On Wednesday, February 19, 2025 at 3:11:21 PM UTC-7 Alan Grayson wrote:
On Wednesday, February 19, 2025 at 8:02:05 AM UTC-7 Quentin Anciaux wrote: AG, while the universe has large-scale structure (filaments, voids, and clusters), it is homogeneous and isotropic on scales beyond a few hundred megaparsecs. The cosmic microwave background (CMB) confirms this—on sufficiently large scales, the density variations average out. So while local structures exist, GR still treats the universe as homogeneous and isotropic for cosmological modeling. *I really don't see how you can claim the universe is isotropic and homogeneous. On the largest scale we see huge filaments containing galaxies in one direction, and in another we see huge voids. I am sympathetic to those who want to simplify E's field equations in order to get solutions, but they're assuming something which is obviously not true. AG * If the universe were contracting toward near-zero volume, local black holes might merge, but this doesn’t mean the entire universe becomes a black hole. The key difference is that black hole formation requires an event horizon surrounding a localized mass. If everything is contracting together, there’s no external region for an event horizon to form around. Instead, it would just result in a Big Crunch, a singularity different from a traditional black hole. *How would the singularity of a Big Crunch differ from a traditional BH? I would assume that as the numerous BH's form on a non-uniform contraction, there would be a small time interval just before all volume decreased to zero, to form the horizon you claim couldn't form. AG * *Would you agree that cosmologists who contemplate a Big Crunch, are assuming a finite universe which collapses to zero volume in finite time? if so, the idea of a finite universe is not beyond the pale. AG* Regarding terminology, infinite space doesn’t imply unchanging volume in any meaningful way. Even in an infinite universe, the concept of expansion/contraction is well-defined in GR through the scale factor, which describes how distances evolve over time. The word choice is not an issue of misinterpretation—it’s how the theory mathematically describes cosmic evolution. *"Infinite space doesn't imply unchanging volume in any meaningful way." Really? If the universe is infinite in spatial extent, the average distance between galaxies might decrease or increase, but the volume, remaining infinite, cannot change. I don't see how you can argue against this pov. AG* Quentin Le mer. 19 févr. 2025, 15:34, Alan Grayson <[email protected]> a écrit : On Wednesday, February 19, 2025 at 6:12:49 AM UTC-7 Quentin Anciaux wrote: AG, black hole formation typically requires asymmetric collapse because it happens within an existing spacetime with a surrounding region that remains unaffected. This allows an event horizon to form around the collapsing mass. In contrast, a globally contracting universe (if it were homogeneous and isotropic) wouldn’t have an external region for an event horizon to form—it would just keep collapsing as a whole. That’s why traditional BH formation and a collapsing universe are different scenarios. However, if the universe were not perfectly homogeneous (which it isn’t at smaller scales), then localized collapses could happen, leading to black holes forming within the universe. But this doesn’t mean the entire universe itself becomes one giant black hole. Instead, it would just mean that density fluctuations might create many black holes during contraction. The universe isn't homogeneous on large scales. We see huge filaments containing galaxies and huge voids between them. and if the volume is actually decreasing close to zero, it's possible these numerous BH's could merge and the entire universe could contract to one giant BH. AG Regarding expansion/contraction, if you think "average distance between galaxies" is a poor descriptor, what would you propose instead? The scale factor in the FLRW metric defines expansion/contraction precisely—it’s not just an "opinion," it’s how GR describes cosmic evolution. An infinite universe can still expand or contract in this framework without implying finiteness. I agree that an infinite universe can have distances between galaxies increasing or decreasing. But since the volume of space is infinite and hence unchanging, it was a poor choice of words to call this increasing or decreasing, which implies changes in volume. Now that's my opinion and it's not the first time a poor choice of words has occurred in science and other disciplines. AG Quentin Le mer. 19 févr. 2025, 13:04, Alan Grayson <[email protected]> a écrit : On Wednesday, February 19, 2025 at 3:56:10 AM UTC-7 Quentin Anciaux wrote: AG, if spacetime contracts to an extremely small volume but remains homogeneous and