Expansion history of the Universe as seen by supernovae
Bruno Leibundgut
European Southern Observatory
Cosmological SN results so far
Acceleration confirmed with all data sets• based on more than 200 SNe Ia
Consistency with flat geometry
No further constraints yet on w• possibility to measure time dependence of ω at
the moment very limited• all analyses with constant ω
Systematics remain the main issue
Concordance
ΩΛ
ΩM
No Big
Ban
g
Empty Universum
Einstein – de Sitter
Lambda-dominatedUniverse
Concordance Cosmology
The distant SN Hubble diagram
Riess et al. 2004
The new challengeIdeally we would like to derive the
expansion history H(z) directly
(Equation of state parameter ω can substitute for the cosmological constant)
with a luminosity distance (here already w=const.)
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assumes Ωtot=1
Miknaitis et al., in prep.
ESSENCEWorld-wide collaboration to find and characterise SNe Ia with 0.2 < z < 0.8Search with CTIO 4m Blanco telescopeSpectroscopy with VLT, Gemini, Keck, MagellanGoal: Measure distances to 200 SNe Ia with an overall accuracy of 5% determine ω to 10% overall
SNLS – The SuperNova Legacy Survey
World-wide collaboration to find and characterise SNe Ia with 0.2 < z < 0.8Search with CFHT 4m telescopeSpectroscopy with VLT, Gemini, Keck, MagellanGoal: Measure distances to 1000 SNe Ia with an overall accuracy of 5% determine ω to 7% overall
Supernova models
Thermonuclear Supernovae
White dwarf in a binary system
Growing to the Chandrasekhar mass (MChand=1.4 M) by mass transfer from a nearby star
The “standard model”
© ESA
How well do we understand SNe Ia?
Ejecta masses and also nickel masses are not uniform
Peculiar SNe Ia with super-Chandrasekharmass (Mejecta=2.2M ?)Howell et al. 2006
Stritzinger et al. 2006
The nearby SN Ia sample and Hubble’s law
Evidence for gooddistances
Are SNe Ia good distance indicators?
Yes!• normalisation through the light curve shape
– still problems with methods!
• Hubble diagram of nearby SNe Ia• peak luminosities of nearby supernovae
Essence Survey Goal: ω
Monte Carlo of
Special attention to systematics
single photometric system
calibrate zero points
understand filter transformations (K-corrections)
spectroscopic classification
A joint analysis, including results from Supernovae, CMB, BAO, and large scale structureshould allow us to determine equation of state parameter ω to 10%.
Controlling systematics
Miknaitis et al., in prep.
The ExperimentCTIO 4m with Mosaic imager:
150 half nights over 5 yrs (2002-2006)
3 lunations, every other night, avoiding full moon
Broadband RI+V filters
32 fields, 0.36 arc-deg per field
12 sq-deg search
200s R, 400s I, + V when needed
Limits at S/N=3 - 24.3 (RI)
Remote observing from La Serena
ESSENCE goals
~ 200 supernovae with 0.25 < z < 0.75
determine a distance modulus in each bin (of Δz = 0.1)
to 2% (statistical)
~3% photometry at peak SN brightness
spectroscopic classification
control systematics as much as possible
understand SNe Ia better
Sources of systematic error(ESSENCE)
Photometry• linearity of photometry
• spatial dependence of PSF on zeropoints
• Transfer of zeropoint across camera –chip-to-chip flux normalisation–aperture corrections
• 0.9m calibration error
• Biases in difference images photometry
K-corrections• Bandpass miscalibration
• Limitations of spectral sample
• “warping” of spectra to reproduce SN colours
• uncertainty in SED of Vega
Distance estimation• Biases in overall recovered cosmology
–e.g. due to light curve sampling, phase coverage, S/N
Cosmological sources• Low-z sample zeropoint error
• Dust–zeropoint of Milky Way maps–Non-standard RV
–Non-standard reddening of SNe Ia
• Lensing
• Sample contamination by Ib/c SNe
Vega is primary celestial calibrator
4%
2%
5000 A 1 m
An alternative calibration approach
Calibrate end-to-end relative system response primarycorrector opticsfilterdetector
relative to Si photodiode.
