# Why is the observable Universe larger than its age would suggest?

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The age of the Universe is estimated at 13.8 billion years, and current theory states nothing can exceed the speed of light, which can lead to the incorrect conclusion that the universe can't have a radius of more than 13.8 billion light years.

Wikipedia deals with this misconception as follows:

This reasoning would only make sense if the flat, static Minkowski spacetime conception under special relativity were correct. In the real Universe, spacetime is curved in a way that corresponds to the expansion of space, as evidenced by Hubble's law. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance. → Ned Wright, "Why the Light Travel Time Distance should not be used in Press Releases"

That doesn't clear the matter up for me, and having no science or maths background beyond high school, further reading into Hubble's law isn't helping much either.

One layman's explanation I've seen offers explanation that the Universe itself isn't bound by the same laws as things within it. That would make sense - insofar as these things can - but the above quote ("Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance") seems more general than that.

Can anyone offer (or direct me to) a good layman's explanation?

The easiest explanation for why the maximum distance one can see is not simply the product of the speed of light with the age of the universe is because the universe is non-static.

Different things (i.e. matter vs. dark energy) have different effects on the coordinates of the universe, and their influence can change with time.

A good starting point in all of this is to analyze the Hubble parameter, which gives us the Hubble constant at any point in the past or in the future given that we can measure what the universe is currently made of:

$$H(a) = H_{0} sqrt{frac{Omega_{m,0}}{a^{3}} + frac{Omega_{gamma,0}}{a^{4}} + frac{Omega_{k,0}}{a^{2}} + Omega_{Lambda,0}}$$ where the subscripts $m$, $gamma$, $k$, and $Lambda$ on $Omega$ refer to the density parameters of matter (dark and baryonic), radiation (photons, and other relativistic particles), curvature (this only comes into play if the universe globally deviates from being spatially flat; evidence indicates that it is consistent with being flat), and lastly dark energy (which as you'll notice remains a constant regardless of how the dynamics of the universe play out). I should also point out that the $0$ subscript notation means as measured today.

The $a$ in the above Hubble parameter is called the scale factor, which is equal to 1 today and zero at the beginning of the universe. Why do the various components scale differently with $a$? Well, it all depends upon what happens when you increase the size of a box containing the stuff inside. If you have a kilogram of matter inside of a cube 1 meter on a side, and you increase each side to 2 meters, what happens to the density of matter inside of this new cube? It decreases by a factor of 8 (or $2^{3}$). For radiation, you get a similar decrease of $a^{3}$ in number density of particles within it, and also an additional factor of $a$ because of the stretching of its wavelength with the size of the box, giving us $a^{4}$. The density of dark energy remains constant in this same type of thought experiment.

Because different components act differently as the coordinates of the universe change, there are corresponding eras in the universe's history where each component dominates the overall dynamics. It's quite simple to figure out, too. At small scale factor (very early on), the most important component was radiation. The Hubble parameter early on could be very closely approximated by the following expression:

$$H(a) = H_{0} frac{sqrt{Omega_{gamma,0}}}{a^{2}}$$

At around:

$$frac{Omega_{m,0}}{a^{3}} = frac{Omega_{gamma,0}}{a^{4}}$$ $$a = frac{Omega_{gamma,0}}{Omega_{m,0}}$$ we have matter-radiation equality, and from this point onward we now have matter dominating the dynamics of the universe. This can be done once more for matter-dark energy, in which one would find that we are now living in the dark energy dominated phase of the universe. One prediction of living in a phase like this is an acceleration of the coordinates of universe - something which has been confirmed (see: 2011 Nobel Prize in Physics).

So you see, it would a bit more complicating to find the distance to the cosmological horizon than just multiplying the speed of light by the age of the universe. In fact, if you'd like to find this distance (formally known as the comoving distance to the cosmic horizon), you would have to perform the following integral:

$$D_{h} = frac{c}{H_{0}} int_{0}^{z_{e}} frac{mathrm{d}z}{sqrt{Omega_{m,0}(1+z)^{3} + Omega_{Lambda}}}$$

where the emission redshift $z_{e}$ is usually taken to be $sim 1100$, the surface of last scatter. It turns out this is the true horizon we have as observers. Curvature is usually set to zero since our most successful model indicates a flat (or very nearly flat) universe, and radiation is unimportant here since it dominates at a higher redshift. I would also like to point out that this relationship is derived from the Friedmann-Lemaître-Robertson-Walker metric, a metric which includes curvature and expansion. This is something that the Minkowski metric lacks.

In short: things can not move faster that light by theirselves, but they can move faster than light due to universal expansion. The more far away, the faster they go away.

I was just thinking about that and here is my layman's explanation. Imagine you're tracing two dots on a crumpled piece of paper, the dots are moving, but as they are moving, so is the paper getting 'uncrumpled', the actual distance between the dots will be more than the sum of distances they have travelled.

The completely unscientific explanation…

Imagine the universe to be a balloon. Two bodies start close to each other but on opposite surfaces. The expansion of the balloon takes them away from each other at equal speed and such a rate that the light from one at its starting point takes almost the entire history of the universe to reach the other. The distance between the two NOW is not twice the age of the universe - because you cannot travel "through" the balloon - but must instead go round the surface of the balloon… 13.8 * PI billion light years = 43 billion light years.

Not strictly correct, but at least avoids too much worrying about astrophysics and cosmology!

I love Ned Wright's cosmology tutorial and I highly recommend it, but that statement by him is at the least very misleading. Superluminal recession speeds plainly can't be related to spacetime curvature because they don't vanish in the limit of zero curvature (zero energy density or zero $$G$$).

