Presentation on theme: "The CMB and the SZ Effect"— Presentation transcript:
1 The CMB and the SZ Effect Current Topics 2008Dr. Katy Lancaster
2 Course Content Introduction Science from the SZ effect Practicalities The Cosmic Microwave BackgroundThe Sunyaev Zel’dovich EffectScience from the SZ effectThe Hubble constantGas fractionsNumber countsPracticalitiesTelescopesIssues
3 Lecture 1 A brief history Production of the CMB Production of primary anisotropiesThe Sunyaev Zel’dovich effect
4 Brief history lesson 1950s, two conflicting cosmological theories Steady State theoryUniverse always has been and always will be in a static, homogenous state.Expanding UniverseHubble’s observations, 1929Gamov - realised that the Universe was once extremely dense and hot, thus must have since expanded and cooled
5 Penzias and Wilson1965, were making sensitive observations of microwave emission from the galaxyDetect ‘annoying level of static’ in all directionsAt the same time, Dicke at Princeton predicted the existence of ‘relic’ radiation from the Big BangNobel Prize, 1978
6 COBE NASA satellite launched late 80s ‘Cosmic Microwave Background radiation’ was found to have a perfect blackbody spectrumImplies Universe was once isothermal. Only Big Bang models predict this.For this (and other reasons), the COBE scientists won the Nobel prize in 2007
7 Primordial Universe Early Universe: Devoid of structure. T > 4000K, entirely ionised - ‘sea’ of electrons, protons, helium nuclei and photonsToo hot for atoms to formPhotons repeatedly Thomson scatter from electrons, unable to propagate freelyAlmost perfect thermal equilibrium due to the ‘coupling’ of matter and radiationPredicted and required by Big Bang modelsUniverse is ‘opaque’
8 ThermalisationAs COBE discovered, the CMB has a perfect blackbody spectrum.Two contributory processes during the first year after the Big Bang, each creates/destroys photons:At very early times, thermal Bremstrahlung radiation / absorption: e+pe+p+. This ceased as the Universe cooled. Produces thermal spectrumLater, double Compton scattering: e+ e+2.Only effective while collision rate > expansion rateSince then there has been no process capable of destroying the spectrum (although there may be tiny distortions)
9 RecombinationEarly Universe filled with free electrons, photon mean- free path small (Universe in thermal equilbrium, ‘opaque’)Impossible for any information about this time to be communicated to us via radiationTemperature of Universe falls to ~4000K, very few photons with energy > Hydrogen binding energy, 13.6eVElectrons and protons combine: e+p H+Vast majority of free electrons disappear. Universe now neutral, photons can free-streamIn fact recombination happens over a time or redshift ‘slice’ (1500 > z > 1200 ) rather than instantaneously.
10 Surface of last scattering The CMB photons were emitted at the same time, and thus underwent their final scattering event at the same timeAll CMB photons move at the speed of light, have travelled the same distance since this timeWe can think of the CMB as being emitted from a fictitious spherical surface, of which we are at the centreLike observing the surface of the sun, although it is the outer reaches (or rather, the very early Universe) that we can not observe, rather than the inner workingsStrictly speaking, recombination is not instantaneous, so we sometimes talk about the ‘thickness’ of the surface
12 The CMB todayThe CMB photons have been significantly redshifted by the Hubble expansion.Photon wavelengths have increased by R(t)=1/(1+z)CMB temperature falls as 1/R(t): a specific prediction of the Big Bang modelTemperature of the CMB today T0= T/(1+z) = 2.73KCan test the relation at other redshifts by observing stellar line emission from CN moleculesActually first measured in circa 1940 but not identified as the CMB until much later!
13 Observing the CMBUniform high energy glow - the sky is not dark at radio frequencies
14 The Dipole Doppler shift introduces ‘hot’ and ‘cold’ regions The Local group is moving at 400km/s relative to the CMB!Also see annual modulation due to Earth’s orbit
15 Primary FluctuationsThe CMB appears isotropic (same temperature everywhere) unless we look very carefullyInitial isotropy actually slightly distorted during / before recombinationWe observe temperature variations, referred to as ‘primordial anisotropies’ or ‘fluctuations’ (see pic)We measure the temperature difference in two directions separated by some angle .Take many measurements and find the mean value for a particular angular scaleAll CMB anisotropies are characterised in this way
16 WMAP, monopole, dipole and galactic emission removed K in the presence of 3K background
17 Primary FluctuationsThe CMB appears isotropic (same temperature everywhere) unless we look very carefullyInitial isotropy actually slightly distorted during / before recombinationWe observe temperature variations, referred to as ‘primordial anisotropies’ or ‘fluctuations’ (see pic)We measure the temperature difference in two directions separated by some angle .Take many measurements and find the mean value for a particular angular scaleAll CMB anisotropies are characterised in this way
18 Measure size of temperature difference for a range of Plot against : ‘Power Spectrum’
21 Sachs Wolfe EffectQuantum fluctuations in the dark matter distribution led to density inhomogeneitiesThese developed under gravityA CMB photon released from a region with a non-zero gravitational potential will experience an additional redshiftIt has to ‘climb out’ of the potential wellThis creates ‘power’ on the scales > 1
22 Acoustic Oscillations Over-dense regions in dark matter amplified during inflation, collapse under gravityBaryons falls into the resulting potential wellsRadiation pressure increases as the material collapsesEventually the pressure overcomes gravity and causes an expansion….…..expansion continues until gravity wins again
23 Acoustic Oscillations This was taking place at recombinationOscillations were happening on all scales. Largest scale: sound horizon. Other scales were not ‘causally connected’.Modes which had reached their extrema by recombination produced enhanced features in the CMBCompressions - hot spots. (Recombined slightly later, thus suffered less cosmological redshifting)Rarefactions - cold spots. (Recombined slightly earlier)
24 Acoustic Oscillations Oscillating modes form harmonic sequence: Largest regions had diameter of the sound horizon, next largest were half this size etcOscillation frequencies corresponded to this: largest region oscillates at half the speed of the next largest etcFirst peak: region which had time to compress exactly once before recombinationSecond peak: region which had time to compress and rarefy, ie one full oscillation before recombiningThird peak, fourth peak….
