On Thursday, February 20, 2025 at 12:22:32 AM UTC-7 Quentin Anciaux wrote:
Le jeu. 20 févr. 2025, 08:05, Alan Grayson <[email protected]> a écrit : On Wednesday, February 19, 2025 at 11:54:52 PM UTC-7 Quentin Anciaux wrote: AG, a Big Crunch scenario does not necessarily assume a finite universe. An infinite universe can also undergo a global contraction—meaning that while distances between galaxies shrink, the universe itself remains infinite at all times. A finite universe collapsing to zero volume in finite time is just one possibility, but it’s not required for a Big Crunch model. The idea of a finite universe is, of course, not beyond the pale—it remains an open question in cosmology. Regarding infinite space and "unchanging volume," the key issue is that volume in an infinite universe is not a meaningful quantity in the way you are describing it. Yes, if the universe is infinite now, it was always infinite, but that doesn’t mean nothing changes—the scale factor determines how distances evolve. The phrase "volume cannot change" is misleading because in an infinite universe, there is no finite, well-defined total volume to begin with. Instead, we talk about the expansion or contraction of distances within that infinite space, which is physically meaningful. Quentin *Do you concede that the universe isn't isotropic or homogeneous? What is the nature of the singularity in the Big Crunch for a finite and infinite universe? How does it differ from the standard BH? AG * AG, on small scales, the universe is neither isotropic nor homogeneous due to the presence of galaxies, filaments, and voids. However, on large scales, it is effectively homogeneous and isotropic, as confirmed by the cosmic microwave background (CMB) and large-scale surveys. The Cosmological Principle—which assumes large-scale homogeneity and isotropy—remains valid for describing the universe at scales beyond a few hundred megaparsecs. *While the CMB is approximately uniform in temperature, but that's at 380,000 years after the BB, but when we observe it at later times, there are huge filaments with a plethora of galaxies, separated by huge voids. This cannot be isotropic since the scale is hugely large. It's also not homogeneous if we consider the property of density. So, IMO, whereas the universe seems to satisfy the CP at 380,000 years, its subsequent evolution contradicts the CP. AG* *Do you agree that the age of the universe is finite, so if it's infinite, that condition could not have evolved over its finite lifetime, but must have been its property as an initial condition? AG* Regarding the Big Crunch singularity, it differs depending on whether the universe is finite or infinite: Finite Universe: If the universe is closed and finite, a Big Crunch would resemble the time-reversed version of the Big Bang—space collapses to a singularity where density, temperature, and curvature diverge. It’s a true spacetime singularity in GR, where classical physics breaks down. *Do you believe in the BB as a specific event, at time defined as T=0, from which the universe emerged from some underlying substratum? IOW, do you believe the universe came into existence at the BB? AG* Infinite Universe: If an infinite universe undergoes a Big Crunch, distances still shrink everywhere, but it remains infinite at all times. The singularity would be a global state of infinite density everywhere rather than a localized point. Unlike a black hole, this singularity isn’t "contained" within an event horizon—it involves the entire universe. A black hole singularity, in contrast, is localized—it forms due to asymmetric collapse, creating an event horizon around a specific region of space. In a Big Crunch, there’s no such horizon enclosing the universe because space itself is collapsing uniformly (on large scales). That’s the fundamental difference: a black hole singularity is localized within spacetime, while a Big Crunch singularity is the entire spacetime itself collapsing. Quentin -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/everything-list/6053dd31-9031-441e-9d46-f736a07c354bn%40googlegroups.com.
