The Cosmic Microwave Background Tomi Ylinen KTH/HIK KTH 5A5461 Experimental Techniques in Particle Astrophysics Outline • Introduction • Theory • Detection • Case studies: COBE, WMAP • The future: Planck Introduction Introduction 3/34 Why bother? • Measurements of the Cosmic Microwave Background (CMB) allow for precise estimations of the age, composition and geometry of the universe • What is the universe made of? How old is it? And where did objects in the universe, including our planetary home, come from? T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Introduction History 4/34 • First discovered by Arno Penzias and Robert Wilson of AT&T Bell Laboratories in 1965, when trying to remove a weird background noise in their radio antenna (they thought it was bird crap). http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html • Received the Nobel Prize in 1978 T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Once upon a time… 5/34 • Early universe composed of a plasma of charged particles and photons • After 380 000 years of cooling, first atoms formed and the universe became transparent to photons W. Hu and M. J. White, "The Cosmic Symphony", Sci. Am., 290N2, (2004) 32 T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Mathematically speaking 6/34 • The anisotropies in the CMB sky can be described by a spherical harmonic expansion T, almYlm , lm • Observations can be divided into three categories: – Monopole (a00): the mean temperature of the CMB – Dipole (l=1): the anisotropy caused by the movement of the solar system relative to the CMB – Higher-order multipoles (l≥2): anisotropy caused by perturbations in density in the early Universe T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Mathematically speaking (2) 7/34 • Many models of the early Universe say that the temperature anisotropies should obey Gaussian statistics All statistical properties of the temperature anisotropies can be computed from a single function of multipole index l, the power spectrum • Thomson scattering of anisotropic radiation at last scattering gives rise to ~5% polarization in the CMB This gives two measurable quantities called the Stokes Q and U parameters These can be decomposed into E- and B-type polarization patterns • The temperature anisotropies can then be characterized by four power spectra CT, CE, CB and CTE T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Monopole 8/34 • Due to the expansion of the universe, the photons have cooled from an initial black-body distribution at 3000 K to a present value of about 2.725 ± 0001 K 2hv3 1 Iv c2 hv e kT 1 • Measured using absolute temperature devices http://www.astro.ucla.edu/~wright/CMB.html T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Dipole 9/34 • Anisotropy with an amplitude of 3.358 ± 0.017 mK, caused by the fact that Earth, our Solar system and Galaxy is moving relative to the CMB. • Can be used for calibration http://map.gsfc.nasa.gov/m_mm/ob_techcal.html T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Higher-order multipoles 10/34 • The temperature variance as a function of the sizes of the hot and cold spots, i.e. the power spectrum, fully characterizes the anisotropies • From this plot a vast variety of information about the early universe can be extracted • Measured using differential temperature Fundamental Overtones Sharp cut-off devices wave, largest due to wave variations dissipation (λ < xmean) W. Hu and M. J. White, "The Cosmic Symphony", Sci. Am., 290N2, (2004) 32 T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Theory Anisotropies 11/34 • Inflation in combination with quantum fluctuations triggered soundwaves in the primordial plasma, which much like a musical instrument had a fundamental wave along with a series of overtones • After recombination, the density anisotropies were frozen into the cosmic microwave background radiation W. Hu and M. J. White, "The Cosmic Symphony", Sci. Am., 290N2, (2004) 32 T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Detection Detection 12/34 • What do we want to detect? – Temperature (energy) of the CMB – Anisotropies in the CMB temperature at different scales – Polarization of the CMB • How can we detect them? – Heterodyne detection Incoherent detection – Detectors pointed in different directions – Polarization sensitive detectors T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Detection Heterodyne detection 13/34 • A horn receiver working like an antenna picks up the radiation http://lambda.gsfc.nasa.gov/product/cobe/COBE_gallery.pdf The pulse is mixed with a different frequency from a local oscillator The output (IF = Intermediate Frequency) is finally fed through a diode which converts the pulse into a proportional voltage • Examples are Dicke-receivers (COBE) and HEMT-based (High Electron Mobility Transistor) detectors (WMAP) T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Detection Incoherent detection 14/34 • Consist of an absorber of heat capacity C, which is connected via a weak thermal link, G, to a heat reservoir with a constant temperature T0 Bolometer • The absorber is exposed to the power of incoming light P and a bias power P . signal bias http://bolo.berkeley.edu/bolometers/introduction.html The temperature of the absorber is then http://www.planck.fr/article227.html T = T0 + (Psignal + Pbias)/G • The energy of an incoming photon is determined by measuring the temperature increase it causes to the absorber. T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Detection Polarization 15/34 • Polarization in the CMB can be measured using a polarization sensitive bolometer, with two layers of absorbers corresponding to perpendicular polarization directions http://www.planck.fr/article228.html T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Detection Complications 16/34 http://www.rssd.esa.int/SA/PLANCK/docs/Bluebook-ESA-SCI%282005%291_V2.pdf • Foregrounds Microwave emission from our Galaxy and from extragalactic sources through synchrotron, bremsstrahlung and dust emission. Observations at several frequencies enable separation • Secondary anisotropies Gravitational lensing, patchy reionization and the Sunayaev-Zel’dovich effect, i.e. Inverse Compton scattering of the CMB photons by a hot electron gas, which gives spectral distorsions • Higher-order statistics Most of the CMB anisotropy information is contained in the power spectra, but weak signals are present in higher-order statistics, which can measure any primordial non-Gaussianity in the perturbations T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Case studies Case studies 17/34 • Choosing to take a closer look at: COBE • WMAP • Planck • Other experiments: ACBAR • ACME/HACME • ACT • AMI • AMiBA • APACHE • APEX • ARCADE • Archeops • ARGO • ATCA • BAM • BaR-SPOrt • BEAST • BICEP • BIMA • BOOMERanG • CAPMAP • CAT • CBI • CG • Clover • COSMOSOMAS • DASI • EBEX • FIRS • KUPID • MAT • MAXIMA • MBI-B • MINT • MSAM • PIQUE • POLAR • POLARBeaR • Polatron • Python • QMAP • QMASK • QuaD • QUIET • RELIKT-1 • SK • SPOrt • SPT • SuZIE • SZA • Tenerife • TopHat • VSA T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Case studies COBE COsmic Background Explorer 18/34 • Operational 1989-1993 • Carried three instruments: FIRAS, DMR, DIRBE • Sensitivity ΔT/T ~ 10-5 Angular resolution ~7° • John Mather and George Smoot received the Nobel Prize for this in 2006 T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Case studies COBE instruments 19/34 • Far Infrared Background Experiment (FIRAS) – A polarizing Michelson-interferometer, designed to obtain a precision measurement between the CMB spectrum and a Planckian calibration spectrum. The energy was measured by bolometric detectors. http://lambda.gsfc.nasa.gov/product/cobe/COBE_gallery.pdf T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Case studies COBE instruments 20/34 • Differential Microwave Radiometers (DMR) – Designed to detect the temperature differences in the CMB. The receiver input is alternately connected to two separate antennas pointing in different directions in the sky – If the two parts of the sky differ in brightness, the signal will change when the switch moves from one antenna to the other – To show that the differences come from the sky and not from the differences in the antennas, the whole apparatus is rotated T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Case studies COBE instruments 21/34 • Diffuse Infrared Background Experiment (DIRBE) – An off-axis Gregorian telescope, designed to make an absolute measurement of the spectrum and angular distribution of the diffuse infrared background. http://lambda.gsfc.nasa.gov/product/cobe/COBE_gallery.pdf – The vibrating beam interrupter allows for continuous comparison between the sky and a cold zero-flux surface inside the instrument T. Ylinen The Cosmic Microwave Background KTH 5A5461 1 October, 2007 Case studies WMAP Wilkinson Microwave Anisotropy Probe 22/34 • Operational 2001-present • Carries dual back-to-back Gregorian telescopes that feed 20 differential polarization sensitive radiometers • Sensitivity ΔT/T ~ 35 . 10-6 Angular resolution ~15’ • 45 times better sensitivity and 33 times better angular resolution than COBE http://map.gsfc.nasa.gov/m_ig.html T. Ylinen The