# Dust filled universe but dust is a real field.

• Spinnor
In summary, tuning the mass density of a closed dust filled universe can result in a long lived closed universe, as predicted by General Relativity. However, if the universe has a non-zero cosmological constant, it is possible for a closed universe to expand forever. In this case, measurements of spatial curvature and topology are necessary to determine the openness or closedness of the universe.
Spinnor
Gold Member
Tuning the mass density of a closed dust filled universe can give us a long lived closed universe?

General Relativity can tell us how that mass density changes with time?

Instead of dust let us have a real field defined at each point of spacetime. Let us assume the field is such that in some frame the field has no momentum and only energy? Assume the energy density of the field changes with time just as the energy density of a dust filled universe. How might the amplitude (and frequency) of this field change with time so that this might happen?

Thanks for any help.

Spinnor said:
Tuning the mass density of a closed dust filled universe can give us a long lived closed universe?

We observe that the cosmological constant is nonzero, so you can't get a closed cosmology; it's guaranteed to be open and long lived. If you're talking about a cosmology with zero cosmological constant, then this is correct.

Spinnor said:
General Relativity can tell us how that mass density changes with time?
Yes, this is what the Friedmann equations do: http://www.lightandmatter.com/html_books/genrel/ch07/ch07.html#Section7.2

Spinnor said:
Instead of dust let us have a real field defined at each point of spacetime.
Do you mean a real scalar field? I could be wrong, but I don't think qm even has interesting real scalar fields. To get a traveling scalar wave in qm, I think it needs to be complex.

Spinnor said:
Let us assume the field is such that in some frame the field has no momentum and only energy?
I don't think you need to add this as an extra assumption. If you're referring to the average momentum, then I think the existence of such a frame follows from homogeneity and isotropy. If you're referring to the pressure, then this is basically the definition of dust: P=0.

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bcrowell said:
We observe that the cosmological constant is nonzero, so you can't get a closed cosmology; it's guaranteed to be open and long lived.

No, with a positive (as measured) cosmological constant, there is no longer a direct correspondence between openness and infinitely long-lived expansion, i.e., it is possible to have a closed universe that expands forever. In fact, current measurements do not rule this out.

George Jones said:
No, with a positive (as measured) cosmological constant, there is no longer a direct correspondence between openness and infinitely long-lived expansion, i.e., it is possible to have a closed universe that expands forever. In fact, current measurements do not rule this out.

Really?? Wow, that surprises me. Thanks for the correction. Can you point me to somewhere that I can read about this? Anything on arxiv.org or livingreviews.org? What measurements are necessary in order to determine whether it's open or closed? Are you talking about the mean spatial curvature of the universe, or about a purely topological thing, like forming a piece of paper into a cylinder with zero intrinsic curvature?

bcrowell said:
Really?? Wow, that surprises me. Thanks for the correction. Can you point me to somewhere that I can read about this? Anything on arxiv.org or livingreviews.org? What measurements are necessary in order to determine whether it's open or closed?

Since the late 90s, measurements indicate: a positive cosmological constant; infinite expansion; a spatial geometry that is very near the borderline between open and closed.

Almost any GR/cosmology book published after 2000 will discuss this.

Click on figure 6 from Sean Carroll's Living Review

http://relativity.livingreviews.org/Articles/lrr-2001-1/

to see a summary of the supernova data from a few years ago. I think we have better data now.
bcrowell said:
Are you talking about the mean spatial curvature of the universe, or about a purely topological thing, like forming a piece of paper into a cylinder with zero intrinsic curvature?

This has to do with both spatial curvature and topology. Closed in this context means that space has positive curvature, and that space is topologically compact.

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George Jones: The question of the geometry... that could be an artifact of the size of the universe and our place in it. I'm sold that negative curvature hasn't existed yet in our universe, but without invoking the anthropic principle, I'm not sold on the rate of expansion or its overall geometry. Too many pieces missing from that puzzle.

Thanks, George Jones -- the livingreviews article was very helpful!

## What does it mean for a universe to be filled with dust?

A universe filled with dust means that the space between celestial bodies, such as stars and galaxies, is filled with particles of matter. This matter can be in the form of dust, gas, or other small particles.

## What is the significance of dust in the universe?

Dust plays a crucial role in the formation and evolution of stars and galaxies. It acts as a building block for larger structures and can also absorb and scatter light, affecting the appearance of the universe.

## What is the dust field in the universe?

The dust field refers to the distribution of dust particles throughout the universe. It is a real physical field that can be studied and measured by scientists to better understand the composition and structure of the universe.

## How does the presence of dust affect our understanding of the universe?

Dust can obscure our view of distant objects in the universe, making it more challenging to observe and study them. However, it also provides important clues about the formation and evolution of celestial bodies, allowing scientists to gain a deeper understanding of the universe.

## Can dust be found in other parts of the universe besides our own?

Yes, dust is a common component of many other galaxies and even intergalactic space. It is estimated that over half of the atoms in the universe are in the form of dust particles, making it a significant contributor to the overall composition of the universe.

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