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

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

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)

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

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 11/11/12)