The Sloan Digital Sky Survey Supernova Search ● THANKS TO THE APO TEAM FROM THE SDSS-II SUPERNOVA SCIENCE TEAM!!

The Expanding Universe ● By measuring galaxy redshifts, we observe that essentially all galaxies are moving away from us ● The speed they are moving is proportional to how far away they are ● This linear expansion (Hubble's law) implies that the Universe is expanding

The Accelerating Universe ● Any matter in the Universe pulls on other matter by the force of gravity, which should cause the expansion to slow down ● Since we know there's matter in the Universe, everyone always expected that the rate of expansion has been decreasing over cosmic time; the big questions were always how fast the deceleration was, whether it would be enough to cause an eventual recollapse of the Universe, and what the inferred age of the Universe was ● But about ten years ago, observations of distant supernovae threw a very unexpected wrinkle into the picture

Supernovae as Cosmological Probes ● Certain types of supernovae – type Ia --can be used as distance indicators ● Out to intermediate redshift (z~1), SN are fainter than expected for decelerating (or even empty) Universe --> they are farther away, so Universe has been expanding faster than expected ● So something appears to be pushing things apart in the Universe! What is is? Who knows ---> DARK ENERGY! ● This conclusion does depends on our understanding of supernovae: are distant (= long ago) supernovae the same as they are today? – But there's other evidence apart from supernovae that suggest the same thing

Dark energy ● Dark energy may be key for understanding basic physical laws: particle physicists are very interested in what it might be! ● Dark energy is usually parameterized by its equation of state: ● Most famous “explanation” is that dark energy is the “Cosmological constant” invented by Einstein, which has w=-1 and unchanging in time. This might be expected from particle physics -- vacuum energy -- but amplitude way off from simple expectations ● Other models, e.g. quintessence, has w that varies with time. Lots of physicists have been coming up with lots of ideas! ● Major observational goal: measure w and its evolution !

The SDSS Supernova Survey: goals ● Existing SN surveys have targetted either nearby or very distant SN – nearby SN via targetted galaxy search – distant SN via small field blind search – neither technique gets intermediate redshift objects ● SDSS telescope/camera has very wide field, moderate depth --> ideally suited for intermediate redshift ● Calibration uniformity is also an issue: cosmology results depend on comparing low and high redshift samples, which are taken with totally different instruments/techniques ● SDSS bridges the gap – look for continuity in redshift-dist relation – uniform calibration – evolution of w – Lots of supernovae --> better understanding of these objects

SDSS SN search techniques ● SDSS uses dedicated 2.5m telescope at Apache Point Observatory with very wide (corrected) field, very large format camera (30 science 2048x2048 CCDs) ● SDSS drift scans across sky in 2.5 degree strip; two strips fill the stripe ● SDSS SN survey looks at equatorial stripe during Sep-Nov , alternating strips each clear night: roughly 50 Gbytes per night

SDSS-SN Discovery ● Lots of supernovae have been discovered!! Goal was 200 supernovae over 3 years, so far we've found more than 400!

SDSS-SN followup spectroscopy ● Multiple larger telescopes used for spectroscopic followup to confirm supernova type and measure redshift

SDSS-SN Cosmology ● No obvious departures from concordance cosmology – No discontinuity in Hubble relation ● Results consistent with w=-1

Other projects: SN Ia rates and hosts ● Understanding SNIa rates important for understanding of nature of Ia progenitors (which is important for using Ia's as cosmological probes!) ● Rate measurement requires accurate understanding of experiment efficiency – detection efficiency obtained by inserting fake SN during initial selection – Sample efficiency from sophisticated light curve simulations – Total low redshift efficiency: / (stat) +/ (sys) ● SDSS sample ideal: large numbers, blind search, well-defined (reasonably) sample definition ● Currently have one of the best rate estimates in the literature! ● Also studying the correlation of SN properties with host galaxy properties

Other projects: self-contained cosmology ● Currently, Ia light curve training done from nearby sample, but this is non- homogeneous and may not have well defined photometry ● Large sample of low-z SDSS SN may allow for self-consistent light curve training and application

Summary ● Survey is just about complete ● Survey has exceeded its goals! ● Initial results not showing any big surprises, but couldn't know that without doing the experiment! – Lots more work remains to be done! ● Ultimate value of SDSS-II SN survey may be in its large, homogeneous data set, useful for understanding type Ia (and other types!) of supernovae, and their use for cosmology as distance indicators

Conclusion ● None of this would have been possible without the support of the APO staff! – 2.5m observers for dealing with all the imaging, with real time decisions about which strip, which portion, moon, seeing, clouds, etc. etc!!! – 3.5m observers for all of their help with the followup spectroscopy, half of which (second half!) was done in service mode! – All the rest of the APO staff for keep the whole operation and telescopes going!