Transcript Dark Energy

Addressing the Dark Energy Crisis
in Fundamental Physics Looking into the Crystal Ball
Christopher Stubbs
Department of Physics
Department of Astronomy
Harvard University
1
Outline
Evidence for accelerating expansion = Dark Energy
Consequence: a crisis for fundamental physics
Tests of gravitation, and connections to Dark Energy
- cosmic scales
- solar system scales
- laboratory scales
Closing thoughts.
2
This is a remarkable time
Our view of the Universe is shifting, yet again
Sun-centered solar system
Galactic structure
Recognition of external galaxies
General Relativity (originally with cosmological constant )
Discovery of Expansion of the Universe
Big Bang + inflationary paradigm
Dark Matter greatly outweighs all luminous matter
Discovery that the rate of cosmic expansion is increasing:
The “Accelerating” Universe
Emergence of a Standard Cosmology
Our geometrically flat Universe started in a hot big bang
13.7 billion yrs ago. It has been expanding ever since.
The evolution of the Universe is increasingly dominated by
the phenomenology of the vacuum, the “Dark Energy”.
“Dark matter”: what is it?
Ordinary matter is a minor component.
Luminous matter comprises a very
small fraction of the mass of the
Universe.
4
3 Pieces of New Fundamental Physics
Inflation: The early Universe grew by a factor of e60. Quantum
fluctuations in this era seeded the large scale distribution of
galaxies we see today. We don’t know what drove inflation, or
how it ended.
Dark Matter: The internal kinematics of galaxies like the Milky Way
indicate more mass than we see in stars and gas. Furthermore,
all measures of mass exceed the baryonic inventory. We don’t
know what the dark matter is… axions, LSP…?
Dark Energy: The Universe has recently entered a renewed phase
of accelerating expansion. Something is driving repulsive gravity
in the vacuum. We don’t understand the physics of dark energy.
5
It’s like living through a bad episode
of Star Trek!
Empty regions of space (vacuum) interact via a repulsive
gravitational force.
This effect will increasingly dominate, leading to all
unbound galaxies eventually being unobservable
At the interface between particle physics and gravity, we’re
in a more sophisticated state of confusion than ever
before…
The accelerating Universe scenario is supported
by multiple independent lines of evidence
Lower bound on age, from stars
Inventories of cosmic matter content
Measurements of expansion history using supernovae
Primordial element abundances
“Baryon acoustic oscillations”: large scale galaxy dist.
Cosmic Microwave Background provides strong confirmation
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The Original Evidence for Accelerating Expansion, 1998
Schmidt et al, High-z SN Team
Far away
Distance
to Supernova


Nearby
0.01
0.1
Redshift = 
8
1.0
Some notation….
There is a “critical density” that would eventually halt
the current expansion, let’s call it crit. This quantity
varies over cosmic time.
crit
3H 2

, where H o =75 km/sec per Mpc
8 G
Measure all densities in units of crit ~ 5 H atoms/m3
i 
9
i
crit
And a cosmic sum rule…
General Relativity and isotropy imply
b  dm  curvature     1
But the relative proportions of these
vary over cosmic time.
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The data drive us to
non-zero 
Why is this a crisis in
fundamental physics?
11
Kowalski et al, ApJ 686, 749 (2008)
The quantum mechanical vacuum is a
seething turmoil…
Lamb shift in Hydrogen (virtual QED process)
Electron (g-2) (Hanneke et al, PRL 100, 1120801 (2008))
Casimir-Polder forces… (Lamoreaux, PRL 78, 5L (1997) & …)
It’s confusing…. So let’s ask the theorists!
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Dark Energy Theory
1010. Well, that can’t be right…
0. Through some profound but not
yet understood mechanism, the
vacuum energy must be cancelled to
arrive at value of identically zero
ummm... Supersymmetry
uhhh ...Planck Mass
0.7, you say??
String landscapes….uhhhh
No, wait! IT’S ANTHROPIC!
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Two possible “natural” values
Vacuum energy integrated up to Planck
120
scale 
10

