Question

a) Explain what are meant by the terms Ωb, Ωm, ΩΛ. Give current best estimates for these parameters. b) How is the deceleration parameter, q0, related to the above terms. c) During what period could the Universe be described as being radiation dominated? d) Why is it only now that dark energy is starting to show its effects in the Universe?

Answer 1

a) Ωb = baryonic density = 0.0227 ± 0.0006

Ωm = total matter density = 0.133 ± 0.006

ΩΛ = constant energy density associated with the vacuum = 0.74 ± 0.03

b) Declaration parameter is given as q0 = 1/2*Ωm - ΩΛ

c) after cosmolabout 47,000 years after the Big bang marks the period when the universe can be described to be radiation dominant.

d) Just after the Big Bang, the universe was much smaller and it composed of an extremely high-energy plasma. During this time the gravity was pulling things together hence slowing the expansion. From somewhere 3 to 7 billion years after the Big Bang, instead of the expansion of universe slowing down, it sped up. This is because now the universe has expanses of "empty" space dotted by dense galaxies of stars. This has reduced the effect of gravity and dark energy is showing it's effect i.e. expansion of universe is accelerating.