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1. Accelerating Universe
In January 1998 two competing teams rocked the cosmological world by reporting that the Hubble diagram of high redshift supernovae indicates that the Universe is accelerating, not decelerating as might be expected if gravity is attractive.
Suppose that the Universe contains only matter (M) energy and vacuum energy (a cosmological constant L), and that it is geometrically flat
 (1.1) 

For the Universe to accelerate, W_{L} should be less/greater (pick one) than ____________________ .
2. Critical density
(a) Critical density
The critical density r_{c} of the Universe, the density which makes it geometrically flat, is

The critical density of the Universe is ____________________ kg/m^{3}.
(b) How big is that?
Estimate approximately how much volume you personally would fill if you were spread out to critical density. If this volume were a cube, roughly how long would be the side of the cube? Compare this distance to that of some familiar object.
I would occupy a volume of about ____________________ m^{3}.
This corresponds to a cube of side about ____________________ m.
This is about ____________________ times as big as ____________________ .
3. Recombination
The Cosmic Microwave Background is thought to have come to us from the epoch of Recombination, when hydrogen and helium in the Universe changed from being ionized to neutral atoms.
(a) Size of the Universe at Recombination
Wien's law states that the peak wavelength l_{peak} varies inversely with the temperature, l_{peak} µ 1/T. General relativity shows that the wavelength stretches in proportion to the cosmic scale factor, l_{peak} µ a. If the temperature of the Cosmic Background was 3,000 K at Recombination, and is now 2.73 K, by what factor has the Universe expanded since Recombination?
Since Recombination, the Universe has expanded by a factor of ____________________ .
(b) Age of the Universe at Recombination
The cosmic scale factor a varies with time t approximately as a µ t^{2/3}. If the current age of the Universe is about 14 Gyr, approximately how old was the Universe at the time of Recombination? [Hint: You will need to use your answer from part (a). Note that a µ t^{2/3} is strictly true only for a matterdominated Universe; the actual relation depends on what the Universe is made of.]
At Recombination, the Universe was ____________________ years old.