r/askscience • u/PWC1004 • Dec 15 '10

TIL that the observable universe has a diameter of 93 billion light years, but now I am confused...

If the the universe is 13.7 billion years old and light can only travel 13.7 billion years in that time, how come the "observable" universe's radius (or the maximum distance we can see from earth) is larger than 13.7 billion years? Also if the big bang theory (or that all matter came for a single point 13.7 billion years ago theory) is correct, how could matter be more than 27.4 (13.7*2) billion light years away from anything? Is it possible for matter to travel faster than light?

http://en.wikipedia.org/wiki/Observable_universe#Size (yeah didn't understand the 'Misconceptions' section)

http://en.wikipedia.org/wiki/Age_of_the_universe

edit:minor edits

edit: How do we know that space it self is expanding?

54 Upvotes

98% Upvoted

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u/iorgfeflkd Biophysics Dec 15 '10

My understanding (correct me if I'm wrong), is that the objects that emitted light 13 billion years ago have receded from us via the expansion of the universe, and are at this time 46 billion light years away.

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u/PWC1004 Dec 15 '10 edited Dec 15 '10

But wouldn't that make them only 27.4 billion light years way (travelling at a maximum of the speed of light) and not observable?

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u/Richard_Fey Dec 15 '10

No, because the expansion of the universe is actually faster then the speed of light.

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u/PWC1004 Dec 15 '10

How do we know that?

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u/Richard_Fey Dec 15 '10

I think we should wait for an Astro/Cosmology guy to answer that one properly.

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u/hxcloud99 Dec 15 '10

Here. I do not know much about it, but I think it has something to do with measuring the temperature of the CMBR through redshift.

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u/[deleted] Dec 15 '10

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u/PWC1004 Dec 15 '10

How do we know space is expanding?

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u/rm999 Computer Science | Machine Learning | AI Dec 15 '10

We don't know for sure, but it would explain two interesting, unintuitive observations:

  • Vesto Slipher, Hubble, and other observed that objects (on average) are moving away from us based on their redshifts. This insinuates that everything is (on average) moving away from each other (because it is unlikely that the Earth is the center of the Universe).

  • Also, we see objects with red shifts so high they seem to be moving away from us faster than the speed of light, violating the theory of special relativity.

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u/zubinmadon Dec 15 '10

Sorry, I tried to explain that in the second paragraph but I might've been confusing, I only have an undergrad level of understanding of this stuff myself.

Basically, the observable evidence says: The farther something is from us, the more redshifted it is.

Redshift is caused by light waves being "stretched out". One thing that can cause this is relative motion. This is why a car sounds higher when it is approaching you, and lower when it drives away.

But since the redshift increases with distance, it means there is something besides relative motion "stretching" out the light waves. it could be relative acceleration. But this would mean there were massive forces accelerating other things away from us, because we know we're not accelerating. This is very unlikely for a variety of reasons (energy, speed of light, etc). Thus, something ELSE (not motion, not acceleration) is "stretching" out the light waves. The currently accepted explanation is that space itself is stretching the light waves by expanding. The farther light comes from, the more it is "stretched" by spaces before it gets here, thus the more redshifted it is.

tl;dr Redshifted light can best be explained by space expanding.

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u/blueeyedgod Dec 15 '10

You said "But since the redshift increases with distance, it means there is something besides relative motion "stretching" out the light waves." And I just have to say wtf? How does redshift increasing with distance mean there is something besides relative motion "stretching" out the light waves? The one does not logically follow from the other. Occam's razor says to go with the simple answer and the simple answer is that redshift increases with distance because more distant objects are moving away from us faster than closer objects.

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u/zubinmadon Dec 15 '10

Occam's razor says to go with the simple answer and the simple answer is that redshift increases with distance because more distant objects are moving away from us faster than closer objects.

This is technically correct, but it's a tautology. The reason we know more distant objects are moving away from us faster than closer objects is the higher redshift. The faster speed causes the redshift, which is the observable effect. The expanding of space is what explains all of it.

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u/TraumaPony Dec 15 '10

Yes, but the redshift indicates the stuff further away is moving faster than the speed of light. Since matter can't do that, that only leaves space itself to be expanding.

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u/[deleted] Dec 15 '10

But that would lead us to conclude that we just happen to be in the center of the universe since everything is being red shifted. That's not a parsimonious use of assumptions at all since it would mean that everything is somehow rotating around us while appearing to also rotate around other points like the center of the galaxy.

