How Does Matter Influence the Universe's Expansion and Dark Energy?

[stever19]

From what I understand it appears that the expansion of our universe is contingent on the amount of matter within.

I have a few questions about matter..

Has the universe always contained the same amount of matter or is new matter created? If new matter is created or could be created what are some accepted ideas on this?

If expansion is contingent on the amount of matter and new matter is not created wouldn't the universe's expansion slowly stop? What would be the implications of this and is this accepted as a possibility?

 

[bcrowell]

Has the universe always contained the same amount of matter or is new matter created? If new matter is created or could be created what are some accepted ideas on this?

The basic answer is that no new matter is created. For example, baryon number is believed to be conserved, so you can't just create new hydrogen atoms out of nothing. There were models like the Hoyle steady-state model in which new atoms were created, but those models have been disproved by observations.

The slightly more subtle answer is that general relativity doesn't have a well-defined notion of "amount of matter." Even if the universe turns out to be finite in volume, there is no well-defined way in GR to define the total amount of mass in the universe.

If expansion is contingent on the amount of matter and new matter is not created wouldn't the universe's expansion slowly stop? What would be the implications of this and is this accepted as a possibility?

You don't need to create more matter in order to have expansion. There are cosmological models in which the universe is expanding but it contains no matter at all; our own universe is fairly well approximated by such a model right now. If matter exists, then its gravitational attraction tends to *decelerate* the expansion.

 

[mathman]

If expansion is contingent on the amount of matter and new matter is not created wouldn't the universe's expansion slowly stop? What would be the implications of this and is this accepted as a possibility?

There is observational evidence that the expansion is speeding up. The explanation is that there is something called dark energy which is doing it.