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Cosmology
Scott Dodelson Penticton Workshop
Two Success Stories: Cosmology & Particle Physics
We know much
We understand little
How you can help
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Scott Dodelson Penticton Workshop
Consider the United States in 1790
Over-densities of order 50
Concentrated in East
Vast Voids with low density
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June 13, 2011 Scott Dodelson Penticton Workshop
Consider the United States Today
Over-densities of order 10,000
Concentration in coasts
Traces of primordial density (Boston-Washington; East > West)
Vast Voids
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Scott Dodelson Penticton Workshop
The story of this evolution is the story of the United States
When we understand the evolution from one map to another, we can understand the sociological, economic, and political forces acting on the US the people, or the constituents, of the US
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Less provincially, we have detailed maps of the Universe at two epochs
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COBE
Sloan Digital Sky Survey
(SDSS)
Map of the photons, electrons, & protons when the Universe was 400,000 years old
Map of the galaxy distribution in the Universe today
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Standard Model of Particle Physics
4 particles which mediate the fundamental forces (photon, 2 W’s, Z)
Two Fermions (Electron and Neutrino) Higgs particle: Responsible for Mass
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Three minor extensions (all of which won Nobel Prizes)
Quarks as well as leptons
Quantum Chromodynamics: gluons and their interactions
Three generations: electron, muon, tau (and similarly for quarks)
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The Standard Model explains everything ever produced in an accelerator
One page (out of 298) from the Particle Data Group Booklet
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and is consistent with all observed phenomena (not necessarily a good thing)
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The Cosmic Story requires new physics
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COBE
Sloan Digital Sky Survey
(SDSS)
At t=400,000 years (z=1000), the photon/baryon distribution was smooth to one part in 10,000.
General Relativity predicts that perturbations have grown since then by a factor of 1000
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Dark Matter Solves Cosmic Structure Problem
ClumpinessDark matter was much clumpier than baryons were at the time of the picture of cosmic microwave background (CMB). Enough time for structure to grow!
Large Scales
Scott Dodelson Penticton Workshop
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Two Stories Complement One Another
Cosmic Story requires Dark Matter, which cannot be explained by Standard Model of Particle Physics
This is the simplest example of how detailed study of cosmological maps leads to (i) a coherent story for cosmic evolution and (ii) understanding of new physicsJune 13, 2011 Scott Dodelson
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Inflation
Dark Matter
Neutrino Mass
Dark Energy
Dark Energy
LHC
Previous Accelerators
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Detailed Analysis of Maps
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Neutrinos
Massive neutrinos contribute to the energy density
This fraction of the matter does not clump on small scales
Produces relatively large changes in the power spectrum of matter
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eV
mf
matter 108.0
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June 13, 2011 Scott Dodelson Penticton Workshop
Even for a small neutrino mass, get large impact on structure: power spectrum is excellent probe of neutrino mass
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Neutrinos
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Current Constraints
Strongest constraint on sum of neutrino masses
Oscillation experiments measure difference of squared masses
Sum of the neutrino masses must be greater than 0.05 eV
Ichiki, Takada, & Takahashi 2009
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Dark Matter
Physics of the CMB
Pressure of radiation acts against clumping
If a region gets overdense, pressure acts to reduce the density: restoring force
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Dark Matter
C note on a guitar
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Dark Matter
CMB is different because …
Fourier Transform of spatial, not temporal, signal
Time scale much longer (400,000 yrs vs. 1/260 sec)
No finite length: all k allowed!
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Dark Matter
Why are there peaks & troughs in CMB Spectrum?
Vibrating String: Characteristic frequencies because ends are tied down
Temperature in the Universe: Small scale modes begin oscillating earlier than large scale modes
Fou
rier
Am
plit
ud
e
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Dark Matter
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)()4/31(3
2
FTk
Tb
Detailed shape depend on matter & baryon densities
Forced Harmonic Oscillator
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Dark Matter
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Dark Matter
CMB and Matter power spectra give consistent results
Require dark matter even for the shape
Total matter density well-constrained
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Dark Energy
Einstein’s Equations applied to the Universe yield the Friedmann Equation
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2
2
0
2
)1())(1(1
3exp zzwz
dz
H
Hk
i
ii
Curvature density (=0 from position of CMB
peaks)
Eqn of state (pressure/energy density)
of species iw=0 for matter
1/3 for radiation
Energy density of species i in units of the
critical density
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Dark Energy
This equation does not hold today!