Initial run at CTIO Jan 2005 was promising
10% 10%
Opotek tunable laserOpotek tunable laser~100 mW~100 mW
SystematicsExtinction by “gray” dust?
Careful multicolor measurements, esp. in IRExploit different z-dependenceLook at SNe behind clusters of galaxies
“Evolutionary” Effects?Use stellar populations of different ages as a proxy
Selection differences in nearby vs. distant samples?
Increase the sample of well-monitored SNeCalibrate detection efficiencies
K-corrections, Galactic extinction, photometric zeropoints....
See Leibundgut, ARA&A, 31, 69 (2001)
Beyond the searches …
light curves• photometric zero-points• extinction• light curve shape corrections
classifications• spectroscopy
K-corrections
Remember: We need 3% accuracy of peak brightness!
Light Curves
Running searches
SN Legacy Survey
ESSENCE spectroscopy - an overview
spectroscopy is vital
one axis on Hubble Diagram
we now have decent spectra…
e.g. from VLT, Keck and Gemini
Matheson et al. (2005)
Redshifts
Checking the redshifts
Blondin et al. 2006Miknaitis et al., in prep.
Investigating evolution
Blondin et al. 2006
Line velocities
No significant differences in the line velocity evolution observed• implies similar density structure and element
distribution• explosion and burning physics similar
Peculiarities observed in nearby SNe Ia also observed in the some distant objects• detached lines
The properties of distant SNe Ia are indistinguishable from the nearby ones with current observations
Remember, we said 3%...
Control of the systematics is the difficult part• understand telescope/camera combination• calibration (external and internal)• K-corrections
More systematics
Reddening• RV≈3 amplifies any photometric uncertaintes
• what is the exact value of RV?– e.g. Astier et al. use this as a free parameter
Evolution• remains difficult
– not much guidance from the models
– no obvious signs (still!)
The effect of absorption
Unknown absorption law • corrections are rather unsecure• assume different absorption priors
SNLS 1st year results Astier et al. (2006)
Based on 71 distant SNe Ia:• for a flat ΛCDM cosmology:
ΩM=0.264±0.042 (stat) ± 0.032 (sys)
• Combined with BAO (Eisenstein et al. 2005)
ΩM = 0.271± 0.021 (stat) ± 0.007 (sys)
w = -1.02 ± 0.09 (stat) ± 0.054 (sys)
ESSENCE cosmology results(preliminary!)
Wood-Vasey et al. (2006)Based on 92 distant SNe Ia• plus 47 nearby ones
Combined with BAO (Eisenstein et al. 2005)• full sample
w = −1.06±0.15 (stat) ‘conservative’
w = − 0.95±0.13 (stat) ‘aggressive’• low extinction sample
w = − 0.87±0.15 (stat) ‘conservative’
w = − 0.89±0.13 (stat) ‘aggressive’
systematics still under investigation
‘conservative’‘aggressive’
Summary
Type Ia Supernovae are fantastic astrophysical laboratories• explosion physics becomes more resolved• investigation of global parameters
– Ni mass
– ejecta mass
• provided some unexpected surprises• standard candle picture is too simple
The SN Ia Hubble diagram
Powerful tool to• establish SNe Ia as good distance indicators
• measure the absolute scale of the universe (H0)
• determine the amount of dark energy• measure the equation of state parameter of
dark energy– current best results are consistent with w=-1
Interesting years ahead
ESSENCE will finish observing in Jan 2007• extension for one season granted
SNLS will finish observing in Fall 2007• also will extend the observing for one year
Results can be expected in the next couple years
Nature of dark energy
Riess et al. 2004
Supernovae will continue …Both the US-interagency Task Force on Dark Energy and the ESO-ESA Working Group on Fundamental Cosmology recommend further studies of supernovae to investigate Dark Energy.
Several surveys are under way or planned• SDSS II (2005-2007; 0.1<z<0.3; >300 SNe Ia)• Carnegie Supernova Project (2004-2009; 0.1<z<0.5)• PanSTARSS (2007 - )• Dark Energy Survey (2010-2015)• LSST (>2013)• DUNE/JDEM/SNAP etc. (>2015)
Time variable ω
Current data sets are not sufficient in• data quality• size• systematic control
Future surveys must concentrate on the above• DUNE, SNAP make use of the stability of
space observatories
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