The discussion centers on whether light can travel in a circle at the edge of the observable universe, specifically in relation to the mass required to form a black hole event horizon. Participants conclude that the observable universe does not contain sufficient mass to create such an event horizon, and light emitted tangentially would not loop back. The Schwarzschild radius, calculated at 2.2*10^26 m for a mass of 1.5*10^53 kg, is deemed irrelevant in the context of the universe's expansion and FLRW spacetime. The edge of the observable universe varies based on the observer's location, further complicating the concept of light traveling in circular paths.
PREREQUISITESAstronomers, physicists, and cosmologists interested in the nature of light, black holes, and the structure of the universe will benefit from this discussion.
No.Devin-M said:Is there sufficient mass within the observable universe’s volume to form a black hole event horizon around the observable universe
No.Devin-M said:and, if yes would light fired tangentially at the edge of our observable universe ever loop back around in a circle or spiral inwards?
The Schwarzschild radius isn't really relevant here. The event horizon is only at the Schwarzschild radius for a non-rotating black hole. And light doesn't do circular orbits at the event horizon anyway.Devin-M said:For an ordinary mass of 1.5*10^53 kg then the Schwarzschild radius would be 2.2*10^26 m, is that correct?
Wouldn't the edge of the observable Universe depend on where you are in the Universe?. In other words, the "edge" we can see is not the same edge as an observer 10 billion light years away can see.Devin-M said:Is there sufficient mass within the observable universe’s volume to form a black hole event horizon around the observable universe and, if yes would light fired tangentially at the edge of our observable universe ever loop back around in a circle or spiral inwards?
Yes, which is perhaps a clearer way of getting at what I was trying to say in #4.Kevin the Kiwi said:Wouldn't the edge of the observable Universe depend on where you are in the Universe?. In other words, the "edge" we can see is not the same edge as an observer 10 billion light years away can see.
There are several previous threads that explain why the early universe was not a black hole. Which I assume is the question that you posed cryptically in your original post?Devin-M said:So at the time the light from the farthest galaxies we can observe was emitted, the mass we can see within the observable universe wasn’t enclosed within that radius?
Yeah, but it's not relevant. Schwarzschild radius and event horizon are not synonymous except in Schwarzschild spacetime, which the large scale universe is almost completely unlike. You can't even define an event horizon in the cosmological FLRW spacetime (at least in the sense of a black hole event horizon - there is an unrelated thing with the same name, unfortunately).Devin-M said:I must have missed those threads can you post a link? I’m confused because for 13.8 billion light years I get a radius of 1.3*10^26 m and for a mass of 1.5*10^53 kg I get a Schwarzschild radius of 2.2*10^26 m.
Not only that, but technically, if you were standing next to me we would each have our own observable universes (with essentially total overlap but technically not exactly the same)Kevin the Kiwi said:Wouldn't the edge of the observable Universe depend on where you are in the Universe?. In other words, the "edge" we can see is not the same edge as an observer 10 billion light years away can see.
Devin-M said:So at the time the light from the farthest galaxies we can observe was emitted, the mass we can see within the observable universe wasn’t enclosed within that radius?
That's cool. I've never had my own observable universe before. First thing I need is a suitable title, then there are going to be some changes around here.phinds said:Not only that, but technically, if you were standing next to me we would each have our own observable universes (with essentially total overlap but technically not exactly the same)
"Lord of all I survey except the bits others survey too"? There's a lot of volume out there, but there's not a lot in it...Kevin the Kiwi said:First thing I need is a suitable title,
How could that possibly be the case?Devin-M said:Had the most distant observable stars & galaxies formed while still within a volume less than the Schwarzschild radius for the combined mass within that volume at the time?
I thought at the time the light was emitted by the most distant galaxies we can see, they were less than 13.8 billion light years distant from us (less than 1.3*10^26 m) and for a mass of 1.5*10^53 kg (the normal mass of the observable universe) I get a Schwarzschild radius of 2.2*10^26 m.PeroK said:How could that possibly be the case?
You are talking about a situation where a large mass is surrounded by a MUCH less dense area. That's not the case with the universe. What is now the observable universe was just a part of a huge (possibly infinite) fairly uniform mass, so there was no center around which a Schwartzchild radius would be meaningful.Devin-M said:I thought at the time the light was emitted by the most distant galaxies we can see, they were less than 13.8 billion light years distant from us (less than 1.3*10^26 m) and for a mass of 1.5*10^53 kg (the normal mass of the observable universe) I get a Schwarzschild radius of 2.2*10^26 m.
You may be right. The Schwarzschild radius increases linearly with mass; whereas, the radius of something massive increases with the cube root of the mass. Eventually, if you have a large enough volume of low density matter in a homogeneous universe, then you must exceed the Schwarzschild radius at some volume.Devin-M said:I thought at the time the light was emitted by the most distant galaxies we can see, they were less than 13.8 billion light years distant from us (less than 1.3*10^26 m) and for a mass of 1.5*10^53 kg (the normal mass of the observable universe) I get a Schwarzschild radius of 2.2*10^26 m.