The real reason that distances can be larger than $$c$$ times the current cosmological time is that the clocks that we use to measure cosmological time are not at relative rest, like the clocks in inertial coordinate systems, but are moving radially away from each other, making cosmological coordinates more like polar coordinates. If we have a family of uniformly distributed clocks, and we define $$t$$ to be the reading on the nearest clock and $$x$$ to be (the number of clocks between that one and the origin) × (the separation between adjacent clocks when they both read the same time), then $$Δx/Δtle c$$ is a true statement if those clocks are at relative rest, but not if they're moving outward from a common origin point. In the latter case, there turns out to be no upper limit on $$Δx/Δt$$, even in special relativity.

In the special-relativistic case, you can think of this as being due to time dilation. If you look at two clocks with respect to inertial center-of-velocity coordinates, they move in opposite directions at some speed $$v$$. After an inertial coordinate time $$t$$, they're an inertial coordinate distance $$2vt$$ apart, but the elapsed time they've recorded is smaller than $$t$$ by a factor of $$γ=1/sqrt{1-v^2/c^2}$$. Since $$γ{ o}infty$$ as $$v{ o}c$$, the ratio of the coordinate distance to the elapsed times on the clocks also goes to infinity as $$v{ o}c$$.

In special relativity, there's a tendency to think of inertial coordinate times as the "real" times and readings on clocks as somehow distorted by time dilation, but that's really just a human prejudice. The universe doesn't care about coordinate systems, and it only "cares" about reference frames if they're actually instantiated by physical objects. There are no naturally occurring inertial reference frames at large scales in the real world, but there is a naturally occurring radial reference frame, given by the averaged motion of matter on large scales, or by the crossing points of wavefronts from the cosmic microwave background. The most natural coordinate system for the universe - and the one actually used by cosmologists - is based on that naturally occurring frame, and, as Ned Wright said, when you define distances and times in that way, the distance/time ratio $$c$$ has no special significance.

(Actually, all three of Ned Wright's sentences are correct. The trouble is that when you take them together, they seem to imply that superluminal expansion is related to spacetime curvature, and that isn't correct.)

## Age versus Size of the universe

The only 'size' we can possibly observe is called the particle horizon, essentially how far light could have moved in the age of the universe. Hence, the two are related, but not trivially. They depend on observed parameters and the expansion dynamics of the universe.

The parameters are not completely separate. For one, there are several ways of dating both the age and the size of the universe. From wiki:

Since the universe must be at least as old as the oldest thing in it, there are a number of observations which put a lower limit on the age of the universe these include the temperature of the coolest white dwarfs, which gradually cool as they age, and the dimmest turnoff point of main sequence stars in clusters (lower-mass stars spend a greater amount of time on the main sequence, so the lowest-mass stars that have evolved off of the main sequence set a minimum age).

In addition to this, you can plug several cosmological parameters into an equation to yield the age of the universe. This assumes that our knowledge of each parameter is correct, which may or may not be true. However, current measurements of these parameters yield an age that is in good agreement with other methods, such as the one above.

The size of the universe can be estimated using the cosmic distance ladder: http://en.wikipedia.org/wiki/Cosmic_distance_ladder
This consists of various measurements for objects at different distances.

With few exceptions, distances based on direct measurements are available only out to about a thousand parsecs, which is a modest portion of our own Galaxy. For distances beyond that, measures depend upon physical assumptions, that is, the assertion that one recognizes the object in question, and the class of objects is homogeneous enough that its members can be used for meaningful estimation of distance.

It's important to understand that our estimates of both the age and the size of the universe depend upon the accuracy of our knowledge of the underlying physics. Since both the size and the age of the universe depend on the underlying physics, I don't see this as a case of "the snake biting its own tail" even if the physics underlying the age and size of the universe aren't completely separate. The basic rules are what determine both of them.

## New Paper Suggests Life Could Be Common Across The Universe, Just Not Near Us

The building blocks of life can, and did, spontaneously assemble under the right conditions. That's called spontaneous generation, or abiogenesis. Of course, many of the details remain hidden to us, and we just don't know exactly how it all happened.

Or how frequently it could happen.

The world's religions have different ideas of how life appeared, of course, and they invoke the magical hands of various supernatural deities to explain it all. But those explanations, while colorful tales, leave many of us unsatisfied.

'How did life arise' is one of life's most compelling questions, and one that science continually wrestles with.

Tomonori Totani is one scientist who finds that question compelling. Totani is a professor of Astronomy at the University of Tokyo. He's written a new paper titled Emergence of life in an inflationary universe. It's published in Nature Scientific Reports.

Totani's work leans heavily on a couple concepts. The first is the vast age and size of the Universe, how it's inflated over time, and how likely events are to occur. The second is RNA specifically, how long a chain of nucleotides needs to be in order to "expect a self-replicating activity" as the paper says.

Totani's work, like almost all work on abiogenesis, looks at the basic components of life on Earth: RNA, or ribonucleic acid. DNA sets the rules for how individual life forms take shape, but DNA is much more complex than RNA.

RNA is still more complex, by orders of magnitude, than the raw chemicals and molecules found in space or on the surface of a planet or moon. But its simplicity compared to DNA makes it more likely to occur via abiogenesis.

There's also one theory in evolution saying that although DNA carries the instructions to build an organism, it's RNA that regulates the transcription of DNA sequences. It's called RNA-based evolution, and it says that RNA is subject to Darwinian natural selection, and is also heritable. That's some of the rationale behind looking at RNA vs DNA.

Double stranded RNA. (Supyyyy/Wikimedia/CC By 4.0)

RNA is a chain of chemicals known as nucleotides. Some research shows that a chain of nucleotides needs to be at least 40 to 100 nucleotides long before the self-replicating behaviour called life can exist.

