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by codethief
1030 days ago
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Thanks for your enlightening comment, raattgift! As always, you know much more about these things than I do, so I won't be able to contribute much to the BC discussion, but I always enjoy learning something new! The Visser paper I already knew from one of your previous comments. :-) |
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Nobody's wanted to talk seriously about "terminal" Big Crunch for many years -- the 1998 type Ia SN light curve redshift work that earned the 2011 Nobel prize in physics pretty much killed off the idea of an inertially-expanding universe, rather than an accelerated expansion. It's much easier to recollapse a inertial expansion: "inertial" means there was an effective single impulse that started the bits flying apart from one another, and the bits continue to fly apart along the lines of Newton's First Law. However, as far as anyone can tell the driver of the acceleration is a small positive cosmological constant (constant everywhere in time and space), and to recollapse you have to either turn off (and even reverse) the cosmological constant or you have to overwhelm it with a long-range "fifth force".
This line of discussion surfaces a Big Crunch paper with a wonderful title. In 1982 Andrei Linde proposed his "new inflation"[1] which introduced a scalar field as the driver of inflation. The scalar field starts with high values and slowly rolls down a potential "hill" taking on lower values as it rolls. Part of the hill is very shallow, "sufficiently flat", and so the potential rolls very slowly there; it steepens later. When the evolution of the scalar field is slow compared to the expansion, inflation occurs. When the scalar field is on the steep part, inflation ends. In 2004, Linde and co-authors wrote a paper that used a slow-rolling scalar field to drive the metric expansion (rather than inflation), and in that it is a "fifth force". Rolling slowly near the top of the potential "hill" the scalar field drives the accelerated expansion of space; rolling quickly in the steep part further down the "hill" the accelerated expansion stops and can even reverse leading to an accelerating contraction of space as the scalar field rolls further down an ever-steepening hill and as formerly-separated galaxy clusters become gravitationally bound to one another.
This leads to a relatively quick accelerated shrinking of space, much quicker than the expansion, motivating the poetic title of
Wang, Kratochvil, Linde & Shmakova, "Current observational constraints on cosmic doomsday" (2004, JCAP vol 12).
https://iopscience.iop.org/article/10.1088/1475-7516/2004/12... (aka <https://arxiv.org/abs/astro-ph/0409264>).
Warning, though, that is really a Bayesian reasoning paper dressed up as cosmology. :-) tl;dr: their scheme would destroy the universe in somewhere between 20 billion and 4 trillion years.
There is an assortment papers exploring "big bounce" where there is a partial contraction of the universe; this is often to try to abolish either the singularity at the early boundary or the extremely low entropy at the early boundary. I prefer a Big Crunch that stays crunched, rather than going all soggy and spongey and threatening to cause me to repeat all my mistakes in life over again. Cyclical expansion and contraction also appears with surprising frequency when diverse types of modifications to General Relativity are made, thus it pops up in lots of quantum gravity approaches. These all seem to struggle with observational support for the cosmological constant (and at the extreme anti-de Sitter space has a cosmological constant, but with the wrong sign, which does weeeiiird things, like Hawking radiation reflects off the boundary -- for a single central black hole that means it never evaporates; the same reflecting boundary conditions in an AdS universe with lots of black holes makes the whole universe unstable; and (very) weak vacuum perturbations lead to black holes after sufficiently long times).
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[1] https://www.sciencedirect.com/science/article/abs/pii/037026... (which is also on sci-hub)