On 7/30/2014 4:57 PM, LizR wrote:
On 31 July 2014 10:26, Russell Standish <[email protected]
<mailto:[email protected]>> wrote:
On Thu, Jul 31, 2014 at 10:08:52AM +1200, LizR wrote:
>
> PS One problem I have with uncountable infinity not being a feature of the
> world is that it appears to scupper eternal inflation, and even universes
> expanding exponentially. Does anyone have any comments on that?
>
Why?
Because if space-time isn't an infinitely divisible continuum, it presumably has some
sort of granularity, and if it's blown up in size (or squashed down) the grain size may
become relevant. It's one of the "end of the universe scenarios" mentioned by Max
Tegmark in his recent book, that the expansion of the universe makes the quantum
granularity too large for matter to continue to exist (in some way).
The evidence however is against spacetime granularity:
arXiv:1109.5191v2 [astro-ph.CO] 18 Apr 2012
*Bounds on Spectral Dispersion from Fermi-detected Gamma Ray Bursts*
Robert J. Nemiroff,1 Ryan Connolly,1 Justin Holmes,1 and Alexander B. Kostinski1
1Dept. of Physics, Michigan Technological University, 1400 Townsend Dr., Houghton MI,
49931, USA
Data from four Fermi-detected gamma-ray bursts (GRBs) is used to set limits on
spectral dis-
persion of electromagnetic radiation across the universe. The analysis focuses on photons
recorded
above 1 GeV for Fermi detected GRB 080916C, GRB 090510A, GRB 090902B, and GRB
090926A
because these high-energy photons yield the tightest bounds on light dispersion. It is
shown that
significant photon bunches in GRB 090510A, possibly classic GRB pulses, are remarkably
brief, an
order of magnitude shorter in duration than any previously claimed temporal feature in
this energy
range. Although conceivably a > 3 fluctuation, when taken at face value, these pulses
lead to an
order of magnitude tightening of prior limits on photon dispersion. Bound of c/c < 6.94 x
10-21
is thus obtained. Given generic dispersion relations where the time delay is proportional
to the
photon energy to the first or second power, the most stringent limits on the dispersion
strengths
were k1 < 1.61 x 10-5 sec Gpc-1 GeV-1 and k2 < 3.57 x 10-7 sec Gpc-1 GeV-2
respectively. Such
limits constrain dispersive effects created, for example, by the spacetime foam of quantum
gravity.
In the context of quantum gravity, our bounds set M1c2 greater than 525 times
the Planck mass,
suggesting that spacetime is smooth at energies near and slightly above the
Planck mass.
I suppose that if you blow up space-time exponentially, you will rapidly reach any
existing grain size. If inflation would have blown up Planck-cell sized chunks to
anything vaguely macroscopic, for example, we wouldn't expect any detail to exist below
the expanded size.
Macroscopic, in the sense of "classical acting, not quantum", is a matter of degrees of
freedom. So I think it would depend on how things were "blown up". Just changing the
Planck length would be simple rescaling and nothing observable would change. So space
quanta would have to get bigger compared to something else fundamental.
Brent
I'm not sure exactly how this works, but once you have a universe in which some sort of
structure size is defined, expanding it a lot might thereafter mean that size can't be
supported anymore by quantum physics.
(If you see what I mean...?)
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