The "observable" universe

Didn’t want to hijack this thread.

I need some clarification concerning the term “observable universe.” Does it literally mean the universe to the extent our current technology can observe it, yet we know (or think) there is more that remains hidden? Or is it “observable” in the sense that this is an estimate of the entire universe, and we believe that’s all there is?

And if we know how long ago the big bang occurred, doesn’t that give us the size, in light years, of the ***entire ***universe, “observable” or not? (leaving aside the possibility of other big bangs)

Looking far away is like looking backwards in time. If we observe a galaxy that’s a million light years away, we’re not seeing it as it looks now. We’re seeing it as it looked a million years ago.

Since the universe is about 14 billion years old, we can’t see anything that’s more than 14 billion light years away. * The light hasn’t had time to reach us yet. You can think of the observable universe as a bubble centered on Earth. Everything inside it is close enough for its light to have made the trip during the lifetime of the universe. Everything outside the bubble is too far away for us to see because the light has had time to traverse the distance.

The full universe is probably much, much larger than the observable universe, perhaps even infinitely large. It’s unclear if we will ever know the true size of the universe because our observations are necessarily limited by the speed of light. Note that this isn’t a matter of technology. Building better telescopes won’t increase the size of the observable universe. The light literally hasn’t had enough time to arrive yet.

(* This is a simplification. Because the universe is expanding the edge of the observable universe isn’t 14 billion light years away but more like 45 billion. Because, as the light from distant galaxies is headed toward us, the galaxies themselves are zooming off in the opposite direction. Still, you get the idea.)

An interesting consequence of this is that as time goes on, our observable universe gets larger, and yet the amount of stuff we can see will decrease.

The distance we can see is just dependent on light travel time, so as more time goes on, more light will be able to reach us. But as the universe expands, light from far away galaxies becomes more and more redshifted, the wavelength becomes essentially infinity, and for all practical purposes it is undetectable.

Trillions of years from now, our observable universe will be huge, but we won’t be able to actually see anything beyond our own galaxy.

Ditto Hamster King and Yumblie. But, add this twist: in a certain sense, the universe beyond 14 billion lightyears away hasn’t started existing yet. However, no doubt in some of the galaxies that are almost that far away, there are intelligent people discussing online the very same issue (or there will be when the age of the Big Bang is about 14 billion years there), and our region of the universe has not existed for long, and Earth is something like ten billion years in the future; and they can say the same thing about points opposite us in their sky.

Another way of looking at it is that the furthest thing we can see is the Cosmic Microwave Background Radiation (CMBR). Furthermore, this will always be the case, no matter how old the universe is.

As best we can tell, the age of the universe is 13.75 billion years. The CMBR was radiated when the universe of 380,000 years old. When we view the CMBR, we see a cross section of the universe where the light just happens to take 13.75 billion (minus 380,000) years to reach us. Someone recently calculated how far away that section of the universe is now. Unfortunately, I forget the number and can’t find it with a quick google, but because of the accelerating expansion, it was much more than 13.75 billion light years away.

I’m not sure I do get this.

I think I understand that during the 14 billion years that it took for light from the ‘edge’ of the universe to reach us, the ‘edge’ has also been traveling (or at least, expanding) away from us. But, at most the speed of the ‘edge’ away from us was ‘c’. In other words, at most, the ‘edge’ has receded 14 billion light years from us during the last 14 billion years. Doesn’t that mean that, at most, the ‘edge’ is ‘now’ 28 billion light years from us*?

*I assume you don’t also include an additional 14 billion l.y. due to the fact that we have also been receding from the edge for 14 billion years. That seems a reasonable assumption to make, since by definition when we talked about the ‘edge’ being 14 billion l.y. away in the first place, its light had already reached us, i.e. we expand away from the edge only after its light has reached us.

No. Space proper is itself expanding, and far away this expansion is (apparently) faster than ‘c’. The expansion is accelerating lately (in eon-terms anyways). I am not up to explaining this coherently, but space is ‘produced/expanded’ across vast distances (call it inserting ‘space’ where less existed before) making things expand faster away from each other - and the sum of some of the distances/speeds of recession is faster than ‘c’ would seem to allow. Overall, the distance(s) are measurably further than ‘c’ alone would provide for. I cannot show the math, but that is how I understand it to regurgitate it back.

I hope I make a modicum of sense, and trust someone will verify me soon.

You’re only limited to the CMB if you insist on looking using light. With neutrinos, you can go back a ways further, and with gravitational waves, much further yet.

And it should be noted that when you’re looking on cosmological scales, you have to be careful to define just what you mean by “distance”. There are many ways of defining distance, and the definitions are calibrated such that they all agree on short scales, but that breaks down when you’re looking at cosmological distances.

