The space component of the FLRW metric is multiplied by the square of this quantity. For 10 points each:
[10m] Name this dimensionless quantity which characterizes the expansion of the universe.
ANSWER: cosmic scale factor [or the Robertson-Walker scale factor]
[10e] Because the densities of radiation and this component scale differently over time, about 50,000 years after the Big Bang the universe transitioned from “radiation-dominated” to dominated by this component, which makes up about 5% of the universe's mass-energy and interacts with all four fundamental forces.
ANSWER: matter [accept baryonic matter or ordinary matter; reject “dark matter”]
[10h] Denoting the scale factor as “a”, the Friedmann acceleration equation relates the sum of an energy density term and a pressure term to what function of a?
ANSWER: the second derivative of a divided by a [or a-double-dot divided by a or a-double-prime divided by a]
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