# Constant density of dark energy

1. May 14, 2004

### kurious

Dark energy has a constant density as the universe expands.
Doesn't this suggest that the baryonic mass of the universe is
expanding in a substance of constant volume and density that occupies a volume far larger than the universe itself currently occupies?

2. May 14, 2004

### hellfire

According to general relativity any space with some energy contained in it will expand or contract. This large volume you are talking about would be also expanding or contracting.

I understand you are asking also about how to explain this dynamics of space considering that the total energy of a given comoving volume would increase (due to the constant energy density) and, thus, how to reconcile this with the assumption of energy conservation.

IMO the same intuitive explanation which is given for the inflationary period can be given for today: assuming that energy conservation can be applied to the universe as a whole, the growing energy (as space is expanding) in any scalar field with constant energy density would be compensated with its negative gravitational potential (this is ‘somehow’ a kind of ‘negative’ energy).

Anyway, I have never seen a quantitative explanation for this, so I would be also happy so see more comments…

Regards.

3. May 14, 2004

### kurious

This large volume you are talking about would be also expanding or contracting.

Not if the gravitational force that all the energy particles exert on one another
is strong enough - the molecules in Earth's atmosphere do not expand in to space
because of gravity.

4. May 15, 2004

### hellfire

I do not understand exactly the analogy you proposed. Anyway, such a static model needs of a fine tuning of the dark energy density and pressure.

Consider first an universe model which contains mainly matter. To get a static universe model one has to introduce a cosmological constant with a very fine tuned value in the Friedmann equations. The cosmological constant is a type of dark energy which has an equation of state $$\inline P = - \rho$$.

But I think the case is different here, since there would be no matter but only dark energy in this external shell of great volume. In such a case, to get a static universe, the Friedmann equations require a different type of dark energy, with a different equation of state:

$$P = - \frac{1}{3} \rho$$

The question arises then for me whether such a kind of dark energy, which - I would say - cannot be generated by a scalar field as in case of the cosmological constant, does also have a constant energy density.

Regards.

5. May 15, 2004

### kurious

I envision dark energy to be massless particles like photons but with the ability to repel normal baryonic matter and photons by virtue of possessing negative energy.
However since in gravity like attracts like dark energy particles all with negative energy would attract one another and form a large ball - larger than the universe we know.After the big bang dark energy would have slowed down the expansion of the universe because there would have been more repulsion outside the universe than in it -think of one sphere inside another - but a time came when the amount of dark energy inside the sphere of the universe exceeded the amount outside and so the net repulsion became in the direction of expansion of the universe and speeded up the expansion.Sometime in the future the baryonic universe will expand beyond the dark energy and the force of gravity will get stronger - somehow the expansion of the baryonic universe will create
more massless gravitons ( perhaps the expansion reduces a pressure which enables
gravitons to be released like an evaporating gas from a liquid) and the universe will contract again.This isn't theory development so I won't continue in this vain!

Last edited: May 15, 2004