# Difference between revisions of "Hubble's Law and Scale Factors"

${\displaystyle {v=Hr}\,\!}$
where ${\displaystyle H}$, the Hubble parameter, is not constant, but can vary slowly with time. By convention, ${\displaystyle H}$ is often expressed as ${\displaystyle H=100\cdot h{km \over s\cdot Mpc}}$, where 1 parsec (pc) ${\displaystyle \approx 3\cdot 10^{18}cm=3.26ly}$, is the distance at which 1 AU appears as 1 arcsec on the sky. The Hubble Space Telescope Key Project (Freedman et al. ApJ 553, 47, 2001) measured the present day value of Hubble Constant ${\displaystyle H_{0}=72\pm 8{km \over s\cdot Mpc}}$, giving us that the current timescale for the expansion of the universe is ${\displaystyle H_{0}^{-1}\approx {h \over 10^{11}}yrs\approx 9.778h^{-1}Gyrs}$.