On Thursday, February 20, 2025 at 3:43:57 AM UTC-7 Quentin Anciaux wrote:
AG, whether the universe is finite or infinite, emerging from absolute nothing is equally miraculous. If a substratum exists, its infinity remains an open question. If it’s eternal, that’s already an infinity in itself. For quantum gravity, if spacetime isn’t quantized at Planck scales, LQG struggles. The real issue: does gravity itself need quantization, or just its interactions? Still unresolved. If spacetime can't be quantized, what can? Interactions? AG Quentin Le jeu. 20 févr. 2025, 11:31, Alan Grayson <[email protected]> a écrit : On Thursday, February 20, 2025 at 2:45:02 AM UTC-7 Quentin Anciaux wrote: AG, while filaments and voids extend across hundreds of megaparsecs, isotropy only breaks down locally, not globally. If you look at one specific region, it may appear anisotropic, but if you average over sufficiently large volumes, the universe still appears statistically homogeneous and isotropic. Observations of the cosmic microwave background (CMB) and large-scale galaxy surveys confirm this—on scales larger than 1 gigaparsec, the universe still obeys the Cosmological Principle. Regarding the universe emerging from nothing, if it were spatially infinite now, then yes—it would have had to be spatially infinite from the very beginning. An infinite universe doesn’t "grow" from a finite state in a finite time. If you assume the universe truly began from absolute nothing, then it must have instantaneously been infinite— *That's why I conclude the universe is finite, to avoid the singularity of intantaneously becoming infinite. Of course, I can't prove a transition from Nothing to Something. However, that Nothing from which it might emerge, isn't really Nothing, but an underlying substratum, which erupts, roughly analogous to a volcano. Yes, my model is speculative, but so are the other models. AG* *What's your take on quantum gravity models? Since measurements show the universe isn't quantize at Planck length, isn't LQG dead on arrival? In general, what are quantum gravity models trying to quantize? AG* which itself raises deep questions about the nature of such a transition. This is one of the reasons why many models (such as eternal inflation or cyclic universes) propose pre-existing states rather than a true emergence from nothing, which in itself is something miraculous. Quentin Le jeu. 20 févr. 2025, 10:14, Alan Grayson <[email protected]> a écrit : On Thursday, February 20, 2025 at 1:56:36 AM UTC-7 Quentin Anciaux wrote: AG, the Cosmological Principle (CP) applies at large scales, not at the scale of individual galaxies, filaments, or voids. While structure formation creates density variations, these variations average out when viewed over hundreds of megaparsecs. The CMB provides the earliest direct evidence of large-scale homogeneity, and while gravitational evolution has produced filaments and voids, the CP still holds statistically when considering the universe at a sufficiently large scale. The fact that structure forms doesn’t contradict the CP—it’s an expected consequence of small initial fluctuations growing under gravity. *When we can observe filaments and voids, aren't we observing way beyond hundreds of megaparsecs, and isotropy clearly breaks down? AG * Regarding the age of the universe, yes, it’s finite (around 13.8 billion years). If the universe is infinite now, then it must have been infinite from the beginning—infinity doesn’t "grow" in a finite time. This is why an infinite universe was already infinite at the Big Bang, just in an extremely dense and hot state. That’s not an opinion; it follows directly from how GR and the FLRW metric describe an infinite expanding spacetime. As for the Big Bang (BB), it is best understood as a transition rather than a singular "event." The BB represents the point where classical GR models break down, and physics needs quantum gravity to describe what came "before" (if that question even makes sense). The universe didn’t necessarily emerge from nothing— *Right, not necessarily, but if it did emerge from Nothing, could it have become infinite instantaneously? AG* the BB marks the beginning of our current phase of expansion, but there could have been a prior state (eternal inflation, a bouncing universe, or some other pre-BB phase). GR alone doesn’t tell us whether the universe "came into existence" at T=0—it just describes its evolution from an extremely hot, dense state forward. Look for hot big bang. Quentin Le jeu. 20 févr. 2025, 09:42, Alan Grayson <[email protected]> a écrit : 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/01c530aa-a9e6-4b05-a555-611c57566266n%40googlegroups.com <https://groups.google.com/d/msgid/everything-list/01c530aa-a9e6-4b05-a555-611c57566266n%40googlegroups.com?utm_medium=email&utm_source=footer> . -- 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/57165c35-0c47-44a5-a67e-e426590319d0n%40googlegroups.com.
