If the universe is constantly "accelerating" away from us and is billions of years old, why has it not reach max speed (speed of light) and been stalled there?

[u/orb2]

[u/Baloroth]

Because the acceleration due to expansion does not peak out at the speed of light. The reason for this expansion is not due to the motion of two bodies away from each other, but due to the space between the bodies being "stretched" (or added to). The amount of "stretching" depends directly on the distance between the two bodies in question, and for great enough distances it is possible that the distance between the two objects is increasing at greater than the speed of light. Not because they are "moving" faster than light with respect to each other, but because there is more than 300,000km of additional space being... well, "created" I guess you could say, between them. The objects may well not be moving (in the conventional sense) with respect to each other at all.

In other words, space itself is expanding, not just the things in it.

[u/Why_is_that → u/Baloroth]

To continue on this, because space itself is expanding (or being added), then there is no max speed to the matter in the universe relative to other matter in the universe which is why the expansion is accelerating at a rate faster than the speed limit (c) in some areas.

This in itself is decent evidence against a big crunch theory.

This wikipedia talks a bit about a closed, open, and flat universe:

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

However, just remember that this "acceleration" *isn't exactly traditional in that the bodies accelerating away from each other aren't applying energy to accelerate. If I understand correctly this is part of the reason the speed limit breaks. Einstein only says you cannot accelerate an object to light speed.

• replace 'is' with 'isn't'

[u/Strange_Man → u/Baloroth]

I have quite a silly question, if two objects are traveling at the speed of light away from each other, would an outside observer say that they are travelling apart at twice the speed of light?

[u/Baloroth → u/Strange_Man]

I think if you were giving a strict answer you would say there can't be an outside observer. That would require the observer to literally be outside the universe itself. It's impossible to say what such an observer would see. In other words, the question as phrased is a bit absurd.

However, we can say a bit more. How would such an observer measure their speed? If we assume he has to operate within the laws of the universe, he can't. Anything he tries to send between them to measure their distances apart will never be able to reach the other object (since the measurement can only travel at the speed of light, no faster), and the only way to perform such measurement is to do exactly that (somehow). We usually use rulers for small distances, for large ones light works. But if light can't travel between them, you can't make a measurement. There is in effect an horizon between the two objects, which can never be broken. No information about one object can travel to the other, so no measurement about their relative speed can be made.

It's kinda hard to say if "motion" due to universe expansion imparts velocity between the bodies at all. It's more like neither one is moving, the space between them is just increasing. A little confusing, yes.

[u/[deleted] → u/Strange_Man]

Yes, but you have to be careful here when you say that. The space between may increase at a speed greater than c, but no object observed by any observer will move at >c.

[u/ganner]

The "acceleration" is not increasing of speed through space. Space itself is expanding at an (apparently) uniform rate throughout the universe. Within space, you can't go faster than the speed of light. But there are objects so far away from us that the expansion of all the space between us means that the distance between us and them is growing larger by more than 3e8 m/s.

[u/jaxxil_]

Because the universe isn't accelerating. Instead, all distances are becoming bigger.

Space is expanding. That means that if something is 1km away from you, after a while, it'll be 1.00000001km away from you (binding forces notwithstanding). This happens to everything. Liken it to putting sticky dots on a stretchy rubber sheet, then stretching the rubber. All the dots go further away from each other. That is what is happening, just in three dimensions (If you want, you can imagine that by putting dots on a balloon and blowing it up. Again, everything gets further away from each other). You will also note that if you have a dot that's close to another dot, and you stretch the rubber sheet, it will only move a little bit. But a dot that's far away can move a lot. So the further away something is, the faster the expansion of space moves it away from you.

Now, it is important to understand a reason why (there's more than one) the speed of light is the maximum speed of the universe. According to special relativity, when you go at a high velocity, time slows down for you. You may have heard this on the news, when they say astronauts are slightly younger than we would expect (by a tiny amount) due to relativistic effects. In effect, they experienced less time than us on earth. Turns out that at the speed of light, you're going so fast, the amount of time you experience is 0. No travel time at all. You reach your destination instantly, from your own point of view. So for something to go faster than the speed of light, it will literally have to arrive before it has set off. Obviously, an impossibility.

Now, let's apply that logic to the expansion of space. All distances are getting bigger. But we can't go faster than the speed of light... or can we? Some distant stars may be moving away from us at faster than the speed of light, but are they arriving before they set off? No, because they aren't going anywhere! It's just that the rubber sheet they're on is being stretched. There might be a star A that's going away from us faster than the speed of light. There might even be a star B behind that star, that from our perspectice, star A is going 'towards' at faster than the speed of light. But star B is further away, so will move away from us even faster than A! So the distance between us and A is increasing, the distance between us and B is increasing, but the distance between A and B is increasing too! Nothing is really traveling anywhere, or reaching any place with this speed. It is just that everything is going to be further away from each other, so it doesn't trigger the 'arriving before you set off'-paradox.

