= Photon brightness perturbation
{title2=$\Delta=\int q^3f_1dq/\int q^3f_0dq=4\Theta$}
The photon brightness perturbation is the fractional frequency-integrated directional energy perturbation. For a small blackbody temperature shift $\Theta=\delta T/T$, $f_1=-qf_0'(q)\Theta$. Integration by parts gives $\Delta=4\Theta$. Its direction average is the photon density contrast. This temperature identification assumes a perturbation of blackbody form; the integrated brightness definition itself is more general.
// Target: cosmology.bigb
Back to article page
