# Cosmological constant

1. Mar 11, 2013

### arunshanker

A positive value for the cosmological constant was found by the studies of Adam G. Riess et al and Pearlmutter et al
In terms of Planck units, and as a natural dimensionless value, the cosmological constant, λ, is on the order of 10−122 or 10−29 g/cm3
The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate.
The universe is accelerating and space is expanding
Does it mean that new space is being created and if so what is the cosmological constant in the new space or expanded space
If suppose an area of space has a cosmological constant value of 10−29 g/cm3 then if it expands what will be the value of consmological constant as the word itself says it is constant
Is dark energy or dark matte created to keep this constant

2. Mar 11, 2013

### Staff: Mentor

You cannot point to anything in space and say "this space is new". Everything expands with the same rate (ignoring some local collections of matter). The cosmological constant itself could vary with time, but no variation has been found yet.

3. Mar 12, 2013

### Naty1

Hi arun....lots of unknowns remain to be discovered...

The energy density is believed to remain constant with the metric expansion of the cosmos. So as the universe expands, the total vacuum energy increases and its repulsive nature causes space to expand even faster…in a run away exponential expansion. In the big bang model, dark energy [the cosmological constant] powers an accelerated expansion after about 7 or 8B years as it overpowers the gravitational attraction of matter. We are already moving from a 'matter dominated' to an 'energy dominated' epoch. Dark matter dominated the past; dark energy will dominate the future.

I get the sense 'dark energy' may be less popular now than in the recent past...at least by some in these forums..... and that perhaps it is just a feature of curved space time that leads to some gravitational repulsion at vast distances....

Dark matter and dark energy are so far two distinct and virtually unrelated 'unknowns'... which is what 'dark' means. We infer the existence of dark matter by its gravitational effects; We infer the existence of dark energy by the accelerated expansion of space....dark matter seems to aid the formation of galaxies a bit; dark energy may slow formation a bit.

Here is what Bianchi and Rovelli have to say about the cosmological constant...dark energy...vacuum energy....call it what you want...

Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Dated: April 13, 2010)

IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY

4. Mar 13, 2013

### Jorrie

Hubble Constant at end of inflation

I have a related question. If we wind the FRW equations back from the time of matter-radiation equality (z~3300), ignoring matter and Lambda densities (they were small before that) to the end of inflation, I get order of magnitude:
$$H(t) \approx H_0\sqrt{\frac{\Omega_{rad}}{a^4}} \approx 10^{56} H_0$$
I used for end of inflation: $a \approx 10^{-28}$ and $\Omega_{rad} = \Omega_m/z_{eq}\approx 10^{-4}$, the present value.

Would that H(t) be roughly the constant H (and related Hubble radius) during inflation? This would mean that the cosmological constant must 'jump' into domination around that point (running time backwards), swamping any radiation density.

Last edited: Mar 13, 2013