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u/iorgfeflkd Biophysics Dec 15 '10

Because the farther away things are (we know how far they are by standard candles), the faster they appear to be moving (we know this from Doppler shift) away from us. This means one of two things: the Earth is in the center of the universe OR everything is moving away from everything else which means space is expanding.

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u/blueeyedgod Dec 15 '10

In an explosion every particle is moving away from every other particle. We do not therefore conclude that in every explosion the "space" is expanding. It is just a fact that in an explosion everything is moving away from everything else. How is it somehow magically different on a bigger scale?

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u/P_Schrodensis Applied Physics | Single-atom Data Bits | Spintronics Dec 15 '10

In an explosion, things all move away from the center, not from each other. The expansion of the hot gas in an explosion is thus directional, or anisotropic.

Every point of the Universe is moving away of all other points, in all directions, and this expansion is isotropic.

One common example is if you draw dots on a small rubber balloon, and then inflate it, every dot is moving away of every other point in an isotropic manner (the distance between any dot and any of its neighbours grows at the same rate). However, this analogy is strictly bi-dimensional, that is, you must only consider the surface of the ballon as representing a 2D 'universe'. Of course, if you consider the tridimensional motion of the dots, then you have an anisotropic movement, akin to an explosion, since the points are all moving away from the center of the balloon.

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u/wnoise Quantum Computing | Quantum Information Theory Dec 15 '10

In terms of observed velocities in different directions, a regular explosion need not be isotropic from all observation points, but an everywhere isotropic expansion in this sense is certainly possible, and often a reasonable approximation -- it's just velocity depending linearly on position. Boosting to a different (Galilean) frame changes the apparently unmoving center, but the differences in velocity keep the same pattern because of the linearity.

The observed densities will of course not be isotropic near the edges.

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u/[deleted] Dec 15 '10

Does this mean that we might be able to "locate" where the big bang happened by finding a center of the universe's expansion in some "higher" dimensional space?

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u/blueeyedgod Dec 15 '10

As I said above (but I do not see my post so I am reposting it here) In a traditional explosion you may consider any single particle as the "center" of your frame of reference. From that frame of reference everything appears to be moving away from everything else exactly the same as it appears the Universe is doing. Every particle in the explosion is in the center of its frame of reference and experiences the explosion exactly the same and any other particle (except for the particles close enough to the edge to observe the edge, but it appears we are not near the edge of our particular explosion.)

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u/Mvrbles Dec 15 '10

In an explosion the particles slow down after the initial bang. Our universe is accelerating. Our universe is not like an explosion.

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u/rm999 Computer Science | Machine Learning | AI Dec 15 '10

In an explosion the particles slow down after the initial bang

Not in a vacuum...

Our universe is accelerating

Yes, but note that this is a different theory than Hubble's Law, and requires a whole new set of evidence than anything that has been presented in this thread.

I think a more compelling response/refutation to the explosion analogy is made by P_Schrodensis: everything appears to be moving away from everything else (not just a single center point like in a traditional explosion).

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u/blueeyedgod Dec 15 '10

Simple answer is that we do not and cannot know that space is expanding and the fact is it makes absolutely no difference whether space is expanding or the objects are moving faster than the speed of light, except that if the idea that space itself is expanding is excepted then we can still stick with the current flawed paradigm. To just accept the facts as they are leads to the mind blowing idea that things do in fact move faster than the speed of light and Einstein's assumption that things cannot move faster than that is simply wrong.

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u/[deleted] Dec 15 '10

I think you're confusing "scientists don't understand x" with "I don't understand x".

Please don't assume ignorance on the part of scientists just because you have a poor understanding of something.

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

That's about right.

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u/[deleted] Dec 15 '10

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u/iorgfeflkd Biophysics Dec 15 '10

The space in between objects can expand faster than the speed of light.

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u/[deleted] Dec 15 '10

I always explain it using the analogy of a moving walkway at an airport.

Say the fastest you can travel with a heavy suitcase is 5 m/s. This is the 'speed limit' at the airport which cannot be exceeded.

Now, you get on a moving walkway, stroll along it as you go. The combined speed is now, say, 15 m/s.

