MSSSO Annual Report 1997

SCIENTIFIC HIGHLIGHTS

The Infinite Universe

The High-Z Supernova Search combines the talents of almost twenty astronomers on four continents to find distant exploding stars. This project is lead by Dr Brian Schmidt and aims to measure the ultimate fate of the Universe: Will the Universe expand forever, or will gravity eventually halt its expansion, and send it crashing into the "gnaB giB" the "Big Bang" in reverse?

Type Ia supernovae (SN Ia) form a fairly homogeneous class of objects. These explosions of white dwarf stars have similar spectra and light curves. Small differences do exist, and by observing many objects, research has demonstrated that the rate at which a SN Ia brightens and fades depends on its intrinsic luminosity.

Measuring distances in the Universe is extremely difficult, but by comparing the relative brightnesses of SN Ia, it is possible to measure a distance to a single object with a precision of about 7%. Although this is as good (or better) than any other method for measuring extragalactic distances, SN Ia are so bright that they can be used to measure distances to objects more than half way across the Universe. By comparing the expansion rate of the Universe nearby and far away, Schmidt and collaborators will measure the rate the Universe is slowing down, and thereby gauge its ultimate fate.

Fig. 1. SN 1997ck, the most distant star yet discovered

If gravity is unable to reverse the expansion, Einstein's equations of General Relativity demand that the Universe curves away from itself and is infinite in extent now, in the past, and in the future. Over the coming year Schmidt and collaborators will dis

Scientific Highlights

cover and analyse another 20 objects. These objects will enable the High-Z Supernova Search to probe the existence of a postulated form of exotic matter/energy, represented in Relativity by Einstein's Cosmological Constant. This form of matter, if it exists, would be a panacea for some current problems in our understanding of Cosmology and the Big Bang.

In 1997 Schmidt and collaborators discovered the most distant star yet. The light from this extremely faint object, designated SN 1997ck, was emitted some 8 billion years ago, as shown in Figure 1, and was discovered by comparing an image taken with the Canada-France-Hawaii Telescope, located on Mauna Kea, in early April with one taken 3 weeks later. This SN and 3 others were followed with the Hubble Space Telescope, so their light curves could be compared to supernovae discovered in the nearby Universe. Figure 2 shows that this comparison strongly indicates that gravity is not significantly decelerating the Universe, and the Universe will expand forever.

Fig.2 In the curved space-time of General Relativity one of the measures of distance is 'luminosity distance' or distance modulus. This is plotted vertically here for supernovae against redshift on the horizontal axis. The expectations for different cosmological parameters are shown by three curves. Distant supernovae appear brighter than the predictions of closed universes, even ruling out the elegant Einstein-De Sitter Universe, long a favourite model of cosmologists.