Over time, enough nucleotides can form a chain to meet that length requirement. But the question is, has there been enough time in the life of the Universe? Well, we're here, so the answer must be yes, mustn't it?

But wait. According to a press release announcing this new paper, "… current estimates suggest that magic number of 40 to 100 nucleotides should not have been possible in the volume of space we consider the observable universe."

The key here is the term 'observable universe.'

"However, there is more to the universe than the observable," said Totani. "In contemporary cosmology, it is agreed the universe underwent a period of rapid inflation producing a vast region of expansion beyond the horizon of what we can directly observe. Factoring this greater volume into models of abiogenesis hugely increases the chances of life occurring."

Our Universe came into being during the Big Bang, a single inflation event. According to Totani's paper, our Universe "likely includes more than 10^100 Sun-like stars," whereas the observable Universe only contains about 10 sextillion (10^22) stars.

We know that life has occurred at least once, so it's not out of the question that abiogenesis occurred at least once more, even if the chances are infinitesimally tiny.

According to statistics, the amount of matter in the observable Universe should only be able to produce RNA that is 20 nucleotides long, well under the 40 to 100 number. But because of rapid inflation, much of the Universe is unobservable. It's simply too far away for light emitted since the Big Bang to reach us.

When cosmologists add up the number of stars in the observable Universe with the number of stars in the unobservable Universe, the resulting number is 10^100 Sun-like stars. That means there is much more matter in play, and the abiogenic creation of long enough chains of RNA is not only possible, but probable, or even inevitable.

In his paper, Professor Totani states the basic relationship under investigation. "Here, a quantitative relation is derived between the minimum RNA length/min required to be the first biological polymer, and the universe size necessary to expect the formation of such a long and active RNA by randomly adding monomers."

Is it getting confusing? Here's a hopefully more manageable summary.

The Universe is larger than its observable portion, and likely contains 10^100 Sun-like stars. For the probability of abiotic creation of RNA on an Earth-like planet to equal 1, or unity, then the minimum nucleotide length must be less than about 20 nucleotides, which is much smaller than the initially stated minimum of 40 nucleotides.

But scientists don't think that RNA only 20 nucleotides long can be self-replicating, at least not from our perspective as observers of terrestrial life. As Totani says in his paper, "Therefore, if extraterrestrial organisms of a different origin from those on Earth are discovered in the future, it would imply an unknown mechanism at work to polymerize nucleotides much faster than random statistical processes."

What would that process be?

Who knows, but this is likely an inflection point where people of faith can chime in and say, "Why God, of course."

Totani's work has by no means provided an answer. But like a lot of scientific work, it helps refine the question, and invites other to study it.

"Like many in this field of research, I am driven by curiosity and by big questions," said Totani.

"Combining my recent investigation into RNA chemistry with my long history of cosmology leads me to realize there is a plausible way the universe must have gone from an abiotic (lifeless) state to a biotic one. It's an exciting thought and I hope research can build on this to uncover the origins of life."

## Age of the universe: observable or entire universe?

The way that the age is usually defined is by taking the classical Big Bang model and extrapolating back in time. Go far enough back in time, and the model says there is a singularity. The age is defined from that point.

However, the singularity should not be understood as being a real thing. It's the point where something happens which we don't quite understand. One popular proposal for dealing with the singularity is cosmic inflation. Cosmic inflation pushes back the time scale of the universe by a tiny fraction of a second before the Big Bang singularity. But its nature is such that it hides whatever happened before.

So the best way to understand it, to me, is that an event happened roughly 14 billion years ago which hides the nature of whatever happened before that point. To put it another way, anything that may or may not have happened before roughly 14 billion years ago is outside our observable universe. It's possible some very interesting things happened prior to inflation, or that inflation was the actual start of our universe. We just don't know.

The way that the age is usually defined is by taking the classical Big Bang model and extrapolating back in time. Go far enough back in time, and the model says there is a singularity. The age is defined from that point.

However, the singularity should not be understood as being a real thing. It's the point where something happens which we don't quite understand. One popular proposal for dealing with the singularity is cosmic inflation. Cosmic inflation pushes back the time scale of the universe by a tiny fraction of a second before the Big Bang singularity. But its nature is such that it hides whatever happened before.

So the best way to understand it, to me, is that an event happened roughly 14 billion years ago which hides the nature of whatever happened before that point. To put it another way, anything that may or may not have happened before roughly 14 billion years ago is outside our observable universe. It's possible some very interesting things happened prior to inflation, or that inflation was the actual start of our universe. We just don't know.

The Big Bang singularity comes from a model without inflation. Inflation changes the early model, removing that singularity. It's possible to prove that inflation had to start at some point (it can't be eternal into the past), but the precise timing of that event is hidden.

The model of inflation unambiguously predicts a past singularity (this is distinct from the Big Bang singularity, but related).

One way to understand this past singularity is that as inflation progresses, the universe becomes exponentially more dilute. This is why inflation explains the horizon problem: it makes a region much larger than the observable universe almost perfectly uniform by its nature. If you instead run time the other way, and ask what inflation looks like into the past, then the answer you get is if there are any contents in the universe at all during inflation, even a single photon, will result in a past singularity. Even just a slight uneveness in the inflaton field itself will cause this. Thus you are saddled with two possibilities:
1. Inflation is perfectly finely-tuned, being perfectly uniform and with no other matter (including photons) in the universe.
2. There is a past singularity in the model (at an unknown time).

Generally the first possibility is considered absurd enough to discount entirely, leaving the past singularity. That past singularity has the same general class of possibilities as the Big Bang singularity. One possibility is that there are some unknown physics that resolve the singularity. Another is that there was an event that occurred that isn't captured by simply extrapolating inflation back in time.

