The Accelerating Universe and the Sloan Digital Sky Survey Supernova Search Jon Holtzman (NMSU) + many collaborators (FNAL, U.Chicago, U.Washington, U. Penn., etc., etc.)

The Expanding Universe ● Recession velocities of astronomical objects can be measured using the Doppler shift ● Applied to galaxies, we find that all except the nearest galaxies are receding ● Recession velocities are proportional to the distance to objects --> Hubble's law

Hubble's Law ● v = H d (locally) ● To see that relation is linear only requires relative distances ● To determine the Hubble constant, H, requires absolute distance measurements ● Hubble's law implies an expanding Universe

Cosmology and Einstein ● Einstein's theory of general relativity combined with assumption of homogeneous and isotropic universe is consistent with an expanding Universe ● Rate of expansion, however, changes with time depending on the contents of the Universe: how much matter/energy there is ● With no matter, expansion rate is constant ● With matter, the expansion rate slows down with time ● Since Einstein didn't know about the expanding Universe, he also noted that an arbitrary term – the cosmological constant -- could be added to the equations to allow for a non-expanding Universe

Expansion rate change with time for different cosmological models: note that different models correspond to different ages of Universe The figure above shows the scale factor vs time measured from the present for Ho = 71 km/sec/Mpc and for Ωo = 0 (green), Ωo = 1 (black), and Ωo = 2 (red) with no vacuum energy; the WMAP model with ΩM= 0.27 and ΩV = 0.73 (magenta); and the Steady State model with ΩV = 1 (blue). The ages of the Universe in these five models are 13.8, 9.2, 7.9, 13.7 and infinity Gyr. The recollapse of the Ωo = 2 model occurs when the Universe is 11 times older than it is now, and all observations indicate Ωo < 2, so we have at least 80 billion more years before any Big Crunch. (from Ned Wright's cosmology page).

The Accelerating Universe ● Since we know there's matter in the Universe, everyone always expected that the rate of expansion has been decreasing; the big question was 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

Distant Supernovae ● Certain types of supernovae can be used as distance indicators (more later!) ● 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 – Possible problem: are SN at earlier times intrisically fainter? Or is there “grey” dust? ● At highest redshifts (z>1), SN are brighter than expected --> probably rules out evolution. – Universe was decelerating a while ago

Cosmological parameters (1) ● Supernovae by themselves indicate the need for acceleration, but don't constrain cosmological parameters uniquely ● Multiple combinations of matter density and cosmological constant match SN data

Cosmological Parameters (2) ● Other observations constrain parameters more ● WMAP observations of cosmic microwave background constrain universe to be nearly flat (total Omega=1) ● Measurements of Hubble constant locally constrain things further ● Baryon acoustic oscillations (structure in matter power spectrum) constrain matter density (Omega_m) to be ~0.3 ● All observations together lead to “concordance model”:

Dark energy ● What causes current acceleration? ● For lack of knowledge, call it “dark energy” ● Dark energy is usually parameterized by its equation of state: ● Cosmological constant has w=-1 and unchanging: could result from vacuum energy but amplitude way off from simple expectations ● Other models, e.g. quintessence, has w that varies with time ● Major observational goal: measure w and its evolution !

The SDSS Supernova Survey: goals ● Existing SN surveys have targetted either nearby or very distance 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

Supernovae as distance indicators ● Several types of supernovae: – core collapse supernovae (type II, Ib, Ic) – binary star supernovae (type Ia) ● None are standard candles; however, type Ia SN are “standardizable” based on light curve shape ● Nagging problem: we don't exactly know what type Ia supernovae are!

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

SDSS-SN Discovery ● Candidate SN identified after subtracting template images ● Automatic and manual identification both play a part – Biggest contaminator is moving (solar system) objects: partly removed by time lag between filters!

SDSS-SN followup ● Identification as type Ia supernovae requires spectroscopic followup ● Candidates identified by color selection: very effective using 5 colors, 2 epochs (~90%)!

SDSS-SN followup spectroscopy ● Multiple larger telescopes used for spectroscopic followup

SDSS-SN results ● 129 confirmed type Ia's from 2005, 193 more from 2006! ● target redshift regime well sampled

SDSS-SN photometry ● Photometry extracted using “scene- modelling” software developed at NMSU ● Light curve fitting in progress using a variety of techniques ● Systematic effects being explored through Monte-Carlo

SDSS-SN Cosmology ● No obvious departures from concordance cosmology – No discontinuity in Hubble relation

SDSS-SN Cosmology (2) ● In conjunction with other measurements (e.g. BAO), should provide constrain on w at moderate redshift

Other SDSS SN projects/plan ● Work in progress (papers nearing submission) – survey overview, search techniques, spectroscopic followup, photometry, initial cosmological results, all from 2005 data – SN Ia rates, important for work on identifying type Ia progenitors – Analysis of peculiar SN that a large sample provides ● Full analysis after 2007 data is collected ● Possible strategy modifications to target more low redshift SN in 2007; self-contained cosmology using SDSS only

Future directions ● Many new projects under development to contribute to understanding of dark energy – JDEM (Joint Dark Energy Mission): space mission ● Mission concepts: SNAP, DESTINY, JEDI – DES (Dark Energy Survey) – SDSS AS2 (After Sloan 2)