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## Cosmological Observations—2004

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### Cosmological Observations—2004

What the data tell us about dark energy and the contents of the universe

DPF 2004, Riverside

August 28, 2004

Joe Fowler

Princeton University

Current Picture of the Universe

- General relativity
- Homogeneous & isotropic
- Began with hot big bang
- Quantum fluctuations grew during inflation
- Galaxies & other structures grew gravitationally from these tiny early fluctuations

HST Images

Evidence for a Hot Big Bang

- Hubble expansion (recession of distant objects)
- Thermal cosmic background radiation
- Light element abundances

Hubble Ultra Deep Field

Released: March 2004

Contents of the Universe

- ΛCDM Model
- At least 96% of the universe is mysterious!

Λ

Note that “Λ” here may be a dynamical field a la quintessence, an Einsteinian cosmological constant, or …?

Evolution in an FRW Universe

Open (Ω=0.3)

Open (Ω=0.3)

Flat (matter)

Flat (matter)

Closed (Ω=5)

Closed (Ω=5)

ΛCDM

- History and fate are determined by proportion of stuff
- Express energy densities as Ω, i.e. scaled by the critical density
- Today, ρcrit = 5000 eV cm-3 = 6 protons per m3

Roles of Inflation

- Solves the “horizon problem” (all visible universe was once in causal contact)
- Explains the source of inhomogeneities
- Flatness is unstable—but inflation drives towards flatness early on

Matter, Energy and Geometry

Current model

Accelerating now

Decelerating now

ΩΛ

Matter only

Closed

Flat

Open

Ωmatter

- Generally only the 2 black lines are considered: flat or matter-only.

Lines of Evidence for Dark Energy

ObservationResultInterpretation

1. Distant supernovae ~25% too dim Expansion accelerating

2a. CMB acoustic peak ℓ = 220 Flat universe

2b. Matter distribution Ωm ~ 25% Rules out flat, matter-only

3. CMB + LSS power (fits) All of the above

spectra

4. Integrated Sachs-Wolfe Mass at z~0.5 If flat Λ>0

(a 2 – 3σ) correlated w/ CMB

What is the Dark Energy?

Gij – Λgij = 8πG TijCurvature of empty space

or

Gij = 8πG Tij + ΛgijVacuum energy

Vacuum energy opens up more possibilities than curvature.

Two key question for observations:

- Does Λevolve?
- What is its equation of state w ≡ P/ρ?

w < -1/3 is required if Λ is repulsive

w = -1 is a true cosmological constant

Baryon Fraction from Big Bang Nucleosynthesis

- Light elements form in first few minutes (D, 3He, 4He, Li)
- Ratio of baryon to photon density determines proportions
- We know photons (CMB)
- Must measure primordial abundance of light elements

Tytler et al, 2000

Ωb = 0.041 ± 0.004

(assuming h=0.7)

Burles, Nollett & Turner, 2004

Dark Matter Distribution in the Universe

- Dark matter clustering drives structure formation on scales larger than galaxies.
- Must be “cold” to support the smallest scales observed.

R. Cen

Techniques for Studying Matter Distribution

Plan: study the evolution of structure by measuring it locally

- Number counts of galaxy clusters
- Velocity fields of galaxies
- Weak gravitational lensing
- Galaxy spatial power spectrum
- Cold intergalactic gas (Lyman-α forest)

Gravitational Lensing of Background Galaxies

Chandra X-ray Observatory

Hubble Space Telescope

Strong lensing shown here

Weak Lensing

- Relies on shear: preferential warping of background galaxies parallel to contours of foreground matter.
- A statistical hunt for ellipticity
- Shape noise (galaxies have ellipticity ~ 0.3; PSF…)
- Shape bias: are some shapes easier to find?

Tyson et al 2002

Large Sky Surveys

Sloan Digital Sky Survey

Galaxy Power Spectrum

Ideally, surveys are flux-limited.

2 degree Field Galaxy Redshift Survey

Sloan Digital Sky Survey

Galaxy Power Spectrum Systematics

Redshift distortion (due to peculiar velocity)

Galaxy “bias”

Tegmark et al 2003 astro-ph/0310725

Seljak et al 2004 astro-ph/0406594

The Lyman-α Forest

Quasar

Clouds containing Neutral Hydrogen

- Absorption by H atoms in bulk IGM (λ=121 nm).
- Test ΛCDM at unique range of z and small size.

