CMB: Current State & Future Prospects Jonathan Sievers (CITA/UToronto)

Discovery of the Cosmic Microwave Background Predicted by Gamow & Alpher because helium exists (most produced in the Big Bang). Guessed it would be between 1 and 5 Kelvin. Canadians actually measured it very early (excitation temperature of gas clouds) but didn’t know what it was. Penzias & Wilson (Bell Labs) discovered it while building & testing antennas. 3 degrees in all directions. The big bang was hot, so there must still be photons left from it. Not a white dielectric

Z ~ 1100,  z~100, t~400,000 yr, light crossing 300 Mpc Distortions in energy <.0001 (Compton cooling of electrons)

Basic Picture  Initial perturbation power spectrum set some time early in the universe, presumably by inflation.  When larger than the horizon, overdensities grow in amplitude – as far as they know, Ω>1, so they collapse.  Photon energy density comparable or larger to baryons, so sound speed is relativistic. When ionized, baryons lock photons in place. Pressure becomes important when modes cross horizon, perturbations become sound waves.  Ionization fraction of hydrogen very sensitive function of temperature. At ~3000K, e - +p H, photons freed from baryons. Transition is fast, so we get a snapshot of the universe when it is 400,000 years old.  Amplitude of a mode is set by initial amplitude, phase at which we see it, and matter/energy contents of universe.  Perturbations small, so physics is linear. We can calculate the expected spectrum to high precision.

Parameters of Cosmic Structure Formation What is the Background curvature of the universe? closed flat open Density of Baryonic Matter Density of non- interacting Dark Matter Cosmological Constant Spectral index of primordial scalar (compressional) perturbations Spectral index of primordial tensor (Gravity Waves) perturbations Scalar Amplitude Tensor Amplitude Period of inflationary expansion, quantum noise  metric perturb. Optical Depth to Last Scattering Surface When did stars reionize the universe?

This is the TT (for temperature-temperature) spectrum

So now… Go out and measure it! Astronomers have gone to some odd places. DASI, ACBAR at South Pole. Boomerang launched from coastal Antarctica. CBI at 16,700 feet (5080 meters) in the Chilean desert. TOCO was on a mountain overlooking the CBI site. Makes for some great stories… Just the CBI (the telescope I have worked on) has had to deal with:

The CBI Adventure…(a less nice day) wo winters a year! With accompanying Two winters a year! With accompanying blizzards, windstorms, impassable roads… Volcano (makes a nice windsock). Also, earthquakes (7.0), landmines, even a lost flamingo.

So now… Go out and measure it! First single-expt. measurement of first Doppler peak done by TOCO. Boomerang, MAXIMA made the first high-precision first-peak measurements, followed by DASI, then VSA. CBI, then ACBAR measured power spectrum up to ℓ~3000 (others go to ~1000, missing the fall in power past third peak). Basic framework in place before WMAP. Accurate measurement of flatness, dark matter, baryons, n s, age… CBI ACBAR Boomerang

WMAP Satellite Full-sky, 10’ resolution, low-noise map provides best current intensity spectrum at ℓ<600. Cosmic variance limited at ℓ< – we have all the first peak info we’re going to get.

Data! (Full disclosure: selective data set use with noisiest points removed for clarity. See also DASI, VSA…

Parameters To get feel for parameter constraints, I take best-fitting standard Λ-CDM cosmology, then vary one parameter at a time. I adjust the overall power spectrum amplitude to the value that best goes through the data. Constraints come from shape of the power spectrum.

How Constrained are Things? Curvature of the universe: (needs some external limit on H 0 ) Universe is flat to an accuracy of 2% (40<h<100) Physical size of Fluctuations is fixed. Curvature sets the apparent angular size. Essentially shifts spectrum to larger or smaller scales.

How Constrained are Things? Dark Matter density of the universe: Ω DM =0.106±0.010 (From Mactavish et al.) Baryons oscillate, DM collapses. Baryons feel more gravity when falling with the DM rather. During expansion, DM keeps baryons from going as far as they’d like. Baryons+DM = enhanced odd peaks. Uptick at low-ℓ happens when Λ takes over the expansion. The more DM there is, the later this happens and the smaller the bump.

How Constrained are Things? Baryon density of the universe: Ω B =0.0233± (in good agreement with deuterium, helium, lithium abundances. Especially with new neutron lifetime). More baryons means more compression of photons before pressure halts collapse, therefore higher first peak. Baryons+DM = enhanced odd peaks. Photons diffuse on small scales before CMB happens, more baryons makes diffusion length shorter. Raises & flattens fall-off past third peak.

