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Why is it concluded that the cosmos is expanding at an ever increasing rate when looking further out and therefore further back in time?

Surely, if the time were reversed to play forwards, the conclusion would be that the expansion of the universe is actually deaccelerating, and the need to postulate 'Dark Energy' would be avoided.

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2 Answers 2

What we directly observe is that the Universe was expanding and the expansion was accelerating during the recent five billions of years or so (it was actually not accelerating before that because the dark energy wasn't dominating). The prediction that it will continue to expand and accelerate results from a "clever scientific extrapolation" – from writing the most convincing equations that describe the past and from solving them in the future.

The past observations imply that the Universe is expanding because we see that distant galaxies were moving away from us in recent billions of years (because the light is redder – has a lower frequency – than it should have according to spectroscopy, a fact we attribute to the Doppler effect). If things are moving away from each other, their distance is increasing as you go from a smaller $t$ to a larger $t$ and this increase is called expansion, by definition. Your argument by which you want to confuse expansion and contraction looks utterly bizarre to me and I am pretty sure that I won't be the only one. Something that looks and behaves like a bomb after it detonates is called "expansion" and something that behaves like the rubber after we create a hole to a rubber balloon with a needle is called contraction or implosion or shrinking and most people know how to distinguish these two processes without asking questions on Physics Stack Exchange.

There's a clear difference between expansion and contraction, a difference that even most of the laymen and kids from the kindergarten probably appreciate. These two opposite processes lead to different observations and one may clearly say that the Universe has been expanding for quite some time.

In an analogous way, one may determine that the expansion was accelerating. This is found by reconstructing the acceleration rate that was valid "very recently" and the acceleration rate that was valid "billions of years ago": one needs to look at objects at very different distances to get the rates at at least two moments in this case. When the two rates are found, we see that the rate (=speed) of the expansion in recent years was faster than the rate (=speed) that applied billions of years ago. This increase of the rate (=speed) is what is called acceleration, by definition.

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Perhaps I could appeal for somebody else who understands my question better to reply by throwing more light on it –  DDD Jan 8 '12 at 12:54
I think DDD's point was that if we see more distant objects moving faster but more distant objects are also older then that is saying that the universe is slowing - older=faster, younger=slower! In fact the expansion of the universe doesn't quite work likethat –  Martin Beckett Jan 8 '12 at 18:02
How does it work then? –  DDD Jan 8 '12 at 18:27
Of course I should have said that my conclusion is that the expansion of the universe is decelerating not contracting (which may come later) –  DDD Jan 9 '12 at 9:22
@DDD: if that's what you meant, edit your question to say so. It'll be easier to give you a helpful answer that way. –  David Z Jan 10 '12 at 0:05

You should try to think of the Universe expanding in terms of the space itself expanding. Not objects moving away from each other in a fixed space. The accelerated expansion refers to the way space in between objects is growing. This is where quantities like proper and comoving distances become very important. If you want more details on that I would refer you to Friedman equations and the scale factor therein. The bottom line is, the space itself between objects is growing therefore the Universe is expanding. The rate at which it is growing is also a positive quantity therefore it is accelerating as well.

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But if it is say expanding x times faster y lightyears away than we see here, why isn't it thought that we are looking at the speed it was y years ago and we are just seeing the faster rate of expansion it used to be. But in actual fact if you could see y light years away in the present time the rate of expansion would be the same as here –  Jonathan. Jan 10 '12 at 0:41
@Jonathan. When we observe a large distance, high redshift, away we are observing the rate of expansion in the past. However, I don't know what you mean by "... if you could see y light years away in the present time ...". When you see y light years away, you are observing the past. The expansion rate of the Universe, or more precisely the scale factor, is a function of time and therefore has changed over the history of the Universe. –  Omar Jan 10 '12 at 1:49
Exactly so the rate of expansion is not what we are seeing. We are seeing the rate of expansion 10 years ago if looking 10 light years away. In that 10 years the rate of expansion could hava slowed. –  Jonathan. Jan 10 '12 at 15:38
@Jonathan. If the rate at which some quantity is changing decreases that does not mean the quantity itself is decreasing. In this case we are talking about the scale factor $a(t)$. The rate at which $a(t)$ is increasing, might decrease, but $a(t)$ itself is still increasing and therefore the Universe expanding and accelerating. The ultimate fate of the Universe, big crunch or otherwise, is determined by many factors and $a(t)$ represents how the space itself is changing. Perhaps this picture will be useful. casa.colorado.edu/~ajsh/cosmo_04/scalefactorold.html –  Omar Jan 10 '12 at 16:44
surely the rate of expansion is the same thing as the acceleration? –  Jonathan. Jan 10 '12 at 16:56

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