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Q & A: cosmic inflation: speed limit, models, evidence

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Most recent answer: 03/30/2014
Q:
Our current model of the happenings in the beginning of the universe suggests, among other things, that matter has to have been moving faster than light. In what way could the matter at that point be excused of the universal speed-limit. Is the idea of negative energy pushing the universe apart a consequensial explanation, appearing when running the evidence we see through the equations we know? Does the idea of the inflation in the early seconds of the universe actually fit in with our current physical theories or do we just accept that it has occured because of the evidence we see of it? (hmm, maby that’s all we can do and what we always do. Sorry if this is turning in to a metaphysical train of thought!)
- Tobias Johansson (age 20)
Chalmers University of Technology, Sweden, Göteborg
A:
That's a nice series of questions, and a sophisticated philosophical afterthought.

" In what way could the matter at that point be excused of the universal speed-limit."

There's a speed limit in Special Relativity, but S.R. only applies to small patches of the universe, not to the largest scales. So nobody can pass in front of you at speeds greater than c in your frame, but in some general coordinates assigned to large regions of the universe, the nominal increase of distance coordinate as a function of some time coordinate can exceed c.


"Is the idea of negative energy pushing the universe apart a consequensial explanation, appearing when running the evidence we see through the equations we know?"

Actually, the energy driving the accelerating expansion is positive. It's different from ordinary energy because it (approximately, at least) keeps a constant density rather than a constant value as the volume of space changes. And yes, putting a constant energy density into the standard General Relativistic equations drives an exponential expansion, without any further fudging or fiddling.

"Does the idea of the inflation in the early seconds of the universe actually fit in with our current physical theories or do we just accept that it has occured because of the evidence we see of it?
(hmm, maby that's all we can do and what we always do. Sorry if this is turning in to a metaphysical train of thought!)"

We try to do both- make some theory that fits what we see and that fits within a broad general framework intended to describe everything we see. As you say, we do the same thing all the time. We hypothesize dogs to fit various tactile, olfactory, visual, and audible impressions. We hypothesize inflation to fit various data on cosmic microwaves and the redshift of light from galaxies, etc. In both cases we try to stick to hypotheses that make sense within our broader framework. It would take more than just a few sense impressions to make us think we'd seen a ghost, and it would take more data to make us believe in inflation if it weren't a possibility raised by General Relativity together with quantum field theory.

Mike W.

(published on 10/22/2007)

Follow-Up #1: cosmic inflation and the speed limit

Q:
Just reading Bill Bryson's book A Short History of Nearly Everything, in it he writes "in a single instant we were endowed with a universe that was vast - at least a hundred billion light years across. How can this be as nothing can travel faster that the speed of light?
- Ken Rogers (age 63)
England
A:
We've more or less  answered this before, so I've marked it as a follow-up. One point to add is that there is now some dispute as to whether inflation actually occurred. There are alternative pictures ("ekpyrotic"), involving collisions in higher-dimensional spaces, described by Paul Steinhardt in a Scientific American article not too long ago.  The Planck satellite is currently gathering data that may help resolve this question.

Mike W.

(published on 11/10/2012)

Follow-Up #2: estimating extent of cosmic inflation

Q:
How is the duration and extent of cosmic inflation calculated/determined/estimated?
- lutz thilo (age 72)
cape town
A:

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)

Follow-Up #3: Quantitative inflation question

Q:
Thank you for your qualitative answer. However, what I actually meant in my question was how numbers were put to the extent[10^20 to 50? ]and the duration [10^-32s].
- lutz thilo (age 72)
cape town
A:

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)

Follow-up on this answer.