Transcript Document

What is the Dark Energy?
David Spergel
Princeton University
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One of the most challenging
problems in Physics
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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
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A brief summary on the contents of the universe
Evidence for the acceleration and the implied Dark Energy
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Supernovae type Ia observations (SNe Ia)
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Cosmic Microwave Background Radiation (CMB)
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Large-scale structure (LSS) (clusters of galaxies)
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What is the Dark Energy?
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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
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The properties of the standard cosmological
model are expressed in terms of various
cosmological parameters, for example:
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H0 is the Hubble expansion parameter today
M  M / c is the fraction of the matter
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energy density in the critical density  3H
c
(G=c=1 units)
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   / c
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
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Standard candles
Their intrinsic luminosity is know
Their apparent luminosity can be measured
The ratio of the two can provide the luminositydistance (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)
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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
bh2 = 0.015,0.017..0.031
also measured through D/H
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Determining Basic Parameters
Matter Density
mh2 = 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 m-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)
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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’ – 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?
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Cosmological Constant
Failure of General Relativity
Quintessence
Novel Property of Matter
 Simon Dedeo astro-ph/0411283
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COSMOLOGICAL CONSTANT??
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Why is the total value measured from
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Is it a fantastic cancellation of a puzzling smallness?
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cosmology so small compared to quantum field
theory calculations of vacuum energy?
48 GeV4
 From cosmology: 0.7 critical density ~ 10 From QFT estimation at the Electro-Weak (EW)
scales: (100 GeV)4
 At EW scales ~56 orders difference, at Planck
scales ~120 orders
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?
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Not useful to discuss creation science in
any of its forms….
Dorothy… we are not in Kansas anymore …
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Quintessence
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Introduced mostly to address
the “why now?” problem
Potential determines dark
energy properties (w, sound

speed)
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Scaling models (Wetterich;
Peebles & Ratra)
V(f) = exp(-f)
Most of the tracker models
predicted w > -0.7
matter
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Zlatev and
Steinhardt
(1999)
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Current Constraints
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Seljak et al.
2004
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Looking for Quintessence
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Deviations from w = -1
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BUT HOW BIG?
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Clustering of dark energy
Variations in coupling constants (e.g., a)
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lfFF/MPL
Current limits constrain l < 10-6
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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
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Measure H(z)
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Luminosity Distance (Supernova)
Angular diameter distance
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Growth rate of structure
Checks Einstein equations to first order in perturbation theory
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What if GR is wrong?
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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
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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
Credit: SNAP WL group
Lensed
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Gravitational Lensing
Refregier et al. 2002
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Advantage: directly measures mass
Disadvantages
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Technically more difficult
Only measures projected massdistribution
Tereno et al. 2004
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Baryon Oscillations
CMB
C(q)
Baryon oscillation scale
q
1o
Galaxy
Survey
Limber Equation
Selection
function
(weaker effect)
C(q)
q
photo-z slices
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Baryon Oscillations as a
Standard Ruler
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In a redshift survey, we
can measure correlations
along and across the line
of sight.
Yields H(z) and DA(z)!
[Alcock-Paczynski Effect]
dr = DAdq
dr = (c/H)dz
Observer
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Large Galaxy Redshift Surveys
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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 (l=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
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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
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Cosmology provides lots of evidence for
physics beyond the standard model.
Upcoming observations can test ideas about
the nature of the dark energy.
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