How Constrained are Things? n s : 0.98±0.04, just like inflation predicts. Stephen Hawking: “the discovery of the century, if not of all time.” n s is a tilt, raising the spectrum at high-ℓ, while lowering it at low-ℓ, or vice-versa. Had been discussion of a running index, dn s /dln(k). Driven largely by Lyman- α forest measurements (statistics of very small gas clouds at redshift of a few). Reanalysis by MacDonald et al. has effectively killed the running index for the moment. Running is second-order in slow-roll inflation, so should be small. n s -1 is an extremely interesting number. It’s a direct measure of slow-roll inflation parameters. We’d like to measure it well.

How Constrained are Things? Optical depth of universe due to re-ionization of hydrogen after stars/quasars turn on. To good accuracy, universe went from neutral to ionized everywhere at the same time. Density of electrons goes like (1+z) 3, so highest depth when universe first ionized. Electrons scatter the average CMB temperature they see, so power on scales smaller than the horizon at reionization gets damped. Astronomers care about optical depth because it tells us when stars turned on. Current best WMAP value is z reion =17±5. Subject to change?

How Constrained are Things? Optical depth, n s, Hubble constant are all degenerate in total intensity, especially at low-ℓ. But we can do better. A fundamental prediction of the standard cosmological model is that there should be a polarized component to the CMB as well. This can be used to measure the optical depth, as well as a powerful consistency check on our basic pictures. Unfortunately, polarization is faint. No more than 1-2% of the power in the CMB fluctuations is polarized, and often quite a bit less. Standard (E-mode) polarization comes about because velocity gradients during recombination produce intensity quadrupoles, and intensity quadrupoles+Thompson scattering makes polarization. Which brings us to… E-Mode polarization detections have been the dominant observational advances since WMAP.

Polarization In our plane wave perturbations, matter flows from peaks into troughs (or the reverse, depending on the phase of the wave). Velocity is always ll to k, so gradient is ll to k as well. Therefore, quadrupole is also parallel to k.

Polarization If the flow is converging, electron moves perp. to k. If diverging, electron moves parallel to k. But it never moves at an angle. This is E- mode polarization (B-mode is the polarization tilted by 45 degrees for the same k). E peaks where velocity is maximum and overdensity zero, so E out of phase with intensity spectrum. Also, waves at neither density of velocity null give rise to T and E, causing a correlation.

Current EE DASI (Leitch et al) was the first to measure E-mode polarization. Boomerang (2003 flight, Montroy et al.) and CBI (Sievers et al.) have multiple-bin EE detections, all have TE measurements as well. So far, everything consistent with TT predictions. Coming soon: CAPMAP already has weak detection, further data coming soon. Quad has data in hand, BICEP just went to the South Pole. Also, B-mode consistent with 0 signal (1.2±1.8μK in CBI. Means things work pretty well!)

Does TT Predict EE? Yes! EE is in excellent agree- ment with prediction from TT. Take the same TT curvature plot from before and then show its EE spectrum against the data. There are 0 free parameters in the EE model yet it agrees extremely well with the data (in fact Χ 2 is a bit too good - but not unreasonably so). EE-only measures the angular scale of the CMB to 3%, and gets the same answer as TT. Other parameters (dark matter, baryons…) from EE agree as well, but precision isn’t great yet (~30-40% accuracies, typically).

Back to Degeneracy Reionization electrons will also see a local quadrupole that will then cause polarized scattering, including a polarization-temperature correlation. This is expected to be the dominant effect on large scales both for EE and TE. TE already measured by WMAP, EE coming ASAP? WMAP TE CBI+DASI+Boom EE Polarization

Were Initial Conditions Adiabatic? Restrict n iso =3 isocurvature seed model CBI EE, CBI EE+TE, CBI+B03+DASI EE+TE CBI+B03 TT Add isocurvature CDM model (i.e. photons overdense=matter underdense, metric flat). P iso / P adi < 0.27 large scale, < 1.7 small scale n iso = WMAP1+B03+CBI+DASI TT+TE+EE Both polarization & temperature pick out chiefly adiabatic components.

B-Mode Polarization: The Holy Grail? Inflation predicts tensor perturbations as well as scalar ones. These fluctuations depend on the details of inflation, so everybody wants to measure them. In particular, measuring the amplitude sets the energy scale of inflation. Unfortunately, they are expected to be weak, and not measurable if inflation happened much lower than GeV. Quiet, r=0.18 Planck, r=0.05Spider People are looking. Currently, r (the ratio of tensor to scalar amplitude) GeV, then r should be measurable soon, perhaps by Planck, or a ground-based expt. (Clover in the UK is the next one up – 500 bolometers, 3 frequencies. Currently being built, scheduled for 2 years from now) Clover

No Tensor SPIDER Tensor Signal Tensor  Simulation of large scale polarization signal  This is what we are after!!

