# Would light travel in a circle at the edge of the observable Universe?

• B
• Devin-M
In summary, the conversation discussed the possibility of a black hole event horizon forming around the observable universe and whether light fired tangentially at the edge of the observable universe would loop back around. It was determined that the Schwarzschild radius is not relevant in this scenario and that the event horizon cannot be defined in the cosmological FLRW spacetime. Additionally, it was noted that the observable universe differs depending on the observer's location and that technically, each individual has their own observable universe. The conversation also touched upon the formation of distant stars and galaxies within a volume smaller than the Schwarzschild radius.
Devin-M
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?

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?
No.

For an ordinary mass of 1.5*10^53 kg then the Schwarzschild radius would be 2.2*10^26 m, is that correct?

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?
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.

Furthermore, an FLRW spacetime is the same everywhere, so light at the edge of the observable universe would have to orbit every point at the appropriate distance at the same time. That doesn't make sense.

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?
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.

Ibix
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.
Yes, which is perhaps a clearer way of getting at what I was trying to say in #4.

russ_watters and Kevin the Kiwi
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?

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?
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?

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.

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.
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).

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.
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)

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?

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)
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.

hmmm27, collinsmark, berkeman and 1 other person
Kevin the Kiwi said:
First thing I need is a suitable title,
"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...

Not bad. But "Lord" seems a bit over used to be honest and not nearly majestic enough. Also, I won't be doing any surveying, I have minions for that.

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?

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?
How could that possibly be the case?

PeroK said:
How could that possibly be the case?
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.

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 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
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.

I must admit I never thought of it that way.

The Schwarzschild radius of the Sun is only 3km and that of the Milky Way is about one light-year. But, if you take a large enough volume of galaxies, then you do exceed the Schwarzschild radius for the total mass.

The same proviso applies that the Schwarzschild radius is not relevant for an expanding universe.

Devin-M

## 1. How far can light travel in the observable Universe?

Light can travel approximately 93 billion light-years in the observable Universe. This is the distance that light has had time to travel since the Big Bang, which is estimated to have occurred about 13.8 billion years ago.

## 2. Does light travel in a straight line in the observable Universe?

Yes, light travels in a straight line in the observable Universe. This is because light is a form of electromagnetic radiation, which moves in a straight line unless it is affected by gravity or other forces.

## 3. Can light travel in a circle at the edge of the observable Universe?

No, light cannot travel in a circle at the edge of the observable Universe. This is because the Universe is constantly expanding, and light would not be able to travel in a circular path without encountering obstacles or being affected by gravity.

## 4. How long would it take for light to travel in a circle at the edge of the observable Universe?

It is impossible to determine how long it would take for light to travel in a circle at the edge of the observable Universe, as this scenario is not possible. Light travels at a constant speed of approximately 299,792,458 meters per second, so it would take a very short amount of time for light to travel a circular path at the edge of the observable Universe if it were possible.

## 5. Would light be affected by the curvature of space at the edge of the observable Universe?

Yes, light would be affected by the curvature of space at the edge of the observable Universe. This is because the curvature of space is caused by the presence of massive objects, such as galaxies, which can bend the path of light through gravitational lensing. However, this would not cause light to travel in a circular path at the edge of the observable Universe.

• Astronomy and Astrophysics
Replies
25
Views
2K
• Astronomy and Astrophysics
Replies
4
Views
1K
• Astronomy and Astrophysics
Replies
2
Views
823
• Astronomy and Astrophysics
Replies
1
Views
1K
• Astronomy and Astrophysics
Replies
4
Views
1K
• Astronomy and Astrophysics
Replies
6
Views
929
• Astronomy and Astrophysics
Replies
6
Views
2K
• Astronomy and Astrophysics
Replies
1
Views
1K
• Astronomy and Astrophysics
Replies
1
Views
1K
• Astronomy and Astrophysics
Replies
5
Views
1K