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i
iik zwz
dzz
H
H))(1(
13exp)1( 2
2
0
2
01 m
Einstein’s Equations applied to the Universe yield
2
2
0
2
)1())(1(1
3exp zzwz
dz
H
Hk
i
ii
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Scott DodelsonPenticton Workshop
Introduce New Substance Dark Energy
Dar
k En
ergy
Den
sity
Matter Density
Dark Energy density remains roughly constant as universe expands Nothing like this in Standard Model It’s not just CMB + Galaxy Map: Supernovae also require dark energy
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Galaxy Map
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Scott Dodelson Penticton Workshop
The Horizon Problem
Hubble Radius (Distance light travels as the Universe doubles in size) at t=400,000 years
How are these two spots correlated with one another?
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Scott Dodelson Penticton Workshop
The Horizon ProblemD
ista
nce
Time
t = 400,000 yrs
Hubble Radius
Distance between 2 spots in CMB
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Scott Dodelson Penticton Workshop
The Horizon ProblemD
ista
nce
Time
t = 400,000 yrs
Distance between 2 spots in CMB
Hubble Radius
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Scott Dodelson Penticton Workshop
The Horizon ProblemD
ista
nce
Time
t = 400,000 yrs
Distance between 2 spots in CMB
Hubble Radius
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Scott Dodelson Penticton Workshop
Dis
tan
ce
Time
t = 400,000 yrs
Inflation Solves the Horizon Problem
Inflation
Distance between 2 spots in CMB
Hubble Radius
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Scott Dodelson Penticton Workshop
Quantum Mechanical Fluctuations
Ubiquitous on the sub-atomic level …
Inflation stretches them to astronomical sizes
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Seeds of Structure
Quantum mechanicalfluctuations generated during inflation Perturbations freeze outwhen distances get larger than horizon Evolution when perturbations re-enter horizon
Dis
tort
ion
s in
Sp
ace-
Tim
e
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Evidence for Inflation
The perturbation corresponding to each mode need not have zero initial velocity.
We implicitly assumed that every mode started with zero velocity.
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At every wavelength (or angular scale), we are averaging over many modes with different directions.
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Interference would destroy peak structure
There are many, many modes with
similar values of k. All have different initial amplitude. Why all they are in phase?
First Peak Modes
t/400,000 yrs
Fou
rier
Am
plit
ud
e
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Infinite number of guitars are synchronized
Similarly, all modes corresponding to first trough are in
phase: they all have zero amplitude at
recombination. Why?
t/400,000 yrs
Fou
rier
Am
plit
ud
e
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First Trough Modes
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Without synchronization:
First “Peak” First “Trough”
We will NOT get series of peaks and troughs!
t/400,000 yrs t/400,000 yrs
Fou
rier
Am
plit
ud
e
Fou
rier
Am
plit
ud
e
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Spectrum of Anisotropies Evidence for Synchronization
Scott Dodelson Penticton Workshop
DASI, Boomerang, Maxima (2001)
WM
AP
(2009)
Coherent series of Peaks and Troughs emerges because all Fourier modes shared the same temporal phase
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We know much
Neutrino masses are light and they affect structure formation at a potentially detectable level
85% of all matter is something new, dark matter, not found in the Standard Model
The Universe is accelerating today presumably because 71% of the energy is a new substance, dark energy
The seeds of structure were planted during inflation, an early epoch of acceleration
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We understand little
Why are neutrino masses so small? What is the sum of neutrino masses (must be >0.05 eV)? Normal or inverted hierarchy?
What is the dark matter? It is not part of the Standard Model. Supersymmetry?
Why and when does the Friedmann equation break down? Dark Energy? Modified Gravity? Something else?
Did inflation happen? What is the physics behind inflation? Grand Unified Theory? Something more exotic?
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Neutrino Mass
Optimistic projections suggest that future 21 cm experiments will detect neutrino mass to 10-sigma
Can distinguish normal from inverted hierarchy (see right)
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Pritchard & Pierpaoli 2008
z=0
z=8
eVm 12.0
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Dark Energy
Squares for Cylindrical Radio Telescope+Planck; Crosses add previous experiments
Fiducial: L=100m; Width=14m; Duty Factor=0.5
For comparison, Dark Energy Survey proposal lists FoM=264June 13, 2011 Scott Dodelson
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Seo et al., 2010
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Dark Energy vs. Modified Gravity
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Masui, Schmidt, Pen, & McDonald 2010
Models have identical expansion histories. Error bars assume a 200mx200m cylindrical telescope
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Non-Gaussianity in Inflation
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Joudaki et al. 2011
Importance of Foreground Removal: x-axis sets largest scale above which foregrounds can be removed
Comparable to Planck
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There is a lot left to explore
Modern cosmology depends on maps. We have explored roughly the same fraction of the universe as Penticton covers of Canada. Surprises are on the horizon!
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