Take the 2-minute tour ×

In a Wikipedia article I read that the "Metric Expansion of Space" exceeds the speed of light. If this is true then we must be being disconnected from very remote parts of the universe since gravity can only travel at the speed of light.

In his book "A Brief History of Time", Hawking seems to still believe in the big crunch. Well, it does appear that way, as he devotes a lot of space about the vector of time reversing due to the big crunch.

But how can the big crunch still be a valid theory if the above statement is true, or is it not true?

This question is not about dark energy,although i do appreciate the replies.

share|improve this question

3 Answers 3

up vote 3 down vote accepted

Well, it's not. To our best present knowledge the cosmological constant is positive, which means the universe undergoes accelerated expansion and never crunches. Hawking's book is a little out of date.

share|improve this answer
@johann: my answer to another question here is relevant: physics.stackexchange.com/questions/24017/…. In particular, the consensus view on cosmology is very recent (last 8 years or so) and the lifetime of popular science books are much longer; unfortunately this means most things outside of the academic literature is simply now known to be incorrect. –  genneth May 30 '12 at 13:54
Thanks for all the answers. I would still like to know if the statement that the expansion of space is exeeding the speed of light is true (it seems to be a good statement) and if that would mean that the universe is uncoupling itself from it's own mass? –  johann Jun 3 '12 at 2:32

I don't know what motivated Hawking, but there have been suggestions that there will be a big crunch. See for example http://www.newscientist.com/article/dn2759-universe-might-yet-collapse-in-big-crunch.html and http://news.stanford.edu/pr/02/universe925.html. I must admit that the physics involved is beyond me, but Linde has claimed that results from supergravity (i.e.combining supersymmetry and gravity) suggest that the dark energy may change and become attractive instead of repulsive. This would cause the universe to contract to a big crunch.

I suspect most physicists regard this as highly speculative!

share|improve this answer

There are two sides to this answer. One is that A Brief History of Time was first published in 1988, and so it pre-dates the discovery of dark energy by several years. (The question explicitly says that it's not about dark energy, but mentioning it is unavoidable, since it's the reason why most cosmologists no longer predict a big crunch.) At the time Hawking was writing, the data was ambiguous about whether there was enough mass in the Universe to cause the expansion to eventually reverse, resulting in a big crunch. In that case the separation of distant space-time regions would not have been permanent - the separated regions would eventually have become accessible again as the universe shrank in the distant future.

The second side is, does this mean that what Hawking writes about the arrow of time is now out-dated? I would say no: there is still an important scientific question of whether the entropic arrow of time necessarily the same as the cosmological-expansion arrow of time. Even though we no longer think the cosmological arrow will be reversed in the future, we can still use a big crunch scenario as a "thought experiment" to reason about whether the second law would also have to be reversed. When read in this way, Hawking's argument about the relationship between the two arrows is still addressing a relevant question.

share|improve this answer

Your Answer


