What Is the Universe's Critical Density?

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The critical density of the universe is the density at which the universe is balanced between collapsing under gravity and expanding forever. It is defined as the density needed for a flat universe, which is neither open nor closed. The discussion highlights that options A and B are correct for a closed and flat universe, respectively, while options C and D are incorrect. Participants emphasize the importance of understanding the concept of critical density in cosmology. The conversation also notes the need for clarity when discussing homework-related questions.
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The density of the universe is
A) equal to the critical density if the universe is closed.
B) equal to the critical density if the universe is flat.
C) greater than the critical density if the universe is flat.
D) greater than the critical density if the universe is open.
E) none of the above
 
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This sounds like a homework problem. Read the rules for posting homework problems.

What do YOU think the answer is and why?
 
What is the definition of "critical density"?
 

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