CMB Theory, Observations and Interpretation Danielle Wills Seminar on Astroparticle Physics 14 May 2010 Physikalisches Institut Rheinische Friedrich-Wilhelms-Universität Bonn
Outline of what will follow This is a generalist talk, not a specialist Section 1: What is the CMB, how is it formed, how do we characterize it? Mainly theory Section 2: What can we learn from the CMB? Standard observations and interpretations Section 3: Can we trust what we learn? Non-standard observations and a few very non-standard interpretations (just for fun).. But with a serious point behind it all! I hope to show that the CMB is far from being a closed-case subject, and that there are still unexplored observational routes, unaccounted for anomalies, and hints of many more surprises yet to come
Prediction and Discovery Fossil record of earliest structure in the universe Snapshot of local properties at recombination z 1100 (density, peculiar velocity, total gravitational potential) related to primordial perturbations so encode information about the perturbations themselves, the matter composition and geometry of the universe In the 1940s Gomav estimated the temperature at which deuterium could form in the early universe, and from this predicted the temperature of the thermal background today Discovery made by accident in 1960s by Penzias and Wilson as they experimented with a new supersensitive microwave antenna
Thermal Spectrum and the Horizon Problem Primordial universe photons and baryons tightly coupled via Thompson scattering photon-baryon fluid Thermalizing processes: double Compton scattering and thermal Bremmstrahlung Effective only while rate of reactions > rate of expansion Problem! Horizon size = age of universe speed of light = 380 000 ly radius at recombination most of sky not in causal contact COBE: perfect black body spectrum at 2.726K e + γ e + 2γ e + p e + p + γ Kinney 2002
Inflation Solves horizon problem universe was once in causal contact (thermal spectrum was formed), then inflation drove all regions out of casual contact As time continues, horizon expands and regions re-enter causal contact Inflation predicts: Scale invariance: fluctuations on all size scales look the same Gaussianity: initial conditions were like white noise BUT current models of inflation allow small deviations from these Hu 2008 Statistical Isotropy: universe is homogenous and isotropic in terms of matter as well as statistics
The last scattering surface Universe expands and cools Recombination: At 380,000 years, temperature has dropped to ~ 4000K photon energies drop below the binding energy of hydrogen Electrons and protons combine Universe becomes neutral (no free electrons) Photons and baryons decouple at the Last Scattering Surface (LSS) Photons are no longer scattered and free stream across the universe At low resolution we see uniform glow in all directions across the sky, and on higher resolution we see tiny µk fluctuations imprinted on this glow
Decomposing the Cosmic Microwave Sky Measure temperature differences on each angular scale, find mean for each angular scale Expand as a sum of spherical harmonics and calculate coefficients Take statistical average for each coefficient and plot this against multipole number l Challinor & Peiris 2009 T T l,, l 1 l m l a lm Y m We plot cl a lm 2
Components of harmonic sequence Monopole: 2.726K BB Dipole: due to motion relative to CMB 400km/s Multipoles: CMB PS starts! ILC, l = 2 l = 3,4 l = 5,6 l = 7,8
Acoustic Oscillations FUNDAMENTAL gives SIZE OVERTONES give ORIGIN Takahashi 2002 Hu 2008
The Power Spectrum Observational power spectrum and theoretical power spectrum for standard ΛCDM model in excellent agreement Features of the spectrum: Komatsu et al. 2010 Integrated Sachs Wolfe rise from low redshift dark energy dominance Sachs Wolfe Plateau frozen part of primordial power spectrum waiting to reenter the horizon Damping tail LSS has some thickness, anisotropies on scales smaller than this thickness are damped Scott & Smoot 2004 Tensors imprint of gravitational waves more on this later!
Summary of ΛCDM Cosmological Parameters from 7-yr WMAP Komatsu et al. 2004 Other parameters measure deviations from simple ΛCDM model, constraining neutrino mass, curvature, gravity waves, parity violation, inflation scenarios.
