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 = slope = current

rate of expansion), 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

H 2= aa 2

=8G a

3

3−k c2

a2=

m

a3

r

a4

k

a2

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

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

Possible problem: are SN at earlier

times intrinsically fainter? Or is there

“gray” 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 constrain cosmological

parameters via the redshift-distance

relation

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) Complementary constraints on cosmological parameters can come from

observations of objects of known size, or alternatively, from statistical power at

some known size, via the redshift-angular size relation

Such a size scale is imprinted on the matter/energy distribution because of

acoustic oscillations in the growth of perturbations in the early Universe

Cosmological parameters: WMAP Wilkinson Microwave Anisotropy Probe

measures the cosmic microwave background

Angular power spectrum measures acoustic

peaks at recombination

Size-redshift relation constrains cosmological

parameters

Hubble constant measurements constrain

things further

Cosmological parameters: BAO Acoustic oscillations are also

imprinted on the large scale galaxy

distribution (baryon acoustic

oscillations), since this evolves

from the initial density

perturbations

Feature in the galaxy power

spectrum has been observed in

SDSS galaxy sample (Eisenstein et

al 2005); typical redshift z~0.35

Location of peak places strong

constraint on matter density

Cosmological Parameters: summary Constraints from:

Supernovae

WMAP

BAO

Hubble constant

All observations together lead to

“concordance model”:

m=0.3, =0.7

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 !

w=P

c2

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

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 2006-2008,

alternating strips each clear night: roughly 50 Gbytes per night

SDSS-SN Discovery Candidate SN identified after subtracting template images taken earlier as part of main SDSS survey

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 Accurate cosmology requires accurate measurements of SN brightnesses against galaxy

background

SDSS SN frames taken in variety of conditions – not photometric

Need to work at low S/N and need accurate error estimates

Traditional SN photometry done on template-subtracted frames

Requires astrometric matching / resampling --> introduces correlated errors

To improve situation, developed a photometry technique: “scene-modelling”

Fit entire stack of images without any resampling

Use galaxy model that is a grid of small (0.4'') uniform surface brightness patches

Determine photometric and astrometric from calibration reference catalog for stripe

82 (Ivezic et al 2007)

Extensive tests based on: treating stars as SN, zero-flux in pre-SN epochs, artificial

SN in pre-SN epochs (of real objects)

Extended to work with non-2.5m photometry as well (MDM, UH88, NMSU1m)

Photometry results: lots of light curves!

Light curve fitting in

progress using a

variety of techniques

Use nearby SN

light curves as

templates (MLCS

2K2)

Modifications for

fitting in flux

Systematic effects

being explored

through Monte-Carlo

Photometry results: lots of light curves!

Light curve fitting in

progress using a

variety of techniques

MLCS 2K2

Modifications for

fitting in flux

Systematic effects

being explored

through Monte-Carlo

SDSS-SN Cosmology

No obvious departures from

concordance cosmology

No discontinuity in Hubble

relation

Lots of SN needed to bring

down scatter, look for

systematics --> SDSS data set is

ideal (final sample will have ~4x

number of objects)

SDSS-SN Cosmology (2)

In conjunction with

other measurements

(e.g. BAO), should

provide constrain on w

at moderate redshift

Other projects: SN Ia rates 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: 0.83 +/- 0.02 (stat) +/- 0.01 (sys)

SDSS sample ideal: large numbers, blind search, well-defined (reasonably) sample definition

Will get better with 3 year sample; possible extension to higher z with “photometric Ias”

Other projects: photometry-only Ia's Significantly more likely Ia

light curves than those for

which we have followup

spectroscopy

Figuring out how to use these

will help with cosmology

statistics, rate evolution, etc.

Potential importance for

future projects/missions

Other projects: host galaxies

Studying relationship of Ia properties to host

galaxy properties may shed light on Ia

progenitors and potential systematics

Large samples, but also spatial resolution,

required

SDSS SN provides good sample

HST and SIRTF proposals for followup also

submitted

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

SDSS SN plan Work nearing completion (papers nearing submission) for 2005 data:

survey overview (Frieman et al.)

search techniques (Sako et al.)

spectroscopic followup (Zheng et al.)

photometry (Holtzman et al.)

initial cosmological results (Lampeitl et al.)

SN Ia rates (Kessler et al.)

analysis of peculiar SN that a large sample provides (Prieto et al.)

Full analysis after 2007 data is collected

Possible strategy modifications to target more low redshift SN in 2007; NMSU

1m could be important

Future directions

Variety of projects underway to understand and use type Ia SN

Note SN Factory and possible NMSU 1m contribution

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) : one of the selected projects is a

study that will find BAO at higher redshift