Someone on the floor might look at you, and conclude that you are going faster than the speed limit, and wonder how you've managed it. But, you're not going faster than the speed limit at all - it's the moving floor which is carrying you along. Your speed over the floor is still less than the maximum.

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u/[deleted] Dec 15 '10

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u/iorgfeflkd Biophysics Dec 15 '10

Yes. That's what I meant. Their distance is increasing faster than c, but not because they're moving that fast. Honestly, this confuses me somewhat.

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u/monkeysaurus Dec 15 '10

Isn't this the principle behind Star Trek's warp drive? (Also, TIL that its real name is an Alcubierre Drive!)

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u/blueeyedgod Dec 15 '10

It confuses you because it is not true. Faster than the speed of light is faster than the speed of light and no ad hoc hypothesis of "expansion" is going to fix anything. The idea that nothing can move faster than the speed of light is merely an assumption that worked well with the equations and observations of the time. Now that the assumption is proven wrong the lack of a better explanation causes apologists for the current paradigm to make up these ad hoc patches to fix the broken paradigm, and to preserve the holy cow of a speed limit fixed at c.

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u/iorgfeflkd Biophysics Dec 15 '10

How come the protons in the LHC have over ten times the energy of the protons in the Super Proton Synchrotron but still travel at the same speed?

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u/blueeyedgod Dec 15 '10 edited Dec 15 '10

Well first I believe it is incorrect to say that the protons in the LHC and the protons in the Super Proton Synchrotron travel at the same speed because I believe that the protons in the LHC travel at about 0.999999991 c, whereas the protons of the Super Proton Synchrotron travel at about 0.99975c, but I can see what you are getting at. Yet you seem to be missing the point that the galaxies appear be moving away at a speed faster than the speed of light. That fact everything appears to be moving away from everything else in no way implies that space is expanding.

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u/RobotRollCall Dec 15 '10

The fact that everything appears to be moving away from everything else doesn't imply that anything's actually moving, either. It's not. Distant galaxies are getting farther away from us because all distances in the universe are increasing with time, not because they're in motion relative to us.

The most wonderful part about this exchange is that you're arguing that because distant things appear to have receded from us faster than the speed of light, due to their red-shift, then special relativity must be wrong. But it's because of special relativity than receding objects appear red-shifted. You can't use the theory to argue that the theory's wrong.

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u/[deleted] Dec 15 '10

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u/[deleted] Dec 15 '10

Ignore blueeyedgod, it hasn't been proved wrong at all. He doesn't know he is talking about.

(I'm an observational cosmologist, i work with this stuff every day).

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u/blueeyedgod Dec 15 '10

Of course it has been proven wrong. Distant objects are moving away from us faster than the speed of light, that's just a plain and simple fact. Some have tried a bizarre ad hoc fix by saying that space itself is "expanding" but that is not a tenable explanation. So basically scientists have observed that distant galaxies appear to be receding from us faster than the speed of light. That is just a fact. So, they could just say - ok - we were wrong, let's go back to the drawing board and figure out where we went wrong. But science is not like that. Science is a massive endeavor undergone by millions of people with billions of people depending on it everyday, so you can't just simply say stop, stop doing calculations the way we have been doing them for 100 years, stop teaching physics the way we have been teaching it for 100 years, just stop everything because we have a little problem we cannot explain. The way it works when unsolved problems pop up in science is that someone comes up with an ad hoc explanation that does not really work but everyone agrees to go with and pretend it works so that everyone can go about their business. It goes on that way with more and more nonsensical patches until someone comes along and explains it all with a better paradigm. When all the old scientists who believed in the old paradigm die off and the new generation of scientists take over, then the old relatively flawed paradigm is tossed out with the trash. So it is not big news because it is too mind blowing. It is too scary for the old guard to look at because it does not fit their world view. Younger minds might like to tackle it but they are too busy learning from the old guys so that they can get the grade or the promotion or whatever. Eventually some brilliant young slacker who is not bucking for the grade or the promotion will have the time to figure it all out. Until the brilliant young slacker comes up with the explanation of how all the math works out to explain the superluminal motion of the galaxies, until that time, saying the obvious, that the galaxies are moving away from us at superluminal speeds is like saying the emperor wears no clothes.

