Le ven. 21 févr. 2025, 09:58, Alan Grayson <[email protected]> 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 > Le ven. 21 févr. 2025, 09:11, Alan Grayson <[email protected]> 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 <[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/2da88b8b-f4e0-49cb-84be-c1d83029f991n%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/2da88b8b-f4e0-49cb-84be-c1d83029f991n%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]. 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