The Dark Energy Survey and The Dark Energy Spectrograph Josh Frieman DES Project Director .
Dark Energy in the Universehome.fnal.gov/~dodelson/slac.pdf · Dark Energy in the Universe Scott...
Transcript of Dark Energy in the Universehome.fnal.gov/~dodelson/slac.pdf · Dark Energy in the Universe Scott...
Dark Energy in the Universe
Scott Dodelson
March 25, 2003
Overview
Evidence for Dark Energy
• Age: Hubble constant + globular clusters
• Distance vs. redshift: Type Ia Supernovae
• Inventory: CMB (Ω = 1) + Many (Ωm ' 0.3)
• Growth function: Weak lensing & Cluster counts
Overview
What is it?
• Cosmological constant Λ: Historical edge (Einstein), very unlikely
• Λ = 0; transient energy, eventually will go to zero: Modern favorite,very unlikely
Evidence for Dark Energy
Expansion determined by Einstein Equations for scale factor a. If theuniverse is flat, then
H2 ≡(
da/dt
a
)2
︸ ︷︷ ︸”kinetic energy”
=8πG
3ρ︸ ︷︷ ︸
”potential energy”
and
d2a
dt2︸︷︷︸Acceleration
= −4πGρa
3
1 +new term︷︸︸︷
3w
︸ ︷︷ ︸
Force per mass
with w ≡ P/ρ.
Decceleration unless w < 0.
Evidence for Dark Energy
ρ ∝ a3(1+w) = (1 + z)−3(1+w)
• Matter density scales asa−3 (w = 0)
• Radiation scales as a−4
(w = 1/3)• Cosmological constant is
. . . constant (w = −1)
Evidence for Dark Energy
Expansion rate was slower in Λ model ↔ The universe is accelerating!
Evidence for Dark Energy What observables depend on H(z)?
• Age of the universe: t =∫∞0
dzH(z)(1+z).
• Luminosity distance:
dL(z) = (1 + z)∫ z
0
dz′
H(z′)
• Angular diameter distance to recombination: d∗ = 11+z∗
∫ z∗0
dzH(z)
• Growth function:
D1(z) =5Ωm
2
H(z)
H0
∫ z
0
dz′ (1 + z′)
(H(z′)/H0)3
Evidence for Dark Energy
Evidence for Dark Energy: dL
Type Ia Supernovae
• Observed flux proportional toL/d2
L. Type Ia SN are stan-dard candles (identical L), sotheir apparent magnitude is ameasure of dL
• H(z) smaller in Λ model →dL larger → fainter SN
Calan/Tololo (Hamuy et al, A.J. 1996)
Supernova Cosmology Project
Perlmutter, et al. (1998)
effe
ctiv
e m
Bm
ag r
esid
ual
stan
dard
dev
iatio
n
(0.5,0.5) (0, 0)( 1, 0 ) (1, 0)(1.5,–0.5) (2, 0)
(ΩΜ,ΩΛ) = ( 0, 1 )
Flat
(0.28, 0.72)
(0.75, 0.25 ) (1, 0)
(0.5, 0.5 ) (0, 0)
(0, 1 )(ΩΜ , ΩΛ) =
Λ =
0
redshift z
14
16
18
20
22
24
-1.5-1.0-0.50.00.51.01.5
0.0 0.2 0.4 0.6 0.8 1.0-6-4-20246
Evidence for Dark Energy: dL
> 50 SN observed by 2 teamsimply ΩΛ 6= 0.
ΩΜ
No Big Bang
1 2 0 1 2 3
expands forever
ΩΛ
Flat Λ = 0
Universe-1
0
1
2
3
2
3
closedopen
90%
68%
99%95%
recollapses eventually
flat
Riess et al. 1999
Evidence for Dark Energy: dL
Systematic Effect?
Ordinary dust reddens the im-age; this is not seen. Graydust leads to lower fluxes asz increases
Riess et al. 1999
Evidence for Dark Energy: dL
Recently SN observed at z = 1.7
arXiv:astro-ph/0104455 v1 27 Apr 2001 arXiv:astro-ph/0104455 v1 27 Apr 2001
0.1 1.0z
-1.0
-0.5
0.0
0.5
∆(m
-M)
(mag
)
Coasting (Ω=0)Grey Dust or EvolutionΩM=0.35, ΩΛ=0.65ΩM=0.35, ΩΛ=0.0ΩM=1.0, ΩΛ=0.0
SN 1997ff Red
deni
ng
Riess et al. 2001
Evidence for Dark Energy: dL
Proposed mission SNAP will ob-serve thousands of distant SN atz ' 1
Evidence for Dark Energy: Inventory
• Hot/cold spots in CMB at z = 1100 are the size of sound horizon.Apparent size depends on geometry of universe.
• There are many estimates of matter density: all yield Ωm = 0.3.
Evidence for Dark Energy: CMB
Angular size of hot/cold spots distinguishes between open, closed or flatuniverse.
