# If the Big Bang started in a state of low entropy then shouldn't the Big Crunch be in a state of low entropy also?

What our current level of thermodynamics does tell us though is that if there is such a thing as a Big Crunch, the universe will look a lot different as it heads towards collapse than it did when expanding – the universe started in a state of low entropy and high order and will end in a state of high entropy and low order.

But shouldn't the order be in a high state and hence entropy becomes a low state in the Big Crunch (assuming a closed universe)?

The reason I write this is because the gravitational forces in a closed universe are so strong they will cause gravitational collapse so the universe will shrink and go backwards, eventually through all the stages of the big bang and become a singularity once more.

It was my understanding that a singularity has a state of high order and low entropy. Have I got this the wrong way round?

If so; Can someone please explain why?

• Right at a singularity the entropy is not well-defined, I think. But with a minor change I think this is a great question. I would probably phrase it in a way like this: In models of the universe that have a Big Crunch without a reversal of the thermodynamic arrow of time, how exactly is the low entropy state of the universe just after the Big Bang different from the high-entropy state just before the Crunch? I have no idea what the answer to that question is, but someone here should... – Rococo Oct 4 '16 at 1:48
• @Rococo Understood, thanks for your reply. I will leave this question phrased like this, but keep that comment you made as it may make more sense to others when phrased the way you put it. In other words, better to have it asked in two different ways as it may be interpreted by others in certain forms. Thanks for your time. – BLAZE Oct 4 '16 at 1:59
• @Rococo. Roger Penrose certainly has some interesting ideas on explaining the low entropy problem, if only I could follow them : ) – user108787 Oct 4 '16 at 2:01
• If it's a cyclical universe it would have to return to low entropy somewhere for each cycle. – Bill Alsept Oct 4 '16 at 2:27
• Penrose regards an increase in the tightness of Weyl curvature (the curvature that was a factor in stars on opposite sides of the sun, during that photographing of them by Eddington during 1919's solar eclipse which confirmed GR, appearing farther from each other than they do at night) as corresponding to an increase in the radius of the universe and a consequent decrease in entropy, which I guess is averaged over its volume. In his cyclic cosmology, all matter must evaporate, which isn't jiving with the observed absence of proton decay, but he could still be right about curvature re entropy. – Edouard Oct 1 '18 at 19:14