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Dark Energy and the
Dynamics of the Universe
Eric Linder
Lawrence Berkeley National Laboratory
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Uphill to the Universe
Steep hills:
Building up Eroding away -
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Start Asking Why, and...
There is no division between the human world and
cosmology, between physics and astrophysics.
...
...
Everything is dynamic, all the way to the
expansion of the universe.
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Our Expanding Universe
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Bertschinger & Ma ; courtesy Ma
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Our Cosmic Address
Earth 107 meters
Solar system 1013 m
Our Sun is one of 400 billion stars
in the Milky Way galaxy, which is
one of more than 100 billion
galaxies in the visible universe.
Milky Way galaxy 1021 m
Local Group of galaxies 3x1022 m
Local Supercluster of galaxies 1024 m
The Visible Universe 1026 m
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The Cosmic Calendar
Inflation 1016 GeV
Quarks  Hadrons
1 GeV
Nuclei form 1 MeV
Atoms form 1 eV
[Room temperature 1/40 eV]
Stars and galaxies
first form: 1/40 eV
Today: 1/4000 eV
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Mapping Our History
The subtle
slowing down
and speeding up
of the expansion,
of distances with
time: a(t), maps
out cosmic
history like tree
rings map out
the Earth’s
climate history.
STScI
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Discovery! Acceleration
data from Supernova
Cosmology Project
(LBL)
graphic by Barnett,
Linder, Perlmutter &
Smoot (for OSTP)
Exploding stars – supernovae – are bright beacons that
allow us to measure precisely the expansion over the
last 10 billion years.
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Acceleration and Dark Energy
Einstein says gravitating mass depends on
energy-momentum tensor:
both energy density  and pressure p, as
+3p
Negative pressure can give negative “mass”
Newton’s
2nd
..
law: Acceleration = Force / mass
R = - (4/3)G  R
..
Einstein/Friedmann
equation:
a = - (4/3)G (+3p) a
Negative pressure
Relation between  and p (equation of state)
is crucial:
w=p/
Acceleration possible for p < -(1/3) or w < -1/3
What does negative pressure mean?
Consider 1st law of thermodynamics:
dU = -p dV
But for a spring dU = +k xdx
or a rubber band
dU = +T dl
Vacuum Energy
Quantum physics predicts that the very structure
of the vacuum should act like springs.
Space has a “stretchiness”, or tension, or
vacuum energy with negative pressure.
Review -Einstein: expansion acceleration depends on +3p
Thermodynamics: pressure p can be negative
Quantum Physics: vacuum energy has negative p
“Tree ring” markers can map the expansion history,
measure acceleration, detect vacuum energy.
Cosmic Concordance
cf. Tonry et al. (2003)
• Supernovae alone
 Accelerating expansion
>0
• CMB (plus LSS)
 Flat universe
>0
• Any two of SN, CMB, LSS
 Dark energy ~75%
Frontiers of Cosmology
Us
STScI
95% of the universe is unknown!
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Dark
Is!!!
DarkEnergy
Energy Is...
!• 70-75% of the energy density of the universe
• Accelerating
95%the
of the
expansion,
universelike
unknown!
inflation at 10-35s
!• Accelerating
Determining the
the fate
expansion,
of the universe
like inflation at 10-35s
Repulsive gravity!
! Determining the fate of the universe
Fate of the universe!
Is this mysterious dark energy the original
cosmological constant , a quantum zeropoint
sea?
What’s the Matter with Energy?
Why not just bring back the cosmological
constant ()?
When physicists calculate how big  should
be, they don’t quite get it right.
Sum of zeropoint energy modes:
/8G = <0> ~  h/2   d3k (k2+m2)
~ kmax4
If Planck energy cutoff, <0> ~ c5/G2h ~ 1076 GeV4
-- If kmax~ QCD cutoff, 10-3 GeV4
-- But need 10-47 GeV4 !
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What’s the Matter with Energy?
They are off by a factor of
1,000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000.
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What’s the Matter with Energy?
This is modestly called the fine tuning problem.
But it gets worse: because the cosmological
constant is constant, it is the same throughout
the history of the universe.
