Ia-type supernovae form a class of very homogenous events from the point of view of their luminosity. In fact, they are so luminous that they can be detected at a distance of billions of light years, far enough away to make the observation of cosmological effects possible. Supernovae can also act as "standard candles" for the estimation of some cosmological parameters. Eleven Ia-type supernovae were observed with the Hubble space telescope between 1998 and 2000. Their light curves and spectrums observed from space form an unprecedented set of data concerning these high-redshift supernovae (in other words, those located at great distances). The conclusions resulting from the observation of these supernovae are very important in relation to the presence of "dark energy" in the cosmos.
First of all, these new observations confirm the results announced in 1998 by the SCP and one of its competitors, the Hi-Z Supernova Team. The surprising discovery stemming from these two studies, based on observations made primarily from the ground, was that the luminosity of very distant Ia-type supernovae was less than what would be expected in a Universe without acceleration but consistent with the presence of a high-degree of acceleration energy. This meant that the Universe was in accelerated expansion due to the effect of a type of energy referred to as "dark energy" present throughout space. Criticism at that time was based on the assumption that this loss of luminosity could be due to the presence of dust in the host galaxies that, in large quantities, absorbs and diffuses light, thus decreasing the apparent luminosity of supernovae. But, if this is the case, this dust must also affect the "color" of the supernovae, causing them to appear "redder" since the absorption and diffusion of light and, as a result, its extinction, preferentially take place in the blue range, a phenomenon that is totally comparable to the reddening of the sky at sunset,, which is even more dramatic on particularly polluted days. These observations could therefore not provide conclusive evidence when made from the ground.
On the other hand, observations made with the Hubble space telescope, far from the influence of the Earth's atmosphere, provide much more precise data. In particular, these data make it possible to quantify the degree of extinction (depending on the wavelength) of the light emitted by a supernova, due to the presence of dust in the host galaxy. The results obtained thus made it possible to do away with any ambiguity: The attenuation of light cannot be attributed only to extinction by the dust of the host galaxy– it must be partially due to the presence of dark energy.
These new results have also made it possible to:
- obtain more precise estimates of the relative density of matter and dark energy present in the Universe: 75% of the density of the Universe would thus consist of dark energy,
- obtain new information about the nature of this mysterious energy (its density/pressure ratio, for example),
- exclude some cosmological models that attempt to explain the nature of this energy (including the simplest ones from among the quintessence models).
Ia-type supernovae are the most direct means of measuring the properties of this energy. Experiments under way such as the SuperNova Legacy Survey and the Supernova Factory, with the participation of teams from the CNRS (IN2P3 and INSU(2
)) and the French Atomic Energy Commission (DAPNIA(3
)) , aim at measuring a thousand of these supernovae up to maximum redshift values (very distant supernovae). They will make it possible to explore the nature of dark energy with the greatest precision possible. In the long term, the “SuperNova Acceleration Probe” (SNAP) satellite project, presently on the drawing board, will become instrumental and make it possible to obtain very precise measurements of cosmological parameters and the evolution of the density of dark energy as the Universe expands.
To be published in Astrophysical Journal (Knop et al, Supernova Cosmology Project*, astro-ph/0309368)
(1) National Institute of Nuclear and Particle Physics
(2) National Institute for the Sciences of the Universe
(3) Department of Astrophysics, Particle Physics, Nuclear Physics and Instrumentation associated with DSM