The memory of the cosmos is not only revealed by the study of its background radiation, but also of its brightest stars, the famous supernovae. An analysis of their luminosity recently provided new data leading to radically new hypotheses about the history of the expanding universe and its physical characteristics.
The observers of antiquity had already noticed that, whereas most stars remained identical night after night (except for the planets), sometimes a new star appeared, burned brightly for a while and then disappeared forever. Mention of these new stars (stellae novae) has also been found in the archives of astrologists at the Chinese imperial court of the first millennium. The founders of modern European astronomy, Tycho Brahe and Johannes Kepler, were also lucky enough to observe, in their lifetime, the phenomenon of the supernovae of our galaxy.
It was in 1993 that the German Walter Baade and the Swiss Fritz Zwicky offered the first explanation of this stellar event: the light emitted by a supernova – the name they chose to distinguish novae distinctive for their brightness – did not indicate the birth of a new star but, on the contrary, the explosion of a stellar body similar to our own Sun, whose hydrogen core had completely burnt out, and ejects its atmosphere to create a planetary nebula. Different types of supernovae were subsequently discovered, in particular binary star systems within which matter can be transferred from one star to another until the latter collapses under its own weight.
Reading by the ‘standard candle’ Cosmologists are interested primarily in the physics and distances of the bright supernovae. The luminosity of some of them is a means of measuring their distance by selecting those which astrophysicists call ‘standard candles’. This strange name is used to refer to star families whose luminosity we know in theory. Ideally, this luminosity should be uniform for all members of the family. As this decreases with the square of the distance, observing the luminosity of a star belonging to a group of standard candles makes it possible to determine the distance. “Stars belonging to the so-called '1a' type have proved to be excellent standard candles as their luminosity variations are very similar, whichever one is observed,” explains Bruno Leibundgut, a member of the ESO European network on supernovae which is headed by Wolfgang Hillebrandt of the Max-Planck Institute of Astrophysics in Garching (DE). “By making small adjustments, it is possible to measure their distance with a precision of around 10%, despite their great distance in the universe. This is totally new to cosmology.”
Given the context of an expanding universe, the further one peers into space the further back in time one is seeing. By delving into the cosmic memory, astronomers have managed to discern supernovae around 9.7 billion years old. Thanks to these observations, we are able to travel back to a time when the universe was just one third of its present age and study its development since this time ‘immemorial’.
This approach led to a surprising discovery in the late 1990s. Observations of the supernovae led to the view that – contrary to what had been thought throughout the past century – the universe, after a period of slowed expansion, has been expanding at an accelerated rate during the past 6 billion years or so. Such a revelation called into question its very continuity and implied the existence of a new energy, known as ‘dark energy’, which was capable of generating this acceleration. This impressive enigma was recently corroborated, in May 2004, by a team of physicists, led by Steve Allen, from Cambridge University (UK). They combined the study – using NASA’s Chandra telescope – of X-rays emitted by 26 distant galaxy clusters and data obtained by the WMAP probe on fluctuations in the cosmic background noise. Dark energy, which is at present undetectable and of an unknown nature, is believed to represent almost 70% of the content of the universe.
When taken to the extreme, such a finding can produce a not impossible but purely speculative scenario of a universe that has an end. If, one day, this mysterious force caused an ‘uncontrolled’ expansion of the cosmos, this could result in the dislocation of galaxies – including our own – and ultimately in the disappearance of any trace of atoms. That is what the cosmologists refer to as the Big Rip.(1)
In the meantime, much progress remains to be achieved in understanding the functioning of the supernovae and in ensuring their validity as standard candles. “There are at least two models describing the explosion of the type ‘1a’ supernovae. Only sets of closely matching data will give us an assurance of their luminosity and increase our confidence in measurements of their distances,” states Leibundgut. “What is more, we must verify that the most distant and, therefore, the oldest of these stars function in the same way as the closer ones. This, too, could distort the luminosity measurements.”
(1) The opposite hypothesis would be that of the Big Crunch, in which the universe would collapse in on itself.
A whole spate of projects to observe the supernovae is currently in progress in an attempt to remove all the uncertainties. In space, first of all, with the SNAP (SuperNova Acceleration Probe). Among other things, this aims to increase by 20 the number ...
A whole spate of projects to observe the supernovae is currently in progress in an attempt to remove all the uncertainties. In space, first of all, with the SNAP (SuperNova Acceleration Probe). Among other things, this aims to increase by 20 the number of supernovae observed. As to the JWST (James Webb Space Telescope), the successor to the Hubble space telescope, this will be able to observe these stars in infrared, a wavelength that is inaccessible from the ground. This will make it possible to study them during their development stages about which we know very little. On Earth, astronomers are using large telescopes, such as the European VLT (Very Large Telescope) in Chile, in order to gather a maximum amount of light within a reasonable time. “Our network has chosen a different path by combining the efforts of observers and of theorists,” stresses Leibund. “We have accumulated data of unequalled precision that will enable us to better understand the supernovae.”