The Physics Nobel Prize winners receive a diploma, a medal, and a cash award. The Nobel Prize is the highest honor that scientists can win for their scientific discoveries.
Physics Nobel Prize for 2011
In 2011 the Nobel Prize in Physics was shared by:
- Saul Perlmutter,
- Brian P. Schmidt, and
- Adam G, Riess.
Perlmutter, Schmidt, and Riess were honored for their observations of type Ia supernovas in distant galaxies that showed the expansion of the universe is accelerating. In 2006 they also won the Shaw Prize for Astronomy for this work.
Expansion of the Universe
In the 1920s and 1930s, Edwin Hubble measured distances to galaxies and discovered that the universe is expanding. By also using Doppler redshifts to measure the velocities at which galaxies are moving away from us, Hubble discovered that the more distant galaxies appear to be moving away from us more rapidly. Hubble interpreted this result, often called Hubble's Law, as the expansion of the universe.
A graph of the recessional velocity versus distance for galaxies, called the Hubble plot, shows an approximately linear relationship between these quantities. Astronomers call the slope of the line in the Hubble plot the Hubble constant, and have been refining the value of the Hubble constant since Hubble's initial work. The Hubble constant tells astronomers how fast the universe is expanding. If the Hubble plot is not perfectly linear because the slope of the line is changing, that tells astronomers how the expansion rate of the universe is changing. The greatest difficulty in measuring the Hubble constant, and hence expansion rate of the universe, is measuring the distances to distant galaxies.
Type Ia Supernovas and the Distances to Galaxies
Supernovas are such extreme stellar explosions that the exploding star can outshine an entire galaxy. Type Ia supernovas have no hydrogen lines in their spectra because they occur in white dwarf stars, which have already burned their hydrogen in nuclear fusion reactions. When a white dwarf becomes too massive to remain a stable white dwarf, it explodes as a type Ia supernova. Type Ia supernovas all have very close to the same energy output, called luminosity, when they are at maximum brightness. Their uniform high luminosity makes type Ia supernovas useful for measuring distances to distant galaxies.
Consider an analogy. A forest fire obviously has a much higher luminosity that a single burning match. However a burning match held very close may appear brighter than a distant forest fire. The apparent brightness of burning matches, forest fires, or astronomical objects depends on both their distance and luminosity. Astronomers use this fact to measure distances. If astronomers know the luminosity and apparent brightness of a celestial object, they can calculate its distance.
Astronomers know the luminosity of type Ia supernovas when they explode in distant galaxies. Their extremely high luminosities also makes supernovas visible in the most distant galaxies. Hence when a type Ia supernova explodes in a distant galaxy, astronomers can use its apparent brightness and known luminosity to calculate the distance to the galaxy. If astronomers also measure the galaxy's Doppler redshift, they can plot the galaxy on the Hubble plot to provide another datum for determining the Hubble constant.
Type Ia Supernovas and Hubble Plot
Perlmutter headed the Supernova Cosmology Project. Schmidt and Riess worked on the High-z Supernova Search Team. The term "high-z" refers to large redshifts and hence distances.
Both teams searched for distant type Ia supernovas. When they found a supernova, they used its known luminosity and apparent brightness to calculate the distance to the supernova and hence its host galaxy. After measuring recessional velocity from the galaxy's Doppler redshift, the teams of astronomers placed another point the Hubble plot.
The teams announced their results in 1998. They both concluded that distant galaxies were receding from us more slowly than would be predicted from a Hubble plot of less distant galaxies. The expansion rate of the universe for the most distant galaxies is slower than for nearby galaxies. When astronomers observe distant galaxies, they are looking backwards in time owing to the time that it takes light to reach us from the distant parts of the universe. Hence, the slower expansion rate for the most distant galaxies tells astronomers that the universe was expanding more slowly early in its history.
The expansion rate of the universe is therefore accelerating.
No one expected this result. Astronomers expected that the total gravitational force of all the mass in the universe would tug inward on galaxies and slow the universe's expansion rate.
Much of the excitement of doing science results from unexpected discoveries. When data do not conform to scientists' preconceived notions, they learn something new about the universe around us. The 2011 Nobel Prize in Physics recognizes an unexpected discovery that shook the foundations of modern cosmology like an magnitude 10 earthquake.
Further Reading
Nobel Prize Foundation, nobelprize.org, accessed October 13, 2011.
Physics Nobel Prize Winners 2010
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
