What Are the Different Horizons and Distance Scales in Cosmology?

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In summary, redshift can help estimate the size of the observable universe, which is the part of the universe that we can currently see. However, there is also a possibility that the entire universe is infinite in size. The estimated size of the observable universe is about 46-47 billion lightyears, based on measurements of the curvature of the observable part. This distinction between the observable universe and the whole universe is often not explicitly stated in popular media, leading to confusion among readers. It is important to note that the estimates for the size of the universe have a range of uncertainty and allow for the possibility of it being infinite.
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hobobobo
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I was wondering how can the redshift tell you the size of the universe when there is a cosmological horizon where scientist can't see since light hasn't traveled here yet.
 
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By "universe", most of the time we actually mean the "observable universe".
 
  • #3
oh ok thanx
 
  • #4
yenchin said:
By "universe", most of the time we actually mean the "observable universe".

Who is the "we" that you are talking about. It is not scientists, in my experience. The astronomers I know are very careful to say observable universe when that is what they mean.

And when they say universe, they do NOT mean only the observable part.

Yenchin, I suspect you do not know what you are talking about. So I would ask you to find a scientific paper in some professional journal where the author agrees with you and says universe when what he actually means is the observable universe.
 
  • #5
hobobobo said:
I was wondering how can the redshift tell you the size of the universe when there is a cosmological horizon where scientist can't see since light hasn't traveled here yet.

Cosmologists can only ESTIMATE the size of the universe. What you point out is perfectly right. They don't try to say what the size is with 100 percent certainty.
There were a couple of papers by David Spergel and Niel Cornish that said it must be AT LEAST so-and-so big, based on WMAP measurements of the Cosmic Microwave Background. And that allows for the possibility that it is infinite in size.

I'd be glad if you would reformulate your question, Hobo. What is it you want to know?
There are a few professional estimates of the size, and they involve a range of uncertainty and allow for it being infinite. Do you want to know how those estimates are derived?

Or do you want to know how they come up with the common figure of 46 or 47 billion lightyears, for the particle horizon (the farthest away things are that we can see, in the sense of getting light from them today.) Do you want to know how they get the figure of 46 or 47?

That is the current radius of the part of universe that we are currently observing. The whole is naturally bigger---and the way they estimate its size is essentially by measuring the curvature of the part we can see. Assuming uniform average positive curvature, the flatter the larger-----and the curveder the smaller.

So what do you want to know about? The size of the part we can see, or the estimates of the size of the (possibly infinite) whole?
 
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That is an interesting point, marcus. I was under the impression cosmologists were referring to the observable universe unless otherwise stated.
 
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Marcus: By "we" I simply mean in the popular treatment of the subject where often the use of universe and observable universe is not explicitly spelled out and requires the readers to judge which is which. I believe the OP was confused by such a vague statement from the popular treatment, and thus the "we" was referring to the sloppy usage that he or she may have encountered somewhere. I certainly am not implying cosmologists are sloppy!

I have also made the implicit assumption that the OP was asking about the size of the current observable radius, and not the whole universe (which might well be infinite though need not be), as most popular accounts and expository articles for lay readers concern with the former rather than the later.

Should have been more careful and explicit, but I was rushing off for lecture this morning when I reply.
 
  • #8
yenchin said:
Marcus: By "we" I simply mean in the popular treatment of the subject where often the use of universe and observable universe is not explicitly spelled out and requires the readers to judge which is which. I believe the OP was confused by such a vague statement from the popular treatment, ...

Yenchin, I totally agree... about the popular treatment. I think you are right about the OP, who may well have been confused by loose talk in the popular media.

Here at PF Cosmology forum this confusion generated by vagueness in the media comes up repeatedly. If we ever have a FAQ, or a sticky thread for basic facts for newcomers, this distinction between the whole and the observed part should probably be fact number one.

And then we'd need to define the various horizons/distance scales, like

Hubble distance about 14 Gly
Event horizon radius about 16 Gly
Distance to CMB origin 45 Gly
Particle horizon 46 Gly

And list various redshifts, and the estimated distances they correspond to in the standard model.

Or maybe that is too much to put in a "basic facts" thread, but at least the distinction between whole and observed part would be made explicit.
Yenchin, you are welcome to try spelling these things out. I think we eventually will get a sticky thread and it will just be a question of who has the time to write the content and edit for brevity.
 

1. What is the size of the universe?

The size of the universe is constantly changing and expanding. Currently, the observable universe is estimated to be about 93 billion light years in diameter.

2. How do scientists measure the size of the universe?

Scientists use various methods to measure the size of the universe, such as observing the cosmic microwave background radiation, studying the redshift of distant galaxies, and using mathematical models based on the expansion of the universe.

3. Is there an edge to the universe?

It is currently believed that the universe has no edge or boundary. The universe is thought to be infinite and expanding in all directions.

4. How long will it take to reach the edge of the universe?

Since the universe is constantly expanding, it is not possible to reach the edge of the universe. Even if we were to travel at the speed of light, it would take an infinite amount of time to reach the edge.

5. Can the size of the universe change?

The size of the observable universe is constantly changing and expanding due to the expansion of space. However, the overall size of the entire universe is still unknown and may be infinite.

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