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Supernovae and Cosmology


compatible with a cosmological constant, WL~0.7. SNe: ... Produces calibrated star catalogs in the CFHTLS Deep fields, in natural Megacam magnitudes. ... – PowerPoint PPT presentation

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Title: Supernovae and Cosmology

Supernovae and Cosmology
Pierre Astier LPNHE IN2P3/CNRS Universités Paris
March 16 2006
Cosmological Principle
foreword 1) at large scales, only gravitation
matters. 2) equivalence
principle - metric theory of gravitation
Universe is homogeneous and isotropic - no
prefered point or direction - ... but no time
invariance - ... and undefined spatial
curvature - Friedmann-Lemaître-Robertson-Walker
Scale factor
Comobile coordinate
k -1,0,1 (negative, null, positive curvature)
Friedmann's Equation
Sufficient in principle, once specified how
densities vary with R. One may otherwise
complement it with
A sufficiently negative pressure may cause an
acceleration of expansion
In a matter and L dominated universe
Initial conditions
present conditions
WM WL 1 - Wk
Distances vs WM and WL ?
r(z) (comobile) distance to a source at a
redshift z.
Source and observer are themselves comobile
Standard candles
Provide luminosity distances as a function of
redshift dL(z)
Directly sensitive to the expansion history. For
a flat universe,
If dominated by matter and dark energy,
Supernovae Ia
Thermonuclear explosions of stars which appear
  • Very luminous
  • Can be identified
  • Transient
  • (rise in 20 days)
  • Scarce (1 /galaxy/millenium)
  • Fluctuations of the peak
  • luminosity 40
  • Can be improved to 14

Type Ia supernovae (SNe Ia) - Riess 1998 (10(6)
SNe Ia) - Perlmutter 1999 (42 SNe
Ia) Acceleration of expansion - Allen
2002, X observations of galaxy clusters - Wm -
Spergel 2003, CMB (WMAP) - WT Concordance
model Flat (/- 0.02) universe dominated
today by a Dark Energy compatible with a
cosmological constant, WL0.7 SNe - Sullivan
2002 Hubble diagrams by galaxy types
(non-evolution test) - Tonry 2003 (8) - Barris
2003 (23 SNe Ia, zmeasured with HST) - Riess 2004 (16 SNe Ia
found and discovered with HST) up to z1.6
Baryon acoustic oscilations - Eisenstein 2005

A varying cosmological constant?(L or something
else ?)
Fluid x caracterised by its equation of state
w Pxwx?x or ??x R -3(1wx)
More recent constraints on w
SNe flat universe Large scale structures
CMB (Knop et al 2003)
w-1.05 0.15-0.20 (stat) /- 0.09 (sys)
Similar constraint in Riess et al (2004) w
-1.02 0.13-0.19
(Knop et al 2003)
Dark energy
In the GR framework, we need a fluid with
negative pressure to accelerate the expansion
(Friedmann) - The Cosmological Constant
(p-r) is one possibility but faces very serious
challenges - One can invoke one more (yet
unknown) scalar field - One can also can also
consider modified gravity. The Equation of State
is the discriminator Or equivalently the
density as a function of z Goal measure On
w(z0), dw/dz or dw/da(aaref) w-1
Cosmological Constant w-1 scalar fields w more exotic physics
Observing Dark Energy(!)
Dark energy plays an important role in the
recent universe (z with increasing z. Particularly sensitive
methods - Supernovae Ia Optical (and
IR) telescopes, imaging and spectroscopy
Figure of merit number of SNe - Weak
gravitational shear Optical telescopes,
imaging Figure of merit surveyed area
on the sky (up to z1) - Baryon wiggles
Optical telescopes, imaging and spectroscopy.
Figure of merit surveyed universe volume
(to learn more, see http//edeninparis.in2p3.
Dark energy properties and dL(z)
At least 4 mathematically equivalent
descriptions - dL(z) or r(z) - WDE(z) - wDE(z)
w(z) - V(f) for a scalar field f
Related via identities such as
No way to get precise w measurements from
distance data without some external WM
comparable size but opposite signs. Same for

