Saul Perlmutter, Brian P. Schmidt, and Adam G, Riess shared the 2011 Nobel Prize in Physics for their discovery, in the late 1990s, that the expansion of the universe is accelerating. One possible explanation for this acceleration is the cosmological constant in Einstein’s general relativity theory. Einstein, however, called the cosmological constant his biggest blunder. Was it really a blunder?
Einstein’s General Relativity Theory
Einstein’s theory of general relativity, which is a theory that explains why gravity works, ranks as one of the crowning intellectual achievements of the twentieth century. One implication of general relativity is the expanding universe. Einstein did not believe this implication of his theory. He initially envisioned the universe as eternal and static.
Einstein therefore forced his theory to conform to his preconceived ideas about the universe. To keep general relativity from predicting either an expanding or collapsing universe, Einstein added a cosmological constant to the general relativity equations. There was absolutely no experimental or observational justification for the existence of this cosmological constant. Einstein’s cosmological constant was a blatant fudge to force his theory to conform to his conception of a static universe. The universe is however not static.
Hubble and the Expanding Universe
In 1924 Edwin Hubble proved that spiral galaxies were indeed outside our Milky Way galaxy, when he measured the distance to the Andromeda galaxy. Hubble continued to measure the distances to galaxies, as well as their Doppler redshifts, which tell us the speeds at which the galaxies are receding from us. When Hubble made a graph plotting the recessional speeds of galaxies versus their distances, he noticed something interesting. The more distant galaxies are receding from us more rapidly. Hubble’s interpretation of this result is that the universe is expanding.
Astronomers now call this graph the Hubble plot. The slope of the line on the Hubble plot is the Hubble constant, and the value of the Hubble constant tells astronomers how fast the universe is expanding. A steeper slope, corresponding to a larger value of the Hubble constant, tells astronomers that the universe is expanding more rapidly.
Einstein learned about Hubble’s result, visited Hubble, and was convinced that Hubble’s work was correct. Einstein then did what any good scientist should do when confronted with data that disagree with the theory. Einstein modified general relativity by removing the cosmological constant and returning the equations to their original form. To Einstein’s credit he admitted his error and called the cosmological constant his biggest blunder.
The Accelerating Universe and the 2011 Nobel Prize in Physics
Ever since Hubble’s time astronomers have been refining the value of the Hubble constant with more accurate measurements of distances to distant galaxies. Two independent teams of astronomers, the Supernova Cosmology Project and the High-z Supernova Search Team, announced the same remarkable discovery in 1998. The rate at which the universe is expanding is accelerating. Perlmutter headed the Supernova Cosmology Project. Schmidt and Riess worked on the High-z Supernova Search Team.
Both teams used type Ia supernovas to measure the distances to very distant galaxies. The name “high-z” in fact refers to the large redshifts and hence large distances of the most distant galaxies. Type Ia supernovas are explosions that occur in white dwarf stars when they become too massive to remain stable white dwarf stars. All type Ia supernovas have very nearly the same luminosity (total energy output) at their maximum brightness.
Their uniform extremely high luminosities make type Ia supernovas ideal for measuring distances to very distant galaxies. How bright a celestial object appears from Earth depends on the object’s luminosity and distance; therefore if astronomers know a celestial object’s luminosity and apparent brightness they can calculate its distance. When these teams of astronomers observed a type Ia supernova in a distant galaxy, they used its apparent brightness and the known luminosity for all type Ia supernovas to calculate the distance to the host galaxy.
After using the type Ia supernova to measure the distance to a galaxy, the astronomers also measured the galaxy’s Doppler redshift, and hence recessional velocity, to add another datum to the Hubble plot. Their work showed that the universe was expanding more slowly in the distant past than it is today. Both teams therefore concluded that the rate at which the universe is expanding is accelerating. The 2011 Nobel Prize in Physics recognizes their remarkable and totally unexpected discovery.
Einstein’s Cosmological Constant Reconsidered
Why is the expansion of the universe accelerating? Both common sense and basic physics suggest the opposite. The net gravitational force from all the mass in the universe should tug galaxies inward and therefore slow the expansion rate of the universe. Astronomers refer to whatever force is causing this acceleration in the expansion of the universe as the dark energy and are puzzled about the nature of the dark energy.
One possible solution is Einstein’s cosmological constant. If cosmologists put the cosmological constant back into Einstein’s general relativity equations, then general relativity equations can predict that the expansion of the universe is accelerating. The value of the cosmological constant would have to be different than the value Einstein originally used to force the theory to predict a static universe. The idea basic is however the same.
Was Einstein’s biggest blunder actually a masterstroke?