How many of you have heard about inflation?
No, it’s not what’s going to happen because of the Fed’s monetary policy. It’s what happened roughly 14 billion years ago during the first moments of the universe.
Or it’s what we use to connect the dots.
One of the great accomplishments of the last 15 years is the amount of detailed data we have on the cosmic microwave background. For those who don’t know, this is the light that came from ‘the last scattering surface’ when the universe cooled enough that atoms started forming. Before that the light had been in thermal equilibrium with the protons and electrons, which means the photons were scattered before they could move any distance. This signal was first observed in the sixties and led to a Nobel Prize in 1978. The thing that is new, or at least more recent, is that there is data on the detailed structure of this last scattering surface.
And it’s ringing.
When one plots the frequency spectrum of the spatial differences, it displays a set of harmonics. Call it the aum of the early universe if you want. Well, that’s fine. Why shouldn’t there be sound waves bouncing around in the plasma of the early universe?
The problem is the phase coupled with the finite speed of light.
This issue is that the scale of these oscillations only recently entered the light cone of the rest of the universe. Put another way, the oscillations which cause this ringing were of such a scale that they could not harmonize, or be in sync at the time of the last scattering surface. Consequently, they should not be in time with each other (random phases), which would not give a set of harmonics. It would just give a noise spectrum.
Here be dragons, I mean inflation.
A classic way around a problem is to change the question. Rather than directly answer how these oscillations could be in phase, inflation postulates that their scale was small enough in the early universe that the oscillations could be in sync (inside the light cone), then something called inflation pushed the scale of these oscillations outside the light cone.
I consider this an example of what I consider the connect the dots approach. Since we know something has to connect what came before to what came after, we postulate its existence and give it a name. A similar story holds for ‘dark energy’. In neither case is there an explanation of a mechanism of what would cause the phenomenon, just that there is a gap in our knowledge which requires a solution with certain properties.
One might wonder why we care about this ringing. The answer is that it ties into all sorts of other questions about why physics works the way it does: quantum mechanics, relativity, etc. I’m not enough of an expert here, but I know enough to know that it’s important.