Type Ia supernovae basic features
- Duration 30 days (FWHM) - MB -19.4 (1010
suns) sometimes brighter than its host
galaxy - Moderate dispersion of peak absolute
luminosity 0.35 mag ... reduced when accounting
for shape/color/luminosity/ relationships
- Max brightness in the blue - Carateristic
spectrum with the Si absorption 6200A. - Wide
lines (large speed) Assumed mechnism explosion
of a white dwarf reaching the Chandrasekar mass
(by accretion from a companion)
Observing supernovae Iafirst generation surveys
steps 1) Detection via subtraction of images
taken 3 weeks apart zSN/deg2/month 1m telescope 5 minutes.
zminutes. z4m telescope 30 mn to 1h. z1.2
Space 2) Identification via spectroscopy. z
measurement. Twice as large telescope, 30mn
to 2h exposures. 3) Photometry to measure the
light curve As for detection but 2 as
much time, for each epoch.... 4) since
there was bad weather, play it again.
More and better measured events
Which z? - We need BOTH nearby and distant
SNe - Measured in similar restframe spectral
bands - Blue is a good choice (400-500
nm) one other band to measure
color. - z1 is a limit from the ground
atmosphere, detectors Nearby SN 0.02- THE difficulty is to discover them. -
Besides anchoring the Hubble diagram, they can be
used to train lightcurve models and
distance estimators. Distant SN 0.1rate 5 to 10 SN/month/deg2.... - With a
large intrument, one can multiplex
Large Mirror area x solid angle
Second generation surveys
  • Statistics o(100) - o(1000) SNe Ia
  • Systematical improvement of photometry
  • - multi-band measurements(2)
  • - single instrument enables precise
    photometric calibration
  • - long integrations precise photometry
  • Two running programs
  • - Essence _at_ CTIO (4m) spectro VLT ( others)
  • targets 200 SNe Ia _at_ z(2002-2007)
  • - SNLS _at_ CFHT (3.6 m) spectro VLT, Keck,
  • targets 700 SNe Ia _at_ z(2003-2008)
  • AIM w (constant) at 0.05

MegaCam at CFHT
MegaCam - 36 CCDs 2k x 4.5k pixels - 1 pixel
0.185'' - field of view 1 deg2 - DAPNIA/CEA -
1rst light at end 2002.
CFHT - diametre 3.6m - Mauna Kea, Hawaii - 4200
m - 0.8''
The CFHT Legacy Survey
CFHTLS - observation program using MegaCam
- over 5 ans, 9 nights/lunation (i.e. 500
nights) - defined by French and Canadian
communities - started june 2003. 3
Components - Very Wide (asteroids) 22
- Wide (weak shear) 33 - SN/Deep (Deeper
and wider Hubble Deep Field) 45 Deep/SN
- 4 pointings spread on the sky. - observed
5 time a lunation in 4 bands g,r,i,z (400 - 1000
nm) -- astro-cinema study of all variable
objects. - For SNe, observations combine
detection and photometric followup
French-Canadian Collaboration to discover,
identify and measure SNe Ia in the CFHTLS(DEEP).
About 40 persons. Spectroscopy 250 h/year on
8m-class (!!) - VLT (Europe) large programme
240 h over 2 ans. renewed. - Gemini
(US/UK/Can) 60 h/semestre - Keck (US) 30
Supernova Legacy Survey design
Objectives - 700 SNe Ia zw.r.t previous studies) - Observed in 4 bands
(g,r,i,z) SDSS - Fair sampling of lightcurves
( 5 points a month) - Spectroscopic
idendification (of all 700 SNe) Justifications -
High statistics provide insight on systematics -
Photometry with a single telescope -
beneficial for photometric calibration (thanks to
repetition) - helps when evaluating selection
biasses - Multi-band observations - mandatory
to follow the same restframe spectral region at
various z (B,V) z0 (g,r)
z0.2 (r,i) z0.4 (i,z)
z0.8 - helps LC modeling at z0.4 - redundancy
when more than 2 bands enable to measure
CFHTLS/Deep Observing mode
- 40 nights/year for 5 years. - Repeated
observations every 4 night (rolling search),
mode service - 4 bands g,r,i,z
- Photometric data before objects are
detected - Multiplexing several SNe
per field in a single exposure -
Repeated calibration of field stars
Detecting Supernovae
- New images (of the previous night) are
subtracted off a reference image of the field
(e.g. a stack of last year images) - before
subtraction one has to align images -
geometrically - photometrically
- PSF (bring to the same star shape). -
Detection of (positive) excesses (typically
above 3 ? ) - Association of detections over
nights/bands to reach 8s - Lightcurves
are fit to a SNe Ia tempate to evaluate a Ia
likelihood - Spectroscopy.
Identification of SNe Ia Redshift (usually of the
host galaxy) Detailed studies of a (small) sample
of SNe Ia/II Telescopes - VLT Large program
(service) 240h in 20032004, idem 20052006
- Gemini 60h/semestre (Howell 2005,
astro-ph/0509195) - Keck 30h/an (spring
Analysis for cosmology of the SNLS first year
data sample August 2003 July 2004
- Differential photometry - Photometric
calibration - Fitting lightcurves - Fitting
cosmology - Systematics
Differential photometry
  • Fit galaxy(i,j) pixels on a stamp
  • Fit a residual sky background on each image
  • Fit a common position of SN to all images
  • Robustify for dead pixels.

