1 what is the dark energy? david spergel princeton university
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What is the Dark Energy?
David Spergel
Princeton University
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One of the most challenging problems in
Physics Several cosmological observations
demonstrated that the expansion of the universe is accelerating
What is causing this acceleration?
How can we learn more about this acceleration, the Dark Energy it implies, and the questions it raises?
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Outline A brief summary on the contents of the universe
Evidence for the acceleration and the implied Dark Energy Supernovae type Ia observations (SNe Ia) Cosmic Microwave Background Radiation (CMB) Large-scale structure (LSS) (clusters of galaxies)
What is the Dark Energy?
Future Measurements
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Contents of the universe (from current observations)
Baryons (4%)
Dark matter (23%)
Dark energy: 73%
Massive neutrinos: 0.1%
Spatial curvature: very close to 0
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A note on cosmological parameters
The properties of the standard cosmological model are expressed in terms of various cosmological parameters, for example: H0 is the Hubble expansion parameter today
is the fraction of the matter energy density in the critical density(G=c=1 units)
is the fraction of the Dark Energy density (here a cosmological constant) in the critical density
cMM ρρ /≡Ω
πρ
8
3 2Hc ≡
cρρ /ΛΛ ≡Ω
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Evidence for cosmic acceleration: Supernovae type Ia
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Evidence for cosmic acceleration: Supernovae
type Ia
Standard candles Their intrinsic luminosity is know Their apparent luminosity can be measured The ratio of the two can provide the
luminosity-distance (dL) of the supernova The red shift z can be measured
independently from spectroscopy Finally, one can obtain dL (z) or equivalently
the magnitude(z) and draw a Hubble diagram
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Evidence for cosmic acceleration: Supernovae type Ia
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Evidence from Cosmic Microwave Background
Radiation (CMB)
CMB is an almost isotropic relic radiation of T=2.725±0.002 K
CMB is a strong pillar of the Big Bang cosmology
It is a powerful tool to use in order to constrain several cosmological parameters
The CMB power spectrum is sensitive to several cosmological parameters
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This is how the Wilkinson Microwave Anisotropy Probe
(WMAP) sees the CMB
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ADIABATIC DENSITY FLUCTUATIONS
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ISOCURVATURE ENTROPY FLUCTUATIONS
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Determining Basic Parameters
Baryon Density
Ωbh2 = 0.015,0.017..0.031
also measured through D/H
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Determining Basic Parameters
Matter Density
Ωmh2 = 0.16,..,0.33
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Determining Basic Parameters
Angular Diameter Distance
w = -1.8,..,-0.2
When combined with measurement of matter density constrains data to a line in Ωm-w space
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Simple Model Fits CMB data
Readhead et al. astro/ph 0402359
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Evolution from Initial Conditions IWMAP team assembled
DA leave Princeton
WMAP completes 2 year of observations!
WMAP at Cape
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Evidence from large-scale structure in the universe
(clusters of galaxies)
Counting clusters of galaxies can infer the matter energy density in the universe
The matter energy density found is usually around ~0.3 the critical density
CMB best fit model has a total energy density of ~1, so another ~0.7 is required but with a different EOS
The same ~0.7 with a the same different EOS is required from combining supernovae data and CMB constraints
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Cosmiccomplementarit
y:Supernovae,
CMB,and Clusters
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What is Dark Energy ?What is Dark Energy ?
“ ‘Most embarrassing observation in physics’ – that’s the only quick thing I can say about dark energy that’s also true.”
Edward Witten
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What is the Dark Energy?
Cosmological Constant Failure of General Relativity Quintessence Novel Property of Matter
Simon Dedeo astro-ph/0411283
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Why is the total value measured from cosmology so small compared to quantum field theory calculations of vacuum energy? From cosmology: 0.7 critical density ~ 10-48 GeV4
From QFT estimation at the Electro-Weak (EW) scales: (100 GeV)4
At EW scales ~56 orders difference, at Planck scales ~120 orders
Is it a fantastic cancellation of a puzzling smallness?
Why did it become dominant during the “present” epoch of cosmic evolution? Any earlier, would have prevented structures to form in the universe (cosmic coincidence)
COSMOLOGICAL CONSTANT??
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Anthropic Solution?
Not useful to discuss creation science in any of its forms….
Dorothy… we are not in Kansas anymore …
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Quintessence Introduced mostly to address
the “why now?” problem Potential determines dark
energy properties (w, sound speed)
Scaling models (Wetterich; Peebles & Ratra)
V() = exp
Most of the tracker models predicted w > -0.7
ρ
matter
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Zlatev and Steinhardt (1999)
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Current Constraints
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Seljak et al. 2004
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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Looking for Quintessence Deviations from w = -1
BUT HOW BIG? Clustering of dark energy Variations in coupling constants (e.g., )
FF/MPL
Current limits constrain < 10-6
If dark energy properties are time dependent, so are other basic physical parameters
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Big Bang Cosmology
Homogeneous, isotropic universe
(flat universe)
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Rulers and Standard Candles
Luminosity Distance
Angular Diameter Distance
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Flat M.D. Universe
D = 1500 Mpc for z > 0.5
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Volume
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Techniques
Measure H(z) Luminosity Distance (Supernova) Angular diameter distance
Growth rate of structure
.
Checks Einstein equations to first order in perturbation theory
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What if GR is wrong? Friedman equation (measured through
distance) and Growth rate equation are probing different parts of the theory
For any distance measurement, there exists a w(z) that will fit it. However, the theory can not fit growth rate of structure
Upcoming measurements can distinguish Dvali et al. DGP from GR (Ishak, Spergel, Upadye 2005)
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Growth Rate of Structure Galaxy Surveys
Need to measure bias Non-linear dynamics Gravitational Lensing Halo Models Bias is a function of galaxy
properties, scale, etc….
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A powerful cosmological probe of Dark Energy:
Gravitational Lensing
Abell 2218: A Galaxy Cluster Lens, Andrew Fruchter et al.
(HST)
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The binding of light
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Gravitational Lensing by clusters of galaxies
From MPA lensing group
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Weak Gravitational Lensing
Distortion of background images by foreground matter
Unlensed LensedCredit: SNAP WL group
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Gravitational Lensing
Advantage: directly measures mass
Disadvantages Technically more difficult Only measures projected mass-
distribution
Tereno et al. 2004
Refregier et al. 2002
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Baryon Oscillations
C()
C()
CMB
Galaxy Survey
Baryon oscillation scale
1o
photo-z slices
Selection
function
Limber Equation
(weaker effect)
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Baryon Oscillations as a Standard Ruler
In a redshift survey, we can measure correlations along and across the line of sight.
Yields H(z) and DA(z)!
[Alcock-Paczynski Effect]
Observer
r = (c/H)zr = DA
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Large Galaxy Redshift Surveys
By performing large spectroscopic surveys, we can measure the acoustic oscillation standard ruler at a range of redshifts.
Higher harmonics are at k~0.2h Mpc-1 (=30 Mpc). Measuring 1% bandpowers in the peaks and troughs requires
about 1 Gpc3 of survey volume with number density ~10-3 galaxy Mpc-3. ~1 million galaxies!
SDSS Luminous Red Galaxy Survey has done this at z=0.3! A number of studies of using this effect
Blake & Glazebrook (2003), Hu & Haiman (2003), Linder (2003), Amendola et al. (2004)
Seo & Eisenstein (2003), ApJ 598, 720 [source of next few figures]
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Conclusions Cosmology provides lots of evidence for
physics beyond the standard model. Upcoming observations can test ideas
about the nature of the dark energy.