In: Statistics and Probability
Cosmic Contraptions
(a) The Friedmann equations describing our universe at large scales are 12(??(?)??)2−4???(?)3?2(?)=−??22ℓ2 and ???(?(?)?3(?))=−?(?)????3(?) for the scale factor ?(?). Explain the meaning of the quantities ?, ℓ, ?, and ? in these equations.
(b) Suppose that the scale factor ?(?)=?0?1/2. If ?=0, find the dependence of ? and ? on ?. What cosmic era does this correspond to?
(c) A theorist comes up with a theory of dark matter that indicates the equation of state of the dark matter is ?=79?. Find the time-dependence of ?(?) and ?(?), assuming ?=0. [2] (d) Which process dominates early times (small ?) -- the one in part (b) or the one in part (c)? Why?