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)
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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
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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
P. Astier (FFP14 15/07/14)
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
P. Astier (FFP14 15/07/14)
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
P. Astier (FFP14 15/07/14)
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)
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Flat CDM
Ωm measurement
independent of CMBand compatible with Planck
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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