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Questions for Discussion: Gravity and the Expanding Universe

What led Einstein to hypothesize the cosmological constant?
What made Einstein wish he could "put the genie back in the bottle"?
What is the evidence for the Big Bang Theory, as presented in the video?
The narrator indicates that our understanding of the impact of gravity on galaxies and stars may be incomplete. What does he mean?
Why were scientists surprised by the evidence that the expansion of the universe was accelerating? What is their explanation in this video for the expansion?