Why didn’t it take over the expansion billions of
years ago, before galaxies (and us) had the
chance to form?
Or why didn’t it wait until the far future, so today
we would never have detected it?
This is called the coincidence problem.
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Cosmic Coincidence
Think of the energy in  as the level of the
quantum “sea”. At most times in history,
matter is either drowned or dry.
Dark
energy
Matter
Size=1/4
Size=1/2
Today
Size=2
Size=4
Key Issue for Physics Today
The universe is not simple:
So maybe neither is the quantum
vacuum (or gravitation)?
On Beyond !
On beyond ! It’s high time you were shown
That you really don’t know all there is to be known.
-- à la Dr. Seuss, On Beyond Zebra
We need to explore further frontiers in high
energy physics, gravitation, and cosmology.
New quantum physics?
Quintessence (atomic particles, light, neutrinos, dark
matter, and…), Dynamical vacuum
New gravitational physics?
Quantum gravity, supergravity, extra dimensions?
We need new, highly precise data
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Type Ia Supernovae
• Exploding star, briefly as bright as an entire galaxy
• Characterized by no Hydrogen, but with Silicon
• Gains mass from companion until undergoes
thermonuclear runaway
Standard explosion from nuclear physics
SCP
Insensitive to initial conditions:
“Stellar amnesia”
Höflich, Gerardy, Linder, & Marion 2003
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Standardized Candle
Brightness
Time after explosion
Brightness tells us distance
away (lookback time)
Redshift measured tells us
expansion factor (average
distance between galaxies)
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What makes SN measurement special?
Control of systematic uncertainties
At every moment in the explosion event, each individual supernova is
“sending” us a rich stream of information about its internal physical state.
Lightcurve & Peak Brightness
Images
Redshift & SN Properties
M and 
Dark Energy Properties
Spectra
data
analysis
physics
History & Fate
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Weighing the Universe
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Cosmic Concordance
cf. Tonry et al. (2003)
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“Stretchiness” (EOS)
Nature of Dark Energy
Matter Density
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What We Know
“ ‘Most embarrassing observation in physics’ – that’s the only quick
thing I can say about dark energy that’s also true.” -- Edward Witten
Dark energy causes acceleration -- “negative
gravity” -- through its strongly negative pressure.
Define equation of state ratio by
w(z)=pressure/(energy density)
Today’s state of the art:
wconst= -1.05+0.15-0.200.09
wconst= -1.08+0.18-0.20?
(Knop et al. 2003) [SN+LSS+CMB]
(Riess et al. 2004) [SN+LSS+CMB]
But what about dynamics? Generically expect time variation w
What We (Don’t) Know
Assuming w is constant can be deceiving, even
to test if dark energy is a cosmological constant .
If we don’t look hard for the time variation w then
we don’t learn the physics!
We have to do it right.
• Longer “lever arm” (higher redshift, more history)
• Many more supernovae, more precisely
• High accuracy
Hubble Diagram
~2000 SNe Ia
10 billion years
0.2
0.4
0.6
0.8
1.0
redshift z
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Understanding Supernovae
Nearby Supernova Factory
G. Aldering (LBL)
Supernova Properties
Astrophysics
Cleanly understood astrophysics leads to cosmology
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High Redshift Supernovae
Discover
Reference Subtract-->SN!
Riess et al./STScI
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Looking Back 10 Billion Years
STScI
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Looking Back 10 Billion Years
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Looking Back 10 Billion Years
To see the most distant supernovae, we must
observe from space.
A Hubble Deep Field has scanned 1/25 millionth
of the sky.
This is like meeting 10 people and trying to
understand the complexity of the entire
population of the US!
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Dark Energy – The Next Generation
Dedicated dark energy probe
SNAP: Supernova/Acceleration Probe
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Design a Space Mission
9000 the Hubble Deep Field
wide
plus 1/2 Million  HDF
HDF
deep
GOODS
• Redshifts z=0-1.7
• Exploring the last
10 billion years
• 70% of the
age of the universe
colorful
Both optical and
infrared
wavelengths to
see thru dust. 37
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Astronomical Imaging
Focus star
projectors
Half billion pixel array
36 optical CCDs
36 near infrared detectors
Guider
Visible
NIR
JWST Field of View
Spectrograph
port
Calibration
projectors
Larger than any camera
yet constructed
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New Technology CCD’s
• New kind of CCD detector developed at LBNL
• Radiation hard for space ; High efficiency
• Able to be combined into large arrays
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Astrophysical Uncertainties
For accurate and precision cosmology,
need to identify and control systematic uncertainties.