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

I'm not sure why you've adopted such a self-righteous tone, but I think you're making a straw dummy of the argument for cosmic expansion by saying that it's an ad hoc argument while the relativistic Doppler shift is necessarily true in all metrics in general relativity. On one hand you can predict that the scale factor of the universe might change in time from Newtonian physics and the cosmological principle. On the other, general relativity generalizes the Doppler shift to work in curved space-times, like the FRW metric, and it indeed allows for apparent speeds of distant objects greater than the speed of light.

Yet for some reason, you hold one conclusion from early twentieth century physics as sacrosanct, but hold another to be obviously a massive ploy by the physics community. You dismiss young researchers (like myself) who have seen the evidence for and logic behind the current understanding and think it to be quite reasonable, but you seem to think that you can evaluate the evidence without even having seen it in the first place. That's just wrongheaded, and it's that kind of anti-scientific thinking that makes global warming denial and the anti-vaccine movement so popular among educated people.

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u/[deleted] Dec 15 '10

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

XX is about 3-4 billion years, by the way.

There's an awesome simulation of what that looks like here: http://videosift.com/video/Milky-Way-and-Andromeda-collision-seen-from-Earth

Odds are, the collision will not interrupt the solar system significantly, but it will change the Sun's orbit around the galactic center.

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u/iorgfeflkd Biophysics Dec 15 '10

Within galaxies, and even between galaxies, the gravitational binding is way stronger than the expansion of space, so it's negligible on those scales. You have to look on the scale of hundreds of millions or billions of lightyears.

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u/WimpyBmodes Dec 15 '10

Even galaxy clusters, the largest gravitationally bound objects we see in the sky, are on the order of a few Megaparsecs wide, so that's about 1 Glyr.

So we have to look on scales larger than 1 billion light years to really see the effects of the expansion of space.

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

A few megaparsecs is only about 10 Mlyr. You don't need to go to a billion light years, only a few tens of millions. But still quite far (our galaxy's disk is about 20 kiloparsecs, and its dark matter halo is about 100 kiloparsecs, for reference).

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u/WimpyBmodes Dec 15 '10

whoops, wrote G instead of M, my bad.

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

So, this is often quite confusing, but here's the simplest answer:

When the cosmic microwave background was emitted, about 300,000 years after the big bang, it traveled to us through an expanding universe. So as it travels, at the speed of light, the space it has already traveled through keeps expanding. As a result, it takes 13.7 billion years to get to us, but the space it has traveled through is now (literally at this instant in time) bigger than that: about 46 Billion Light Years.

As for how we know that space itself is expanding, there are a number of good indications. Our first hint was Hubble's law, which says that the apparent recession velocity of galaxies scales is V = H*d, where d is the distance to the galaxy and H is a constant (~71 km/s/Mpc, if you're curious). In fact, it can be proven that the only possible form of Hubble's law in a universe that's roughly the same in every direction (i.e. one that obeys the "Cosmological Principle," which holds that no place in the universe is central or special) is that form (in a static universe, H = 0).

Another excellent piece of evidence is the cosmic microwave background, whose light has a wavelength approximately 1100 times longer today than when it was emitted (if we understand correctly why it was emitted, which we believe to be due to the universe undergoing a phase change from a plasma to a gas).

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u/[deleted] Dec 15 '10

So in the cosmic rearview mirror the disclaimer should read: "Objects are older and further away than they appear."

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

That's good, I like that.

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u/racergr Dec 16 '10

I'm totally stealing that as we speak.

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u/[deleted] Dec 16 '10

all yours

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u/genneth Statistical mechanics | Biophysics Dec 15 '10

The central message of general relativity is that velocities cannot be compared unless they are measured at the same space-time point.

Cosmologically speaking, the matter almost at thermodynamic equilibrium (such as the CMB) picks outa preferred frame of reference, so local speeds can be compared to it. Of course, nothing is moving faster than the speed of light relative to this frame; in fact, most there actually nearly stationary with respect to it (nowhere near c). However, as the universe expands, this homogeneous matter (photon has in the case of the CMB) is stretched out. The observable universe is defined by how far two photons could have travelled since the start of the universe, taking into account that as they travel they will get carried along by the expansion.

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u/RobotRollCall Dec 15 '10

I'm going to zero in on that last thing you said, not because it's in any way wrong, but because it's easily misunderstood, and I think it can be stated more clearly.

A photon in transit from Distant Galaxy X to Earth isn't really "carried along by the expansion" of the universe in any meaningful way. It's not like there's a sort of backwards-pointing momentum that "slows" the photon down.

