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.
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. 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/fee93dd9-e889-429d-8984-1a1a1d30aba8n%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/fee93dd9-e889-429d-8984-1a1a1d30aba8n%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/8b1080ee-31a3-4b39-9ad5-54cfc0ce9b5fn%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/8b1080ee-31a3-4b39-9ad5-54cfc0ce9b5fn%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/CAMW2kApaeTgAPiYx2%3DbGdKNSQQmgaKKR9VhQhABNDsrtsbMuTw%40mail.gmail.com.
