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Expanding space ?

  1. Oct 20, 2011 #1
    This might be in the wrong section. If all the matter and energy in the universe was localized into a small region at some point in time. why would it expand, the gravitational field would seem to be very strong and prevent this. But then I think they say that space itself is expanding, OK but why wouldn't the matter just stay in place and have the boundary of the universe expand.
  2. jcsd
  3. Oct 20, 2011 #2


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    Why would it expand depends somewhat on which model is used to eliminate the singularity.
    Typically we are talking about quantum cosmology. The old classical (pre-quantum theory) model blew up, so you could not say anything except for after the expansion had already begun. If you pushed the old model back as far as the very start, it broke and would not compute.

    So you are asking about an active area of research called quantum cosmology (abbreviated QC).
    Here, not to read but just to realize the existence, are the QC papers from 2009 onward:
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2008&FORMAT=WWW&SEQUENCE=citecount%28d%29 [Broken]

    In what is probably the most widely studied type of QC the Einstein law of gravity (GR) is quantized and it is found that at very high density gravity is REPELLENT instead of attractive. So there is a simple answer to the question why it all didn't stick together. It wanted to expand.
    After a short while the density was lower and gravity was behaving normally but by then expansion was under way and intituitively it had a kind of momentum.

    There are other ways of explaining the start of expansion. Some day hopefully we will be able to test them against observations going far back in time, and judge which are the most likely. In the approach I mentioned, our big bang (really a big bounce) resulted from the collapse of a prior universe which actually achieved the extreme density needed for quantum effects to make gravity repellent. So there was a rebound producing a whole new region of space and time.

    They actually have been running computer simulations of this bounce cosmology. It may be possible to test it---confront it with data from the next generation of observatory spacecraft---and maybe it will fail the tests and be discarded--or maybe not. Right now it is just an untested theory on the drawing boards.
    Last edited by a moderator: May 5, 2017
  4. Oct 21, 2011 #3
    thanks for your reply marcus, thanks interesting that gravity can become repulsive at high densities, is this similar to the nuclear force.
  5. Oct 21, 2011 #4
    My question probably also has simple answer, but anyway, why black holes don't explode then?
  6. Oct 21, 2011 #5
    I have often thought something similar to this.

    Without being overly speculative I have often wondered:

    If the requirement for an LQG model "bounce" is a classically contracting spacetime then I have wondered if it is possible that Bh's represent a turtles all the way up and all the way down. My thought was that while I dont think BH's are right now Universes, but within their own FoR they "bounce" from a classically contracting spacetime. From our FoR this would never happen as it is the mapping of a finite observer time to an infinite coordinate time.

    I am not promoting this as personal theory itts just something if I wondered had ever been discussed or considered?
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