Cancellation via tooth fairy:
  0.0000000000000000000000000000....
But it’s measured to be around 0.7!
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Why Dark Energy Constitutes A
Crisis in Fundamental Physics
Puzzle #1: why is  so small?
Puzzle #2: why is  so large?
Puzzle #3: what’s the underlying physics?
Understanding the nature of the Dark Energy is arguably the
most profound outstanding problem in contemporary physics.
Are the properties of the Universe we see the result of some
beautiful (but as yet not understood) underlying symmetry
principles OR just an anthropic accident/selection effect?
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Three philosophically distinct possibilities...
A “classical” cosmological constant, as envisioned
by Einstein, residing in the gravitational sector.
A “Vacuum energy” effect, arising from quantum
fluctuations in the vacuum, acting as a “source”
term.
Departure from GR on cosmological length scales.
Regardless, it’s evidence of new fundamental physics!
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The Next Step: Dark Energy’s Equation of State
w=
w=
w=
w=
P = w
0, matter
1/3 ,radiation
- 1, 
- N/3, topological defects
c(1  z )
3
3(1 w )


DL ( z ) 
(1

)(1

z
)


(1

z
)
dz 



H0 0
z
• For a flat Universe, luminosity distance DL depends z, , w.
• Evolution of Dark Energy density depends on w.
• Any value of w other than -1 excludes cosmological constant
• Any evolution in w excludes cosmological constant
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Narayan et al, 2010
33
70
104
112
62
14
18
(1  w)  0.008  0.07(stat)  0.13(syst)
Evolution of Precision, Convergence
Kowalski et al, ApJ 686, 749 (2008)
All observations to
date are consistent
with Dark Energy
being the
cosmological
constant originally
envisioned by
Einstein:
w = -1
What’s missing from
this plot?
19
An analogy from the past…
We’ve seen something like this before:
~ 1880’s - early 1900’s physics faced three profound experimental puzzles:
1.blackbody spectrum
2. discrete
atomic spectra
.
.
E4
E3
E2
E1
3. Photoelectric
effect
e-
20
But that’s not all…
The challenge posed by the dark energy has shaken
the reductionist and fundamentalist philosophy that
has served us so well….
Physics has tried to determine a simple set of rules that
govern the Universe, with the expectation that these
rules and their associated parameters are both
uniquely determined by some underlying principle.
L=0
Vacuum energy
melectro
n
mproton
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The Anthropic Perspective
An alternative to the unique fundamentalist approach is
the claim that the most basic scientific observable is
that we’re here, and that fact restricts the possible
values of physical parameters.
Proponents of the anthropic approach contend that the
dark energy saturates the allowed upper bound that
could give rise to life as we know it.
All physical parameters (masses, charges, interaction
strengths…) are essentially accidental, apart from the
constraint imposed by an anthropic selection effect.
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This is a vibrant ongoing debate
Skeptics debate whether the anthropic
approach is actually science, as opposed to
philosophy.
Is it falsifiable?
I don’t know.
So let’s return our attention to
measurements we can make to better
understand the dark energy.
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Probing the Nature of Dark Energy
• Higher precision supernova data
• All-sky gravitational lensing
• Mapping evolution of structure
– abundance of galaxy clusters
– large scale distribution of galaxies
with
• Dedicated spacecraft mission (JDEM)
• Next-generation ground-based projects
– especially surveys…
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Dark Energy status… and 3 questions
Measurements are “out of pace” with theoretical understanding.
This is a Bad Thing. (Same as string theory, but with opposite sign.)
Numerous next-generation projects are coming on line:
e.g. PanSTARRS-1, SPT, BOSS, DES, LSST?, JDEM?…
Current data favor w -1 (to 10%), and no evolution with cosmic time.
1.
What if this is the real answer?
cosmological observations?
When do we quit with
2.
If cosmology has thrown down this challenge to our understanding
of fundamental physics, how long must we wait until it’s resolved?
3.
What other experimental anomalies might shed light on the DE?
Exotic Physics in the Dark Sector?
OM
DM
DE
Ordinary matter sweg g+w?+?? g + ?
Dark matter
g+w?+?? g + ??
Dark energy
g + ??
Essentially everything we think we know about Dark
Energy and Dark Matter comes from their
gravitational influence on ordinary matter.