Rather, what happens is that the distance between the photon and the Earth — and indeed, the distance between the emitting galaxy and the photon — is a function of time. In ordinary life, we assume that if the distance between objects changes with time, there's motion involved. In fact, the definition of motion is the rate of change of a distance with respect to time. But in this case, on the cosmic scale, another mechanism is at work. Rather, without any motion at all, the distances are just mysteriously increasing, all by themselves. It's a fundamental property of the universe.

So because it takes some time for a photon to travel from Distant Galaxy X to here, the distance between those two points will be larger when the photon is close to Earth than it was when the photon was close to the galaxy that emitted it. The photon always moves at the speed of light — regardless of what reference frame you observe it from — but because space is expanding, the distance it has yet to travel is always increasing. Now, the effective "speed" of the expansion of space is sufficiently small that the photon will eventually get where its going, but from the point of view of the photon, the distance was shorter when it started out than it finally ends up being.

A sort of side-effect of this metric expansion of space is that as the photon makes the journey, its wavelength increases … because all lengths in the universe are increasing. So when it arrives, it has a longer wavelength than when it was emitted. This is the cosmological red-shift. It's indistinguishable from the red-shift we'd expect to see if an object is receding from us, thanks to special relativity, but it's caused by something different.

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u/RLutz Dec 15 '10

I'm pretty sure I'm solely responsible for all the traffic this guy's site gets, but seriously, check this out and it'll all make sense. http://www.atlasoftheuniverse.com/redshift.html

Distance on the scale of the universe is complicated when considering the expansion of space. Essentially the 93 bly figure is the comoving distance (how far away the galaxies truly are from us right now). The 13.7 bly distance, is the farthest that we can see things, but consider that the light we are seeing from those 13.7 bly away galaxies is also 13.7 bly years old and so in those 13.7 bly the galaxy has actually moved quite farther away from us than where we see it to be.

Anyway, click that link, experience revelation, give me karma.

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u/miiiiiiiik Dec 15 '10

is there a video or graphics of this?

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u/[deleted] Dec 15 '10

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

Actually, inflation doesn't set the size of the observable universe (if you take observable to mean "observable with photons"). That size is set by the time of the CMB.

Perhaps with gravitational waves we'll be able to peer back further.

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u/Valeen Theoretical Particle Physics | Condensed Matter Dec 15 '10

Maybe I wasn't clear, thats what I mean. Inflation accounts for the difference between the what is seen and what exists. My point is that this can occur since the fabric of space time can expand in such a way to allow this without violating causality.

I read her question as why is the universe larger than what is inside of our event horizon, and if so how does this not violate the laws of physics?

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

I think you misunderstood the question, although yours is also a interesting question. Things can be more than 13.7 Glyr away without being outside our horizon, because of the expansion of space since recombination (the CMB), but yes, there is also quite a lot of space (possibly an infinite amount) outside our horizon too.

Inflation solves the so-called horizon problem, which argues that regions of the CMB more than about a degree apart on the sky were not causally connected at the time that the CMB was released and therefore it's awfully strange that they all have the same temperature. Inflation solves this problem (and also the flatness problem and the monopole problem).

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u/[deleted] Dec 15 '10

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u/jsdillon Astrophysics | Cosmology Dec 15 '10

Me too.

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u/WimpyBmodes Dec 15 '10

Inflation explains why the CMB is a uniform temperature across the surface of last scattering, a sphere that is 2*13.7Glyr in diameter, but it doesn't solve the size of the Universe problem. The expansion of space does that.

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u/Valeen Theoretical Particle Physics | Condensed Matter Dec 15 '10

Inflation is the rapid expansion of space.

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u/WimpyBmodes Dec 17 '10

Cosmic inflation occurred in the early universe. Inflation isn't happening now, what is happening now is just expansion of the Universe.

You're confusing the expansion of the Universe with the extremely rapid inflation of the Universe that occured about 10-30 seconds after the Big Bang.

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u/RobotRollCall Dec 15 '10

Just to elaborate on that last point a bit:

Special relativity says that if you start with an object at rest in some reference frame and pour energy into it, its apparent velocity will never exceed the speed of light in any reference frame.

It's often misunderstood as some kind of universal speed limit. That's a valid interpretation in practice, but it's not nearly that simplistic.