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.)

243ln1)(32

02 )1()( aaaeHaH totrM

adaw

V

zdzzS

H

czD

z

iiL

i

21

0

)1(32

0

)1()1()1(

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