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There are Numerous Reasons to Test the
Foundations of Gravity
The Dark Energy sits right along the intellectual fault line between quantum
mechanics and gravity.
Numerous speculative scenarios invoke modifications to General Relativity to
account for the accelerating expansion.
Growing interest in exotic couplings in the dark sector, involving some
combination of dark matter and dark energy.
The evidence for dark matter comes from assuming normal law of gravity.
This might be inappropriate.
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Tests of Gravitation
Tests of the Equivalence Principle
Do all forms of mass-energy experience
same free fall in a gravitational field?
Are there new long-range forces?
Does the growth of cosmic
structure behave as expected?
Measuring Geffective
as a function of
environment
Tests of Inverse Square Law
Do interactions in lowdensity regions look the
same as in high-density
regions?
F ~ 1/r2 ?
Is there any evidence for extra dimensions?
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But… there is history here
1960s
Claim of GW detection
Positrons didn’t fall down
1970s Rotation curves imply missing
force problem*
1980s Claim of 5th force, a Yukawa
coupling to Baryon #
Gyroscope L vs. R anomaly
MOND claims to fit rot. curves
1990s “Pioneer” anomaly
Claims of grav. Shielding
Claim of accelerating Universe*
MOND claims to fit rot. curves
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2000s Antigravity for v/c > 1/sqrt(3)
MOND claims to fit rot. curves
The first anomalous gravity
Pioneer anomaly
experiment?
…
…
Testing gravity on the cosmic scale
Testing gravity in the solar system
Testing gravity in the laboratory
30
Millenium
simulation
team,
Raul Angulo
m=0.25
=0.75
514 Mpc
on each side
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Opportunities for Cosmic Tests of Gravity
Isotropy of Cosmic Expansion
- Test SN Hubble diagram in different directions. DE uniformity?
Kinematics of galaxies
– Test CDM paradigm, comparing with N-body simulations
– Halo streams as tracers of Galactic gravitational potential
Cosmological parameters
 m +  + k =? 1
– w = P/ = -1, or w(z)?
Evolution of Cosmic Large Scale Structure
– compare gravitational potentials sensed by light, matter, and dark matter
– probe the transition from granular to uniform
Overconstrained systems
– H0, luminosity distances, lensing, geometry, time delays, mass distr.
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The “Chameleon” Idea
A new long-range interaction, repulsive between like
objects, takes effect on cosmic scales.
But it’s “screened” in some way near mass concentrations.
One fashionable mechanism is to introduce a term that
depends on the local scalar curvature, f(R).
This, in turn, depends on both the environment (e.g. a void
vs. a cluster) as well as properties of the object under
consideration.
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Comparison of
galaxy-galaxy
lensing, galaxy
clustering, and
velocities
Reyes et al, Nature 464,7286
(2010)
also Lombriser, Slosar, Seljak
and Hu, arXiv:1003:3009 (2010)
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Comparing
galaxy
kinematics, in
clusters and in
voids
Hui, Nicolis & Stubbs,
PRD 80, 104002, (2009)
NGC 5907 (Credit: R. Jay Gabany)
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Is the accelerating expansion the same in
different directions?
A. Diercks’ PhD thesis, UCSB,1999
Cook & Lynden-Bell, MNRAS 401, 1409 (2009), and references therein
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The PanSTARRS Survey
1.4 Gpix camera
3.3 degree FOV
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1.8 meter telescope
PanSTARRS supernovae: > 300 in 6 weeks
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Testing gravity on the cosmic scale
Testing gravity in the solar system
(with some slides from Tom Murphy, UCSD)
Testing gravity in the laboratory
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Lunar Retroreflector Arrays
Corner cubes
Apollo 11 retroreflector array
Apollo 14 retroreflector array
Apollo 15 retroreflector array
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Apache Point Observatory Lunar Laser-ranging Operation
Mapping the lunar orbit to 1 mm
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APOLLO: Reaching 1 mm
3.5 meter telescope
1 arcsec median seeing
Avalanche Photodiode Array
(lenslet gives full fill factor)
 = 30 m
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-Courtesy of Lincoln Lab
Record-setting photon detection rates
Reflector
APOLLO max APOLLO max APOLLO max
photons/run photons/5-min photons/shot