Evidence for Dark Energy: CMB
BoomerangDASIMaximaTOCOCBIPythonMSAMViper VSAQMAP
Prior to 2003, ∼ 10 experiments have verified position of first peak. Ouruniverse is flat → Total energy density is equal to the critical density.
Evidence for Dark Energy: CMB
Now WMAP has measured the spec-trum with exquisite precision
Bennett et al. 2003
Evidence for Dark Energy: CMB
• Small scalemodes enter hori-zon earlier; haveundergone moreoscillations.
• Observers todayseee mode ampli-tude at recombi-nation (η∗)
Time →
Evidence for Dark Energy: CMB
• There are manywavevectors ~k whichcontribute to anisotropieson fixed angular scale
• Their amplitudes vary,but their phases (all startwith constant δT : cosinemode) are fixed
• First peak mode haslarge dispersion at re-combination
Time →
Evidence for Dark Energy: CMB
First trough mode hassmall dispersion at re-combination
Time →
Evidence for Dark Energy: CMB
With random phases ...
”First peak” ”First trough”
Inflation sets the phases
Evidence for Dark Energy: CMB
• Structure of peaks andtroughs depends on fre-quency of oscillation anddriving force.
• The CMB is very sensi-tive to Ωmh2
Evidence for Dark Energy: CMB
The CMB plus a mild con-straint on Hubble constant im-plies dark energy.
Spergel et al. 2003
Evidence for Dark Energy: Ωm
• Direct counting givesΩm = 0.3
• Also: Large scale struc-ture, velocities, Clusters. . . All give Ωm = 0.3
Mass-to-Light Ratio vs. Scale
H0 = 100
Ω = 1
Ω = 0.3
1000
100
Rich Clusters (med)
Morgan Groups (med)
Hickson Groups (med)
CFA Groups (med)
X-ray Groups
The Local Group
M101, M31, Milky Way
Cor Bor Supercluster
Spirals (med)
Ellipticals (med)
Shapley Supercluster
Cosmic Virial Theorem
Least Action Method
Virgo Infall (range)
Bulk Flows (range)
M/L
B (
M(
/L(
)
10
1
0.01 0.1 1
R (Mpc)
Sp
E
10
Bahcall et al. 2000
Evidence for Dark Energy
All data agree
SNCMBCMB+HSTALL
Lewis & Bridle 2002
Evidence for Dark Energy: Growth function
• Less growth in a Λ uni-verse
• Clustering was compara-ble at z ∼ 0.5−1 to now
• Roughly same number ofclusters
Evidence for Dark Energy: Growth function
How can we measure mass?
Gravitational Lensing!
Evidence for Dark Energy: Growth function
What can be done with lensing?
• cluster masses• galaxy-galaxy• lensing by lss• lensing of cmb
SDSS: Fischer et al. 2000
Evidence for Dark Energy: Growth functionWittman et al. 2000
• In 2000, four groups detectedweak lensing of distant galaxiesby large scale structure
• Lensing by LSS today is whereCMB was eight years ago
Evidence for Dark Energy: Growth function
• Tomography: Can seehow structure grows withredshift
• Growth sensitive to darkenergy and neutrinomass
• Accelerator ν experi-ments will teach us aboutdark energy
Abazajian & Dodelson, 2003
Niels closed the conversation with
one of those stories he liked to tell on
such occasions: “One of our neigh-
bors in Tisvilde once fixed a horse-
shoe over the door to his house.
When a common friend asked him,
‘But are you really superstitious?
Do you honestly believe that this
horseshoe will bring you luck?’ he
replied, ‘Of course not; but they say it
works even if you don’t believe in it.’”
Heisenberg 1927
What is it?
Why now? Now is the onlytime when ρΛ ' ambient den-sity. Need fine tuning initiallyto one part in 10128 to getpresent value.
What is it?
Quintessence
• True value of Λ = 0
• Some other form of energy non-zero today, will eventually relaxto true vacuum.
• Most popular incarnation: singlescalar field with V (φ)
What is it? My (Favorite) Quintessence Model
Exponential potential leads to ρφ
tracking ambient density. Useinstead V (φ) = e−λφ [1 +
A sin(νφ)]
10−11
10−10
10−9
10−8
10−7
10−6
10−5
10−4
10−3
10−2
10−1
100
a
10−2
10−1
100
101
102
λ = 4.0, A = 0.98, ν = 0.51
Ωφ
φ
Dodelson, Stewart, & Kaplinghat (2001)
Conclusions
• Several pieces of independent evidence for dark energy: Type Ia su-pernovae and Cosmic Inventory. Efforts to hunt down systematics andincrease statistics are ongoing.
• Another class of evidence growth function (gravitational lensing, clus-ters) will play a key role in near future
• Modern Cosmology encompasses not only smooth universe, but alsostructure. Need to learn about dark energy, weak lensing, polarization,inflation, galaxy surveys, velocities, clusters, . . .