1. 1
What is the Dark Energy?
David Spergel
Princeton University
2. 2
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?
3. 3
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
4. 4
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
5. 5
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
π
ρ
8
3 2
H
c ≡
cρρ /ΛΛ ≡Ω
7. 7
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
10. 10
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
16. 16
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
18. 18
Evolution from Initial Conditions IWMAP team
assembled
DA leave
Princeton
WMAP completes
2 year of
observations!
WMAP at Cape
19. 19
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
21. 21
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
22. 22
What is the Dark Energy?
Cosmological Constant
Failure of General Relativity
Quintessence
Novel Property of Matter
Simon Dedeo astro-ph/0411283
23. 23
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??
24. 24
Anthropic Solution?
Not useful to discuss creation science
in any of its forms….
Dorothy… we are not in Kansas anymore …
25. 25
Quintessence
Introduced mostly to address
the “why now?” problem
Potential determines dark
energy properties (w, sound
speed)
Scaling models (Wetterich;
Peebles & Ratra)
V(φ) = exp(−φ)
Most of the tracker models
predicted w > -0.7
ρ
matter
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Zlatev and
Steinhardt
(1999)
26. 26
Current Constraints
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Seljak et al.
2004
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
27. 27
Looking for Quintessence
Deviations from w = -1
BUT HOW BIG?
Clustering of dark energy
Variations in coupling constants (e.g., α)
λφFF/MPL
Current limits constrain λ < 10-6
If dark energy properties are time dependent, so
are other basic physical parameters
32. 32
Techniques
Measure H(z)
Luminosity Distance (Supernova)
Angular diameter distance
Growth rate of structure
.
Checks Einstein equations to first order in perturbation theory
33. 33
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)
34. 34
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….
35. 35
A powerful cosmological probe of Dark Energy:
Gravitational Lensing
Abell 2218: A Galaxy Cluster Lens, Andrew Fruchter et al. (HST)
39. 39
Gravitational Lensing
Advantage: directly measures mass
Disadvantages
Technically more difficult
Only measures projected mass-
distribution
Tereno et al. 2004
Refregier et al. 2002
41. 41
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
δr = (c/H)δzδr = DAδθ
42. 42
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
(λ=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]
43. 43
Conclusions
Cosmology provides lots of evidence for
physics beyond the standard model.
Upcoming observations can test ideas about
the nature of the dark energy.
Editor's Notes
Obviously a puzzling and far reaching question that has many unexpected consequences
The acoustic oscillations are also quantitatively useful, because they can form a standard ruler.