Norbert Straumann, Uni Zürich
"The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2011 with one half to Saul Perlmutter and the other half to Brian P. Schmidt and Adam G. Riess 'for the discovery of the accelerating expansion of the Universe through observations of distant supernovae'."
It has long been recognized that supernovas of type Ia are excellent standard candles and are visible to cosmic distances. In 1979 Tammann  and Colgate  independently suggested that at higher redshifts this subclass of supernovae can be used to determine also the deceleration parameter. In the 1990's this program became feasible thanks to the development of new technologies which made it possible to obtain digital images of faint objects over sizable angular scales, and by making use of big telescopes such as Hubble and Keck.
In the standard scenario of Type Ia supernovae a white dwarf accretes matter from a nondegenerate companion until it approaches the critical Chandrasekhar mass and ignites carbon burning deep in its interior of highly degenerate matter. This is followed by an outward-propagating nuclear flame leading to a total disruption of the white dwarf. Within a few seconds the star is converted largely into nickel and iron. The dispersed nickel radioactively decays to cobalt and then to iron in a few hundred days. (A lot of effort has been invested to simulate these complicated processes, but for the cosmological applications this may not be so relevant.) In view of the complex physics involved, it is not astonishing that Type Ia supernovae are not perfect standard candles. Their peak absolute magnitudes have a dispersion of 0.3-0.5 mag, depending on the sample. The research teams of the Nobel Prize winners have, however, learned over the years to reduce this dispersion by making use of empirical correlations between the absolute peak luminosity and light curve shapes. Examination of nearby SNe showed that the peak brightness is correlated with the time scale of their brightening and fading: slow decliners tend to be brighter than rapid ones. There are also some correlations with spectral properties. Using these correlations it became possible to reduce the remaining intrinsic dispersion, at least in the average, to - 0.15 mag. Other corrections, such as Galactic extinction, have been applied, resulting for each supernova in a corrected (rest-frame) magnitude. The redshift dependence of this quantity is compared with the theoretical expectation within the class of cosmological models independently found by Friedmann and Lemaître.
The Nobel laureates are leaders of the two major teams investigating highredshift SNe Ia: the 'Supernova Cosmology Project' (SCP) and the 'High-Z Supernova search Team' (HZT) ¹. Each team has found a large number of SNe, and both groups have published almost identical results. In 1998 the Hubble diagram for Type Ia supernovae gave, as a big surprise for many cosmologists, the first serious direct evidence for an accelerating Universe. After the classic papers , ,  significant progress has been made by both teams. (For up-to-date information, see the home pages  and .) Possible systematic uncertainties due to astrophysical effects have been discussed extensively in the literature. The most serious ones are (i) dimming by intergalactic (grey) dust, and (ii) evolution of SNe Ia over cosmic time, due to changes in progenitor mass, metallicity, and C/O ratio. Because of such worries, astronomers were for quite some time sceptical and doubted that the observations definitely established a non-vanishing cosmological constant or some more general (dynamical) form of Dark Energy. By now the results from the supernovae are well established, and are in addition strongly supported by other completely independent cosmological observations, in particular by precision measurements of the microwave background and large scale surveys of galaxy distributions. The evidence that about 70% of the matter-energy density of the Universe consists of what is called (somewhat misleadingly) "dark energy" is now really strong. There are, however, attempts to explain the observations differently, for instance by modifying general relativity.
1. Most cosmologists repeat again and again that it was Hubble who discovered the expansion of the Universe. This is a myth that was first propagated by Humason in 1931. After first tentative steps toward the discovery of the velocity-distance relationship by Wirtz and Lundmark in the early 1920's, it was Lemaître who established in 1927 the expansion of the Universe and interpreted it as a consequence of general relativity (GR). In his crucial paper  he derived the general redshift formula for an expanding Universe, and showed that it leads for small distances to a linear relation, known as Hubble's law. He also estimated the Hubble constant H0 based on Slipher's redshift data. Two years before Hubble he found a value only somewhat higher the one of Hubble from 1929. (Actually, Lemaître gave two values for H0.) In a public talk on January 1929 in Brussels, Lemaître employed the same balloon model we use today. Hubble, on the other hand, nowhere in his famous 1929 paper even mentions an expanding Universe. In addition, Hubble never claimed to have discovered the expanding universe, he apparently never believed this interpretation. That Hubble was elevated to the discoverer of the expanding universe belongs to sociology, public relations, and rewriting history. These and related facts are in detail documented in the recent excellent book  of our Swiss colleagues Harry Nussbaumer and Lydia Bieri.
2. It is well-known, that Einstein realized the freedom to introduce the cosmological term in his field equations only when he applied GR to cosmology. GR is a classical field theory with two free constants – the gravitational constant G and the cosmological constant Λ – that have to be determined from observations. This was the point of view of several leading cosmologists, among them Lemaître, Eddington and Tolman. When Einstein dropped in 1931 the cosmological term, just for simplicity reasons, many influencial colleagues (for instance Pauli) followed him.
Lemaître was the first who associated in 1933 the cosmological constant with vacuum energy. Today, this confronts us with the profound mystery that the energy scale belonging to Λ is so tiny by particle physics standards. From quantum fluctuations in known fields up to the electroweak scale, contributions to the vacuum energy density are expected to be vastly larger (more than 50 orders of magnitude) than the observed dark energy density. In spite of many attempts, no convincing proposal out of this dilemma has emerged. Einstein's so called "biggests blunder" was not that he introduced the cosmological term – actually he should have done that already in his 'final' field equations in November 1915 – but that he did not realize that even with this term there are no stable static cosmological solutions (for dust).
In the next issue of this journal, we shall indicate the interesting early history of cosmology, beginning with Einstein's model of 1917, up to the point when the majority of cosmologists accepted Lemaître's ideas on the expanding Universe.
¹ An active member of this team – since its constitution – is the Swiss astronomer Bruno Leibundgut (ESO).
 G. A. Tammann. In Astronomical Uses of the Space Telescope, eds. F. Macchetto, F. Pacini and M. Tarenghi, p.329. Garching: ESO
 S. Colgate, Astrophys. J. 232, 404 (1979).
 S. Perlmutter, et al., Astrophys. J. 517, 565 (1999).
 B. Schmidt, et al., Astrophys. J. 507, 46 (1998).
 A. G. Riess, et al., Astron. J. 116, 1009 (1998).
 SCP-Homepage: http://www-supernova.LBL.gov
 HZT-Homepage: http://cfa-www.harvard.edu/cfa/oir/Research/supernova/HighZ.html
 G. Lemaître, L’univers en expansion. Ann. Soc. Sci. de Bruxelles 47, 49 (1927). Translated in MNRAS 91, 483 (1931).
 H. Nussbaumer and L. Bieri, Discovering the Expanding Universe, Cambridge University Press (2009).
Picture sources: 1: www.oe24.at © AP Photo; 2: www.nzz.ch © Keystone/epa; 3: www.epochtimes.de © Gail Burton/The John D. and Catherine T. MacArthur Foundation/AP/dapd; 4: www.spiegel.de © Corbis; 5: unendlicheweiten.wordpress.com
[Released: January 2012]