Why didn't the Big Bang form a Black Hole?

[peterraymond]

TL;DR Summary

Why didn't the big bang form a black hole?

(pop science) It seems like the mass in the universe at the point of origin was way higher and the size way smaller than required to form a black hole, so why didn't our universe just sit forever as a single black hole? From a figure for the mass of the universe you could calculate the event horizon diameter. I can imagine that maybe it was a universe expanding, not just a massive object, but shortly after there was plenty of mass inside that universe that was very closely packed, so there could still be very many black holes forming shortly after the big bang, or a single one sitting out there someplace.

 

[PeterDonis]

(pop science)

Is not a good source for learning actual science.

why didn't our universe just sit forever as a single black hole?

Because it was rapidly expanding. All of the calculations cited in pop science sources about how much mass there was in how small a volume vs. what would be required to form a black hole are only valid for a static object surrounded by vacuum. They are not valid for a rapidly expanding universe which is roughly the same density everywhere.

 

[PeterDonis]

See this Insights article for a somewhat longer answer:

https://www.physicsforums.com/insights/universe-black-hole/

 

[peterraymond]

Thanks for the replies. It did seem like it would be a FAQ - I just hadn't seen it. My "(pop science)" label was included not to say I had learned something from pop science, but just a warning that that was the level of the questioner.

One part of the answer leads naturally to another question I've been thinking about, but that will take a little longer to craft.

Thanks again.

 

[Orodruin]

My "(pop science)" label was included not to say I had learned something from pop science, but just a warning that that was the level of the questioner.

This is what the thread level is for. If you only have pop science knowledge about the subject you should pick thread level B. Level I indicates undergraduate level knowledge.

 

[Devin-M]

All of the calculations cited in pop science sources about how much mass there was in how small a volume vs. what would be required to form a black hole are only valid for a static object surrounded by vacuum. They are not valid for a rapidly expanding universe which is roughly the same density everywhere.

I have a question about this answer. I believe a few millions of years after the big bang, the observable universe, if it was surrounded by vacuum, had enough density for an event horizon. But larger black holes get less dense as the size increases. So does it mean that there are only 2 possibilities: that either A the universe is infinite and consistent density therefore no event horizon or B the universe is not infinite, but has an event horizon since the part we can see had sufficient density in the past to have an event horizon, and if the part we can’t see has the same density, it must be surrounded by an event horizon, since larger black holes are less dense?

 

[PeterDonis]

I believe a few millions of years after the big bang, the observable universe, if it was surrounded by vacuum, had enough density for an event horizon.

You left out the "static object" part. Our observable universe has always been expanding. It has never been static. So even if we assume (contrary to our best current models) that outside our observable universe is vacuum, it still would not have formed a black hole. The expansion makes a difference.

does it mean that there are only 2 possibilities

No. Again, you're leaving out expansion, which is crucial. See above.

 

[Orodruin]

if it was surrounded by vacuum

Also ... It wasn't. At least not according to the basic cosmological models, which assume the entire universe to be homogeneous - that includes regions outside the currently observable universe.