Question

Fluid approximation for inflaton?

Answer 1

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can be described by energy density and pressure. So I am wondering whether it is also true for inflaton and why? It seems so since people can define density, pressure and even speed from the energy momentum tensor for inflaton as functions of coordinates. And what about those matter contents with polarizations like photon?

Also, do we consider inflation to be adiabatic? the equation of motion of inflaton can also be derived from the conservation equation of fluid ρ˙+3H(ρ+p)=0ρ˙+3H(ρ+p)=0 and the zero on the right hand side makes me assume so. Actually I am really confused about how to interpret the term conservation in this case. Even though the conservation of energy momentum of the universe as a whole ∇μTμν=0∇μTμν=0must be true since the universe is closed or simply from Einstein equation, I couldn't make sense of the ever increasing total vacuum energy during inflation.

hope this helps...

Answer 2

The inflaton is a hypothetical scalar field that is supposed to be responsible for cosmic inflation in the very early universe. A quantized particle for this field is expected, similar to other quantum fields, called an inflaton. The field provides a mechanism by which a period of rapid expansion from 10−35 to 10−34 seconds after the initial expansion can be generated, forming the universe.