Knocking on Heaven's Door - Lisa Randall [170]
Einstein had actually derived an expanding universe from his equations of general relativity. At that time, however, no one had yet measured the universe’s expansion, so he didn’t trust his prediction. Einstein introduced a new source of energy in an attempt to reconcile his theory with a static universe. After Hubble’s measurements, Einstein dispensed with the fudge he had made, calling it “his biggest blunder.” This modification was not entirely erroneous, however. We will soon see that more recent measurements show that the cosmological constant term he added is actually necessary to account for recent observations—although the measured magnitude, which accounts for the recently established acceleration of its expansion, is about an order of magnitude bigger than the one Einstein proposed to merely stall it.
The expansion of the universe was a nice example of a convergence of top-down and bottom-up physics. Einstein’s theory of gravity implies that the universe expands, yet only with the discovery of the expansion did physicists feel confident they were on the right track.
Today, we refer to the number that determines the rate at which the universe expands at present as the Hubble constant. It is a constant in the sense that the fractional expansion everywhere in space is the same. However, the Hubble parameter is not constant over time. At an earlier time, when the universe was hotter and denser and gravitational effects were stronger, it expanded at a far more rapid rate.
Measuring the Hubble constant precisely is difficult, since we face exactly the problem we raised earlier of disentangling the past from the present. We need to know how far away the red-shifting galaxies are, since the red shift depends both on the Hubble parameter and distance. This imprecise measurement was the source of the factor-of-two uncertainty in the age of the universe that I mentioned at the beginning of this chapter. If the Hubble parameter measurements were uncertain by a factor of two, so too would be the universe’s age.
That controversy is now pretty much resolved. The Hubble parameter has been measured by Wendy Freedman of the Smithsonian Astronomical Observatories and her collaborators and others, and the expansion rate is about 22 kilometers per second for a galaxy a million light-years away. Based on this value, we now know the universe is about 13.75 billion years old. This might under- or overestimate the age by 200 million years, but not by a factor of two. Although this might still sound like a good deal of uncertainty, the range is too small to make any great difference in our understanding today.
Two other key observations that agreed nicely with predictions further confirmed the Big Bang theory. One class of measurement that relied on both particle physics and general relativity predictions and therefore confirmed both was the density of various elements in the cosmos, such as helium and lithium. The amount of these elements that the Big Bang theory predicts agrees with measurements. This is in some respects indirect proof, and detailed calculations based on nuclear physics and cosmology are required to compute these values. Even so, this agreement of many different element abundances with predictions would be an unlikely coincidence unless physicists and astronomers were on the right track.
When the American Robert Wilson and the German-born Arno Penzias accidentally discovered the 2.7-degree microwave background in 1964, it was further confirmation of the Big Bang theory. To put this temperature in perspective, nothing is colder than absolute zero, which is zero degrees kelvin. The universe’s radiation is less than three degrees warmer than this absolute limit to how cold anything can be.
The collaboration and adventure of Robert Wilson and Arno Penzias (for which they won the 1978 Nobel Prize) was a superb example of how