The Hubble Constant: A Historical Review

For 100 years since galaxies were found to be flying apart from each other, astronomers have been trying to determine how fast. The expansion, characterized by the Hubble constant, \(H_0\) , is confused locally by peculiar velocities caused by gravitational interactions, so observers must obtain accurate distances at significant redshifts. Very nearby in our Galaxy, accurate distances can be determined through stellar parallaxes. There is no good method for obtaining galaxy distances that is applicable from the near domain of stellar parallaxes to the far domain free from velocity anomalies. The recourse is the distance ladder involving multiple methods with overlapping domains. Good progress is being made on this project, with satisfactory procedures and linkages identified and tested across the necessary distance range. Best values of \(H_0\) from the distance ladder lie in the range \(73-75\)  km s \(^{-1}\) Mpc \(^{-1}\) . On the other hand, from detailed information available from the power spectrum of fluctuations in the cosmic microwave background, coupled with constraints favoring the existence of dark energy from distant supernova measurements, there is the precise prediction that \(H_0 = 67.4\) \(\pm 1\%\) . If it is conclusively determined that the Hubble constant is well above 70 km s \(^{-1}\) Mpc \(^{-1}\) as indicated by distance ladder results then the current preferred \(\Lambda \) CDM cosmological model based on the Standard Model of particle physics may be incomplete. There is reason for optimism that the value of the Hubble constant from distance ladder observations will be rigorously defined with \(\sim 1\%\) accuracy in the near future.