Theoretical and observational cosmology: Light cones

The Past Light Cone in the Expanding Universe


This animated diagram (referred to on p. 2-10) shows our past lightcone in an expanding homogeneous isotropic universe, with proper distance to a point on the light cone plotted horizontally, and the age of the universe as a fraction of the current age plotted vertically. The lightcone curves back on itself on account of the expansion of the universe. It would completely close off at t=0, with one point smeared over our whole sky. However, the visible universe is limited by the surface of last scattering (the cosmic microwave bakground radiation) represented here by the horizontal axis.

The inside of the light cone is the observable universe: as time goes by more and more of the universe becomes observable, and the volume of the past lightcone increases. The limit to our observable universe at any time, defined by the finite extent of the past light cone, defined our particle horizon (Notes section 2.15).

Out worldline tracks up the central time axis at the apex of the lightcone. Worldlines of distant galaxies are represented by the curves moving upwards whose proper distance relative to us is increasing as the universe expands. We receive light signals from these galaxies sent along the lightcone; at any time we see the nearer galaxy as it was some time in the past, and the further galaxy as it was even more distant in the past.