(published on 10/22/2007)
(published on 11/10/2012)
That's a very timely question. Until the last few weeks, there were only some weak lower bounds on the extent. The universe is very nearly flat and uniform. If inflation is the explanation, then the extent of it had to be enough to take a tiny piece of space, one small enough to be very uniform, and blow it up to much bigger than the observable universe.
Just now data on polarized cosmic microwave background radiation have been reported from the Bicep2 radio telescope at the South Pole. These data, unless there's some error, seem to show a signature of quantized gravitational waves, blown up by the inflation process. We're told (I don't understand the math, see link below) that the size of the tensor component (a sort of twisty pattern) of the polarization can be used to set the energy scale of the inflation process. That scale turns out to be around the same scale at which the quantum chromodynamic force is suspected to be unified with the electroweak force. It's not a whole lot smaller than the Planck scale, at which we think the smooth structure of spacetime must break down due to quantum gravity effects.
Here's a link to one of several good discussions that have come out:
Mike W.
(published on 03/26/2014)
This paper has a nice argument:
Since we aren't very familiar with it, it's best for now to just pass it along rather than to risk making a mistake by trying to summarize it. I should point out that at the time that paper was written, it looked like the ratio of the B-mode to E-mode amplitudes was under 0.11, but the new data indicate a value closer to 0.2. Maybe that indicates that something is wrong with the inflation interpretation, or maybe it's just from the usual experimental uncertainties.*( It was an experimental problem- see below.)
Mike W.
*The BICEP2 collaboration claimed to have seen what looks like the leftover effects of the quantum zero-point spread of gravitational waves, left as tiny "B-mode" ripples in the polarization of the cosmic microwave background radiation. It now looks like they really saw somethig more local and less interesting. If one of the more sensitive versions of this experiments works, then it will be fair to say that a real quantum gravity effect will have been seen. /Mike W.
(published on 03/30/2014)