Title: The%20Accelerating%20Universe%20and%20the%20Sloan%20Digital%20Sky%20Survey%20Supernova%20Search
1The Accelerating Universe and the Sloan Digital
Sky Survey Supernova Search
- Jon Holtzman (NMSU)
- many collaborators (FNAL, U.Chicago,
U.Washington, U. Penn., etc., etc.)
2The 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 --gt Hubble's law
3Hubble'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
4Cosmology 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
5Expansion 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 Oo 0 (green), Oo 1 (black), and Oo
2 (red) with no vacuum energy the WMAP model
with OM 0.27 and OV 0.73 (magenta) and the
Steady State model with OV 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 Oo 2 model occurs when the Universe is
11 times older than it is now, and all
observations indicate Oo lt 2, so we have at least
80 billion more years before any Big Crunch.
(from Ned Wright's cosmology page).
6The 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
7Distant Supernovae
- Certain types of supernovae can be used as
distance indicators (more later!) - Out to intermediate redshift (z1), SN are
fainter than expected for decelerating (or even
empty) Universe --gt 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 (zgt1), SN are brighter than
expected --gt probably rules out evolution. - Universe was decelerating a while ago
8Cosmological 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
9Cosmological Parameters (2)
- Other observations constrain parameters more
- WMAP observations of cosmic microwave background
constrain universe to be nearly flat (total
Omega1) - 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
10Dark 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 !
11The 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 --gt 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
12Supernovae 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!
13SDSS 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
14SDSS-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!
15SDSS-SN followup
- Identification as type Ia supernovae requires
spectroscopic followup - Candidates identified by color selection very
effective using 5 colors, 2 epochs (90)!
16SDSS-SN followup spectroscopy
- Multiple larger telescopes used for spectroscopic
followup
17SDSS-SN results
- 129 confirmed type Ia's from 2005, 193 more from
2006! - target redshift regime well sampled
18SDSS-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
19SDSS-SN Cosmology
- No obvious departures from concordance cosmology
- No discontinuity in Hubble relation
20SDSS-SN Cosmology (2)
- In conjunction with other measurements (e.g.
BAO), should provide constrain on w at moderate
redshift
21Other 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
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23Future 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)