Sunday, 14 January 2018

Will the Universe Inevitably Contract?



In Could the Universe have Created Itself?, I commented that to the effect that the universe, or at least the part of the universe that matters, will inevitably contract.  This sounds like a big claim given that there are cosmologists out there arguing for at least four different end states of the universe: Big Freeze (the universal destiny previously known as heat death), Big Crunch, Big Rip and Big Slurp (which is a rather ridiculous term referring to a catastrophic vacuum metastability event, but I guess it’s in keeping with the other terms).  Note that the Big Bounce is just a cycling of Big Bangs and Big Crunches, so each instance of the universe would end in a Big Crunch.

I’m not arguing for a Big Crunch per se, but rather a combination of the Big Crunch and the Big Freeze (and maybe even aspects of the Big Rip, but I don’t think so).  We could call it the Big Hole because it would be one black hole in an otherwise empty, eternal universe.

So, why is this inevitable?

First, I need to clarify my assumptions.  I’m working with my model of the universe, which is expanding with time (at the speed of light) in which the expansion is retarded by concentrations of mass-energy (and this manifests as gravity).  We know that the universe isn’t expanding evenly because if it did, then we wouldn’t have noticed it happening.  What we do notice is that the gaps with next to no mass-energy in them (between galaxies) are expanding more than the regions with higher concentrations of mass-energy.  I’m also assuming that this recession is not the only form of motion in the universe – for example, despite being some distance away, the Andromeda galaxy is not receding from us but is rather approaching us at high speed.  There’s some recession going on for sure, but galaxies also have motion on space as well.  Some galaxies are in orbit around other galaxies, such as the Large Magellanic Cloud which is a satellite galaxy of the Milky Way.

Imagine a circle on which there are evenly spaced points and then expand that circle:



Then imagine instead a circle on which two points are marginally closer to each other, and that the closer two points are, the less the distance between them increases (note that I have amplified the effect here by not modifying any of the blue points, focussing instead on only the green and orange points):

Then imagine that the points are all moving around, even if only a little.  Eventually, you’ll get clumping.  It might take a very long time, but in what I am modelling, we have (future) eternity to work with.  Eventually, everything in the universe will be clumped together, and eventually that will all be compressed into a(n in-universe) black hole.

I agree that, if the universe was simply expanding, then everything would end up just receding away from everything else and we’d end up with either a Big Freeze or a Big Rip.  But the universe is not simply expanding – there’s something about concentrations of mass-energy that makes the expansion uneven.

It’s possible that what I have described here is pretty much a Big Crunch, but I don’t envisage a scrunching up of all spacetime in the black hole, or into a singularity, just the mass-energy.  I lean instead towards a final equilibrium state with just enough space to maintain the appropriate density for whatever mass the black hole ends up with (the original mass-energy, plus whatever additional energy, if any, was added via quantum processes during the “life” of the universe).

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