A cosmology overview

Martin White

UC Berkeley

Cosmology

• Physical cosmology– The large-scale properties of space-time– The formation and evolution of large-scale structure

• Particle cosmology– Inflation– Baryogenesis– Dark matter searches

• Astrophysical cosmology– Epoch of reionization, star formation

• String cosmology

The golden age?

• Advances in detectors and computing power have transformed cosmology from a data-starved to a data-rich subject.

• We now have an incomplete but highly predictive (phenomenological) theory of the evolution of the Universe from the earliest instants to the present.

• Comparison between theory and experiment is now done with high precision, comparing complex models with large data sets using sophisticated algorithms.

The paradigm

The Universe is expanding

from a hot big bang

in which the light elements were synthesized.

There was a period of inflation

which led to a “flat” universe today.

Structure was seeded by gaussian irregularities

which are the relics of quantum fluctuations

and is dominated by cold dark matter.

Martin Rees (Princeton, 1996)

The cosmic microwave background

• One of the pillars of the hot big bang model was the discovery (Penzias & Wilson) of the cosmic microwave background radiation.

• Relic radiation left over from a hotter, denser phase of the universe.

• Blackbody radiation, redshifted by the universal expansion to peak in the microwave region of the spectrum.

CMB spectrum

1 parameter fit: T = 2.725 0.001 K

COBE-FIRAS

(Mather et al.1999)

The cosmic microwave background

WMAP 1st year data

Subtracting off the monpole component and removing the Doppler shift due to the Earth’s motion reveals tiny anisotropy …

The power spectrum

The WMAP 1st year dataare in excellent agreementwith the theoreticalpredictions for the anisotropyspectrum in a dominatedCDM model.

The cartoon• At early times the universe was hot, dense and ionized.

Photons and matter were tightly coupled by scattering.– Short m.f.p. allows fluid approximation: baryon-photon

fluid• Initial fluctuations in density and gravitational potential

drive acoustic waves in the fluid: compressions and rarefactions.

• A sudden “recombination” decouples the radiation and matter, giving us a snapshot of the fluid at “last scattering”.

[harmonic wave]

Acoustic oscillations seen!First “compression”,at kcstls=Density maxm, velocity null.

First “rarefaction” peak at kcstls=2

Velocity maximum

Acoustic scale is set by the sound horizon at last scattering: s = cstls

Fit to models

• The angular power spectrum contains rich structure which depends upon– Initial spectrum of density perturbations– Properties of the baryon-photon fluid– Geometry of the universe– Expansion rate of the universe

• This allows us to simultaneously fit for numerous cosmological parameters with high precision.

Of the “dozen” parameters in our cosmological model:

1 parameter known to better than 1%

5 parameters known to better than 10%

(independently)

Other inferences• Structure forms through gravitational instability.• From the narrow first peak we know that whatever

“rang the bell” was sharp and of short duration, not a continuous driving.

• The fluctuations are dominated by large-scale structure (not vorticity modes or gravity waves).

• The universe was not “weird” at z~103.• Inflation is in good shape.

– Limits on specific models of inflation– e.g. 4 inflation is ruled out at >3.

Inflationary origin for perturbations?Shapes of the peaks suggest an early universe origin from perturbations outside the horizon at last scattering.

Hu, Spergel & White (1997)

Peak spacing is unchanged, but peak positions out of phase (sine vs cosine).

Missing “first” peak in polarization.

Polarization

Hu & White (1997)

In the presence of anisotropy we expect scattering to generate (linear) polarization.

Consequence of electro-magnetic gauge invariance!

Polarization provides a prediction, a cross-check and further information about conditions at last-scattering and reionization.

DASI CMB Polarization Three-year Results

6.3 detection

Consistent with zero

2.9 detection

Single shaped l-bin Five l-bins

Leitch et al., 2005

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New information from polarization

WMAP 1st year data

Adiabatic/inflationarypeak

The first stars formed earlier than previously thought!

