Hubble constant ($H_0$)

The Hubble constant is the present value $H_0=H(t_0)$ of the Hubble parameter. The word "constant" fixes the epoch; it does not mean $H(t)$ stays unchanged throughout cosmic time. It sets the present instantaneous proportionality in Hubble's law and the approximate nearby observed redshift-distance relation.

For illustration, stipulate $H_0=70\mathrm{km}{\mathrm{s}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-1}$; this is an example input, not a claim about a measured consensus value. A million parsecs is $3.085677581\dots \times 10^{19}\mathrm{km}$, givingHere a year means $365.25$ days of $86400\mathrm{s}$, and $\mathrm{Gyr}$ denotes a billion such years. The parsec conversion follows ^[ref]. This Hubble time is a present expansion timescale; the age of an FLRW universe also depends on the earlier evolution of the scale factor and need not equal $H_0^{-1}$.