This line of argument is why some physicists claim that inflation doesn't solve certain problems it claims to solve. The hope of people advocating for inflation models is that the uncertainties about how inflation began are somehow easier to explain than the Big Bang singularity. This isn't proven, but it's reasonable on the surface: the event that started inflation would have had to occur over a much smaller region of space, lending hope that it's easier to come up with a physical process that will do that. At this point, though, exactly what would explain Inflation's past singularity is speculation.

The model of inflation unambiguously predicts a past singularity (this is distinct from the Big Bang singularity, but related).

One way to understand this past singularity is that as inflation progresses, the universe becomes exponentially more dilute. This is why inflation explains the horizon problem: it makes a region much larger than the observable universe almost perfectly uniform by its nature. If you instead run time the other way, and ask what inflation looks like into the past, then the answer you get is if there are any contents in the universe at all during inflation, even a single photon, will result in a past singularity. Even just a slight uneveness in the inflaton field itself will cause this. Thus you are saddled with two possibilities:
1. Inflation is perfectly finely-tuned, being perfectly uniform and with no other matter (including photons) in the universe.
2. There is a past singularity in the model (at an unknown time).

Generally the first possibility is considered absurd enough to discount entirely, leaving the past singularity. That past singularity has the same general class of possibilities as the Big Bang singularity. One possibility is that there are some unknown physics that resolve the singularity. Another is that there was an event that occurred that isn't captured by simply extrapolating inflation back in time.

This line of argument is why some physicists claim that inflation doesn't solve certain problems it claims to solve. The hope of people advocating for inflation models is that the uncertainties about how inflation began are somehow easier to explain than the Big Bang singularity. This isn't proven, but it's reasonable on the surface: the event that started inflation would have had to occur over a much smaller region of space, lending hope that it's easier to come up with a physical process that will do that. At this point, though, exactly what would explain Inflation's past singularity is speculation.

## How can the observable universe be larger than the age of the universe?

I was reading the Wikipedia article on the universe and it states two things that seem contradictory to me.

The radius of the observable universe is 47 billion light years.

The age of the universe is 13.7 billion years.

How do we observe things that occurred more than 13.7 billion years ago? Here's the page:

The universe is expanding.

The most distant thing we can see is the Cosmic Microwave Background. The light from it traveled 13.7 billion light years to get to us. However, the matter that emitted that light is now 45 billion light years from us. This is because the metric expansion of space can result in the distance between two objects increasing at more than 299,792,458 meters per second.

As the others have said, it's because of the expansion of space itself - a concept that's really hard to grasp if you've never taken a general relativity class (I've taken a class that did GR for the last 3 weeks or so and I don't really understand it)

it's expanding at an accelerated rate. so it's not a continuous rate

This would be true even if it were a constant rate. All that is required is that the object be moving away from us, and it will be farther than its light travel distance when the light gets here.

In the words of Professor Jim Al-Khalili - "Nothing moves through space faster than light, but space itself can stretch at any speed."

The thing is that when an object moves at slower speed rates (non-comparable to light) our classic physics model works pretty fine. But the thing that made Einstein famous was that when experiencing greater speeds (comparable to lightspeed) time is actually perceived different. And not in a biased "I feel like time goes faster" perception but it actually compresses for you along with distances. So the faster you go the shorter the distances you actually see and the less time it actually transcurs, leading to the premise that if you would be able to reach lightspeed (which would be impossible with that model since we would become an infinite-mass object) you wouldn't notice because time would not have been passed for you. That is how can the universe age slower than it "really" is (although this concept makes you doubt about that "really").

## How Many Atoms In The Universe?

When you consider that a dot like this . contains roughly 125 million atoms, you'd expect the total atoms in the observable Universe to be a very large number indeed. Believe it or not, scientists have been working on estimating what this total might be.

The answer they came up with was anything between 10 78 and 10 82 . Remember, the little numbers are how many zeros go after the 1.

Yes, these are very large numbers, but what about those that go a step further. I promised you one that was actually too big to fit into the Universe. Well, read on.

## Universe Might be Bigger and Older than Expected

A projectaiming to create an easier way to measure cosmic distances has instead turnedup surprising evidence that our large and ancient universe might be even biggerand older than previously thought.

Ifaccurate, the finding would be difficult to mesh with current thinking abouthow the universe evolved, one scientist said.

A researchteam led by Alceste Bonanos at the Carnegie Institution of Washington has foundthat the TriangulumGalaxy, also known as M33, is about 15 percent farther away from our ownMilky Way than previously calculated.

Thefinding, which will be detailed in an upcoming issue of AstrophysicalJournal, suggests that the Hubble constant, a number that measures the expansionrate and age of the universe, is actually 15 percent smaller than other studieshave found.

Currently, mostastronomers agree that the value of the Hubble constant is about 71 kilometersper second per megaparsec (a megaparsec is 3.2 million light-years). If thisvalue were smaller by 15 percent, then the universe would be older and biggerby this amount as well.

Scientistsnow estimate the universe to be about 13.7 billionyears old (a figure that has seemed firm since 2003, based on measurementsof radiation leftover from the Big Bang) and about 156 billionlight-years wide.

The newfinding implies that the universe is instead about 15.8 billion years old and about180 billion light-years wide.

A newway to measure distance

Theresearchers reached their surprising conclusion after using a new method theyinvented to calculate intergalactic distances, one that they say is moreprecise and requires fewer steps than standard techniques.

"Wewanted an independent measure of distance--a single step that will one day helpwith measuring darkenergy and other things," said study team member Krzysztof Stanek fromOhio State University.

The newmethod took 10 years to develop and relied on optical and infrared measurementsgathered from telescopes all around the world. The researchers looked at abinary star system in M33 where the stars eclipsed each other every five days.Unlike single stars, the masses ofpaired stars can be preciselycalculated based on their movements. With knowledge of the stars' masses,the researchers could calculate their true luminosities, or how bright theywould appear if they were nearby.

