’ve seen “filament rotation” before: in the filaments that are created in star-forming regions within individual galaxies. But in a surprise to some, even the largest-scale filaments in the Universe, the ones that trace the cosmic web, appear to be rotating as well, at least on average. Their speeds are comparable to the speeds at which galaxies move and stars orbit within the Milky Way: up to

hundreds of kilometers per second. Even though there’s a lot we still have left to unpack about this phenomenon, these large-scale cosmic filaments, which typically extend for hundreds of millions of light-years, are now the largest known rotating structures in the Universe.

Why are they rotating, however? Is it something that can truly be explained by tidal torques and nothing else? The early evidence points to “yes,” as the presence of large masses near the filaments — what cosmologists identify as “haloes” — seems to intensify the rotation. As the authors note, “the more massive the haloes that sit at either end of the filaments, the more rotation is detected,” consistent with gravitational torques inducing these motions. Nevertheless, more study is needed, as temperature and other physics may also play a role.

The big breakthrough is that we’ve finally detected rotation on these unprecedentedly large scales. If all goes well, we’ll not only figure out why, but we’ll be able to predict how quickly each filament that we see ought to be spinning, and for what reason. Until we can predict how every structure in the Universe forms, behaves, and evolves, theoretical astrophysicists will never run out of work to do.