isotropic, it doesn’t form a black hole in the traditional sense. A black hole requires an asymmetric collapse of mass within an already-existing spacetime, leading to an event horizon. The early universe, however, wasn’t collapsing into an external space—it was space itself evolving. The conditions for a black hole simply don’t apply when everything is contracting uniformly rather than collapsing toward a single point within a larger spacetime. Why does BH formation requires asymmetric collapse? How can you know that a collapsing universe would do so uniformly, particularly since its mass distribution is not uniform? AG Regarding your second point, homogeneity and isotropy are not the same. A universe can be homogeneous but not isotropic (same properties everywhere but looks different in different directions). Likewise, it can be isotropic but not homogeneous (looking the same in all directions but with variations in density at different locations). Together, they imply a universe that has no special center or direction, but one doesn’t strictly require the other. Your assumption that spacetime contraction implies finiteness isn’t correct. It was what you seemed to be implying in your last post. AG An infinite universe can still contract or expand without needing to be finite. If you want to consider average distances between galaxies as your criterion, then concepts of contract and expand are poor descriptors IMO. AG The FLRW metric allows for infinite spatial extent while still having a changing scale factor over time. Quentin Le mer. 19 févr. 2025, 11:37, Alan Grayson <[email protected]> a écrit : On Wednesday, February 19, 2025 at 2:46:42 AM UTC-7 Quentin Anciaux wrote: A black hole forms when mass collapses within an external spacetime, creating an event horizon. The early universe wasn’t a localized collapse within surrounding space—it was the entire spacetime itself contracting or expanding. That’s why an event horizon doesn’t form around it. But if we're referring to a collapse not quite to zero volume, spacetime exists for a BH horizon to form. Also, if you grant that spacetime can contract or expand, are you not implicitly assuming a finite universe? AG The difference between homogeneous and isotropic is simple: homogeneity means the universe has the same properties everywhere on large scales, while isotropy means it looks the same in all directions. Together, they describe a universe that doesn’t have a preferred center or edge, unlike a collapsing object forming a black hole. ISTM that they're essentially equivalent, that one implies the other and vice-versa. AG Quentin Le mer. 19 févr. 2025, 10:20, Alan Grayson <[email protected]> a écrit : On Wednesday, February 19, 2025 at 1:40:13 AM UTC-7 Quentin Anciaux wrote: AG, the key issue is that the universe isn’t collapsing into a localized region—it’s expanding/collapsing everywhere. I am not being sarcastic to ask if that's a fact based on solid physics or your opinion? AG A black hole forms when mass collapses within a surrounding spacetime, creating an event horizon. The early universe, however, was homogeneous and isotropic on large scales, meaning there was no "outside" region for an event horizon to form around it. Firstly, I have a hard time distinguishing between homogeneous and isotropic. Also, if we assume the volume shrinks close to, but not equal to zero, there would be a region where an event horizon could form. AG In GR, a universe that contracts to extremely high density doesn’t necessarily become a black hole— "Doesn't necessarily", but is it possible or absolutely precluded? AG it follows different equations that describe a hot, dense state rather than a localized collapse. The FLRW metric describes a global evolution of spacetime, not a local gravitational collapse like a black hole. That’s why the early universe could be dense without forming a black hole—it didn’t have a surrounding spacetime to collapse into. As I posted earlier on another thread, there could be other metrics that predict a BH result close to the BB. AG Quentin Le mer. 19 févr. 2025, 09:33, Alan Grayson <[email protected]> a écrit : On Tuesday, February 18, 2025 at 11:19:45 PM UTC-7 Alan Grayson wrote: On Monday, February 17, 2025 at 11:59:45 PM UTC-7 Alan Grayson wrote: [image: Alan Grayson's profile photo] Alan Grayson 11:56 PM (1 minute ago) to Everything List Running the clock backward, and assuming the physical size of the universe converges to a singularity with zero volume at T=0, will it form a Black Hole? TY, AG Let me pose the problem differently; if the entire *universe* contracted to almost zero volume, is there anything we know that would prevent it from becoming a BH? AG -- 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/bbd61657-233c-45c5-b907-bdf2ef3354a2n%40googlegroups.com.