25 Doppler shifts Also related to the acoustic oscillations At times inbetween the extrema of expansion for each oscillation region, the motion of the fluid reached its maximum velocityThis resulted in a Doppler shift of the photons released when the plasma recombinedThis contriubutes power inbetween the locations of the acoustic peaks: the power spectrum does not go to zero
26 Silk DampingOn the smallest scales, the effect of photons ‘escaping’ from the oscillating region becomes importantThe loss of these photons ‘damps’ the power on the smallest angular scales
27 What can we learn?The power spectrum is a complicated function which depends on the values of the various cosmological parameters: H0, ΩM, Ωb, Ω, Ωk, zre, t0….and many more.We observe the CMB and then try fitting the powere spectrum to the data. We tweak the parameters to find the best fit.And hey presto, we have our very own measurement of the cosmological model
29 Galaxy formationThe anisotropies in the CMB are widely regarded as imprints of the ‘seeds’ of structure formationThat is, those oscillating regions from the early Universe grew and developed under gravity into the stars and galaxies we see todayIf we take the CMB and compare it to observations of large scale structure, we can constrain structure formation scenarios
31 Secondary Anisotropies Majority of CMB photons have travelled through the unimpeded since last scatteringHence observed power spectrumSome have interacted with ionised matter on their path towards usThis imprints structures on the observable CMB - ‘Secondary Anisotropies’Also contribute to the power spectrum
32 Sources of anisotropy Integrated Sachs-Wolfe effect Gravitational lensingRees-Sciama effectOstriker-Vishniac effectCosmic stringsSunyaev Zel’dovich effect - by far the largestMany more postulated…..
34 Galaxy ClustersRich Clusters - congregations of hundreds or even thousands of galaxiesSee cluster galaxies and lensing arcs in the opticalBut only around 5% of a cluster’s mass is in galaxiesMost of the mass is in Dark MatterBut a sizable fraction is found in baryonic gas......
35 Chandra Image of the Coma cluster X-rays - see hot gasvia Bremstrahlung emission10-30% of total massChandra Image of the Coma cluster
36 Cluster Gas Clusters of galaxies have masses ~ 3x1014M Deep potential wells, gas temperatures ~7keVIonised and energeticConstitutes ~30% of the cluster massGas characteristics may reflect those of the Universe as a whole - interesting to study
37 Compton ScatteringCompton scattering: Photon loses energy on interacting with matterInverse Comptin scattering: Photon gains energy on interacting with matterIn the SZ effect: low energy CMB photon scatters from high energy cluster electronPhoton energy is boosted
38 SZ Effect basics CMB photons incident on a galaxy cluster Scattering probability is smallThose which do collide receive energy boost due to inverse Compton scatteringSpectrum shifted to higher frequencyDecrement - null - incrementNeed a new name for this?
39 Optical DepthFor a cluster atmosphere with electron density ne(r), the optical depth for scattering along a particular line of sight is:Where Tis the Thomson cross sectionThe cluster gas is optically thine<<1, ie the probability of scattering is small
40 ComptonisationThe degree to which the CMB is affected by inverse Compton scattering is described by the Comptonisation parameter:Or for the isothermal approximation (often employed in the past):
41 Brightness Temperature Often used in Radio / CMB astronomyDefined as: ‘The temperature of a blackbody that would be observed with the same intensity as the observed source, at a particular frequency’From the Planck law:For the low frequency Rayleigh-Jeans region:
42 Temperature Decrement The change in the brightness temperature of the CMB due to the thermal SZ effect is given by:Where the frequency dependence is given by:For the Rayleigh Jeans region:
43 Intensity Change In units of specific intensity: With frequency dependence given by:
45 Kinematic SZ EffectAdditional spectral distortion caused by cluster velocity component along line of sight, zCollective motion of cluster gas modifies CMB spectrum via Doppler shiftObserve decrement:Frequency dependence:
46 SZ Intensity Spectra g(x), h(x) Thermal: decrement, null, increment Kinematic: Near maximum at the thermal null
47 The KSZ effect is < 10% of the thermal effect at low freq. Thermal vs KinematicSpecific intensity changes:Spectral dependence similar at low freq.i.e for a typical cluster:Proportionalities, typical values. Highlight this!The KSZ effect is < 10% of the thermal effect at low freq.
48 Decrement - Null - Increment ACBAR produced these nice images of a galaxy cluster at 150, 220 and 275 GHzMulti-frequency observations useful for eliminating primordial CMB contamination (as well as detecting the kinematic effect)
49 Summary 1 CMB - blackbody spectrum, primordial features Fit ‘power spectrum’ to deduce values of the cosmological parametersCMB photons incident on a galaxy cluster may be inverse Compton scattered by hot gasThis ‘Sunyaev Zel’dovich Effect’ manifests itself as a decrement - null - increment depending on observing frequencyCluster peculiar velocity also modifies the radiation via the smaller ‘Kinematic’ effect