## Wednesday, November 2, 2016

### cosmology works?

a freely-falling observer experiences the metric $g_{\mu\nu}$ of generated by any energy-momentum distribution $T_{\mu\nu}$ as being of Minkowskian shape $\eta_{\mu\nu}$. the observer would conclude that there is no curvature and consequently, no gravitational fields and that the energy-momentum is vanishing.

but isn't that what we're doing in cosmology? as a freely-falling observer we aim to determine the energy-momentum of all cosmological fluids $T_{\mu\nu}$ expressed in terms of their density parameters $\Omega$ and their equations of state $w$. but how can one measure the energy-momentum-tensor of a gravitating matter distribution with a gravitational experiment?

#### 1 comment:

1. In the first paragraph, this refers to a local measurement (the equivalence principle applies only locally), while in the second case non-local observations are involved.