seeing dark energy (or the cosmological constant which is the simplest form of de) professor bob...
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Seeing Dark Energy(or the cosmological constant which is the simplest form of
DE)
Professor Bob Nichol(ICG, Portsmouth)
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Overview
1. Cosmology Primer2. Standard Candles (Supernovae)3. Standard Rulers (CMB)4. My role in all this (SDSS)
ISW effect BAO
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COSMOLOGY PRIMER
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FRW Equation
• Assuming homogeneous and isotropic universe (RW metric), then GR gives:
Hubble Parameter
Average density of
matter
‘a’ is the scale factor (“radius”) of the Universe relative to
today
k is the curvature of space-time of the
Universe (a constant)€
H 2 ≡a•
a
⎛
⎝
⎜ ⎜
⎞
⎠
⎟ ⎟
2
=8π
3ρ −
k
a2−
Λ
3
Cosmological constant, but could be fn of time & space
w=p/=-1
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3 Solutions to FRW equation (=0)
R
timeBangBang
Never stop!
Stop at infinity
Big crunch!
Value of decides the fate of Universe! Like throwing a stone into space
Larg
er
univ
ers
e
Later in Universe
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Search for two numbers (H0 and 0)
Subscript “0” means today (R=1), but formula holds at other cosmic times. Total
energy density ()
€
0 ≡ρ 0
ρ c=
8πG
3H02ρ 0
€
0 = Ωm +ΩΛ +Ωk =1
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Standard Candles
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Luminosity Distance
• We can’t measure distances in the Universe directly, so hard to measure geometry and expansion rate directly
• dL is the luminosity distance and depends on the cosmological parameters, z is the redshift
€
dL =cz
H0
(1+1
2(1−
Ω0
2)z+.....)€
L =f
4πdL2
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Supernovae II
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(lookback time)(lookback time)(lookback time)(lookback time)
(dis
tan
ce)
(dis
tan
ce)
Supernova are Supernova are 20% fainter 20% fainter than they than they should beshould be
Supernova are Supernova are 20% fainter 20% fainter than they than they should beshould be
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Standard Rulers
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Baryon Acoustic Oscillations
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Initial fluctuation in DM. Sound wave driven out by intense pressure at 0.57c.
BaryonsBaryons PhotonsPhotons
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CMB
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After 105 years, we reach recombination and photons stream away leaving the
baryons behind
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Preferred scale imprinted on CMBPreferred scale imprinted on CMB
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€
dA = dL (1+ z)2
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My research
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Sloan Digital Sky Survey
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Integrated Sachs-Wolfe Effect
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What we measure
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SDSSSDSSSDSSSDSS
WMAPWMAPWMAPWMAP
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No Signal - No DENo Signal - No DE
Positive Signal - DE!Positive Signal - DE!
Most direct evidence yet
that dark energy exists we see it’s repulsive
force counteracting gravity directly
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baryons photons
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TodayToday
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Sullivan et al. (2003)
m=0.256+0.019-0.023
Percival et al. (2006)
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• Supernovae
•CMB • SDSS/LSS
• Supernovae
•CMB • SDSS/LSS
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So is w=-1?So is w=-1?
99.74% detection
Percival et al. (2006)
143k + 465k
79kz~0.35
z~0.2
Percival et al. 2007
Measure ratio of angular-diameter distance between these redshifts (D0.35 /D0.2)
D0.35 /D0.2 = 1.812 ± 0.060
(ratio should be 1.67 for cosmological constant)
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Future Questions
• Is it a Cosmological Constant? Better measurements, specifically control of systematics (new experiments)
• Is it just a breakdown of GR on large scales? Probe universe using different measures (growth of structure). Again limited by systematics
• Better theory (any theory!)• Parallels with HEP - large careful experiments
worrying about large datasets and systematics
DES, SDSS-III, WFMOS, DUNE, SPACE, SNAP, ADEPTDES, SDSS-III, WFMOS, DUNE, SPACE, SNAP, ADEPT