Density of matter in the expanding Universe

In summary, the Universe is expanding and the density of matter is decreasing, but matter is not being created at a constant rate. This contradicts the previous belief that matter should be created to maintain a constant density. The steady state model has been abandoned in favor of a model where the universe has an origin and changes with time. The expansion of the universe only affects the space between clusters of galaxies and has no impact on the gravitational field balance between galactic components. The possibility of universal inflation, where baby universes are constantly popping off, is a return to the steady-state model. The idea of matter entering the universe from outside sources has some problems.
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petrushkagoogol
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As I understand it, the Universe is red-shifted (emission spectra) from any point of reference looking outwards. The Universe is expanding, but is matter being created at the same rate ? Does this mean that the density of matter in space is decreasing ? (density = mass / volume). What does this imply for the gravitational field balance between it's galactic components ?
 
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petrushkagoogol said:
As I understand it, the Universe is red-shifted (emission spectra) from any point of reference looking outwards. The Universe is expanding, but is matter being created at the same rate ? Does this mean that the density of matter in space is decreasing ? (density = mass / volume). What does this imply for the gravitational field balance between it's galactic components ?
Matter is not being created. Why would it be? The density of matter is decreasing outside of bound systems (galactic clusters and smaller) inside of which there is no expansion.
 
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petrushkagoogol said:
As I understand it, the Universe is red-shifted (emission spectra) from any point of reference looking outwards. The Universe is expanding, but is matter being created at the same rate ?
Some famous physicists, including Hoyle, felt that matter should be being created as you suggest, so that the mass density could be conserved. But this is because they felt the universe should not evolve with time-- it should be in a "steady state." I think that sprung from a mostly philosophical commitment to the idea that the universe could not begin or end. But with the discovery of all kinds of evidence (including the cosmic microwave background) that the universe does change with time and did have an early state that was dense and hot, the steady state model has been abandoned in favor of one where the universe does have an origin and does drop in density with time. There is no important impact on galactic potentials, they come from their own local matter distributions that are not affected by the expansion-- the expansion is seen only between clusters of galaxies, on scales unbound by the gravity of local mass distributions.

By the way, modern views of the possibility of "universal inflation,", whereby baby universes are popping off all over the place in a vast universe that is otherwise in a steady state on the largest imaginable scales, are a kind of return to the steady-state model that Hoyle himself could not resist making comment on.
 
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The conditions necessary to originate matter [iow, to convert energy to matter, to be specific] only existed briefly and in the very early universe, therefore no known physical process could account for the matter creation rate necessary to maintain the universe in a state of constant matter density. The alternative to matter creation is for matter from 'outside' the universe to 'enter' it from 'elsewhere' [e.g., via wormholes or as a consequence of expansion]. But, any such explanation has some severe and rather obvious problems.
 

1. What is the density of matter in the expanding Universe?

The density of matter in the expanding Universe is approximately 5% of the critical density. This means that the amount of matter in the Universe is not enough to halt the expansion, causing the Universe to continue to expand at an accelerating rate.

2. How is the density of matter in the expanding Universe measured?

The density of matter in the expanding Universe is measured using various methods, such as studying the cosmic microwave background radiation, observing the large-scale structure of the Universe, and measuring the gravitational lensing of distant galaxies.

3. What is the difference between baryonic and non-baryonic matter in relation to the density of the expanding Universe?

Baryonic matter, which is composed of atoms and makes up all visible matter in the Universe, only accounts for about 15% of the total matter density in the expanding Universe. Non-baryonic matter, which is still largely unknown, is thought to make up the remaining 85% of the matter density.

4. How does the density of matter in the expanding Universe affect the formation of galaxies?

The density of matter in the expanding Universe plays a crucial role in the formation of galaxies. Higher matter density leads to more gravitational pull, causing matter to clump together and form structures like galaxies and galaxy clusters. The lower density of matter in the Universe today is one of the reasons why galaxy formation has slowed down compared to the early Universe.

5. Can the density of matter in the expanding Universe change over time?

Yes, the density of matter in the expanding Universe can change over time. As the Universe continues to expand, the matter density decreases. Additionally, the conversion of matter to energy and vice versa, through processes like nuclear fusion and matter-antimatter annihilation, can also affect the overall density of matter in the Universe.

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