The Universe Is Expanding
As we saw in Section 38-10, it is possible to measure the relative speeds at which galaxies are approaching us or receding from us by measuring the shifts in the wavelength of the light they emit. If we look only at distant galaxies, beyond our immediate galactic neighbors, we find an astonishing fact. They are all moving away (receding) from us! In 1929 Edwin P. Hubble established a connection between the apparent speed of recession v of a galaxy and its distance r from us- namely, that they are direct proportional. That is,
v = Hr (Hubble’ s law).
“in which H, the proportionality constant, is called the Hubble constant. The value of H is usually in tile unit kilometers per second-megaparsec (Ian/s . Mpc), where meppanec is a legit unit commonly used in astrophysics and astronomy:
1 Mpc = 3.084 X 1019 km – 3.260 X 1()6ly
The Hubble constant H has not had the same value since the universe began. Determining its CUl1’eDl”value is extremely· difficult because it involves meaSurements for very distant galaxies. Recently, one study put the Current value of Hat 70 ::t 7 Ian/s • Mpc, whereas another study stJooaIy argued for a value of 58 kIn/s . Mpc intensity no matter where their antenna was pointed. It soon became clear that Penzias and Wilson were observing a cosmic background radiation, generated in the early universe and filling all space almost uniformly. This radiation, whose maximum intensity occurs at a wavelength of 1.1 mm, has the same distribution in wavelengths does radiation in an enclosure whose walls are held at a temperature of 2.7 K. In this situation the enclosure is the entire universe. Penzias and Wilson were awarded the 1978 Nobel prize in physics for their discovery.This radiation originated about 300000 years after the big bang, when. the universe suddenly became transparent to electromagnetic waves (the waves were not immediately absorbed by particles). The radiation at that time corresponded to radiation in an enclosure at a temperature of perhaps 105 K. As the universe expanded, however, the temperature dropped to its present value of 2.7 K.