Question about mass density of the universe

[michel123456]

Having nothing better to do, I was rambling over the Net* and found this page.

Where I wondered about this statement under the first article
Inflation on the back of an enveloppe, exponential expansion, 3rd paragraph

(To persuade yourself that this is at least roughly the right equation, you should note that a similar equation applies to an expanding spherical ball of radius a(t) with uniform mass density V. But in the case of the ball, the mass density would decrease as the ball expands. The universe is different — it can expand without diluting its mass density, so the rate of expansion does not slow down as the expansion proceeds.)

Where it is stated that "The universe is different — it can expand without diluting its mass density".

Question: does that mean that the mass density of the universe is not time dependent, IOW that today's mass density of the universe is the same than the mass density before expansion began?

 

*following Swansont's footsteps.

Edited May 4, 2014 by michel123456

[Orodruin]

This depends on the equation of state of the matter that dominates the Universe. The equation of state is typically given on the form [math]p = w \rho[/math] where [math]\rho [/math] is the energy density of the Universe and p the pressure. For non-relativistic matter w is 0 and for radiation 1/3. However, for dark energy or a cosmological constant w = -1 or even less (in the case of more exotic dark energy contributions). If this dominates, the energy density of the Universe can be constant or even increase.

[michel123456]

This depends on the equation of state of the matter that dominates the Universe. The equation of state is typically given on the form [math]p = w \rho[/math] where [math]\rho [/math] is the energy density of the Universe and p the pressure. For non-relativistic matter w is 0 and for radiation 1/3. However, for dark energy or a cosmological constant w = -1 or even less (in the case of more exotic dark energy contributions). If this dominates, the energy density of the Universe can be constant or even increase.

 

Is that a formal "we don't know" answer?

[Orodruin]

Well, yes and no. We do not know what the dark energy is made of or its exact equation of state - it could be increasing in energy density and it could be decreasing as the Universe expands.

 

We do know that the normal forms of matter and radiation (including all particles we have seen so far) dilute their energy densities as the Universe expands.

[Mordred]

Having nothing better to do, I was rambling over the Net* and found this page.

 

Where I wondered about this statement under the first article

Inflation on the back of an enveloppe, exponential expansion, 3rd paragraph

 

 

Where it is stated that "The universe is different — it can expand without diluting its mass density".

 

Question: does that mean that the mass density of the universe is not time dependent, IOW that today's mass density of the universe is the same than the mass density before expansion began?

 

*following Swansont's footsteps.

This statement is a bit misleading, The universe expansion and contraction is determined by the energy-mass density (positive pressure) of matter and radiation and the cosmological constant. (negative pressure). The corresponding energy-density relation is determined by the equation of state. This relation also determines the universes overall geometry. This is essentially a geometric description of the universes overall energy-density distributions compared to its critical density. As the universe expands. The energy density of matter reduces, however the cosmological constant does not reduce (it remains constant).