# nLab comoving time

Contents

### Context

#### Gravity

gravity, supergravity

# Contents

## Idea

In cosmology the radiation and matter-content of the observable universe may typically be approximated (in particular before any relevant structure formation has taken place and FRW models serve as a good approximation) by a dust? or fluid? which fills all of space. The proper time seen by a particle in this (possibly idealized or even hypothetical) dust?/fluid? is then called comoving time, i.e. the time as measured by an observer (i.e. along a lightlike curve in the spacetime pseudo-Riemannian manifold) whose motion is at rest with respect to the ambient dust/fluid.

Often the adjective “comoving” is suppressed when the context is assumed to be clear. For instance whenever any event in primordial cosmology (such as baryogenesis, nucleosynthesis, etc.) is said to occur at a given “time after the big bang”, then what is meant is always comoving time.