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.
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. An infinite universe can still contract or expand without needing to be finite. 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/d533d2d5-c478-45c3-8a18-d2f17b23653bn%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/d533d2d5-c478-45c3-8a18-d2f17b23653bn%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/CAMW2kAp1rv7P3kgJneF%3DskKiByEHG_%2BCJ-FMX3qNkY6O5m_2iA%40mail.gmail.com.