Thedifference between the true luminosity and the observed luminosity gives thedistance between the stars and Earth. The team's results suggested that the starswere about 3 million light-years from Earth--or about half-a-million light-yearsfarther than would be expected using the commonly accepted Hubble constantvalue.

'Notimpossible'

LawrenceKrauss, a professor of astronomy and chair of the Department of Physics at CaseWestern Reserve who was not involved in the study, said the idea of asignificantly reduced Hubble constant would be hard to accommodate.

"Thingsfit right now very well for a Hubble constant of a low 70s," Krauss saidin a telephone interview. "It corresponds very well with the age ofglobular clusters as we've determined them and the age of the universe. Itwould be hard, although not impossible, to change things by 15 percent."

Stanek saidhis team plan to follow up their finding with distance measurements for eitheranother binary star system in M33 or to look for a binary system in another galaxy, perhaps Andromeda.

## We Were Just Thrust Into a New Age in Astronomy

This past fall, a paper was published by a team from the University of Nottingham that claimed our universe harbors two trillion galaxies – ten times more than the previously calculated two hundred billion. But what’s the big deal? And how did we get the number so wrong to begin with?

The findings are pretty significant, although not for the reasons that many people think…

First off, many publications misinterpreted the findings, which is pretty easy to do if you’re not well-versed in cosmology. One article from the Independent went as far as to claim the findings meant the universe was much larger than previously thought. When cosmologists and astrophysicists talk about the universe, what they really mean is the bit of the universe that we can see right now. Only so much light has been able to reach us since the big bang. We call the edge of this light the cosmic horizon. Beyond that? We don’t really know what’s beyond that, but we’re pretty sure that the universe extends far beyond that. But the observable bit of the universe? That’s not going to get bigger anytime soon. In fact, most models predict that the observable universe is going to get smaller as time goes on, as the expansion velocity of galaxies accelerates beyond the speed of light.

And those extra galaxies that we had not counted? They aren’t what we might think of as galaxies in the more modern universe. When I think of a galaxy, I think of the whirlpools of solar systems and interstellar gas, such as our own Milky Way galaxy, or the neighboring Andromeda. But galaxies can be incredibly small. Take, for example, the small satellites to our galaxy. Some of these galaxies can have only tens of millions of stars, and look more like a clump than a spiral with arms. Near the beginning of time, many smaller galaxies existed that hadn’t yet collided to form the massive clouds of star we see today. There isn’t anymore stuff in the universe, it’s just distributed differently than we thought.

So why was the answer so wrong? Well, the best way that we can see galaxies at the beginning of the universe is with the Hubble telescope. Getting some of our deepest views into the universe can take exposure times of days. It’s really hard to get pictures of early galaxies. So right now, the technology just isn’t there. But in 2018, NASA plans to launch the James Webb Space telescope, a much more powerful successor to the Hubble. This telescope will allow us to probe the early universe, view some of the dimmest galaxies, and even take a peek at a few exoplanets. There’s a lot of stuff out there to study, and as telescope technology advances, we’ll get to see more and more of the universe that we live in.

## Universe age = size?

The visible universe is only 10 billion light years across. But there is no reason that there can't be space beyond the distance we can see.

### #4 Mike K

Vendor - Celestial Teapot Designs

I think the currently accepted number is something like 13.7 billion years, and Jarad is correct the universe may be much larger than 13 billion light years across, but there hasn't been enough time for light from that far away to reach us yet.

It would be interesting to come back in a billion years and re-examine areas of sky where we currently see the most distant visible galaxies, looking to see if new ones have appeared that are even farther away.

And, if space is a closed shape with a finite size, one day we may be able to look out and see OURSELVES far in the past, as the light might have made it all the way around the universe and back to us.

### #6 Mike K

Vendor - Celestial Teapot Designs

### #7 HiggsBoson

400,000 years later some 13.3 Billion years ago. Theory suggest that the universe was opaque and at a temperature of 3000K. Our observations of the CMB correspond to a temperature of

3K due to cosmological red shifting as predicted by General Relativity. This suggest that the universe has expanded by a factor of 1000 since emitting the CMB.

I conclude that the universe much be much larger than 27 Billion Light-years across. The problem is not linear so it is not as simple as multipling 27 billion by 1000.

Universe Measured: We're 156 Billion Light-years Wide!
By Robert Roy Britt
Senior Science Writer
posted: 06:30 am ET
24 May 2004

Stretching reality
The universe is about 13.7 billion years old. Light reaching us from the earliest known galaxies has been travelling, therefore, for more than 13 billion years. So one might assume that the radius of the universe is 13.7 billion light-years and that the whole shebang is double that, or 27.4 billion light-years wide.

But the universe has been expanding ever since the beginning of time, when theorists believe it all sprang forth from an infinitely dense point in a Big Bang.

"All the distance covered by the light in the early universe gets increased by the expansion of the universe," explains Neil Cornish, an astrophysicist at Montana State University. "Think of it like compound interest."

Need a visual? Imagine the universe just a million years after it was born, Cornish suggests. A batch of light travels for a year, covering one light-year. "At that time, the universe was about 1,000 times smaller than it is today," he said. "Thus, that one light-year has now stretched to become 1,000 light-years."

All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger.

"It can be thought of as a spherical diameter is the usual sense," Cornish added comfortingly.

Additionally, if the universe was opaque prior to emitting the CMB, then I will not be able to see farther away than this. Ever!

It is also true that there exist a point before the CMB where visible light has been red shifted out of the visible range due to the expansion of space. This light can only be seen in the Infrared. This is the reason the James Webb Telescope is optimized for IR.

