I'm trying to compare the two most popular big bang theories, the big freeze and the big crunch or big bounce theory, and apply Occam's razor, which says the more assumptions a solution has, the more likely it is wrong.

First of all, if the universe had a beginning, then what was the cause of that beginning of time, if by definition, the lack of time means the lack of events?

Is it plausible to assume that dark energy remains at constant density with the expansion, and why doesn't that break the first law of thermodynamics? Why do they assume that, without being able to even see it?

Up until a couple of decades ago, the big crunch theory was most popular, what we would expect to happen, and do observations of an accelerating expansion really change that, given all the assumptions that need to be made?

Dark energy could be decreasing in density, and in the future when all mass is in black holes, wouldn't this suck in dark energy with everything else, converting it's energy into a pure pulling force of gravity?


1 Answer 1


I'm not sure exactly what you're referring to as the three different theories, but recollapsing, accelerating and anything in between universes are not different theories. They are all examples of a solution to the equations of general relativity called the FLRW metric.

If we make a few physically plausible assumptions about the distribution of matter in the universe then we obtain a spacetime described by the FLRW metric. However this metric has various adjustable parameters: principally the density of matter and the cosmological constant (i.e. dark energy). If the density of matter is high we get a universe that expands then recollapses (Big Crunch). If the density of matter is low we get a universe that expands forever but at an ever decreasing rate (Big Freeze?). If the cosmological constant is high we get a universe that expands forever but at an increasing rate.

In all cases the beginning of the universe (and for a recollapsing universe the end) is a singular point. That means general relativity cannot tell us what happens at that point. If you want a bouncing universe then you have to move to a more complex theory that includes GR as a subset. For example Loop Quantum Cosmology predicts a bounce. I've seen claims that String Theory also predicts a bounce, though in both cases the conclusions are highly speculative and far from accepted by everyone.

It has also been suggested that dark energy is not a cosmological constant but varies with time. These theories are generally known as quintessence. However these theories are all speculative, and I'm not sure Occam would go with them over a simple cosmological constant.

In the old days it was popular to believe in a recollapsing universe for aesthetic reasons, but measurements showed the density of matter was far too low for a recollapsing universe (even including dark matter). I'm not sure where your claim Up until a couple of decades ago, the big crunch theory was most popular comes from as I'm sure this wasn't the case in 1994, or even in the decades before that.

The point of all this is that I don't think the situation is ameanable to an analysis using Occam's razor because the various options are all equally simple.

  • $\begingroup$ The simple cosmological constant you're talking about breaks the first law of thermodynamics, noone has explained how it doesn't, if dark energy is the same energy we are made of, in a different form. That is a larger assumption, that it's a completely different type of energy. The people who found the universe was expanding faster expected the opposite. $\endgroup$
    – rowanman28
    Apr 3, 2014 at 10:03
  • $\begingroup$ @rowanman28: Energy is not conserved in GR because it does not respect time shift symmetry. This makes the first law of thermodynamics inapplicable. No-one knows what dark energy is. If it's a cosmological constant that can be interpreted either as a fluid with an unusual equation of state, or as a geometrical property of the universe and not energy at all. It depends which side of the = sign you put $\Lambda$. Dark energy was indeed a surprise to it's discoverers, but then so was special relativity back in 1905 and we all accept it now. $\endgroup$ Apr 3, 2014 at 10:07
  • $\begingroup$ I don't get what the first thing you said means. I've read that general relativity is so complex that most physicists don't understand it. $\endgroup$
    – rowanman28
    Apr 3, 2014 at 11:38
  • $\begingroup$ See Is the law of conservation of energy still valid?. I can't speak for most physicists, but GR is not as hard as the popular myth suggests. $\endgroup$ Apr 3, 2014 at 12:13
  • $\begingroup$ I read it, I still don't know what time shift symmetry is, and how it could change the amount of energy, which must be measured in numbers of strings or whatever. What I think is it's an unproven theory, called fact, just because it's Einstein. $\endgroup$
    – rowanman28
    Apr 4, 2014 at 12:29

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