Hubble Space Telescope

Keck HIRES

Figure: Bill Keel

Power Spectrum Results

- Completely consistent with ΛCDM model
- Dark Energy
- Consistent with pure cosmological constant
- Inflation
- Simplest possible scenario
- Primordial slope n=0.98 ± 0.02
- Tensor/Scalar ratio r < 0.36 (95% CL)
- Neutrinos
- Massive reduce structure on small scales
- 3 ~degenerate families: Σ m < 0.42 eV
- 3 massless + 1 (LSND): m < 0.79 eV ruling out LSND solutions at 2σ

Max Tegmark + SDSS

Seljak et al, 2004

Cosmic Microwave Background

- As universe cooled below 3000 K, became transparent.
- Most thermal photons last scattered then (at z=1089).
- CMB is the most distant light we’ll ever be able to see.
- Probes the initial conditions for structure formation.

CMB Basic Facts

- Thermal blackbody at T=2.725±0.003 K
- Emitted at T~3000 K
- Isotropic to ~30 x 10-6

2.731

FIRAS spectrum

Residuals

2.721

Fixsen et al 1996

Wilkinson Microwave Anisotropy Probe

- Twin telescopes facing 140o apart.
- Always measuring differences of Temp.
- Amplifiers kept at 90 K without refrigeration.

Once the “Princeton Isotropy Experiment” = “PIE in the sky”

NASA/WMAP Team

WMAP Goal

Map entire mm-wave sky

- 5 frequencies
- 35 μK noise per 0.3° square pixel
- 0.5% absolute calibration

Tegmark & Efstathiou

WMAP Radiometers

Each “differencing assembly” measures ΔT in analog.

Both signals go through all amplifiers!

Pospiezalski, NRAO

Other figures: NASA/WMAP Team

WMAP Mission Profile

- Launched June 30, 2001
- 3 months to L2 (1,500,000 km distant)
- Survey for 2—5 yrs

At L2, WMAP can keep the sun, moon, and earth behind it at all times.

All figures: NASA/WMAP Team

WMAP Sky Maps in 5 Frequencies

22 GHz

30 GHz

40 GHz

+200 μK

Lowest frequency

(galactic electrons)

Highest (some dust)

-200 μK

90 GHz

60 GHz

All figures: NASA/WMAP Team

Temperature Power Spectrum

- Spherical harmonic power spectrum—a radical compression of the map for cosmological purposes.

NASA/WMAP Team

Acoustic Peaks

- Peaks correspond to a well-understood physical size (145 Mpc): they are “standard rulers.”
- Peak at ℓ=220 indicates no global curvature from z=0 to z=1089.
- Ratio of peaks #1/#2 constrains baryon density.

Temperature-Polarization (TE) Cross-power

- Cross-power spectrum sensitive to ionization resulting from early hot stars.
- Data at ℓ>20 fit the cosmology dictated by the TT power spectrum.
- Only DASI has detected polarization anisotropy (EE) as of August 28, 2004.

WMAP Interpretation

Extremely strong support for:

- Hot big bang model
- Existence of baryons, dark matter, and dark energy (4/23/73 ratio)
- Gaussian primordial fluctuations + inflation

WMAP Surprises

NASA/WMAP Team

- The first stars ignited much earlier than thought: 200 Myr (1.5% of current age).
- Very low quadrupole

- How can WMAP tell?
- Early stars massive
- Massive stars hot (UV)
- UV ionizes nearby gas
- Ionized atoms polarize CMB
- Polarization correlates with T

WMAP Results by the numbers

- Age of the universe: 13,700,000,000 years (± 1.5%)
- Age when stars first shone: 200,000,000 years
- Age at last scattering: 379,000 years (z=1089±1)
- Expansion rate (Hubble constant): 71 km s-1 Mpc-1 (± 5%)
- Flatness: Ωt = 1.02±0.02
- Optical depth to last scattering τ = 0.17±0.04
- Apparent fate of the universe: Expand forever (?)
- These figures include constraints from, for example, 2dF galaxy redshift survey and Supernovae Ia.