### #8 matt

Uh, no Michael. That "point" is much closer to us. When you talk of redshifts above 1, which is relatively close in cosmological terms, you already have a lot of "visible" wavelengths "infra"redshifted, and most of the visible light spectra you get show lines which are normally in the ultra-violet.

And before the CMB, space was opaque to all radiation, not just light visible to out eyes.

### #10 Dave Mason

I was under the impression that there are already parts of the universe moving away from us at an apparent greater than light speed due to the expansion of the universe. The expansion figure I heard was 70km/s per megaparsec of space. At some certain distance away, the universe's expansion appears to be faster than light as the space between there and here expands, and therefore the light from areas beyond there will never reach us, regardless of time spent waiting. This appears to suggest to me that the things we see in the universe will eventually get less as more of it expands past the point of apparent light speed. Maybe I got it all wrong though!

### #11 Qkslvr

We can see light some 13B yr's old in all directions, But in that time, space between mass has expanded iirc by a factor of about 15-20 (z, Redshift). The mass itself isn't moving all that fast, but the space between is expanding at a pretty good clip. According to the above text the visible universe is about 156B light years across.

The CMB shows by redshift to have expanded by z

1,100. So the physical Universe has to be bigger than the 156 Bly we can see. ie the farthest galaxies have a z of about 15-20, while the CMB's is

Matt, If I follow what you're saying, expansion faster than light speed if it happened, had to happen before the Universe cleared, if expansion happened faster than light speed after that time, the cmb would be beyond the edge of the visible (EM Radiation) universe, and we'd never see it.

### #13 Qkslvr

How can the universe even have a diameter? Wouldn't that imply an edge?

The visible Universe has an edge, and I think it's reasonable to say the space enclosed by the cmb has an edge, We can't see beyond the cmb, and so far don't know if it's finite or infinite beyond that point.

If you could only see a short distance into the plasma before clearing, say 100'. I think it's reasonable to say the cmb we see is the same 200' diameter sphere inflated to it's current size. If our space represents a mear 200' diameter sphere of early plasma, imagining that plasma might have been solar system or galaxtic in size the amount of Universe beyound the cmb would be very very large even if finite.

### #14 llanitedave

As I understand it, space can expand at faster than light speed. However, objects moving through space must travel at less than light speed.

This implies to me, among other things, that space is actually a viscous medium, in certain respects, and can apply drag to objects that are moving fast enough.

### #15 Qkslvr

As I understand it, space can expand at faster than light speed. However, objects moving through space must travel at less than light speed.

I know there are some energetic events that are ejecting jupiter sized chunks of mass at a lot of nines % speed of light (.99999. ), If we can find single jets at that speed, what if we find one that has 2 opposite jets at that speed? That'd be just under twice the speed of light between the 2 ends.

I'm not sure if there's any point of view that's allowed to exceeds C in special relativity is there?

### #16 matt

Matt, If I follow what you're saying, expansion faster than light speed if it happened, had to happen before the Universe cleared, if expansion happened faster than light speed after that time, the cmb would be beyond the edge of the visible (EM Radiation) universe, and we'd never see it.

Well, in the fractions of a second after the big bang, the universe expanded faster than c. It was called inflation (I was not there, but smart people agree on that), but that has little to do with it.

The CMB was emitted everywhere at the same time it permeates the universe, it does not just lie on the edge of what we perceive as the visible universe. I might try an analogy: the universe is a volume of gas in a tank of some sort. If you expand the tank's volume, the gas will cool as a result of the lowered pressure, and everywhere at once.

### #17 matt

I know there are some energetic events that are ejecting jupiter sized chunks of mass at a lot of nines % speed of light (.99999. ), If we can find single jets at that speed, what if we find one that has 2 opposite jets at that speed? That'd be just under twice the speed of light between the 2 ends.

I'm not sure if there's any point of view that's allowed to exceeds C in special relativity is there?

### #18 Qkslvr

Matt, If I follow what you're saying, expansion faster than light speed if it happened, had to happen before the Universe cleared, if expansion happened faster than light speed after that time, the cmb would be beyond the edge of the visible (EM Radiation) universe, and we'd never see it.

Well, in the fractions of a second after the big bang, the universe expanded faster than c. It was called inflation (I was not there, but smart people agree on that), but that has little to do with it.

The CMB was emitted everywhere at the same time it permeates the universe, it does not just lie on the edge of what we perceive as the visible universe. I might try an analogy: the universe is a volume of gas in a tank of some sort. If you expand the tank's volume, the gas will cool as a result of the lowered pressure, and everywhere at once.

I agree, but when it goes from opaque to clear at t=0, When t = 1 year, disregarding expansion you can only see 1 light year in each direction, and what you see is the plasma expanding away from you, and redshifting, 13 some billion years later we have the cmb. The decoupled light for however far we could see into the plasma, is the cmb.

### #19 Qkslvr

I know there are some energetic events that are ejecting jupiter sized chunks of mass at a lot of nines % speed of light (.99999. ), If we can find single jets at that speed, what if we find one that has 2 opposite jets at that speed? That'd be just under twice the speed of light between the 2 ends.

I'm not sure if there's any point of view that's allowed to exceeds C in special relativity is there?

What does the third guy watching from a far but about in the middle see?

### #20 sergius64

I know there are some energetic events that are ejecting jupiter sized chunks of mass at a lot of nines % speed of light (.99999. ), If we can find single jets at that speed, what if we find one that has 2 opposite jets at that speed? That'd be just under twice the speed of light between the 2 ends.

I'm not sure if there's any point of view that's allowed to exceeds C in special relativity is there?

What does the third guy watching from a far but about in the middle see?

