Shape of Universe question

In summary: But keep in mind that these are just analogies, not exact descriptions. In summary, the universe can either be infinite or finite and there is a consensus that it is unbounded. There are multiple cosmological models with different shapes and curvatures, and the sign of the curvature does not change within a single model. The terms "saddle" and "horn" are analogies used to describe the shape and time evolution of these models, but they are not exact descriptions.
  • #1
Helios
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I want to get this simplest cosmology question asked once and for all because I don't get it. We have "cosmologicial models" which have the curvature of the universe going from positive to negative ( or maybe visa-versa ) at some stage ( or not? ). Now elsewhere we hear that a negatively curved universe is infinite and a positive one is finite. Now any student is going to ask "How can finitude become infinite?" ( or visa-versa? ) . Is what I've said flawed? Is there a contradiction? I am the amateur so the fault is surely mine. This must be a page one cosmology question with an elementary answer so can someone please explain? Someone can throw in some names of different universe shapes if they feel like it. Thanks.

edit--send to cosmology pls
 
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  • #2
In response to one part of your question, physical things that are finite cannot become infinite and things that are infinite cannot become finite. The universe either started out infinite or it started out finite. No one knows, but however it started out, that's how it is now. If it is finite, there is a very strong consensus that it is unbounded because a universe that is finite and bounded would have a center and an edge and our universe has neither.
 
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  • #3
No one is ever going to see "bounds" regardless of consensus. I did not mention bounds.
 
  • #4
Helios said:
We have "cosmologicial models" which have the curvature of the universe going from positive to negative ( or maybe visa-versa ) at some stage

Which models are you talking about? I'm not aware of any models where the sign of the spatial curvature of slices of constant time changes.
 
  • #5
Helios said:
We have "cosmologicial models" which have the curvature of the universe going from positive to negative ( or maybe visa-versa ) at some stage ( or not? ).
This is not correct the way you are thinking of it. A better way to say this is that there are a family of models where some have positive and some have negative curvature.

This is related to the fact that conic sections are a family of figures which include ellipses (positive curvature, finite) and hyperbolas (negative curvature, infinite).
 
  • #6
DaleSpam said:
A better way to say this is that there are a family of models where some have positive and some have negative curvature.

And in each model in the family, the sign of the curvature is fixed; it doesn't change from one spatial slice to another. (I know you know this, but I want to make sure the OP understands it.)
 
  • #7
Thanks. It must be acceleration vrs deceleration that has the sign change. This
https://www.physicsforums.com/threads/accelerating-expansion-and-mediocrity-priniciple.850180/
confused me because I read
"when curvature is crossing from negative to positive"
and I misconstrued the meaning. I take it that flat or negative curvature is the most popular belief. Now are there multiple negative shapes or one, because I've heard "saddle" and also "trumpet or horn" as descriptive of negative curvature?
 
  • #8
Helios said:
I read
"when curvature is crossing from negative to positive"

The "curvature" referred to here is not spatial curvature; it refers to the curvature of the line on the graph describing our actual universe's deceleration/acceleration over time.

Helios said:
I take it that flat or negative curvature is the most popular belief.

Our best current model is that the universe is spatially flat.

Helios said:
are there multiple negative shapes or one

Multiple.
 
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  • #9
Helios said:
Now are there multiple negative shapes or one, because I've heard "saddle" and also "trumpet or horn" as descriptive of negative curvature?
There aren't really good words in English for the shapes of 4D pseudo Riemannian manifolds. I am not sure that any word will convey the geometry correctly, that is why we use math.

Saddle refers to the spatial shape and horn refers to the time evolution, so both are legitimate descriptions.
 
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What is the shape of the universe?

The shape of the universe refers to the overall geometry or structure of the universe on a large scale. It is a complex and ongoing topic of debate among scientists.

What are the three possible shapes of the universe?

The three main possibilities for the shape of the universe are flat, closed, and open. A flat universe would have a Euclidean geometry and infinite size, while a closed universe would have a spherical geometry and finite size. An open universe would have a hyperbolic geometry and infinite size.

How do scientists determine the shape of the universe?

Scientists use various methods and observations, such as measuring the cosmic microwave background radiation and studying the distribution of matter in the universe, to try and determine the shape of the universe. However, due to the vastness and complexity of the universe, the exact shape is still not fully understood.

What is the significance of understanding the shape of the universe?

Understanding the shape of the universe can help us better understand its origins and evolution, as well as the fundamental laws of physics. It can also provide insights into the ultimate fate of the universe.

Is the shape of the universe constant or can it change over time?

The shape of the universe can change over time, as it is affected by factors such as the expansion of the universe and the distribution of matter and energy within it. However, the rate and extent of these changes are still being studied and debated by scientists.

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