P. Astier (FFP14 15/07/14)

Supernovae and Dark EnergyPierre Astier

LPNHE / IN2P3 / CNRS , Universités Paris 6&7.Frontiers of Fundamental Physics - July 2014.

P. Astier (FFP14 15/07/14)

Lemaître (1927), Hubble (1929)

Vel

ocit

y(f

rom

red

shif

t)

Distance (from flux)

V = H d

From redshift

From apparent flux

This “Hubble diagram”uses “nebulae” as tracers

The expansion of the universe

“The farther, the fainter”

P. Astier (FFP14 15/07/14)

The expansion of the universe

D D

VV VV

Us

Isotropy:If distant galaxies are moving away from us,

their escape velocity can only depend on distance

P. Astier (FFP14 15/07/14)

The expansion of the universe

D D

VV VV

Us

D 2D

VV 2V2V

UsThem

Let us change the point of view

P. Astier (FFP14 15/07/14)

The expansion of the universe

Cosmological principle :

No special direction nor special position

Velocity and distance are proportional(at least not too far from us)

P. Astier (FFP14 15/07/14)

So,

Velocity

Distance

● V = H d tells us that the universe expands.● It is consequence of symmetries: no dynamics get encoded there● Dynamics (i.e. influence of content) show up at higher orders:

e.g: different hypotheses formatter density

P. Astier (FFP14 15/07/14)

● General relativity relates trajectories of test particles to the content of the universe

● Einstein Equations + cosmological principle

→ Friedman equation(s)

Expansion rate

Energy densities Cosmological constant

Curvature

The real theory

P. Astier (FFP14 15/07/14)

Evolution of distances with redshift is sensitive to

content

Evolution of distances as a function of z

P. Astier (FFP14 15/07/14)

1996: A Supernova Hubble diagramCalan-Tololo Survey (Hamuy et al, 1996)

Velocity

Log

(dis

tanc

e)

Distances to ~ 7%

Excellent distances !

but redshift rangetoo short to go beyondThe Hubble law

P. Astier (FFP14 15/07/14)

Type Ia supernovae

● Very luminous● Can be identified (spectroscopy)● Transient (rise ~ 20 days)● Scarce (~1 /galaxy/millennium)● Fluctuations of the peak luminosity : 40 %● With luminosity indicators : ~14 %

Thermonuclear explosions of starswhich appear to be reproducible

P. Astier (FFP14 15/07/14)

Measuring supernovae

peak flux

multi-band photometry => distance

spectroscopy:- identification- redshift

z

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So, at the end of the 90's...

● Distances to Type Ia supernovae were the best hope of measuring the distance-redshift relation

● The idea was to constrain the matter density:

● In a matter-dominated universe q0 =

M /2

Matter density, today (in some unit)

P. Astier (FFP14 15/07/14)

1998: the twin papers

Riess et al, 1998[High-z team]

Perlmutter et al,1999[SCP]

P. Astier (FFP14 15/07/14)

DE density varies slowly (or not at all) with time

Perlmutter et al (99) Static density(i..e )

Free curvature

Zero curvature DE density variation :

x ~ (1+z)3(1+wx) (P x = wxx )

Because distant supernovae are fainter than in a matter-dominated universe ==> Postulate a two-component universe : matter & dark energy

Acceleration !?

acceleratedecellerate

P. Astier (FFP14 15/07/14)

Fall 2011

The Nobel Prize in Physics 2011 was divided, one half awarded to Saul Perlmutter, the other half jointly to Brian P. Schmidt and Adam G. Riess "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

P. Astier (FFP14 15/07/14)

Scale factor RD

ensi

ty matter

dark energy

now

??

0

-1/3

-2/3

-1

w : equation of state

Matter

w tells how the density evolves with expansion ● Matter : w = 0 (follows expansion)● Cosmological constant w = -1 (ignores expansion)

Cosmological constant , or what ?

Constraints from SNe (Perlmutter et al 1999)

P. Astier (FFP14 15/07/14)

From the discovery of acceleration to the characterisation of

dark energy

Betoule et al (2014)

P. Astier (FFP14 15/07/14)

Getting more efficient

Rolling searches on large CCD mosaics

Observing steps:● Discovery in image subtraction● Spectroscopic ID● Measure light curves● Get an image without the SN

From the same images !

