The critical density of the universe is the ________
A) density of water
B) average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist
C) actual average density of the universe
D) density of dark matter in the universe
B
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The resolution of Olbers' paradox suggests that it gets dark at night because
a. the universe is not infinite in age. b. the universe is static. c. the universe is closed. d. a and b e. all of the above
Before the decoupling,
A) deuterium produced electrons and positrons. B) protons and electrons combined to form atoms. C) the universe was opaque to radiation. D) there was more helium than hydrogen. E) the universe was transparent to radiation.
Which statements are true about a VIRTUAL image? (There may be more than one correct choice.)
A) Its location can be calculated, but it cannot be viewed directly by your eye without using auxiliary lenses. B) It cannot be photographed. C) It cannot be viewed on a screen. D) A plane mirror always forms a virtual image. E) A concave lens always forms a virtual image.
An astronaut has landed on Planet N-40 and conducts an experiment to determine the free fall acceleration on that planet
She uses a simple pendulum that is 0.640 m long and measures that 10 complete oscillations 26.0 s. What is the free fall acceleration on Planet N-40? A) 4.85 m/s2 B) 1.66 m/s2 C) 3.74 m/s2 D) 2.39 m/s2 E) 9.81 m/s2