The spectrum beyond the CMB• A fit to the CMB data allows us to constrain the amplitude

and local slope of the density/curvature perturbations

• In addition to dramatic advances in CMB research we have information on the spectrum of perturbations over a wide range of times and length scales from– Galaxy surveys (AAT 2dF, SDSS, …)– Abundance evolution of rich clusters of galaxies– Gravitational lensing– Spectra of QSOs - clumping of intergalactic H

• The theory is successful in predicting the shape and evolution of the perturbations of 4 decades in length and ~ 10 Gyr of time.

The power spectrum

Courtesy M. Tegmark

The background cosmology

• There have been dramatic advances recently in understanding the fluctuations which make up large-scale structure and form stars and galaxies.– New, large-scale galaxy and QSO surveys

– Powerful new theoretical ideas and tools

• Ironically the “homogenous” term, the background cosmology, while much better measured has become even more of a mystery ...

A Universal Census

Courtesy WMAP

h2 ~ 1/2

The constituents• Baryonic material

– Mostly dark.

– Know where it is at z~3, but not today.

• (Cold) dark matter– Fundamental properties unknown.

– Cold, dark and weakly (self-)interacting.

– Can weigh it and make maps of it!• e.g.m=2.53(16) x 10-30 g/cm3

– Know T very well.

• Dark energy (the rest)

Dark matter map! (Pen, van Waerbeke, Mellier, in prep)

Accelerated expansion!Dark energy was first “found” by measurements of cosmic expansion, although models containing it had been around for several years.

Want a way to measure distance as a function of expansion (redshift). Use standard candles …

Dark energy• Observed accelerated expansion of the universe

requires– Violation of General Relativity

• Extra-dimensions, string theory, …

– Violation of large-scale homogeneity and isotropy• …but what about CMB?

– Violation of strong energy condition• Fluid with p < -(1/3) e.g. vacuum energy

• Usually described by an extra term on the RHS of Einstein’s equations modeled as a fluid with equation of state p=w(z) and p().– e.g. scalar field with potential V()

Dark Energy II• The nature and evolution of the dark energy driving

the accelerated expansion of the Universe is one of the most important unsolved problems in cosmology (and fundamental physics).

• There are many proposed methods and experiments:– Supernovae– Weak gravitational lensing– Cluster evolution– Baryon oscillations

• Currently all observations are consistent with – Models ruled out: w>-0.8, original tracker models, first

braneworld models, …

A standard ruler?• The SNe method exploits the idea of a standard candle: an object

whose luminosity we know and whose brightness we can measure at cosmological distance.

• A standard ruler is an object whose size we know, and whose apparent size we can measure at cosmological distance. The CMB has provided us with such a ruler:

• Knowledge of the physical scale, s, and the angular scale of the peaks, A, gives the distance to last scattering D through s=D A.

Dominated by uncertainty in m.

Eisenstein (2002)

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Matter power spectrum: P(k)Just as we have peaks in the photon (CMB) spectrum from oscillations in the baryon-photon fluid, so too do we have peaks in the matter (CDM+baryons) spectrum:

A calibrated “standard” ruler

Meiksin, White & Peacock

Both the large-scale “peak” in the matter power spectrum and the fine-scale “wiggles” are well calibrated by the CMB.

Another prediction verified!!

Eisenstein et al. (2005) detect oscillations in the SDSS galaxy (r) at z~0.35! Knowing s determines D(z=0.35)

Regular oscillations in P(k) make a single peak in (r)

Future measurements at higher redshift and accuracy will provide critical constraints on DE

Conclusions• Physical cosmology is a data-rich subject in which

detailed theories can be quantitatively compared to precise observations.

• The big picture is in place, but the major constituents and many of the detailed physical mechanisms remain to be understood.

• Further progress in cosmology will require and enable progress in our understanding of fundamental physics.

• Our ability to measure, model and explain the formation of large-scale structure, galaxies, … has advanced dramatically in the last 5 years.