Transcript Document 7217384

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
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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
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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
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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
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With no matter, expansion rate is constant
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With matter, the expansion rate slows down with time
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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
2
H 2=
ȧ
a
=
8 G
3
a
3
−
k c2
a2
=
m
a3
r
k
a4
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
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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
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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
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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.
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Universe was decelerating a while ago
Cosmological parameters (1)
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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)
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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
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Wilkinson Microwave Anisotropy Probe
measures the cosmic microwave background
Angular power spectrum measures acoustic
peaks at recombination
Size-redshift relation constrains cosmological
parameters
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Hubble constant measurements constrain
things further
Cosmological parameters: BAO
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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
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Constraints from:
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Supernovae
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WMAP
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BAO
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Hubble constant
All observations together lead to
“concordance model”:
m
= 0.3 ,
= 0.7
Dark energy
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What causes current acceleration?
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For lack of knowledge, call it “dark energy”
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Dark energy is usually parameterized by its equation of state:
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P
w=
Cosmological constant has w=-12and unchanging: could result from
c
vacuum energy but amplitude way off from simple expectations
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Other models, e.g. quintessence, has w that varies with time
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Major observational goal: measure w and its evolution !
The SDSS Supernova Survey: goals
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Existing SN surveys have targetted either nearby
or very distant SN
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nearby SN via targetted galaxy search
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distant SN via small field blind search
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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
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look for continuity in redshift-dist relation
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uniform calibration
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evolution of w
Supernovae as distance indicators
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Several types of supernovae:
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core collapse supernovae (type
II, Ib, Ic)
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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
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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
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Candidate SN identified after subtracting template images taken earlier as part of main SDSS survey
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Automatic and manual identification both play a part
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Biggest contaminator is moving (solar system) objects: partly removed by time lag between filters!
SDSS-SN followup
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Identification as type Ia supernovae requires spectroscopic followup
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Candidates identified by color selection: very effective using 5 colors, 2 epochs (~90%)!
SDSS-SN followup spectroscopy
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Multiple larger telescopes used for spectroscopic followup
SDSS-SN results
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129 confirmed type
Ia's from 2005, 193
more from 2006!
target redshift regime
well sampled
Photometry results: lots of light curves!
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Photometry extracted
using “scenemodelling software
developed at NMSU
Light curve fitting in
progress using a
variety of techniques
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MLCS 2K2
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Modifications for
fitting in flux
Systematic effects
being explored
through Monte-Carlo
Photometry results: lots of light curves!
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Light curve fitting in
progress using a
variety of techniques
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MLCS 2K2
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Modifications for
fitting in flux
Systematic effects
being explored
through Monte-Carlo
SDSS-SN Cosmology
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No obvious departures
from concordance
cosmology
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No discontinuity in
Hubble relation
SDSS-SN Cosmology (2)
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In conjunction with
other measurements
(e.g. BAO), should
provide constrain on w
at moderate redshift
Other projects: SN Ia rates
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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
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detection efficiency obtained by inserting fake SN during initial selection
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Sample efficiency from sophisticated light curve simulations
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Total low redshift efficiency: 0.83 +/- 0.02 (stat) +/- 0.01 (sys)
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SDSS sample ideal: large numbers, blind search, well-defined (reasonably) sample definition
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Will get better with 3 year sample; possible extension to higher z with “photometric Ias”
Other projects: photometry-only Ia's
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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
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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
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HST and SIRTF proposals for followup also
submitted
Other projects: self-contained cosmology
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Currently, Ia light curve training done from nearby sample, but this is nonhomogeneous and may not have well defined photometry
Large sample of low-z SDSS SN may allow for self-consistent light curve
training and application
Future directions
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Complete 2005 SDSS analysis, prepare for full sample analysis
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Variety of projects underway to understand and use type Ia SN
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Note SN Factory and possible NMSU 1m contribution
Many new projects under development to contribute to
understanding of dark energy
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JDEM (Joint Dark Energy Mission): space mission
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Mission concepts: SNAP, DESTINY, JEDI
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DES (Dark Energy Survey)
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SDSS AS2 (After Sloan 2) : one of the selected projects is a
study that will find BAO at higher redshift