Playing with Parameters, n s and Ω Λ n s 1.1 Ω Λ 100% Flatness 1.00 Age 13.7 Ga Flatness 1.26 Age 15.2 Ga n s 0.65 Ω Λ 4% Flatness 1.00 Age 13.7 Ga Flatness 0.30 Age 11.3 Ga Flatness depends on the density parameters Age of the universe depends on the Hubble constant and the density parameters
Matter density and the relation between the Peaks First peak gives curvature Ratio of odd/even peaks, eg 1 st /2 nd tells us about the density of baryons (baryons enhance compressions) Ratio of 2 nd /3 rd peak tells us about the density of dark matter Radiation era: higher amplitudes from decaying potentials that drive oscillations Challinor & Peiris 2009 Dodelson 2003
Secondary Anisotropies Integrated Sachs Wolfe effect constrains dark energy CMB photons gain energy as they enter a gravitational potential and keep some on the way out CMB photons lose energy on the way into a void and do not get it all back on the way out Thermal Sunyeav Zelovich effect constrains cluster gas fractions, universal baryonic fraction CMB photons gain energy as they travel through a cluster via inverse Compton scattering in the X-ray gas, spectrum shifted to higher frequency ACBAR
Constraining inflation: Features of Primordial fluctuations Amplitude: Fluctuations etched in CMB are just the right amplitude to produce the large scale structure we observe today, if we assume dark matter Scale-invariance: Nearly scale-invariant, consistent with a slowly rolling inflation field and a slowly varying Hubble parameter during inflation (Kinney 2003) (scale invariance of density perturbations described by n s parameter gives tilt of power spectrum). Gaussianity: Gaussian statistics to at least 1 part in 1000 (inflation predicts nearly Gaussian distribution). Non-gaussianity NOT contained within power spectrum must consider higher-order correlation functions. Can probe eg interactions of field(s) driving inflation. Non-gaussianity from violation of any of the following conditions: Single field, canonical kinetic energy, slow roll, initial vacuum state (Non-gaussianity described by f NL function measures amplitude of second order correction to a Gaussian primordial field.)
Can we learn more? YES! Gravitational Waves! Direct signal of physics that occurred at 10-35 seconds after the Big Bang Inflation generated gravity waves in space inflation predicts these ripples These leave imprint on the CMB Amplitude of gravity wave is proportional to expansion rate H, H is proportional to the energy of inflation squared A detection would : Eliminate alternatives to inflation Pinpoint energy scale of inflation Give hints about symmetries of new physics at ultra-high energy scale (Carroll et al. 2009)
So far we have looked only at temperature anisotropies, but CMB contains polarization anisotropies too we are just beginning to explore these! CMB photons can become polarized by Thompson scattering: Radiation from one direction causes polarization Isotropic radiation No net polarization Dipole anisotropic radiation No net polarization Quadrupole anisotropic radiation Net polarization
Producing the Polarization Signal Small window for polarization plasma must be optically thin enough, and there must be enough free electrons around! Processes that form quadrupoles in the plasma: E-modes from scalar or tensor δg µν = h µν scalar + h µν vector + h µν tensor B-modes from vector or tensor
Polarization Power Spectra E-modes are produced by: CMB primordial temperature anisotropies Scattering after reionization TE spectrum: evidence for adiabatic perturbations, constraints on thickness of LSS B-modes are produced by: Gravity waves during inflation Lensing of E-modes by large scale structure First detection of E-modes by Dasi in 2002 confirmed by WMAP1 B-mode signal still not detected by WMAP7 signal very weak Hu & Donelson 2002
CMB anomalies: lack of large scale power Copi et al. 2010 The Angular Correlation Function measures structure in real space rather than Fourier space: We see lack of correlated structure for angles >60 Possible interpretations: There is no large scale power could tell us about geometry/topology of spacetime (eg Dodecahedral space topology, Luminet 2003) Low multipoles are cancelling each other (Copi et al. 2010) violates statistical isotropy as the C l s no longer independent (this could be a broken symmetry) We live in an extremely unlikely realization of a ΛCDM universe (Important to note: this is not a violation of the ΛCDM model) More skeptical: foreground effects, poor choice of statistics
More anomalies: The Axis of Evil Is there a preferred direction in the CMB fluctuations? Could signal anisotropic expansion, non-trivial cosmic topology or an intrinsically inhomogeneous universe (Land et al. 2005) Alignment of quadrupole and octopole inconsistent with Gaussian, statistically isotropic skies to the 99% c.l. (Copi et al. 2010) Alignments of low multipoles and solar system features even worse violates Copernican principle and in fact provides evidence for the Ptolemaic system! Foreground contamination?
From the Ptolemaic to the Creator: the problem with looking for patterns It was noted, when the WMAP sky maps were first released, that the initials of Stephen Hawking appear in the CMB temperature map The letters are: Roughly the same font size Roughly the same font Aligned neatly along a line of fixed Galactic latitude Bennett 2010: This highlights the problem with a posteriori statistics often employed to study CMB data Serious take home point: If something as unlikely as Hawking's initials can be found in the CMB, then the chances of finding other unlikely patterns may also be quite high On the other hand, if a serious anomaly persists, it should be used to improve and expand our theories
Conclusions The CMB is a rich data source that: Continues to provide us with tighter constraints on ΛCDM and gives insight into physics beyond the simple standard model Offers new possibilities for pushing our observational grasp further back into the early universe Prompts us to search for new improvements and revisions to the current model BUT, we should be about careful how we read the data (in particular, the assigning of statistics), and this is a lesson for observational science with diverse data sets in general
Thank you Any Questions?
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