Friday, January 13, 2012

dark energy

what do the cosmological constant and dark energy have in common? why are they different? why is it wrong to say that Lambda is just DE with $w=-1$? could there be a cosmology with Lambda and DE at the same time?

1 comment:

1. 1. DE and Lambda have in common that they both drive the accelerated expansion of the universe which can be seen in Friedmann's second equation.

2. The cosmological constant is either a mathematical degree of freedom given in the field equations of general relativity or "true constant of nature". Dark energy is an additional (perfect) fluid component with an equation of state p = w \rho with w<-1/3 in the today's state of the universe.

3. Saying w=-1 produces a fluid component that mimics a cosmological constant, but is not the constant itself, because it keeps it's fluid
properties and it can still take part in the dynamics of the system. For example, in case of virialization with dark energy, a fluid with w=-1
can take part in it whereas a cosmological constant would just affect the background potential.

4. Therefore it is possible to have a dark energy fluid component (RHS of the field equations) and a cosmological constant (on the LHS of the
field equations) at the same time.