Wednesday, March 11, 2015

mass content of the Universe

can you estimate and quote an error on the estimate of the total mass inside the Hubble-volume? what assumptions do you make and which measurements are needed?


  1. Up to the usual factors, Omega times the square of the Hubble constant is the density, and the mass is the density times the volume. To know the volume, we need to know the proper distance to the Hubble sphere, which depends on lambda and Omega as well as H.

  2. thanks a lot, Phillip, for your answer. with a measurement of $\Omega_m$ (from e.g. supernovae) and of $H_0$ (from Cepheid stars) one could estimate the mass to be $m=\Omega_m\rho_\mathrm{crit}\times 4\pi\chi_H^3/3$, with $\chi_H=c/H_0$ and $\rho_\mathrm{crit}=3H_0^2/(8\pi G)$. numerically, this yields $M\simeq 10^{52}~\mathrm{kg}/h$ or $10^{22}M_\odot/h$.