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Can the Universe be a Black Hole?

Many years ago, after reading a book by Isaac Asimov (not a novel, but I forgot the title) I started wondering if it is be possible that the Universe itself is a black hole. I first started reading up some (older) books from my dad. Now I am not a physicist, so I had to approach all this with 'simple' Newtonian calculations.

So I hear about the event horizon. The imaginary shell around a body where the escape velocity exceeds the speed of light. Here my first questions were raised. Why would people refer to a Black Hole as a dense object? I do understand that the mass and density are high, but since we can not look inside, it may very well be such that the actual mass is concentrated in an even smaller space with the remaining space all the way up to the event horizon being virtually empty. The Universe is virtually empty in the grand scheme of things I figured.

An other reason I started to wonder was the following. In several books I saw, references to both the Sun and the Earth where claims were made that if the Sun were to be compressed into a black hole it would have a diameter of several kilometers. In case of earth this would be only a few centimeters. Here I saw immediately that the size of the event horizon for the Sun and the Earth was not in the same ratio anymore as the actual dimensions of both the Sun and the Earth.

So now I took pen and paper and my calculator. I also went to the library (pre-internet days...) and gathered me some books with a little more background. Being aware that to do this properly I would likely need to know a little more about Einstein's theory of relativity, I figured I should at least compare the Newtonian calculations. And to my surprise my calculations gave the same results for earth and the Sun as I found in the books. That was encouraging, but at the same time I know full well that extrapolating this from the earth and the Sun is probably not very accurate... But this was the best I could do.

So now I had to look up the mass for the Universe. This was a real challenge and I found various numbers and they all differed significantly. I took the 4 numbers I found and repeated my calculations for all 4 obtained numbers. With only 1 given mass for the Universe it would not have been a Black Hole, but the remaining 3 masses for the Universe all indicated that it would be. What further strengthened my belief was that the number for which the idea had failed was the oldest mass of the Universe I found and I noticed that every new number was a higher mass. So this was how I got to the conclusion that the Universe could be a Black Hole.

Now, well over 25 years later, I understand that the universe is not a Black hole. But that is simply, because if it was, someone would surely have published something to this regard by now. I had, back than, mentioned this to several people and without an exception, everyone told me that this was not possible, but nobody has ever explained to me why.

So maybe I should rephrase my question to; Can someone explain to me why the Universe is not a black hole?

Joey

Joey Joystick
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  • To clear up a misconception, the event horizon is not a physical boundary in the sense that something happens when you cross it. The mass of the black hole in concentrated down into a singularity, and so upon crossing it, you would travel in empty space for a time, and nothing would look different per se, since all the light from the rest of the universe can still get to you, the light bouncing off of you just can’t get to them. – Justin T Feb 27 '22 at 18:51
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    Also, in this paper https://arxiv.org/abs/1309.1487, but for reasons slightly different than what you mention, a similar conclusion is reached, involving white holes forming universes from black holes in other universes – Justin T Feb 27 '22 at 18:55
  • No, sorry that does not really answer my question GrapeFruitIsAwesome. it is going in that direction, but since the mass of the Universe is soo much bigger, I think answering it in this context is fair to ask. If that turns out to be the same answer, I apologize, but I am not knowledgeable enough to make that determination. – Joey Joystick Feb 27 '22 at 18:58
  • I am aware that it is not a physical boundary Justin. I think because of how things are often portrayed in popular science, I simply assumed that a black hole was considered an object reaching out to the event horizon. And I thought this was strange and wrong. Now I understand why... – Joey Joystick Feb 27 '22 at 19:06
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    Related question on our sister site: https://physics.stackexchange.com/questions/23118/are-we-inside-a-black-hole, which has many links, including https://physics.stackexchange.com/q/247168/123208 – PM 2Ring Feb 27 '22 at 22:06
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    BTW, the whole universe is probably infinite, as I mention here. Values of the mass or size of the universe are generally in reference to the observable universe. – PM 2Ring Feb 27 '22 at 22:16
  • Thanks for those links 2Ring. That definitely explains a lot more on the substance. A very interesting read and it clearly shows that there is not yet a single conclusion as well. My simplified projection is obviously out of the window. On a slightly different note, I see many references to Physics. should I have posted this over there instead? – Joey Joystick Feb 28 '22 at 01:37

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