Uses up to 300 images geometrically aligned One
SN flux measurement per exposure
Photometric calibration
  • Relies on repeated observations of Landolt
  • Calibration in Landolt (Vega) magnitudes
    because nearby SNe are
  • calibrated this way
  • Produces calibrated star catalogs in the CFHTLS
    Deep fields,
  • in natural Megacam magnitudes.
  • Comparison of synthetic and observed color terms
  • (Megacam/Landolt Megacam SDSS 2.5m)

-Zero points _at_ 0.01 (0.03 in z) -Repeatability
better than 0.01 (0.015 in z)
Mesurement of distances with SNe Ia
We look for ratios of distances
Reference spectrum associated to the magnitude
system (Vega/AB)
Observed magnitude of the SN (photometrycalibrati
k-correction (uses a model of SN luminosity as a
function of time and wavelength)
Instrument modelling (throughput as function of
Known relations between photometry observables
Brighter - slower
Brighter - bluer
stretch time-scale parameter of the (B)
lightcurve, corrected for (1z) or decline rate
decrease of flux at 15 (RF) days from max
color B-V (rest frame) at peak.
Correlation between U-B color and stretch (at
fixed B-V) enables to reduce brightness
scatter to 13 (0.13 mag)
Fitting Lightcurves
Spectral Adaptive Lightcurve Template
  • Model of the SNe Ia Spectral Energy Density as a
    function of
  • phase (date w.r.t the B maximum)
  • lambda (SN restframe wavelength)
  • stretch (dilatation factor of the time axis in
  • color E(B-V) at the B maximum
  • Tuned on a set of very nearby Sne (not
    necessarily in the Hubble flow)

Guy et al, astro-ph/0506583,AA
Cardelli Extinction law
A-A(B) for E(B-V)0.1
Lightcurves for several values of stretch between
0.7 and 1.1
SNLS-04D3fk z0.358 All bands can be
fitted, because they lie within U to R
restframe. g, r, and i roughly match UBV
SALT fits
SNLS-04D3gz z0.91 g' and r' bands are too blue
in the restframe to be fitted (no model!). r an i
roughly correspond to B and V restframe
First year SNSL sample(July 2003 - July 2004)
- 2 events don't match a SNe Ia lightcurve - 2
events are outliers to the Hubble Diagram
Hubble Diagramme
Magnitude in a restframe filter (sliding for
Cosmological fit
Correlations of luminosity with stretch and
color. Parameters M, a et b fitted together
with cosmology.
Confidence Contours
BAO Baryon Acoustic Oscillations (Eisenstein et
al 2005, SDSS)
68.3, 95.5 et 99.7 CL
Cosmological fit side parameters
sint comes out at 0.13 (mags) - the distance
is measured at 7 (r.m.s) per event - SALT (the
lightcurve model) works well ( Riess et al 2004
sint 0.22)
b comes out at 1.5 it describes the relation
beween B-V color and brightness. If due to
dust, it is supposed to be RB 4.1 Color
variation of SNe Ia is not only due to dust.
Sources of systematic uncertainty
- photometry, photometric calibration, modeling
of the bandpasses - Evolution of the
composition of the progenitor (white dwarf)
of the environment. - study of properties
as a function of host galaxy - comparaisons of
distant and nearby SNe - Empirical modeling of
lightcurves Restframe region used fit (B,V)
- (U,B) at large redshift - tests on
intermediate z SNe - Selection biasses When
close to the sensitivity limit, only the
brightest objects are measured.
bluest and/or slowest SNe - simulation of the
detection pipeline - contamination SN II, SN
Ib/c are sometimes difficult to reject
- cross-check spectral identifications with
photometry - extinction by intergalactic
dust - Quasar colors as a function of
redshift. - gravitational lensing broadens
peak magnitude distribution (but preserves the

