Constraining Inflationary Models

I am going to write a series of posts that will both clarify why I am currently researching non-Gaussianity and explain how we can distinguish between inflationary models.  (The posts are already written so this this promise will be fulfilled. :) )

Without rehashing details, we know that inflation happened.  It makes several specific predictions with nearly all of them being verified better than people had a right to suspect. (The other predictions should be verified soon.)

Inflation is defined as a period where the universe experienced a superluminal expansion, or more technically, a phase in which the comoving Hubble radius shrank.  (See picture).   You can think of the Hubble radius as your light cone.  If space expands faster than the speed of light, the radius of influence of the light coming from you shrinks.

For example, if after a split second light has influenced everything within 100 meters of you, then the space expands superluminally, soon the light is only influencing objects that were once only 5 meters from you.  It's as if it's effective radius of influence shrinks.  (Again, look at the plot and try to make sense of it.)

What we don't know is the exact mechanism that drove inflation.  The problem is there are many models that do the trick: single scalar fields, multiple scalar fields, fields that are not scalars, branes from string theory, etc...  Interestingly, the model that best fits the data is the simplest one: a single scalar field and so this is the one most people study.

So in the next few posts I am going to explain ways we can distinguish between these models.  There are a few parameters that each model of inflation seems to predict different values for that can be used to rule out inflationary mechanisms.  Traditionally these the two parameters were the spectral index n_s and the ratio of tensor to scalar modes r.  Recently, it has been discovered that there are 5 more parameters coming from what is known as non-Gaussianity.  Non-Gaussianity therefore has the potential to constrain and rule out models of inflation better than anyone could have expected a decade ago.