# Half Closed Universe: Is It Finite?

• sea100
In summary: The size of the universe is still a theoretical question that has yet to be answered.It is not known whether...the universe is infinite or finite. The size of the universe is still a theoretical question that has yet to be answered.
sea100
if the universe is not infinite in size is that mean a closed universe ?

infinity is a nice number for mathematicians but in physics you can put a finite number on everything, big or small, if not then there is a singularity.
From current theories we can explain that the universe has a finite size, which can be calculated approximately from speed of light(c) and age of the universe(t). approximately the radius of a spherical universe is 'ct' and hence the volume.
You can do the same rigorously using FLRW metric and GR.
cosmologists use the term closed universe to explain on of the possible end of the universe. a closed universe is one which will contract and end in a big crunch.

what is a closed universe?

balaonspace said:
From current theories we can explain that the universe has a finite size, which can be calculated approximately from speed of light(c) and age of the universe(t). approximately the radius of a spherical universe is 'ct' and hence the volume..

nonsense. we can do no such thing. you are talking about the OBSERVABLE UNIVERSE while using the term "universe" (incorrectly) which is not what the question was about, it was about the universe.

sea100 said:
if the universe is not infinite in size is that mean a closed universe ?

Yep, has to be. The actual question of WHETHER the universe is infinite or not is an open one.

nitsuj said:
what is a closed universe?

Think of the surface of a baloon. NOT the ballon, JUST the surface. If you are limited to traveling only on the surface, then traveling in any straight line brings you back to where you started and that's a closed surface. "Closed" for the universe can be more complicated, but one POSSIBLE closed universe is such that if you travel in a straight line, you get back to where you started (this is a bit more complicated than I'm making it sound, but that's the fundamental idea).

phinds said:
Think of the surface of a baloon. NOT the ballon, JUST the surface. If you are limited to traveling only on the surface, then traveling in any straight line brings you back to where you started and that's a closed surface. "Closed" for the universe can be more complicated, but one POSSIBLE closed universe is such that if you travel in a straight line, you get back to where you started (this is a bit more complicated than I'm making it sound, but that's the fundamental idea).

This is not what is usually meant by closed. In usual terminology, closed universe is one which has enough energy in it to stop expanding and collapse back to itself.

In cosmology, the term 'closed' refers to the topology of the universe. Phinds characterization is essentially correct.

phinds said:
Yep, has to be. The actual question of WHETHER the universe is infinite or not is an open one.

You are relating the concept of infinite universe with constant Ωk or k in RW metric. Whatever the value of the constant is it will only tell you the topology of the universe and not the size of the universe. The size will always be finite

balaonspace said:
You are relating the concept of infinite universe with constant Ωk or k in RW metric. Whatever the value of the constant is it will only tell you the topology of the universe and not the size of the universe. The size will always be finite

Well, you clearly know something that is not known by most physicists, since most consider it an open issue as to whether the universe is infinite or finite. You should spread the word that it is DEFINITELY finite. Some will be astounded. I WOULD suggest that before you do that you find some physics to back it up.

phinds said:
Well, you clearly know something that is not known by most physicists, since most consider it an open issue as to whether the universe is infinite or finite. You should spread the word that it is DEFINITELY finite. Some will be astounded. I WOULD suggest that before you do that you find some physics to back it up.

Are you suggesting that the size of the universe is infinite since dawn.
because according to my understanding if big bang created the time and space then at this epoch an accelerate expanding space should be a finite value and cannot be infinite. I might be wrong but i want to be convinced that i am indeed.

balaonspace said:
Are you suggesting that the size of the universe is infinite since dawn.
because according to my understanding if big bang created the time and space then at this epoch an accelerate expanding space should be a finite value and cannot be infinite. I might be wrong but i want to be convinced that i am indeed.

It is not known whether the universe started as finite or infinte. Certainly if it started finite, it is still finite and if it started infinite, it is still infinite. I make no statement that it is/was one or the other, but I challenge your statement that you know it is/was finite. What is your source for such a belief?

phinds said:
Yep, has to be. The actual question of WHETHER the universe is infinite or not is an open one.

Not necessarily. If the universe is flat, it can still be finite by being in the shape of the surface of a 3-torus.

phinds said:
It is not known whether the universe started as finite or infinte. Certainly if it started finite, it is still finite and if it started infinite, it is still infinite. I make no statement that it is/was one or the other, but I challenge your statement that you know it is/was finite. What is your source for such a belief?

Thank you. I get it. Its just my idea that infinity makes no sense in physics. But I don't know how one can check the hypothesis that the universe started infinite.

balaonspace said:
Thank you. I get it. Its just my idea that infinity makes no sense in physics. But I don't know how one can check the hypothesis that the universe started infinite.

We can't check it. We can make certain assumptions though, we can use the cosmological principle of homogeneity and we can make a pretty good assumption that past the OU is more of the same - whether that is applicable to infinity or just a much larger U than the OU (their are studies which place a lower bound on the size of the Universe if finite.)

Curvature has a big role to play in the topological "flavour" of the universe, whether open/closed or finite/infinite.

If the Universe is finite now, then it had a finite beggining, if the Universe is infinite now, then it has always been infinite. Remember as well than when talking about the "size" and "shape" of the Universe that if you treat any part of it differently (imbue some area of space with a conecpt of a center or an edge) then you invalidate the cosmological prinicpal.

Now I do not often give a personal opinion, but my gut feeling is that beyond our OU is just more of the same for infinity. I think there are probably Hubble volumes totally devoid of mass (which does not invalidate homogeneity when assuming infinity.)

Anyways that's just my two cents!

Mathematically, you can have an open space that's not infinite, for example, the inside of a ball, not including the shell. But, there are some restrictions on what shapes the universe can be. People usually assume homogeneity and isotropy--rather strong assumptions. These assumptions basically constrain an open space to an infinite space. But, if you don't assume homogeneity then you can have open finite spaces.

## 1. What is a half closed universe?

A half closed universe is a theoretical model of the universe in which the shape of the universe is curved but finite. It is also known as a closed saddle universe.

## 2. How does a half closed universe differ from other models?

A half closed universe differs from other models, such as a flat or open universe, in terms of its curvature. In a half closed universe, the universe is curved in a saddle shape, whereas in a flat or open universe, the universe is either flat or curved in a sphere-like shape.

## 3. Is there evidence to support the idea of a half closed universe?

Currently, there is no conclusive evidence to support the idea of a half closed universe. The shape of the universe is still a topic of debate and research among scientists, and more evidence is needed to determine the true shape of the universe.

## 4. How would a half closed universe affect our understanding of the universe?

If a half closed universe were proven to be the true shape of the universe, it would have significant implications for our understanding of the universe. It would suggest that the universe is finite and has boundaries, which could have implications for the expansion and ultimate fate of the universe.

## 5. What are some potential challenges of studying a half closed universe?

Studying a half closed universe would present challenges for scientists, as it would require advanced technology and techniques to accurately measure the curvature of the universe. It would also require further research and observations to gather enough evidence to support this model.

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