This year’s Nobel Prize for Physics was announced this October and it has been awarded to Adam Riess, Brian Schmidt, and Saul Perlmutter for their discovery of the accelerating expansion of the universe. Three astronomers taking home the Nobel for Physics this year is of course a welcome NEWS for all the Universe-worshippers in our department (and of course everywhere in the world).
This is in no way a review of their paper, but merely a historical approach on how our understanding of this expanding universe took shape.
In 1919, the way we see the world changed – Einstein’s General relativity was verified by Eddington. Einstein was in favour of the Static Universe Idea, but when he applied his relativity equations to Cosmology, he found that without introducing a new ‘cosmological constant’ term, it is not possible to predict one. The repulsive effects of the cosmological constant exactly balanced the gravitational attraction of the matter, keeping the model static. However, in 1922, Friedmann published a paper to show that the universe must be dynamical, using Einstein’s equations without taking into account the Cosmological Constant and said that Space itself expands, Universe starts with a big bang and expands – Galaxies move apart as space between them expands. Hubble observed in 1929 that galaxies really were moving apart just and hence discovered expansion of universe. Einstein then called the cosmological constant his “biggest blunder”.
In 1948 Gamow, Herman, and Alpher used Friedmann’s Model to predict that the early universe should be hot and that the hot radiation present back then should be around today as a microwave background and in 1965 Penzias and Wilson discovered this CMBR. But this radiation was very uniform and due to finite speed of light, it was impossible to see how same temp could be seen in different parts of the early universe. In 1981, Alan Guth proposed a theory of inflation to explain this. Inflation causes the big bang explosion. For a tiny amount of time the universe was very small which allows the different parts we see to come into contact with each other, thereby explaining the isotropy of the universe we observe.
Scientists wanted to determine the energy density of the universe. As Friedmann said, if the density were above a critical density, the universe would continue slowing down in its expansion due to the gravitational attraction of the galaxies for each other, and finally the whole universe would collapse. If the universe was below that critical density, it would continue expanding forever. Two teams set out to measure this using distant large-redshifted supernovae. Saul Perlmutter led one team at Berkeley and Brian Schmidt formed a team at Harvard, including Bob Kirschner and Adam Riess. They determined the distances to these Type Ia supernovae by measuring their brightness. Moreover, the wavelengths of light coming from the supernova stretch with space as the universe expands. So, if we measure the redshift of lines in the specturm of a supernova, it is possible to find out how big the universe was (relative to today) when the supernova exploded. So it’s possible to see the expansion history of the universe. When Adam Riess first looked at the results, he surprisingly saw that the universe is accelerating – it is expanding faster and faster! The most likely explanation was that old cosmological constant term of Einstein! This cosmological constant is now a measure of the dark energy of the universe, which has a negative pressure and hence causes the accelerating expansion. The two teams came to the same conclusion and reported their results in 1998.
Anyways, Later Adam Riess of Harvard Team observed very distant supernovae using Hubble to see that at earlier times the universe was decelerating just as expected. We have studied last semester in Nicola’s class in Rome, that as the universe expands, dark energy stays at nearly constant energy density while Radiation thins out as 4th-power and matter as 3rd-power. So a radiation-dominated universe gives way to a matter-dominated one and as the matter in the universe thins out the dark energy begins to dominate. Once that occurs, the universe goes from an expansion that is slowing down to an accelerating expansion. The repulsive effects of dark energy seem to guarantee that the universe will continue to expand forever.