Systematic
Control
Host-galaxy dust
extinction
Wavelength-dependent absorption identified with high S/N multiband photometry.
Supernova evolution
Supernova subclassified with high S/N light curves and peakbrightness spectrum.
Flux calibration error
Program to construct a set of 1% error flux standard stars.
Malmquist bias
Supernova discovered early with high S/N multi-band photometry.
K-correction
Construction of a library of supernova spectra.
Gravitational lensing
Measure the average flux for a large number of supernovae in each
redshift bin.
Non-Type Ia
contamination
Classification of each event with a peak-brightness spectrum.
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SN Population Drift
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Controlling Systematics
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Weighing Dark Energy
SN Target
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Exploring Dark Energy
Current ground based compared with
Binned simulated data and a sample of
Dark energy models
Dark energy theories
Needed data quality
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The Fate of Our Universe
Size of
Universe
Looking
back
10 billion
years
to look
forward
40 billion
Fate
History
0
Future Age of
Universe
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Frontiers of the Universe
What is dark energy?
Will the universe expansion accelerate forever?
Does the vacuum decay? Phase transitions?
How many dimensions are there?
How are quantum physics and gravity unified?
What is the fate of the universe?
Size of
Universe
Fate
History
0
Uphill to the Universe!
Future Age
of Universe
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The Next Physics
The Standard Model gives us commanding
knowledge about physics
-- 5% of the universe (or 50% of its age).
That 5% contains two fundamental forces and 57
elementary particles.
What will we learn from the dark sector?!
How can we not seek to find out?
Frontiers of Science
Let’s find out!
1919
1998
Breakthrough
of the Year
2003
The Next Physics
Cosmic Archaeology
CMB: direct probe
of quantum
fluctuations
Time: 0.003% of
the present age of
the universe.
(When you were
0.003% of your
present age, you
were a 2 celled
embryo!)
Cosmic matter
structures:
less direct
probes of
expansion
Pattern of ripples,
clumping in space,
growing in time.
3D survey of
galaxies and
Supernovae:
direct probe of
cosmic
expansion
Time: 30-100%
of present age
of universe
(When you
were 12-40
years old)
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Cosmic Background Radiation
WMAP/
NASA
Photon density 407±0.4 cm-3
Baryon density bh2=0.023±0.001
Snapshot of universe
at 380,000 years old,
1/1100 the size
Planck satellite (2007)
nb/n=6 x 10-10 ; consistent with
primordial nucleosynthesis
Matter-antimatter asymmetry?
Baryogenesis?
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Gravitational Lensing
Gravity bends light…
- we can detect dark matter through its gravity,
objects are magnified and distorted,
we can view “CAT scans” of growth of structure
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Gravitational Lensing
Lensing measures the mass of clusters of
galaxies.
By looking at lensing of sources at
different distances (times), we measure
the growth of mass.
Clusters grow by swallowing more and
more galaxies, more mass.
Acceleration - stretching space - shuts off
growth, by keeping galaxies apart.
So by measuring the growth history,
lensing can detect the level of
acceleration, the amount of dark energy.
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Fundamental Physics

Astrophysics
SN
CMB
LSS
a(t)

Cosmology 
Equation of state w(z) 
Field Theory
V()
V ( ( a(t) ) )
The subtle slowing and
growth of scales with
time – a(t) – map out the
cosmic history like tree
rings map out the Earth’s
climate history.
Map the expansion history of the universe
STScI
Cosmic Archaeology
Inflation sets seeds of
structure, patterning both
radiation (CMB) and
matter (galaxies)
CMB
}
NASA GSFC/COBE
Large scale structure,
Dark Energy, Acceleration