# Will space always expand when the space has vacuum energy and zero mass?

1. Apr 23, 2012

### james.goetz

I am examining various models of eternal inflation and I want to double check one of my assumptions, which is that space always expands when the space has vacuum energy and zero mass. Is this correct? Will space always expand when the space has vacuum energy and zero mass? Thank you.

2. Apr 23, 2012

### Mark M

James, space will expand when there is a negative vacuum pressure, such as there was during inflation.

Inflation posits that the inflaton field, while in a false vacuum, exerted a negative pressure and decayed to a true vacuum. Once it reaches the true vacuum, it decays into a hot bath of radiation which would give birth to the particles of the standard model. In eternal inflation, the decay rate of the inflaton field is lower than the rate of expansion, so that the universe continually undergoes inflation, producing numerous 'pocket universes'.

Last edited: Apr 23, 2012
3. Apr 23, 2012

### bapowell

This is not correct. The vacuum energy during inflation was positive. Are you mistakenly referring to the negative pressure of the vacuum?

James, a vacuum energy-dominated universe will expand always.

EDIT:
Also, I think this statement is a little confusing, since you seem to be implying that it is a result of the negative pressure that the field finds its way to the true vacuum. This is also not the case -- it is simply that the true vacuum is of lower energy than the false vacuum, and the inflaton, through quantum/thermal fluctuations, tends to find its way down to the true vacuum.

4. Apr 23, 2012

### james.goetz

Hi bapowell, thank you for clarifying that postive vacuum energy causes space to expand. By the way, I never said "...while in a false vacuum, exerted a negative pressure to decay to a true vacuum."

5. Apr 23, 2012

### Mark M

Thanks for correcting, that is what I actually meant. I'll fix that now.

The negative pressure results because of the inflaton field being a state of false vacuum, correct? I'll change my wording to make sure I get this point across. Thanks.
He was speaking to me, I used the word 'to' which implies the decay was a direct cause of the negative pressure. And as bapowell said, the negative pressure is related to the inflaton field being in the false vacuum, and decayed because of standard processes.

6. Apr 23, 2012