Big bang/Talk

"distant galaxies can actually move apart faster than the speed of light"

I admit that I am not a physicist, but this really surprises me. Is this actually true?

Yes - nothing can travel "through" space faster than the speed of light. However if the space itself is expanding then two objects which are at rest (relative to their local environment) can move away from each other at speeds greatly in excess of the speed of light. To conceptualise this - imagine a balloon with dots drawn all over it. Blow up the balloon. Now none of the dots have moved relative to the balloon itself, yet they are now further apart from each other. Read Lawrence Krauss's book "The Physics of Star Trek" - he explains this beautifully (exploitation of this was the justification for how the warp drive worked) - MMGB

In the [observable universe]? (that sphere around us of radius n light years, where n is the age of the universe in years) I do not think it is possible for two objects to recedes faster than the speed of light. Due to the initial inflation the universe is much larger than the observable universe and perhaps the example here is the relative motions of galaxies that are farther apart than n light years and so are not mutually observable -- however such objects can never be seen from each other and in generaal it is impossible for either to have any influence on the other ever (I think). --Eob

Eob - you're absolutely correct. Take Galaxy A (at the edge of our sphere of light perception, and receding at just below c, relative to us) and Galaxy B (likewise at the edge of our sphere of light perception, but diametrically opposed to Galaxy A, with the Earth at the centre of the imaginary line connecting all three). The light from Galaxy A and B is travelling at c and hence we can just perceive each. But relative to each other, they are receding at the '''linear addition of their recession velocities". They would not ever be able to perceive each other. Hence the "observable universe" of Galaxy A would have the earth at the extreme fringe of it (or "where the Earth was going to be when it formed" relative to their timeline) and nothing beyond it. Galaxy B would not exist, nor would it have ever existed, in their reference frame. Likewise, there is probably a Galaxy C beyond Galaxy B that we cannot perceive, and never will.

Even if C keeps moving away from us faster than c, there is a priori no reason to assume that we will never see C. Think of the light emanating from C as an ant walking towards us on a rubber band extending from us to C. The ant is walking at speed c relative to the rubber band, the rubber band is stretched, but the ant will still reach us eventually (unless you keep stretching faster and faster, in an exponential way, which is not what galaxies do). --AxelBoldt

The definition of our "observable universe" is "all the stuff that is moving away from us at less than the speed of light, hence we can still see it. Some versions of the inflationary hypothesis speculate that we can only see 1 millionth to a billionth of the real cosmos. Not that it matters, we cannot (by definition) ever verify it one way or the other (barring the discovery of wormholes, and let's NOT go there). - MMGB

No, objects in the observable universe can be receding faster than the speed of light. It works because what we see lies in the distant past - just because the objects are too far away for light to reach us now, doesn't mean they were when the light was emitted.

I see. But what does "now" mean? Under relativity there is no concept of simultaneity.

Simultaneity is irrelevant - we are only considering our reference frame. "Now" means "our now".

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Again I have to prefix this with "I am not a physicist and I don't play one on TV", but...even with the "balloon" analogy, I am having a hard time believing that it is possible for the dots on the balloon to move apart faster than the speed of light under the constraints of relativity. The fact that they are locally "at rest" doesn't really mean anything anyway, does it? After all, in Einsteinian space-time there is no absolute sense in which anything is at rest anyway.

I am not saying that it is wrong that galaxies can move apart faster than the speed of light--but it is completely alien to my (admittedly limited) understanding of relativistic physics. Egern

I'm not sure of the which is right, but perhaps this issue is being conflated with the apparent superluminal velocity of distant galaxies? [1] -- DrBob

Nah, it's quite definite that some galaxies are receding at true superluminal velocities. This works because limits on relative velocities - you're quite right that local rest doesn't come into it - only apply to objects starting at the same point in space and time. The possible paths of an object there make up a cone, called the point's future, and contain no relatively superluminal paths. In flat static spacetime all cones are oriented the same way, so the same applies, but with the expansion of the universe the cones are relatively tilted. Remember relativity is a local theory. -- Josh Grosse

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Replaced the following

Although the Big Bang Theory is widely accepted, it probably will never be proved; consequently, it leaves a number of tough, unanswered questions.

with

The Big Bang Theory is now the accepted scientific view of the origin of the Universe.

because it gave the wrong impression -- no scientific theory is ever fully "proved". Statements like this just gives fodder to the creationists and their ilk. (Woops, stepping a little outside NPOV there). Perhaps the original author could elaborate on the "number of tough, unanswered questions" -- Eob

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I changed the wording a bit. I'd say that the big bang model is accepted by about 98% of the cosmologists out here, but there are a few here and there who don't accept it, and there are a few models here and there which attempt to challenge big bang. I don't think that they will get anywhere, but they exist. I also added some text about cbr. -- Chenyu

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Here is a reasonably well-written (though VERY biased) web page regarding "opposition" to the Big Bang theory. The author is well informed (which is not to say correct). Even though I personally think he's way off-target, it still merits reading and critical consideration. [2] - MMGB

For the record, the stars older than the universe thing was more or less resolved a while ago. The linked page doesn't appear that well informed, I'm sure we can do better.

OH absolutely, but it provides a good launching point for addressing the criticisms. The pages presents an appallingly distorted view of the evidence. - MMGB

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Why is this article at Big bang rather than Big Bang? The article itself consistently writes "Big Bang". --Zundark, 2001 Nov 20

Half an explanation - I wrote most of the article, and used "Big Bang" because that seemed proper to me. I'll ask LMS to make the call. - MMGB