Wikipedia:Reference desk/Archives/Science/2024 June 28

= June 28 =

Time and the Big Bang

 * Moved here from the Miscellaneous section of the Reference desk. --Lambiam 15:42, 28 June 2024 (UTC)

Time as we know it started with the Big Bang and following the Big Bang the universe continues to expand at an accelerating rate. Additionally based on the astrophysical presumption that the expansion of the universe will cease to accelerate and will lead to what astrophysicists are calling “the big crunch” whereby the universe will start to retract back to a point of singularity leading to repeated Big Bang events; my question is that if the universe was started to retract would time started to flow backwards? 149.12.2.131 (talk) 13:15, 28 June 2024 (UTC)
 * No, even if the Big Crunch were the fate of the universe. Entropy only goes one way. Supernovae won't reassemble themselves. People will not start sdrawkcab gniklat. Clarityfiend (talk) 13:44, 28 June 2024 (UTC)
 * If the arrow of time reverses, so does the direction of the increase of entropy. There is nothing per se contradictory with a model of the universe with a timelike parameter $$\tau$$ such that its entropy increases monotonically with $$\tau$$ for $$\tau<0$$ and decreases monotonically with $$\tau$$ for $$\tau>0.$$ On one side of $$\tau=0$$ the arrow of time points in the positive direction and on the other side it points in the negstive direction – towards $$\tau=0.$$
 * The picture is complicated by quantum physics. The von Neumann entropy, which may be a more fundamental measure, should be invariant for a closed system. The universe as a whole is a closed system. The fact that we observe increasing entropy may be due to our inability to access the information that is "somewhere". As Special Agent Mulder would say, "the truth is out there". --Lambiam 17:01, 28 June 2024 (UTC)
 * If the universe becomes all or mostly black holes, might they coalesce? ←Baseball Bugs What's up, Doc? carrots→ 19:22, 28 June 2024 (UTC)
 * No-hair theorem Sagittarian Milky Way (talk) 22:28, 28 June 2024 (UTC)

So, the consensus is that we have to wait and see? DOR (ex-HK) (talk) 21:25, 28 June 2024 (UTC)


 * There have been and are many theories about the end of the universe. Given our present understanding of cosmology, the observations do not yet allow us to settle on a single one. We may need to keep observing for a couple of trillions of years. I don't think it can be definitely ruled out that the universe is an ergodic system that happens to be in a very low state of entropy. If it is, the Poincaré recurrence theorem promises us an eternal return. --Lambiam 08:50, 29 June 2024 (UTC)


 * Re Lambiam's comment 17:01, 28 June: Entropy increases in both directions Reference desk/Archives/Miscellaneous/2017 February 10. 2A02:C7B:204:8E00:E0E4:8C0D:4571:6A6F (talk) 14:57, 5 July 2024 (UTC)