# Kolb and Turner's expresion for number density Eqn. (3.46) and pressure in Eqn. (3.48)

Having defined the phase space distribution function $f(\textbf{r},\textbf{p},t)$ in $\mu-$space, one can express the information that there are $N$ particles in the volume $V$ through the condition $$\int f(\textbf{r},\textbf{p},t) d^3\textbf{r}d^3\textbf{p}=N$$ as given in, for example, Kerson Huang, Statistical mechanics, second edition, Sec. 3.1, Eq. 3.4).

In equilibrium, the distribution function is independent of $t$ so that $$\int f(\textbf{r},\textbf{p}) d^3\textbf{r}d^3\textbf{p}=N.$$ If the particles are uniformly distributed in space, so that $f$ is independent of $\textbf{r}$, then the number density is given by $$n=\frac{N}{V}=\int d^3\textbf{p}f(\textbf{p})$$

1. The expression for number density as given in Cosmology book by Kolb and Turner, they have an extra factor of $\frac{1}{(2\pi)^3}$: $$n=\frac{N}{V}=\frac{g}{(2\pi)^3}\int d^3\textbf{p}f(\textbf{p})$$ where $g$ counts the number of internal degrees of freedom. But where does the factor $\frac{1}{(2\pi)^3}$ come from? Why is this factor missing in Huang's expression for number density?

2. Kolb and Turner also writes an expression for the pressure for the relativistic particles satisfying $E^2=|\textbf{p}|^2+m^2$ (units in which $c=1$) in equilibrium as $$p=\frac{g}{(2\pi)^3}\int \frac{|\textbf{p}|^2}{3E}f(\textbf{p})d^3\textbf{p}.$$ What should the starting point in deriving this formula for pressure? In other words, what is the general formula for equilibrium pressure?

• This also has to do with how you do the Fourier transformation. Jun 22 '17 at 1:10

1. The extra factor of $\frac{1}{(2 \pi)^3}$ just comes from a different convention for the normalization of $f({\bf p}, {\bf r})$. The one in Kolb & Turner corresponds to Fermi-Dirac and Bose-Einstein distributions on the form: $$f({\bf p}) = \frac{1}{e^{(E({\bf p})-\mu)/T} \pm 1}.\tag{1}$$
2. In general the energy-momentum tensor is given by: $$T^{\mu\nu} = \frac{g}{(2\pi)^3} \int d^3{\bf p} \frac{P^\mu P^\nu}{E({\bf p})} f({\bf p}). \tag{2}$$ For an ideal fluid, you can compare this to $$T^{\mu\nu} = (\rho + P)u^\mu u^\nu - g^{\mu \nu} P.\tag{3}$$ to check the expression for the pressure. (Note that I use the mostly minus metric convention here).