Think he just sees them both moving at near c.

### #21 Mike K

Vendor - Celestial Teapot Designs

Matt, If I follow what you're saying, expansion faster than light speed if it happened, had to happen before the Universe cleared, if expansion happened faster than light speed after that time, the cmb would be beyond the edge of the visible (EM Radiation) universe, and we'd never see it.

You're assuming that the CMB radiated out from a single point. The CMB was being radiated from hot plasma at every point within the early universe, so you're always seeing that portion that is just now arriving at our location from the edge of the currently visible universe.

Mike, if the universe is 13.7 billion years old, then surely it won't be 27.4 across. I thought that due to relativity then 2 particles could not go away from each other at over light speed.

Relativity puts a limit on how fast matter can accelerate. It says nothing about how fast the space in between that matter can expand. Space itself is not comprised of matter, so it is not bound by E=MC^2.

### #22 LesB

Must admit to some confusion. Space can expand faster than light. The space between us and the object in question can expand faster than light. But space is the matrix that holds matter and matter cannot exceed c. So if space is expanding faster than c then how does matter not move with it since matter forms the discontinuities in space via gravity?

Is there a gravitational wind?

### #23 HiggsBoson

Uh, no Michael. That "point" is much closer to us.
And before the CMB, space was opaque to all radiation, not just light visible to out eyes.

### #24 HiggsBoson

This thread has raised so many issues that I hesitate to address them sequentially.

‘c’ is a fundamental constant of the universe. Light and gravity may not propagate at any other speed and matter my not achieve this speed. It is helpful to make this point by calling it ‘c’ rather than the speed of light.

Space and time are two different aspects of a single entity. Space-time, singular, is a thing. It has properties and it is dynamic. It stretches and curves. Its properties are described in General Relativity, the current theory of gravitation.

The properties of space-time limits the speed of things with non-zero rest mass, including all forms of matter, to less than c. Things with zero rest mass, including light and gravity, must propagate at c.

The two particles moving an .9999c away from a star in opposite directions will be observed traveling at a velocity less than c relative to each other. This is a basic result of Special Relativity. Simply adding the velocities will yield the wrong answer. If this is new information, an over view of Special Relativity is required. It also suggest that the issues surrounding the expansion of Space-time will also be confusing. This is a prediction of General Relativity.

The expansion of the universe observed by Hubble is due to the expansion of space-time not movement of mass through space. For this reason the rate at which an object may be receding away from the earth is not limited by c and does not violate Relativity.

Just after Time=zero. Space-time appears to have expanded at truly profound rates. Clearly much faster than c. It appears that the rate slowed to a very slow rate of expansion. Observations now suggest that the rate of expansion is increasing. This is what gives rise to the dark energy postulate.

Does the universe have a diameter? Real answer, Insufficient data. Practically, when we say “The Universe” sometimes we mean ‘the observable universe’. The observable universe has a diameter.

The CMB is not the edge of anything physical. This is simply the light that was at the correct distance at the time of the CMB such that it is arriving here today. If we were 11 billions light-years away we would still see a CMB 13.4 billion light-years away.

Spatial Expansion: A 2 Dimensional analogy.

Consider two bugs that live on the surface of a large spherical balloon. Each bug knows that it is 50 paces from one’s home to the other and it is 100 paces from either to the nearest grocery store. The bugs have no knowledge of the interior of the balloon and they can not fly. They live in a two dimensional universe.

One day one bug notices that it is 105 paces to the store and 106 paces to walk home from the store. The bug concludes that his universe is expanding. The may not know why or in to what, recall the bug has no concept of 3 dimensions. The expansion is inexplicable and unimaginable to him. But the expansion is real. He can see the additional effort required to reach his friend’s home and his friend has noticed the extra steps as well. This is similar to our situation. One of the bugs make a postulate that the world is a sphere in some strange 3rd dimension and that someone is adding dark gas to the sphere.

Due to the curvature of their world it is possible to walk in any direction and return to the start point without a change in direction. This is easy for us earth travelers to imagine but this has not always been the case. Once we know the geometry it is easy to the point of being obvious. Without the knowledge we are stuck with ‘dark gas’ postulates.

## Ask Ethan: How Big Was The Universe When It Was First Born?

Image credit: NASA, ESA, R. Windhorst, S. Cohen, and M. Mechtley (ASU), R. O’Connell (UVa), P. . [+] McCarthy (Carnegie Obs), N. Hathi (UC Riverside), R. Ryan (UC Davis), & H. Yan (tOSU).

You might think of the Universe as infinite, and quite honestly, it might truly be infinite, but we don't think we'll ever know for sure. Thanks to the Big Bang -- the fact that the Universe had a birthday, or that we can only go back a finite amount of time -- and the fact that the speed of light is finite, we're limited in how much of the Universe we can see. By time you get to today, the observable Universe, at 13.8 billion years old, extends for 46.1 billion light years in all directions from us. So how big was it all the way back then, some 13.8 billion years ago? Joe Muscarella wants to know:

I have read very different explanations about the size of the universe immediately after cosmic inflation ended. One source says it was about 0.77 centimeters, another says about the size of a soccer ball, while yet another says larger than the size of the observable universe. So which is it, or is it something else in between?

It's been a very good year for questions about Einstein and the nature of space and time since this is the 100th anniversary of General Relativity, that's quite fitting. Let's start by talking about the Universe we can see.

Image credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute.

When we look out at the distant galaxies, as far as our telescopes can view, there are some things that are easy to measure, including:

• what its redshift is, or how much its light has shifted from an inertial frame-of-rest,
• how bright it appears to be, or how much light we can measure from the object at our great distance,

These are very important, because if we know what the speed of light is (one of the few things we know exactly), and how intrinsically either bright or big the object we're looking at is (which we think we know more in a second), then we can use this information all together to know how far away any object actually is.