CMB Future: Secondary Anisotropies

Cosmic Microwave Background

Study structure as it forms

Clusters “heat” the CMB (SZ Effect)

Early stars

Massive clusters

distort CMB maps

CMB seen now has passed through all these objects!

Primary CMB Ionization effects Grav lensing of CMBCluster surveys

0.4 Myr

~200 Myr

3000—

13,700 Myr

1000-5000 Myr

now

CMB Future: Polarization from Gravity Waves

Polarization B modes are “handed” and not produced by scalar perturbations.

A strong signature of inflation.

But at what level?...

DASI collaboration, 2002

Hu & Dodelson, 2002

E modes B modes

Wayne Hu

Matter Distribution Imprinted on CMB

Cosmic Microwave Background

The “Late-time integrated Sachs-Wolfe effect”

- CMB blue shifts entering large overdensities.
- In matter-only universe, red shift on exit cancels this out.
- In a Λ-dominated universe, expansion outweighs clustering.
- Higher T correlates with high mass density.

Several 2 to 3σ ISW Detections

Boughn & Crittenden, 2004

- In a flat universe, any ISW implies dark energy.

X-ray catalog / CMB angular correlation function

- Need a tracer of mass.
- WMAP +
- SDSS (red) 2.0σ
- NVSS (radio) 2.2σ
- HEAO-A1 (X-ray) 2.5σ
- Combined analysis of last 2 yields
- (1.13±0.35) x Λ CDM prediction.
- ISW alone rejects @ 3σ an allowed WMAP solution with no Λ and high matter content.

Xray x CMB data

Λ CDM Model

1σ, 2σ range of null MC

Hubble’s Diagram and the Expanding Universe

- Uniform expansion v=Hod
- But the next order is interesting! Trace the dynamics of the expanding universe.
- Requires an extremely bright light standard: Supernovae

“Distance modulus”

Δm = 5 factor of 10 in luminosity distance

Type Ia Supernovae

- Type I = deficient in Hydrogen; Ia have Si+ absorption
- Requires “real time” data analysis
- Can now find SN Ia on demand and pre-schedule the follow-up spectroscopy

3 HST discoveries before / after

SN2002hp ( ~ 2 months) HST-ACS

Type Ia Supernovae as Standard Candles

- Model is an accreting white dwarf, passing the Chandrasekhar limit
- Actually, a 1-parameter family in:

- Peak brightness
- Rate of decline
- Color

Can reduce dispersion 3x

N.B.: evolution slows by (1+z)

1 month

Evidence for Recent Accelerated Expansion

- Hubble diagram curvature consistent with universe that’s accelerating now
- Effect is only a ~25% dimming of SNe around z=0.5
- Possible confounding effects:
- Evolution
- Extinction (by very gray, homogeneous dust)
- No evidence for either, but we must be very sure.

Evidence for Earlier Deceleration

- 16 new SN from HST at typical z~1
- As expected from ΛCDM, dimming trend reverses!
- Strongly suggests not evolution or dust

Jerk

Riess et al 2004

Supernovae Interpretation

- SN Hubble diagram constrains (ΩΛ-1.4Ωm)
- If flat universe, then Ωm=0.29±0.04

Cosmic Concordance

- The model may be crazy, but everything is consistent (so far):
- Flat universe
- Dark energy (~70%)
- Still need non-baryonic DM (and not neutrinos)

(Pre-2004)

Probing Inflation (and is it correct?)

- CMB degenerate in:
- n the primordial perturbation spectral index
- τ the optical depth through reionized universe
- r the ratio of scalar to tensor fluctuations (the upper limit 0.35 is already approaching what some simple models predict)
- Large scale structure surveys and E-mode CMB polarization can help break these.
- Detect the B-modes of CMB polarization (next decade?)
- B-modes would rule out ekpyrotic (cyclic) scenarios WMAP should soon tell us how hard this will be (foregrounds)

Probing Dark Energy

Require more studies covering the z<2 range:

- Supernovae—need dedicated, wide-field, fast camera
- Cluster counts—need a distance-independent probe (S-Z effect surveys coming online in 2-3 years)

Summary

The leadingΛCDM model (dark energy + cold dark matter) is consistent with all the data!

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