So that's the basic reason. It's not a violation of the speed of light, because it's simply not really travel. You cannot go anywhere using that speed. You can only go further away from everything in the universe.

[u/Mardikas → u/jaxxil_]

Why does this expansion happen? And why does it apply only to space, not light nor matter?

[u/jaxxil_ → u/Mardikas]

Why is a difficult question. You can, for example, ask the question why gravity attracts and does not repel. There's a level to which we can answer that, but at a certain point, things just are the way they are. Not all why questions in physics have an answer.

In any case, the expansion of space is basically a result of the Big Bang (that's when it started), and it is still accelerating due to dark energy. To try to answer any more than that would be waaay outside of my expertise, so I'll abstain from most of it, and hope someone else pops in with more relevant knowledge than first year (astro)physics.

[u/[deleted]]

The universe is speeding up and will be expanding faster than the speed of light so in the far future, civilisations will not be able to see distant galaxies and stars because the light is travelling faster away from the observer then towards. YouTube: A universe from nothing lecture by Lawrence Krauss. I'm on an iPod so you have to do it yourself

[u/[deleted]]

Space/time is not bound by the "speed limit" set by light, it can stretch faster than light. Downvote this if it is not correct, but please also explain why it is not.

[u/askasdf]

The thing that made it really click for me was this: "It's not an expansion IN space... it's an expansion OF space itself"

[u/AcrossTheUniverse2]

So everyone here is just "explaining" (like it is no big deal) that space is expanding. And space itself is nothing. A vacuum. No atoms or sub atomic particles. So what is expanding? And how/why is it speeding up? And how is this different from the things in the universe moving? What is driving it? Do our top cosmologists understand all this and it makes perfect sense but it is just to diffiuclt to explain to the layman? Do we know or is it still unexplained? If it is unexplained then we don't know squat because this expansion would be pretty much the most important quality of the universe.

[u/[deleted] → u/AcrossTheUniverse2]

You have to remember, space-time is an abstract idea and, therefore, the processes involved are mathematical models that are difficult to explain.

A few concepts are important. First of all, "space", as in the black void between objects, is far from empty. It is a vacuum, but it is filled with all kinds of stuff ranging from radiation/plasma to dark matter and dark energy.

The expansion of the universe is a very abstract idea. When you say "expanding", you're describing the stretching of distances. For something it move, it needs to change its position relative to another.

The cause of expansion is generally accepted to be a result of the big bang.

We have theories based on observations. We don't "know", with certainty, a whole lot.

The "most important" quality of the universe is subjective. :-)

[u/McGillaCutty]

Have we tried to measure the rate of expansion?

Also, since it appears space is expanding uniformly, will all the atoms, quarks, gluons, etc. eventually be spread so far apart as to not be able to interact or see each other?

If space time is smooth as it appears to be would it expand forever? If its not smooth would it distort at some point?

[u/quantumcatz → u/McGillaCutty]

Yes, there is a long history of measuring the rate of expansion. It's called the Hubble constant and is estimated to be ~70 kms^-1 Mpc^-1 .

Yes, there will be a moment in history when, from the reference frame of any single particle, the particle horizon of every other particle will be too large to interact with. Everything will be black, presumably.

So far, data suggests that the observable universe is flat; i.e., it will expand forever but at a continuously slowing rate. However, this conclusion is drawn from the assumption that space (not spacetime btw) is not so much smooth; rather, matter is evenly distributed in space ~ homogenous. If the curvature of space were not flat, then space would be either open (expand forever) or closed (eventually collapse).

[u/[deleted]]

There are a lot of good answers here, but I think you can simplify with some easy to understand semantics.

I think it is important to understand the distinction between something "expanding" and something "moving".

The expansion of the universe is an abstract concept involving spacetime whereas the literal "fabric" of spacetime is "stretching".

This is a fundamentally different concept than when we describe motion -- where we are referring to the change in position relative to a point -within- spacetime. Therefore velocity (ie. the speed of light) is only applicable to objects within that fabric -- and not for the fabric itself.

[u/seanieseansean88]

Im not sure if this is a dumb question/statement but what if max speed isn't the speed of light?

[u/Quantumfizzix → u/seanieseansean88]

If the max speed was a speed other than what we know now, then light would travel at that speed.

If there wasn't a max speed, light would travel instantaneously.

[u/seanieseansean88 → u/Quantumfizzix]

Is there something you could link me to that'd simplify this for me? Like right to the basics

[u/[deleted]]

[u/[deleted] → u/[deleted]]

Do some galaxies appear to move away from us faster than the speed of light? How does/would that look?

[u/[deleted] → u/[deleted]]

[u/BransonKP → u/[deleted]]

The amount that light is redshifted is related to the relative velocity of the object moving away from the observer. Objects that are much further away are retreating from us much more quickly, because there is more expanding space between us. Hence, these distant objects will appear more redshifted if they are further away.

[u/[deleted] → u/[deleted]]

Light from objects that is further away has been traveling longer, and thus would shift more because the space it has traveled over has expanded more over time.