Implemented on 3 major surveys… with “classical spectroscopy”

P. Astier (FFP14 15/07/14)

Major rolling searchesThe SDSS SN Survey The SNLS survey @ CFHT

300 deg2 x 3 years0.1<z<0.45~2000 SNe~500 spectra

4 deg2 x 5 years0.3<z<1~1000 SNe~500 spectra

P. Astier (FFP14 15/07/14)

The current SN sample (for cosmology)

Low-z supernovae (z<0.1) : dominated by 2 samples: - CfA (Hicken et al 2009, 2012)

- CSP (Contreras et al 2010, Strizinger et al 2011)

Rolling surveys at 0.1<z<1 - ~ 2000 Sne - ~ 1000 with spectroscopic ID

High z events with the HST: - About 40 events in total today - About 50 % at z>1.

~200 SNLS events stillunpublished...

P. Astier (FFP14 15/07/14)

The current SN sample (for cosmology)The current SN sample (for cosmology)The current SN sample (for cosmology)

>700 SNe

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Cosmological information

Overall brightness

Related to SN intrinsic luminosityand distance scale: → No cosmological information

Slope (and beyond)

Ratio of distances across redshifts: → This is what constrains dark energy

P. Astier (FFP14 15/07/14)

We are interested in the ratio of SN luminositiesat different redshifts

… for similar restframewavelengths

measurements

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measurements

Each SN is measured relative to surrounding stars

P. Astier (FFP14 15/07/14)

measurements

Field stars are measured Relative to “calibrators”

...derived from stellar models

Vega: historical foundation of photometric System (too bright and … variable...)

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accuracies

~10-3

~ 4 10-3

Blue vs red knownto ~ 4 10-3

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Distant vs nearby SNbrightnesses are typically measured to~ 6 10-3

accuracies

(Betoule et al, 2013)

P. Astier (FFP14 15/07/14)

Current cosmological results (1)

● A joint effort between the two main SN surveys– Direct cross-calibration (of field stars)

– Redundant paths to standard stars

(Betoule et al 2103)

● A careful assessment of lightcurve

empirical modelling: impact is well below calibration

(Mosher et al 2014)

P. Astier (FFP14 15/07/14)

Current cosmological results (2)

● 118 nearby SNe● 366 SDSS● 242 SNLS● 14 HST

740 events in total

Betoule et al (2014)

P. Astier (FFP14 15/07/14)

Flat CDM

Ωm measurement

independent of CMBand compatible with Planck

P. Astier (FFP14 15/07/14)

Flat wCDM

Planck + BAO: w = −1.01 ± 0.08

Planck + SN: w = −1.018 ± 0.057

Best EoS constraint.

Improvements w.r.t previous results : - improved calibration. - additional SDSS data - direct cross-calibration

Betoule et al (2014)

P. Astier (FFP14 15/07/14)

What's next ?

● ~130 SNe at z<0.7 from PanSTARss (2014)● Nearby searches still running● ~ 200 more SNe from SNLS (out in 2015)● ~500 SNe/y from DES (z<1, 2013-2017)● From 2020 onwards:

– LSST (could cover the whole range to z=1)

– Euclid and WFIRST could target the z>1 régime

P. Astier (FFP14 15/07/14)

From 1999 to 2011

Perlmutter et al (1999)Guy et al (2010), Conley et al (2011),Sullivan et al (2011)

P. Astier (FFP14 15/07/14)

Outlook

● The second round of SNe surveys have significantly improved the cosmological constraints.

● CDM is doing fine as far as dark energy is concerned : w= -1 +/- 0.057

● We will shortly go below +/- 0.05. SNe could reach 0.02 by the next decade.

● Sizeable efforts are devoted to improving the probe.

P. Astier (FFP14 15/07/14)

More Slides

P. Astier (FFP14 15/07/14)

Joint SDSS-SNLS calibration

- Short and redundant paths - Direct SDSS-SNLS calibration- Direct observation of HST standards

(Betoule et al, 2013)

Uncertainties validated through redundancy