Image credit: NASA/JPL-Caltech.

In reality, we can only make estimates of how bright or big an object truly is, because there are assumptions that go into this. If you see a supernova go off in a distant galaxy, you assume that you know how intrinsically bright that supernova was based on the nearby supernovae that you've seen, but you also assume that the environments in which that supernova went off was similar, the supernova itself was similar, and that there was nothing in between you and the supernova that changed the signal you're receiving. Astronomers call these three classes effects evolution (if older/more distant objects are intrinsically different), environmental (if the locations of these objects differ significantly from where we think they are) and extinction (if something like dust blocks the light) effects, in addition to the effects we may not even know are at play.

Image credit: Sloan Digital Sky Survey (SDSS), including the current depth of the survey.

But if we're right about the intrinsic brightness (or size) of an object we see, then based on a simple brightness/distance relation, we can determine how far away those objects are. Moreover, by measuring their redshifts, we can learn how much the Universe has expanded over the time the light has traveled to us. And because there's a very well-specified relationship between matter-and-energy and space-and-time -- the exact thing Einstein's General Relativity gives us -- we can use this information to figure out all the different combinations of all the different forms of matter-and-energy present in the Universe today.

If you know what your Universe is made out of, which is:

• 0.01% — Radiation (photons)
• 0.1% — Neutrinos (massive, but

you can use this information to extrapolate backwards in time to any point in the Universe's past, and find out both what the different mixes of energy density were back then, as well as how big it was at any point along the way.

So for you, Joe, I went and did these things. (And plotted them on logarithmic scales, where they're more informative.)

Image credit: E. Siegel, of the different energy components in the Universe at different times.

As you can see, dark energy may be important today, but this is a very recent development. For most of the first 9 billion years of the Universe's history, matter -- in the combined form of normal and dark matter -- was the dominant component of the Universe. But for the first few thousand years, radiation (in the form of photons and neutrinos) was even more important than matter!

I bring these up because these different components, radiation, matter and dark energy, all affect the expansion of the Universe differently. Even though we know that the Universe is 46.1 billion light years in any direction today, we need to know the exact combination of what we have at each epoch in the past to calculate how big it was at any given time. Here's what that looks like.

Image credit: E. Siegel, of the size of the Universe (in light years) vs. the age of the Universe . [+] (in years).

Here are some fun milestones, going back in time, that you may appreciate:

• The diameter of the Milky Way is 100,000 light years the observable Universe had this as its radius when it was approximately 3 years old.
• When the Universe was one year old, it was much hotter and denser than it is now. The mean temperature of the Universe was more than 2 million Kelvin.
• When the Universe was one second old, it was too hot to form stable nuclei protons and neutrons were in a sea of hot plasma. Also, the entire observable Universe would have a radius that, if we drew it around the Sun today, would enclose just the seven nearest star systems, with the farthest being Ross 154.
• The Universe was once just the radius of the Earth-to-the-Sun, which happened when the Universe was about a trillionth (10 -12 ) of a second old. The expansion rate of the Universe back then was 10 29 times what it is today.

If we want to, we can go back even farther, of course, to when inflation first came to an end, giving rise to the hot Big Bang. We like to extrapolate our Universe back to a singularity, but inflation takes the need for that completely away. Instead, it replaces it with a period of exponential expansion of indeterminate length to the past, and it comes to an end by giving rise to a hot, dense, expanding state we identify as the start of the Universe we know. We are connected to the last tiny fraction of a second of inflation, somewhere between 10 -30 and 10 -35 seconds worth of inflation. Whenever that time happens to be, where inflation ends and the Big Bang begins, that's when we need to know the size of the Universe.

Image credit: NASA / WMAP science team. This is slightly out-of-date the Universe is 13.8, not 13.7 . [+] billion years old.

Again, this is the observable Universe the true "size of the Universe" is surely much bigger than what we can see, but we don't know by how much. Our best limits, from the Sloan Digital Sky Survey and the Planck satellite, tell us that if the Universe does curve back in on itself and close, the part we can see is so indistinguishable from "uncurved" that it much be at least 250 times the radius of the observable part.

In truth, it might even be infinite in extent, as whatever the Universe did in the early stages of inflation is unknowable to us, with everything but the last tiny fraction-of-a-second of inflation's history being wiped clean from what we can observe by the nature of inflation itself. But if we're talking about the observable Universe, and we know we're only able to access somewhere between the last 10 -30 and 10 -35 seconds of inflation before the Big Bang happens, then we know the observable Universe is between 17 centimeters (for the 10 -35 second version) and 168 meters (for the 10 -30 second version) in size at the start of the hot, dense state we call the Big Bang.

Image credit: U.S. Marine Corps photo by Gunnery Sgt. Chago Zapata.

The 17 centimeters answer, by the way, is about the size of a soccer ball! So if you just wanted to know which of those estimates was closest to right, based on what we know, go with that one. The less-than-one-centimeter estimate is too small we have constraints from the cosmic microwave background that inflation couldn't have ended at energies that high, meaning that a size for the Universe at the start of the "bang" is ruled out. The larger-than-the-Universe-today version must be talking about the unobservable Universe, which is probably right, but which doesn't offer any hopes of being measured in any foreseeable way.

So how big was the Universe when it was first born? If the best models of inflation are right, somewhere between the size of a human head and a skyscraper-filled city block. Just give it time -- 13.8 billion years in our case -- and you wind up with the entire Universe.

Joe Muscarella (and all previous winners who haven't contacted me yet), you have orders to get in touch with me with your address, because you just won a Year In Space 2016 Calendar! Happy holidays to all the lucky winners!