Photometric calibration uncertainties
Use of color-color relations - offset
calibration itself - slope sensitive to
difference of filter sets Comparison of
measured relations with synthetic one (from the
assumed instrument bandpasses) tests the
central wavelengthes of the simulated
instruments. Offsets conservatively defined to
0.01 (0.03 in z) Slopes uncertainties
translate to 1.5 nm in central wavelengthes.
Evolution test comparing distant (znearby SNe
Stretch, color and relations with luminosity are
essentialy compatible between nearby and distant
3 bands compatibility of colors(UBV at rest)
DU3 U(measured) U(guessed from B and V)
  • UBV relations of SNe Ia
  • seem reproductible
  • Redundancy of distances (SNLS première)
  • Robustness and small uncertainty of SALT
  • Color fluctuations are a small source of

Selection bias from simulated SNe
- Generate Mock SNe from observed stretch and
color distributions, and constant rate -
Account for correlations with luminosity and tune
limiting mag tp reproduce observed peak i'(AB)
and z distributions.
black SNLS data red Simulations
Biais on the distance modulus 0.02 _at_ z0.8
0.05 _at_ z1.
Impact on Wm (flat LCDM) SNe
proches 0.019 0.012 SNe SNLS -0.020
Grey dust - Colors of SDSS quasars (as a
function of z) provide a limit on the dust
column density as a function of Rv s(Wm)s(w)al lensing. Weak lensing asymetric
distribution of residuals, but conserves the
average flux. Strong lensing net loss of
light when multiple images Stringent limits on
multiple images from the CLASS survey
(Myers et al 2003) s(Wm)s(w)(Ib/c mainly) 0.5 SNIb/c expected within the
Systematics summary
Photometric calibration dominates Improvement
expected from the observation of secondary
standards with Megacam observation of field
stars of nearby SNe Uncertainty on the selection
bias will decrease for SNLS Will soon be
limited by the uncertainty of selection bias of
nearby SNe
SNLS cosmological results
Flat LCDM cosmology,
Flat WM,w cosmology
Combined with Baryon Acoustic Oscillations(Eisenst
ein et al, SDSS, 2005)
- Confirms acceleration of expansion with 71 new
distant SNe Ia. - Results fully compatible with
L - Distance estimator accounts for color. -
Systematics can be improved. - Photometric
calibration will improve. - In one year of
operation, SNLS already has the best precision to
date. - The statistics on disk is about 3 times
larger yet.
SNLS status by Nov. 2005
Public list of candidates http//legacy.astro.uto
226 SNe Ia/Ia?
SNLS identified eventual limitations
  • SNLS hardly needs a larger nearby sample
  • SNLS cannot collect it
  • Projects underway
  • - Supernova factory 0.03
  • - SDSS 0.03
  • need to setup a common photometric system.
  • Cosmology requires a good control on the
    selection bias
  • Photometric calibration
  • Distance comparisons involve the spectrum of a
  • reference star Vega spectrum is known to 1.
  • SNe cosmology (and other fields of astronomy)
  • benefit a lot from the replacement of this
    reference star
  • by lab standards
  • Evolution, dust, extinctions, contamination
  • Nothing alarming seen, but we should keep our
    eyes wide open

SNLS and then?
  • SNLS x 10?
  • What for? w(z) !
  • What about spectroscopy?
  • SNLS uses 250 h/year on 8m-class telescopes
    VLT, Gemini and Keck for 140 SNe Ia/year.
  • Multiply that by 10 ?
  • - more than one dedicated 8m telescope
  • - (extremely) unlikely to happen shortly
  • Mandatory improvements for O(10000) cosmology/SNe
  • Photometric identification of Ia's, from
    lightcurve shapes and colors
  • Efficient wide-field (1 deg2) MOS spectroscopy
    to measure redshifts of host galaxies

