Supernovae Observations of the Expanding UniverseHistory and MotivationExpanding HistorySearch for a Standard CandleThe Great Supernovae HuntThe Great Supernovae HuntDiscovery of AccelerationSystematic ErrorsTheoretical ImplicationsDark EnergyFuture ExperimentsSupernovae Observations of the Expanding UniverseKevin TwedtPHYS798GApril 17, 2007Overview• How do we measure expansion?• Use of supernovae 1a as a good measuring stick• Techniques for observing supernovae• Discovery of acceleration• Theoretical implications and Dark EnergyHistory and Motivation• 1929 – discovery of expansion by Edwin Hubble.• The Hubble constant, H0, gives the current rate of expansion. •H0can be measured by looking at how “nearby” objects move. • Measurement of acceleration rate requires use of much more distant objects.Motivation – Acceleration measurements give mass and energy density values for the universe. Needed to distinguish between cosmological models.presentdtdHα=0• How do we measure expansion rates?– Measure magnitude of astronomical standard candles to get accurate distance measurements – This gives the look back time (t = distance to Earth / c) - the amount of time that has elapsed since the light left the standard candle. – Also measure the amount of redshiftz = Δλ/λwhich gives the amount of expansion of space, α. – Measure magnitude and redshift for many standard candles over a wide range of distances.– Construct α(t)Expanding History22dtdonacceleratiα=Search for a Standard Candle• Standard Candle - any distinguishable class of astronomical objects of known intrinsic brightness that can be identified over a wide distance range.• Early attempts (Hubble and others) tried to use galaxies as standard candles. – Too much variation. • Supernovae– Simple radiative properties– Intrinsically bright– Found everywhere in early and recent universe• Subclass SNe type 1a provide best standard candle.Supernovae 1a• Nearby type 1a SNe show a simple relationship between their peak brightness and the time scale of their light curve. • Determine distance by comparing observed magnitude of distant supernovae to absolute magnitude.The Great Supernovae Hunt• Problems –– Supernovae are rare (only 1 or 2 per galaxy per millennium)– Unpredictable – Need to be measured immediately after they are found, as they will pass their peak of brightness within a period of a few weeks.• But, telescope time is assigned months in advance on the basis of research proposals. • Often, supernovae measurements had been made on other people’s telescope time.The Great Supernovae Hunt• 1990’s – astronomers developed systematic discovery process. – Supernovae Cosmology Project (SCP) of LBL – High-Z Supernovae Search of Australia’s Mount Stromlo observatory• Used wide-field imagers to view a large section of sky in one night.• Can search up to a million galaxies a night – ensure discovery of at least a few supernovae.Discovery of Acceleration1998 – Results published by SCP and HZSNSSystematic Errors• Dust extinction – At high redshift, dust in host galaxies can dim the light in unpredictable ways. • Malmquist Bias – selection bias due to the fact that brighter supernovae are more likely to be observed. • Gravitational Lensing• K-Correction – uncertainty involved with fitting observed light curves to templates. • Due to the relatively small number of supernovae measured to date, statistical errors are still dominant –need more data!Theoretical Implications• The supernovae measurements are consistent with the results of galaxy cluster and CMB measurements: Ω0= ΩM+ ΩΛ1 = 0.3 + ΩΛ• ΩΛ > 0 implies the existence of dark energy. • ΛCDM theory predicted the expansion acceleration.Dark Energy• Even distribution and limited interactions – no laboratory detection.• give different expansion rates, so Dark energy equation of state:w = p/ρρ ~ 1/R3(1+w)• Different values of w would more precise acceleration measurements would differentiate dark energy theories. • Note: w = -1 is the equation of state for the vacuum energy.Future Experiments• SNAP (SuperNovaeAcceleration Probe ) - DOE proposed satellite– Increase discovery rate to 2000 / year– Able to find more distant supernovae (z~1.7).– Use of large arrays of CCD’s• Search for deceleration epoch.Summary• Supernovae 1a make good standard candles• Use wide-field imaging to discover distant supernovae• Magnitude and redshift measurements indicate an accelerating rate of expansion• Acceleration implies the existence of some form of dark