eh

This is news to me. What observations say it's slowing down or staying the same?

I think I understand what you're saying, but the expansion of the universe does not depend on how fast gravity can propagate. Space itself is expanding, regardless of the speed of light or gravity.

cfgauss

Maybe it'd help if you think about it like this: gravity moves at the speed of light because that's the *only* speed that a wave *of* space can move, kind of like how the speed of sound is the *only* speed a wave of air can move. But it's more than this, space can only move (wave, stretch, contract, etc) at that speed. Because it can only move at that speed, the universe can only be expanding at that speed.

If the speed of light changed there would be definite observable changes. For example, you may have hear a while ago about some astronomers making some observations that showed that the fine structure constant has changed over time. But I don't know if we know enough to make any kind of good predictions yet.

And, remember, velocity is relative, so if something's moving away at 3/4 c one way, and something else is moving at 3/4 c the other way, they'll be moving away from each other at less than c.

You could try searching google to look for some of the articles on the expansion of the universe.

I'd love to write more, but I've gotta get ready for work.

Hawkingfan

I have never heard that the speed of light changes. To the best of my knowledge, it always stays the same. But I do agree that the expansion of the universe cannot exceed the speed of light. This is because of the equivalence of mass and energy (E=mc^2) which can be used to show the law that states nothing may travel faster than the speed of light. Because energy and mass are equivalent, the energy which an object has due to its motion will add to its mass. In other words, it will make it harder to increase its speed. For example, at 10 percent of the speed of light an object's mass is only 0.5 percent more than normal, while at 90 percent of the speed of light it would be more than twice its normal mass. As an object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its mass would have become infinite, and by the equivalence of mass and energy, it would have taken an infinite amount of energy to get it there. For this reason, any normal object is forever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves that have no intrinsic mass, can move at the speed of light. See "A Brief History of Time" p. 21 for more.

eh

I'm not sure we're on the same page. The expansion of space and gravity waves are not the same thing. It is not light or graviton that is expanding, as it is the flat space itself that expands. Observations suggest the universe is accelerating, and I don't know any that show it to be slowing down. Perhaps citing a source would help.

I've read several articles floating around the web, but could not find anything that claimed the expansion rate depends on the speed of light or gravity waves. Any URL's?

Hawkingfan

Even though I disagree with just about everything cfgauss says, I agree that the universe cannot expand faster than the speed of light. As I understand it, flat space is made up of particles left over from particle/antiparticle annihilations. Those particles have mass, so they cannot travel as fast or faster than the speed of light or else their mass would be infinite and the energy to push them would be infinite as well (see my previous post). Light particles and gravitons have no intrinsic mass, so they can travel at the speed of light.
Also, I have only heard that the farther away something is from us, the faster it is moving away. But that does not mean it is accelerating. It should be slowing down and cooling, yet travelling at a critical rate in order to prevent a collapse.

NumberTenOx

Here is a good tutorial on cosmology. If you look at the FAQ page under "Can objects move away from us faster than the speed of light?", you find this:

"... this is a question that depends on which of the many distance definitions one uses. However, if we assume that the distance of an object at time t is the distance from our position at time t to the object's position at time t measured by a set of observers moving with the expansion of the Universe, and all making their observations when they see the Universe as having age t, then the velocity (change in D per change in t) can definitely be larger than the speed of light." (emphasis mine)

Here is what I think is the problem. A far away galaxy looks like it's moving away from us at some fraction of the speed of light. But it's actually not moving. Anyone else in the local vicinity of the galaxy would see it at rest. It only appears to be moving because space is expanding.

How can I say that it's "not moving"? Because there is a universal frame of reference - the cosmic background radiation. As we look at the CBR, we see that it looks incredibly uniform, always having the same value no matter which way we look. If we had any velocity, we would see a blue shift in the frequency of the CBR in the direction we were moving, and a red shift in the opposite direction. In fact, we see this, which leads us to believe that we're moving about 370 km/sec with respect to this frame of reference.

The apparent motion of far away galaxies is not because they are moving, it is because the universe is expanding. They also see themselves at rest with respect to the CBR.

I agree that gravity waves probably move at the speed of light. It does not follow that space can only expand at the speed of light. Of course, we can never actually see anything moving faster than c, but we can deduce that this is happening.

I'm still taking cfgauss' advice and trying to read more about this. I have a lot to learn. One thing that I still don't understand is the assertion that the universe is infinite is scope if the critical density is less than 1 (i.e. if there will be no big crunch). Assuming that the extent of space was finite at and just after the big bang, when did it make the transition to infinite? Does "infinite space" really mean finite space that is always expanding, such that you will never be able to "see" it all because there will always be more than you can experience? Or does it really mean infinite, in that it is infinite right now? That, I don't get.

eh

Oh well, cosmologists say otherwise. NumberTenOx posted a link to Ned Wrights cosmology tutorial, which should about cover it.

cfgauss

Hawkingfan:
Mass does *not* increase with velocity, mass-energy does! There's a difference, but unfortunately, many popular-science books neglect to mention it. If *mass* increased, then you'd have some real problems on agreeing on what a black hole is! And to say E=mc^2 implies mass and energy are equivalent is misleading at best! Does d = rt imply that distance and speed are the same? Does F = kx imply that displacement and force are the same? No. What's really going on (more or less) is that it *acts* like it has more mass, minus the increase in gravitational force. You can think of this simply as the mass staying the same, but the inertia increasing.

eh:
No, they aren't the same thing, but I never said they were. They are, however, manifestations of the nature of the same thing!
I know there've been some good articles in Scientific American a year or two ago on the expansion of the universe, though I can't think of the month, and don't enjoy in going through the huge pile looking for the article . Most of what you'll find on the internet will probably be on the accelerating universe observations, because it was the most recent, and the most conflicting. Being a student, and there being a Star Trek marathon on at the moment, I don't have time to look for you. However, if I do get some time, and am able to track some good ones down, I'll let you know!

Hawkingfan:
No. Almost none of the particles are made up from particle / anti-particle annihilations! When a particle and its anti-particle annihilate, they create two high-energy photons, not more particles. All(most) of the (heavy) particles today were created by nuclear fusion in stars, fuel courtesy of the Big Bang.

NumberTenOx:
You can have an apparent velocity faster than light really easy. Even something so simple as a shadow could move faster than light! However, no information can be transmitted faster than light, and no object can move faster than light.
There is no "universal frame of reference"! That would contradict special relativity. The CBR happens to be everywhere, but it's no special frame any more than our atmosphere is!
To understand what the critical density means, you have to... understand what it means... It talks about curvature. 1 is flat, <1 is negative curvature, and >1 is positive curvature. Now check out what curvature is, and the answer will be apparent!

Answerer

Well Cf, the universe's expansion is not limited to the speed limit or speed of light. Furthermore, with acceleration of the universe expansion increases exponentially and the value of the speed of light decreases, it is only a matter of time before the speed of the universe expansion exceed the light speed.

By the way, when we are talking about the expansion of spacetime, we are thinking in terms of its sketching rate not translation movement. Therefore, special relativity will not be violated at all.

Shadowy Man

Actually momentum does.

Well, this relationship is for rest mass anyway, so it's ok.

Actually, most of the heavy elements were created either in nuclear fusion in stars or in supernova blast waves. The particles (i.e. protons) that make up these elements were most likely created in the big bang.