The effect on the universe when it is dominated by the matter.

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Chapter 16, Problem 16QAP

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The effect on the universe when it is dominated by the matter.

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1. The current (critical) density of our universe is pe = 10-26kg/m³. Assume the universe is filled with cubes with equal size that each contain one person of m = 100kg. What would the length of the side of such a cube have to be in order to give the correct critical density? How many hydrogen atoms would you need in a box of 1 m³ to reach the critical density? The matter we know, which consists mostly of hydrogen, constitutes only 4.8% of the current critical energy density of our universe. So how many hydrogen atoms are actually in a box of 1 m3 in our universe? Deep space is very empty and a much better vacuum than we can obtain on earth in a laboratory.

What can change the expansion rate of the Universe over time? Gravity: Attractive forces increase the expansion rate by pulling mass together making gravity stronger. Gravity decreases the expansion rate, while electromagnetic forces are always repulsive and speed-up expansion. Attractive forces such as gravity decrease the expansion rate. Repulsive forces would speed-up expansion by pushing spacetime apart. Nothing, since the expansion rate remains always constant, and it is measured from Hubble's law.

The matter density in the Universe today is ?m=2.7×10−27kgm−3. What would be the value of the density parameter, Ω0, if the Hubble constant had the value H0 = 38 km/s/Mpc?