The size and age of our Universe is not only a critically important issue in cosmology, but is also among the most controversial and delicate of the cosmological questions. Infrared observations made using NASA’s Spitzer Space Telescope have now given us the most precise estimate yet of the rate at which our Universe is expanding. The key was not the discovery of a new method for measuring distance. Rather, astronomers discovered how to measure brightness more accurately. The new value for the Hubble constant, good to within three percent, is 74.3 kilometers per second per megaparsec (km/s/Mpc).
The size of the redshift-distance relationship is an expansion rate known as the Hubble parameter. (Once called the Hubble constant, we now know it changes with time.) The size of the Hubble parameter has been remarkably difficult to pin down, to the extent that as recently as 1996 a formal debate was carried out in the astronomical community over observational estimates ranging from 50 to 100 km/s/Mpc.
The new value (74.3 km/s/Mpc) for the Hubble parameter is the result of a number of Hubble space telescope measurements, now refined using the Spitzer infrared observations, and confirmed by an independent approach using the Wilkinson Microwave Anisotropy Probe (WMAP), a satellite that measures tiny anisotropies in the 3K cosmic microwave background radiation. The expansion rate is presently only 2.41 parts in a billion billion per second. This seems a tiny effect, but the Universe is a big place.