What is the Dark Energy?

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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(Gc1 units)
• is the fraction of the
  Dark Energy density (here a cosmological
  constant) in the critical density

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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
  T2.7250.002 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 Wbh2 0.015,0.017..0.031 also
measured through D/H
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Determining Basic Parameters
Matter Density Wmh2 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 Wm-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 I
WMAP team assembled
WMAP completes 2 year of observations!
DA leave Princeton
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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Cosmic complementarity Supernovae, CMB, and
Clusters
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What is Dark Energy ?
Most embarrassing observation in physics
thats the only quick thing I can say about dark
energy thats 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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COSMOLOGICAL CONSTANT??

• 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)

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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(f) exp(-f)

matter
r
Zlatev and Steinhardt (1999)
Most of the tracker models predicted w gt -0.7
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Current Constraints
Seljak et al. 2004
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Looking for Quintessence

• Deviations from w -1
• BUT HOW BIG?
• Clustering of dark energy
• Variations in coupling constants (e.g., a)
• lfFF/MPL
• Current limits constrain l lt 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 gt 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 Lensed Credit SNAP WL group
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Gravitational Lensing
Refregier et al. 2002

• Advantage directly measures mass
• Disadvantages
• Technically more difficult
• Only measures projected mass-distribution

Tereno et al. 2004
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Baryon Oscillations
CMB
C(q)
Baryon oscillation scale
q
1o
Galaxy Survey
Limber Equation
C(q)
(weaker effect)
Selection function
q
photo-z slices
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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

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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 k0.2h Mpc-1 (l30 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
  z0.3!
• 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.