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Q & A: cosmic inflation and relativistic transforms

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Most recent answer: 07/28/2012
Q:
As I understand it in a nutshell, the theory of inflation states that post-big bang spacetime expanded at a more or less constant velocity, but early in its life the expansion of spacetime experienced a large acceleration (presumably followed by a deceleration to our current relatively modest rate of accelerating expansion.) Let me postulate an alternate explaination, and ask you to tell me where my thinking is astray or there is evidence to the contrary. Either I will learn something, or think I had a really good day. I postulate that the perception of inflation is the result of relativistic affects of the region of the universe from which we are taking all of the data that supports inflation. (In other words, there was no inflation, just an acceleration in our frame of reference.) I was thinking about what Brian Greene describes as "now" slices in a spacetime loaf. If two distant objects were stationary relative to each other they would have the same "now" slice, but if they started moving toward each other their "now" slices would then include different things. So if I existed on some other distant creatures' "now" slice (and he eventually got this data via light), but he starting moving toward me, and in almost an instant in this creatures' perception mankind lived on Mars and I was long dead (he got this data too) wouldn't that be tantamount to the perception of "inflation"?
- Scott (age 43)
Denver, CO
A:
I'm afraid that the sorts of standard relativistic effects you're thinking about were long since part of the non-inflationary picture. They weren't adequate to explain what we see, but it's good you brought them up.

First, a minor point on inflation. The standard inflationary picture doesn't involve any deceleration at the end.  You don't need to slow anything down. You just observe the things that are close to you, which are automatically selected to be the ones that aren't moving away too fast.

As I said, understanding the relativistic "now slices" was already a basic part of any picture, with or without inflation. Here's a little story to show why that matters, which I'll tell in a somewhat inaccurate special relativistic way. Say you have some simple Hubble expansion (e.g. post-inflation). If you now see light that came straight to you from something which was receding from you at some v<c, then in a Newtonian space-time picture you'd get that the time at which the light started was t=c/(c+v)T, where T is the time since the big bang. Notice that t > T/2, so it would sound as if you could never directly see anything from times shortly after the BB. However, even applying a special relativistic correction to get the time of the light-emission in the frame of its source you get t*sqrt(1-v2/c2), which can be very close to zero as v gets close to c. Hence, even without using general relativity, you can get an idea of how understanding the different time slices in different frames allows us to see very early events.

None of that has anything to do with inflation, which was originally devised to explain why there's homogeneity on scales which seemed to lack any causal connections and also to explain the rarity of magnetic monopoles. There may be some problems with the inflationary picture, but the main alternatives don't seem to include a simple big bang.

Mike W.

(published on 07/28/2012)

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