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> the Big Crunch, where gravity would pull the universe back to a single point Just as the big bang didn't happen in a single place, a Big Crunch wouldn't be a

> the Big Crunch, where gravity would pull the universe back to a single point Just as the big bang didn't happen in a single place, a Big Crunch wouldn't be a contraction back to a single point. Rather, the scale factor [1] that characterises cosmic expansion would simply(!) decline back to zero. [1] https://en.wikipedia.org/wiki/Scale_factor_(cosmology) https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

raattgift 9y ago
Well, "we don't know" applies here. To a first order approximation (e.g. in the FLRW perfect fluid model), a Big Crunch would resemble a time-reversal of the standard big bang cosmology. In our universe, time reversal results in galaxies appearing at a comoving observer's horizon, and everything inside the horizon becoming denser and hotter. Ultimately the density and heat is expected to result in beyond-the-Standard Model physics in the matter sector, and (hopefully) new physics in the gravitational sector. We don't really know what those physics will be. Everyone hopes for something that prevents a gravitational singularity from forming, but so far what we have in terms of possible stabilizers are conjectural at best. The time-reversals of Big Crunch and the usual-forward-time big bang cosmology with structure formation relate to the BH information loss problem. For a non-eternal BH when we time-reverse from i+ we have a gas of Hawking radiation that collapses into a time-reversed BH which eventually emits matter with much less (Boltzmann) entropy -- ions, molecules, dust, stars and even the things orbiting them. (A time-reversed stellar BH likewise will eventually spit out a white dwarf or neutron star with their respective complex layerings). How does a time-reversed BH "know" how to spit out whole stellar-mass objects when only time-reversed Hawking radiation fell into it? Most quantum gravity programmes hope that strong gravity[1] produces an environment where this sort of (time-reversed) structure formation is likely. Flipping the arrow(s)-of-time is extremely useful for reasoning in a setting in which strong gravity is important, and extra-underlines the question of why we have the arrow(s)-of-time we do in the first place. "Boundary conditions did it" is pretty unsatisfying. - -- [1] i.e., where the radius of curvature is on the order of the Planck length; this mostly comes from the non-renormalizability of perturbatively quantized gravity (where we quantize perturbations of a background metric) and from work in finding the effective field theory limit of semiclassical gravity; generally it's very close to a gravitational singularity and -- depending on the censorship conjecture -- always invisible to outside observers.

raattgift 9y ago

Well, "we don't know" applies here. To a first order approximation (e.g. in the FLRW perfect fluid model), a Big Crunch would resemble a time-reversal of the standard big bang cosmology. In our universe, time reversal results in galaxies appearing at a comoving observer's horizon, and everything inside the horizon becoming denser and hotter. Ultimately the density and heat is expected to result in beyond-the-Standard Model physics in the matter sector, and (hopefully) new physics in the gravitational sector. We don't really know what those physics will be. Everyone hopes for something that prevents a gravitational singularity from forming, but so far what we have in terms of possible stabilizers are conjectural at best. The time-reversals of Big Crunch and the usual-forward-time big bang cosmology with structure formation relate to the BH information loss problem. For a non-eternal BH when we time-reverse from i+ we have a gas of Hawking radiation that collapses into a time-reversed BH which eventually emits matter with much less (Boltzmann) entropy -- ions, molecules, dust, stars and even the things orbiting them. (A time-reversed stellar BH likewise will eventually spit out a white dwarf or neutron star with their respective complex layerings). How does a time-reversed BH "know" how to spit out whole stellar-mass objects when only time-reversed Hawking radiation fell into it? Most quantum gravity programmes hope that strong gravity[1] produces an environment where this sort of (time-reversed) structure formation is likely. Flipping the arrow(s)-of-time is extremely useful for reasoning in a setting in which strong gravity is important, and extra-underlines the question of why we have the arrow(s)-of-time we do in the first place. "Boundary conditions did it" is pretty unsatisfying. - -- [1] i.e., where the radius of curvature is on the order of the Planck length; this mostly comes from the non-renormalizability of perturbatively quantized gravity (where we quantize perturbations of a background metric) and from work in finding the effective field theory limit of semiclassical gravity; generally it's very close to a gravitational singularity and -- depending on the censorship conjecture -- always invisible to outside observers.