(5 min avg)
APOLLO max
photons/shot
(15 sec avg)
Apollo 11
4497 (26)
5395 (65)
0.90
1.4
Apollo 14
7606 (36)
9125 (69)
1.52
2.0
Apollo 15
15730 (26)
18875 (67)
3.15
4.5
750 (11)
900 (31)
0.15
0.24
Lunokhod 2
APOLLO has greatly surpassed previous records
(relative to pre-APOLLO record)
 max rates for French and Texas stations about 0.1 and 0.02, respectively
APOLLO can operate at full moon
 other stations can’t (except during eclipse), though EP signal is max at full
moon!
Often a majority of APOLLO returns are multiple-photon events
 record is 12 photons in one shot (out of 12 functioning APD elements)
 APD array (many buckets) is crucial
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LLR through
the decades
Station
Dish
Seeing
Rate (/min)
MLRS, USA
0.8 m
3”
1
OCA, France
1.5 m
1-7”
4
APOLLO, USA
3.5 m
1”
600
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Equivalence Principle Signal, Illustrated
moon close
moon far
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Graphic excerpt from San Diego Union Tribune
LLR is a precision probe of
fundamental gravity physics
With one millimeter range uncertainty:
Weak EP
a/a
Strong EP
=4-3-
Gravitomagnetism
(dG/dt)/G
Geodetic precession
Long range forces
LLR currently provides the best
limit on all but WEP
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10-14
3×10-5
10-4
10-13 yr -1
3×10-4
10-11 × the
strength
of gravity
The Strong Equivalence Principle
Earth’s energy of assembly amounts to 4.610-10 of its
total mass-energy
The ratio of gravitational to inertial mass for this self energy is
The resulting range signal is then
Currently  is limited by LLR to be ≤4.510-4
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The Inverse
Square Law
r
mM
V (r)  G
(1   e  )
r
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Monitoring the Strength of Gravity, G
• If G changes with time, Kepler’s law is broken
• Range signal (semi-major axis) and period (phase) no longer
run in lock-step
• The rate of phase slippage grows linearly in time
• The phase offset grows quadratically in time, so sensitivity to
G-dot grows rapidly as data accumulate
• LLR sensitivity now limits change to ≤10-12/yr variation
• Less than 1% change over age of Universe
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Compact Extra Dimensions?
Gravity might not be so feeble, intrinsically.
Perhaps most of the gravitational force is
confined to compact extra dimensions…
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Dark Energy + Extra Dimensions require both
G and w to vary, for broad classes of theories
If compact extra dimensions exist, accelerating expansion
causes their size to vary. This happens independent of
the scale, even down to the to Planck length!
The strength of gravity (which is feeble because it “leaks”
into these extra dimensions) therefore changes
Steinhardt and Wesley show (arXiv:1003:2815) that the
joint constraints on G(t) and w(t) do better than either
does alone.
51
Steinhardt and Wesley (arXiv:1003:2815)
CRF= conformal Ricci-flat metrics
NEC= null energy condition
52
Steinhardt and Wesley (arXiv:1003:2815)
53
Testing gravity on the cosmic scale
Testing gravity in the solar system
Testing gravity in the laboratory
(with slides from Eric Adelberger, Univ. Of Washington)
54
Compact Extra Dimensions?
Gravity might not be so feeble, intrinsically.
Perhaps most of the gravitational force is
confined to compact extra dimensions…
55
The Planck length is the “natural scale” for
the size of these compact dimensions
Planck
hG
-35


1.6

10
meters
3
c
Ouch. This is one reason why testing String Theory is so tough.
Planck length is 1020 times smaller than a proton
This corresponds to an energy of 1019 Gev, or ~1015 times the LHC
String Theory = Ouch15
56
Does dark energy define a new
fundamental length scale in physics?
a second “Planck length”?
57
the 42-hole inverse-square law pendulum
58
PhD project of Dan Kapner, at Univ. of Washington
59
Mary Levin photo
95% confidence
upper limits on
Inverse Square
Law violation
r
mM
V (r)  G
(1   e  )
r
size of any compact
dimension < 44 m
D.J. Kapner et al., PRL 98,
021101 (2007)
E.G. Adelberger et al.,
PRL 98, 13104 (2007)
60
Summary
The discovery of the Dark Energy poses a profound challenge to our
understanding of fundamental physics.
It’s difficult to understand =0.7.
So far, the data are consistent with the cosmological constant scenario:
w=-1, w(a)=0.
We are in a situation analogous to the era before quantum mechanics.
Testing gravity on all accessible scales and energies is a fishing
expedition, but we should be unapologetic about this.
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