EE BB T mKmK T/S=0.1 QUAD ACBAR SPIDER l T/S=0.01 SPIDER Angular Power Spectra

3-Colour Foregrounds 30 GHz 44 GHz70 GHz 100 GHz Synchrotron Bremsstrahlung (Free-Free) Thermal Dust 143 GHz217 GHz 353 GHz545 GHz857 GHz  T =  f/df cmb /dT in deg K, linear in sqrt(  T), 1K threshold Foregrounds will make BB tough to measure. All planned expts. have many freqs. to try to fit foregrounds. Spinning dust?

More Planck Goodies We’ll get other parameters much better as well, especially from ESA’s Planck Satellite (launch in ’07, supposedly). In particular, n s to an accuracy of , optical depth to 0.006, and running index to

CBI , WMAP, ACBAR, BIMA Readhead et al. ApJ, 609, 498 (2004) SZESecondary CMBPrimary +Boom03; Acbar05: very nice TT, Oct05. parameters & new excess analysis as SZ Clusters at small scales contribute to CMB PS. Perhaps already seen by CBI, ACBAR, BIMA. Correlation with optical will nail down origin – I have data for CBI in hand, answer coming soon! This will be important for measuring n s. Cluster signal comes from hot gas scattering CMB photons from low to high frequencies, called the Sunyaev-Zeldovich effect.

High-ℓ Signal Red line is a power spectrum with galaxy cluster signal. Accurate measurements of n s will require this to be removed. SZA has some data already, CBI being upgraded, should have blue points in a year. ACT,APEX being built, will do this very well. Level is currently quite uncertain, will be something Planck has to worry about.

Cosmic Strings Topological defects can imprint themselves on the CMB. They are ruled out as a major component of structure formation, but rare strings may still be allowed. Can find them either by lensing of galaxies, or temperature edges in the CMB with scale ~2 o, ΔT/T=8πGμβγ, string of Gμ=10 -6 will have temperature of 70 μK. String limits: Gμ<3.4e-7 from SDSS+WMAP (Wyman, Pogosian, and Wasserman) <1e-5 from WMAP alone (Lo and Wright)… Planck will make things better – smaller noise +better resolution=less CMB noise. Lo and Wright claim factor of 2, perhaps will be better. Small scale telescopes (e.g. ACT) can do much better job if they know where to look. μK level confirmations should not be overly difficult. Candidate CSL-1. Alas, turns out to be a pair of galaxies.

Neutrino Masses CMB spectrum mildly sensitive to neutrino mass. Large-scale galaxy structure more sensitive, but requires CMB input to know what it should look like. Combination of current CMB+current LSS gives limit m ν <0.7eV (Elgaroy et al., Spergel et al., Tegmark et al.) Planck will give better constraints. Should give Σ (m ν ) to 0.2 eV with SDSS (Eisenstein et al.). Massive neutrinos will also lens the CMB, could push limit down to 0.15 eV. CMB feels relativistic neutrinos, can limit number of light, sterile species. Current w/ big-bang nucleosynthesis gives N v =2.6±0.4. Planck should give 0.24, without external help. Caveat: High precision requires getting a lot of small-scale astrophysics correct. Not simple.

And Since Inquiring Minds Want to Know… Red=WMAP-1 Blue=Possible WMAP-3 (weeks away?) One man’s guess as to what the upcoming WMAP TT spectrum might look like.

Summary  CMB measures cosmological parameters with unprecedented accuracy. Theory is very robust, so surprises unlikely.  New generation of detectors coming soon, featuring large numbers of sensitive bolometers. Planck, APEX, ACT, SPT, Spider, Clover. First round of results should be within ~3 years.  Many inflation models predict B-mode polarization that should be measurable in the next few years. If we don’t see it then, we may never get it. Foregrounds are critical  Measuring n s different from unity also a window on inflation, and may be coming soon as well (perhaps even pre-Planck).  CMB also an excellent laboratory for testing new physics, e.g. cosmic strings, new neutrinos…

pattern shift parameter WMAP1+CBI+DASI+B03 TT/TE/EE Evolution: Jan00 11% Jan02 1.2% Jan03 0.9% Mar03 0.4% EE: , phase check of CBI EE cf. TT pk/dip locales & amp EE+TE CBI+B03+DASI (amp= ) L vs A s CBI+B03+DASI EE,TE cf. CMB TT

EE – A Separate View Excellent check on consistency of stan- dard cosmological model. One example: path- ological primordial spectra with very different params can mimic TT. However, EE changes dra- matically.

Sachs-Wolfe Acoustic Oscillations Drag Damping Curvature Doppler Tensors Reionization Sound & Light in the Early Universe