SNe surveys historical view
First generation SCP, HiZ (1993-2004) - No
partly dedicated telescopes. - Not that great
photometry - Weak or missing color
measurements. - Large inefficiency
(weather) Second generation Essence, SNLS, SDSS
SNe - Partly dedicated telescopes. - Good to
excellent photometry - Colors are measured -
High efficiency Third generation (space) -
Space based photometry - Tries to get nearby
SNe as well.
Generation 2.5 Wide-field imaging
facilities coming online LSST, DES DarkCam,
Pan-StarSS, ....
SNe surveys historical view (2)
First generation SCP, HiZ (1993-2004) -
discovered Dark Energy - constraints on
constant e.o.s - possibly large
systematics Second generation Essence, SNLS,
SDSS SNe - Want to measure w (assumed constant)
to photometry - Fight systematics with multi-band
measurements higher
statistics Third generation (space) - Get rid
of atmospheric uncertainties. - Can tackle z1.
The DUNE project
Mission proposed to CNES (French Space Agency) -
Wide-field imaging mission - Focused on weak
lensing - May (should?) host SN survey - Being
developed by French groups (SAP, IAP,
LPNHE). Currently extending. - Concept studies
just finished. Review at CNES next week

Science Case
  • Primary science case
  • Measurement of the evolution of the dark energy
    equation of state parameter (w0) and its
    evolution (wa) from z0 to 1, with a precision
    better than 5 and 20, respectively.
  • Measurement of statistics of the dark matter
    distribution and its evolution from z 0 to 1
    from linear to non-linear scales (power spectrum,
    high order correlation functions)
  • Constraints on inflation via the reconstruction
    of the primordial power spectrum
  • Secondary science case
  • Mass-selected halo catalogue out to z1
  • Galaxy formation relation between galaxy light
    and mass, galaxy morphology
  • Core Collapse supernovae constraints on the
    history of star formation up to z1
  • Fundamental tests test of gravitational
    instability paradigm, distinguish dark energy
    from modification of gravity, Dark energy

Requirements for Supernovae
  • Statistical Requirements
  • Survey 260 deg2 in 2 distinct regions for 9
    months, yielding 10000 Type Ia supernovae out
    to z1
  • Measurement of rest frame U and B peak
    luminosity with an average of 2 statistical
    uncertainties. This can be achieved by
    measurements in UBVRIZ bands of supernovae light
    curves every 4 days.
  • Identification of supernovae from their
    multi-color light curves. This requires
    photometric measurement of at least 2 rest frame
    bands, from about 2 weeks rest frame before
    maximum to about 3 weeks rest frame after maximum
  • Spectroscopic redshift of the supernovae host
    galaxies (differed and from the ground)
  • Systematics Requirements
  • Control systematics such as, malmquist bias,
    extinction by host galaxy, evolution of the
    supernovae luminosity and gravitational lensing
    to an average level of 2 per ?z 0.1 bin. This
    requires precise photometry of the supernovae
    lightcurves in at least 3 bands from 2 weeks rest
    frame before maximum to about 6 weeks rest frame
    past maximum.

Baseline DUNE Phase 0.1
Baseline (WL oriented) 1.2m
telescope 0.50 deg2 FOV PSF FWHM
0.23 20000 deg2 for WL in one (ri) filter 2x60
deg2 times 9 months for SNe (6 bands) Ground base
complements - photometric redshifts for WL
(wide-field multi-band imaging) -
spectroscopic redshifts of SN host galaxies for
SNe Timeline - launch 2012 - 5 years of
observing (3 WL 1.5 SNe) - assumes fast
approval (concept proposed to CNES, ESA and
DUNE Forecasts
constant e.o.s
Varying e.o.s
Models assumed systematic limited for SNe,
statistical for WL
dL (z)
Distance measurements to SNe Ia
differentiation w.r.t z
differentiation w.r.t z
Need other constraints to break degeneracies
Summary (2)
-Current best constraints assume a constant
w. - Twice (?) as good by the end of
SNLS - Requires improvements on both distant
and nearby SNe sample - Constraints on w(z) are
expected from space