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Name: Joseph
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: Summer 2012


Question:
My son asked me how it is known that the universe and the apparent dark energy accelerating its expansion is known not to be the result of the entire universe being drawn into an enormous black hole. To wit: If we are in free fall, then we would not experience the effect of gravitational attraction. Galaxies further into the gravity well would be accelerating away from us (blue shifted) and we would be accelerating away from galaxies that not as far into the gravity well (again blue shifted). I am not able to calculate the shape generated in relativistic gravity for the galaxies that are an equal distance into the hypothesized gravity well as the Milky Way, but I noted that the Andromeda galaxy is the only known red shifted galaxy, and it is close to us. As I understand it, it is not clear to me that his conjecture is known to be untrue. Is the distribution and apparent velocity of the many retreating galaxies well enough understood to rule out the possibility that the entire universe is either falling towards a gigantic black hole?


Replies:
How could a black hole draw everything everywhere away from us? A black hole surrounding the entire universe would actually not exert any net interior gravitational force in any direction. It would, however, collapse in on itself.

Quick correction: objects moving away from us are red-shifted; objects moving toward us are blue shifted. The Andromeda galaxy, moving toward us, is blue-shifted.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming



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