Credit: O. Krause (Steward Obs.) et al., SSC, JPL, Caltech, NASA South Africa and the SDSS-II Supernova Survey Ed Elson (SAAO/NASSP) Bruce Bassett (SAAO/ICG) Kurt van der Heyden (SAAO) Zebalon Vilakazi (UCT)

Dark energy What constitutes our Universe? What constitutes our Universe? Simply radiation and baryonic matter? Simply radiation and baryonic matter? Is there more than meets the eye? Is there more than meets the eye? Up to 95% of its energy content is thought to be exotic. Up to 95% of its energy content is thought to be exotic.

Dark energy 2 leading DE models: cosmological constant (  ) and quintessence. 2 leading DE models: cosmological constant (  ) and quintessence.  has constant negative pressure, causing expansion of the Universe to accelerate.  has constant negative pressure, causing expansion of the Universe to accelerate. Quintessence is a dynamical field in which negative pressure DE varies in space and time. Quintessence is a dynamical field in which negative pressure DE varies in space and time.  is the most economical solution to cosmic expansion.  is the most economical solution to cosmic expansion.

Dark energy and the Universe DE has many implications for the history and fate of the Universe. DE has many implications for the history and fate of the Universe. The more DE, the older the Universe and the expansion increases with time. The more DE, the older the Universe and the expansion increases with time.

Type Ia Supernovae SNIa are semi-detached SNIa are semi-detached binary systems. binary systems. When the Chandrasekhar When the Chandrasekhar mass limit is reached, mass limit is reached, the white dwarf explodes violently. the white dwarf explodes violently. SNIa have a characteristic light curve. SNIa have a characteristic light curve. Important feature: decay rate is related to peak intrinsic luminosity of SN. Important feature: decay rate is related to peak intrinsic luminosity of SN.

Credit: High-Z Supernova Search Team, HST, NASA (Riess et al. 1996a, 1998a)

Supernova 1994d, type Ia.

Supernova 1987A, thought to be type II.

Evidence from SNIa of acceleration A SNIa survey in the late 1990’s provided some of the 1 st evidence of cosmic acceleration. A SNIa survey in the late 1990’s provided some of the 1 st evidence of cosmic acceleration. SNIa were used as standard candles to measure the expansion history of the Universe. SNIa were used as standard candles to measure the expansion history of the Universe. Results indicated acceleration, i.e. large DE content. Results indicated acceleration, i.e. large DE content.

SDSS-II Aim: Study structure of Milky Way Aim: Study structure of Milky Way and nature of dark energy. and nature of dark energy. Expected to discover ~200 new SNIa by Expected to discover ~200 new SNIa by Already over 100 new confirmed SNIa have been announced. Already over 100 new confirmed SNIa have been announced. After each night’s observing, hand scanners work through ~5000 objects to try identify SNIa candidates. After each night’s observing, hand scanners work through ~5000 objects to try identify SNIa candidates.

SDSS-II Performance of the scanners is continuously monitored. Performance of the scanners is continuously monitored. Spectroscopic follow up observations and multi-band light curve fitting are carried out on SNIa candidates. Spectroscopic follow up observations and multi-band light curve fitting are carried out on SNIa candidates.

South Africa’s Involvement SALT will study the Universe at very early times. SALT will study the Universe at very early times. The high res. Spectrograph could be used to obtain SNIa spectra. The high res. Spectrograph could be used to obtain SNIa spectra. Multi-wavelength observations of SNIa will help determine extinction effects of galaxies. Multi-wavelength observations of SNIa will help determine extinction effects of galaxies. Significant results would yet again change out view of the Universe! Significant results would yet again change out view of the Universe!

The end. (Ed (Bruce (Kurt van der (Zebalon