Can expansion be due to large-scale curvature?

[claycravens]

I wanted to say I love the posts on this Cosmology forum - very good reads.

My question is whether there is enough to large-scale curvature alone to cause the expansion of the universe as we measure it? Why can't galaxies accelerating away from us be moving in curved space?

I agree with the idea of infinite matter-energy density at some time in the past and also with inflation. I believe I understand homogeneity and isotropy.

The question in part is whether large-scale curvature satisfactory to move galaxies in ever increasing velocities with further distances would be consistent with observations like COBE data and also would it be symmetrical.

Clay Cravens
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[zhermes]

Expansion isn't due to 'curvature', per se. Curvature is defined at a point (or region) of spacetime, while expansion is the behavior of curvature over time---the 'velocity' of the curvature, if you will.
Accelerated expansion is a level further away from curvature itself, its like the 'acceleration' of the curvature (again, a loose metaphor) and requires a driving force beyond the curvature itself.
But galaxies certainly are moving in curved space.

 

[Chalnoth]

Expansion isn't due to 'curvature', per se. Curvature is defined at a point (or region) of spacetime, while expansion is the behavior of curvature over time---the 'velocity' of the curvature, if you will.
Accelerated expansion is a level further away from curvature itself, its like the 'acceleration' of the curvature (again, a loose metaphor) and requires a driving force beyond the curvature itself.
But galaxies certainly are moving in curved space.

Actually, it's entirely the other way around. The expansion of space is quite precisely and accurately described as space-time curvature. It is not, however, spatial curvature. The spatial curvature of our observable universe is very close to zero. But it does have space-time curvature, and that manifests itself as expansion.

 

[zhermes]

Yeah, @Chalnoth is definitely correct; not sure what I was thinking on that one...

 

[sardar]

Thank you for your kind words about the posts on this Cosmology forum. I am always happy to hear that people are interested in and engaged with the concepts and ideas in our field.

To answer your question, it is important to understand the difference between local and global curvature in the universe. Local curvature refers to the curvature of space in a small region, such as the curvature around a massive object like a planet or star. This type of curvature is what causes the motion of planets and other objects in our solar system.

On the other hand, global curvature refers to the overall curvature of the entire universe. This type of curvature is what determines the expansion of the universe and the motion of galaxies on a large scale.

Based on our current understanding of the universe, it is not possible for large-scale curvature alone to cause the expansion of the universe as we observe it. This is because the expansion of the universe is driven by a combination of factors, including the distribution of matter and energy, the cosmological constant, and the initial conditions of the universe.

While it is true that the motion of galaxies could be affected by curved space, this curvature would have to be very large and asymmetrical in order to explain the observed expansion of the universe. This is not supported by our observations, including the COBE data, which show a high degree of homogeneity and isotropy in the universe.

Overall, while large-scale curvature may play a role in the dynamics of the universe, it is not the sole cause of the expansion we observe. It is important for scientists to continue studying and exploring all possible explanations for the expansion of the universe, including the role of large-scale curvature, in order to gain a deeper understanding of the nature of our universe.