On Friday, February 21, 2025 at 4:07:49 AM UTC-7 Quentin Anciaux wrote:
AG, an always-infinite universe doesn’t necessarily remove the need for a Big Bang (BB)—it just reframes it. The BB isn’t just about spatial finiteness; it’s about the hot, dense early state that led to the Cosmic Microwave Background (CMB) and the observed expansion. Even in an infinite universe, the BB still marks a transition from a uniform, high-energy state to the structure we see today. In the always-infinite universe, the BB is not a singularity, but a state of maximum density and temperature. In this scenario, it would seem possible that we could use GR to determine that state precisely, but it's not the case. Do you have an explanation for this situation? AG In my view, the universe had a beginning. It emerged from something else, and has been expanding ever since. AG If the universe was always expanding, that raises the question: expanding from what? Even in models like eternal inflation, there’s still a "beginning" to our observable region, even if the entire multiverse has no start. A contraction phase remains hypothetical, but the BB framework remains necessary because it explains key observations—CMB, nucleosynthesis, and large-scale structure—not just the origin of a finite space. Quentin Le ven. 21 févr. 2025, 11:51, Alan Grayson <agrays...@gmail.com> a écrit : On Friday, February 21, 2025 at 3:20:16 AM UTC-7 Quentin Anciaux wrote: AG, some cosmologists consider a changing spatial volume because it's the natural outcome of General Relativity applied to a finite universe with curvature. In a closed (positively curved) universe, the volume changes as the universe expands or contracts. However, assuming an always-infinite universe is simpler mathematically but conceptually non-trivial, it requires explaining how an infinite universe emerges or evolves without contradictions. The Big Bang isn’t an explosion into pre-existing space, but rather an expansion of space itself, which makes defining an initial condition for an infinite universe more subtle. In an always-infinite universe, there is no need to assume a BB. It's superfluous. It applies to a spatially finite universe which has a creation event (called the BB) and implied by the existence of CMR. This universe has always been expanding, and the contraction phase is hypothetical and will occur under certain specific conditions. AG Many models, including eternal inflation and some interpretations of the ΛCDM model, favor an always-infinite universe, while others consider a finite but vast one. The debate persists because both scenarios have theoretical and observational challenges. Quentin Le ven. 21 févr. 2025, 10:47, Alan Grayson <agrays...@gmail.com> a écrit : On Friday, February 21, 2025 at 2:31:28 AM UTC-7 Quentin Anciaux wrote: Le ven. 21 févr. 2025, 09:58, Alan Grayson <agrays...@gmail.com> a écrit : On Friday, February 21, 2025 at 1:24:18 AM UTC-7 Quentin Anciaux wrote: Try to use the internet sometimes.... https://www.astro.rug.nl/~weygaert/tim1publication/cosmo2019/cosmology2019.lect3a.cosmological_principle.pdf https://pages.uoregon.edu/jschombe/cosmo/lectures/lec05.html Interesting. TY.The Gamma Ray Bursts are pretty convincing to establish isotropy and homogeneity. One other thing. In Penrose's oscillating universe model, does he assume the volume expands and contracts periodically, or does he assume it remains infinite in volume throughout, and that the average distances between galaxies increases and decreases periodically? AG It assumes it is infinite and remains infinite, only density varies. Quentin Why do you think some cosmologists posit a universe which is changing in spatial volume, when it could be simplier to assume it's infinite and has always been infinite? Mostly, I've heard the former, which is hard to explain, that it's not like an explosion expanding into a pre-existing space. AG Le ven. 21 févr. 2025, 09:11, Alan Grayson <agrays...@gmail.com> a écrit : On Thursday, February 20, 2025 at 8:51:53 PM UTC-7 Alan Grayson wrote: 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. Can you cite a paper which supports your claim? Deep space surveys which show the filaments and voids at distances greater than hundreds of megaparsecs. So, since seeing is believing, at huge distances the CP seems to fail. Despite my skepticism, I am willing to read any paper that substantiates your claim. AG To observe filaments and voids, how far out are they to be viewed? If you go out 500 megaparsecs, that's slightly less than 1.6 billion light years, a small fraction of the radius of the visable universe, which 46 billion light years. At that distance, if they can be viewed, the universe doesn't seem to be isotropic. AG 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—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. Every thing about the universe is miraculus if you think about it more deeply. I tried to form a model of the early universe which is simplest, and a finite bubble, arising from an infinite substratum, approximately spherical in shape, at an ultra-high temperature, seems to fill the bill. AG Quentin Le jeu. 20 févr. 2025, 10:14, Alan Grayson <agrays...@gmail.com> 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 <agrays...@gmail.com> 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 <agrays...@gmail.com> 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 everything-list+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/everything-list/bd681ba5-3df7-470f-a6d0-15292ee4563fn%40googlegroups.com.