If the observable universe was say, a 1,000 times smaller than we have estimated it to be, how would it look different?

Then the universe would only be 14 million years old, not 14 billion. The first stars didn’t form until about 400 million years after the Big Bang, so if the observable universe was 1000 times smaller than we think it is, all that would exist would be a black void filled with diffuse plasma.

And if it was 10 times smaller, would it appear radically different then?

ps. Also, how can you make anything but educated assumptions about a one-time event?

My understanding is that neutrinos generated in the Big Bang are so low energy (due to the expansion of the universe) that it is impossible to detect them. That of course may change with better neutrino detection tech in the future. If we could, we’d be able to see back to about 2 seconds after the BB.

As far as I know, we haven’t detected gravity waves at all, except maybe indirectly. But even if we could, we still couldn’t “see” them further away than the Big Bang.

Generally when talking about cosmological distances, astronomers actually say how long ago the object is. If they give a distance, it’s how far the light traveled to get from there to here. This is less than the distance the object currently is from us but greater than the distance to the object at the time the light was emitted.

However, that’s usually only given in popularizations. In scientific papers, they usually just give the redshift factor(z) of the object. Cosmologically, z is not a linear function. A z=1 is about half the age of the universe. The furthest object so far seen is a gamma ray burst with z=8.2. That translates to 13.1 billion years ago (i.e. the light travelled 13.1 billion light years to get to us). By comparison, the CMBR has a z of over 1000. Wiki page on redshift for those interested in the technical details.

(my bolding)

The light from anything beyond the observable edge of the universe will never reach us. I believe that as time goes on, less and less of the stuff in the universe will be observable by us. Intelligent creatures far in the future may think that their galaxy is the only one in existence because space has expanded so much that every other galaxy is beyond the observable edge. I think I got that right, but I’m sure someone will correct me if I’m wrong.

I don’t understand - you can observe the one-time event. We’re observing the Big Bang right now.

I don’t think you’re really getting what the observable universe is. The size of the observable universe is determined by only two things: the age of the universe, and the speed of light.

When you ask “What if the observable universe were smaller?” you’re really asking “What if the speed of light were slower?” or “What if the universe were younger?”

Making the speed of light slower would have all sorts of complicated effects. However, it’s easy to speculate about what things were like when the universe was younger. 12.6 billion years ago the observable universe WAS 1/10 the size it is now. (Although there was no life around to observe it.) However, the first stars were beginning to form, so at least the universe wasn’t a black void.

No, you’re right. The accelerating expansion of space will gradually move more and more stuff “outside” the observable universe. However, I thought that detail would get in the way of understanding the basic idea of Earth’s light bubble.

I believe the current thinking is that the Local Group of galaxies is bound together tightly enough by gravity to not be pulled apart by the expansion. However, eventually everything else will redshift off into invisibility.

I cannot wrap my head around this. Doesn’t this mean that the “edge” of the universe is expanding faster than the speed of light? I thought that was impossible. :confused:

One simple way to look at is to pretend that the speed of light is 100mph. You’re in a car traveling 50 mph in one direction, and your friend is in a car traveling 50mph in the opposite direction. Neither of you is violating anything, but the space between you is getting greater at 100mph. Still no violation. Now add a mysterious energy to the expansion of space that makes the expansion speed up. Call it Dark Energy. It’s making space expand at 10mph. No violation, but now the space between you and your friend is getting bigger at 120mph? Turn your telescopes toward your friend and you’ll find he is outside your observable universe, because the light coming from him can only go 100mph. It will never get to you. He’s gone from you forever.

Be sure to read my upcoming book: Stretching Highways and the Observable Universe. Humor aside, that’s one of the best analogies I’ve heard for expanding space, Lev. Thanks.

Space itself has no constraints. It can expand at any rate.

Anything inside of space is constrained by the speed of light. “Anything” includes gravity, light, energy, mass, and particles, which are all manifestations of the same thing, all the stuff we normally think of when we think of stuff.

Space is not stuff. That literally makes all the difference in the universe.

In addition to the above realize that no particular part of space (or anything) is moving faster than light speed.

The effect is cumulative.

Every little part is expanding but not very fast. Imagine adding 1 inch per year to a yardstick. Pretty slow. However, if you have enough yardsticks laid end-to-end and each one adds one inch per year eventually someone at the far end of that line of yardsticks would seem to be receding from you at faster than light speed (and from their perspective you would seem to be receding at FTL speeds). That said neither of you are actually moving FTL so there is no violation.

ETA: Check out this page for a nifty animated picture of how expanding space affects the observable edge of the universe and a simple explanation how different distance calculations get different answers.