Background The Cosmic Microwave

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The Cosmic Microwave Background

Tomi Ylinen KTH/HIK

KTH 5A5461

Experimental Techniques in Particle Astrophysics

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Outline

• Introduction

• Theory

• Detection

• Case studies: COBE, WMAP

• The future: Planck

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Introduction

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?

Introduction

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History

• 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).

• Received the Nobel Prize in 1978

Introduction

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http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html

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Once upon a time…

• 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

Theory

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W. Hu and M. J. White, "The Cosmic Symphony", Sci. Am., 290N2, (2004) 32

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Mathematically speaking

• The anisotropies in the CMB sky can be described by a spherical harmonic expansion

• 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

Theory

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Mathematically speaking (2)

• 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 C^T, C^E, C^B and C^TE

Theory

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Monopole

• 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

• Measured using absolute temperature devices

Theory

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http://www.astro.ucla.edu/~wright/CMB.html

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Dipole

• 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

Theory

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http://map.gsfc.nasa.gov/m_mm/ob_techcal.html

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Higher-order multipoles

• 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 devices

Theory

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Fundamental wave, largest

variations

Overtones Sharp cut-off due to wave

dissipation (λ < x_mean)

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Anisotropies

• 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

Theory

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Detection

• 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

Detection

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Heterodyne detection

• A horn receiver working like an antenna picks up the radiation 

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)

Detection

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http://lambda.gsfc.nasa.gov/product/cobe/COBE_gallery.pdf

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Incoherent detection

• 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_signal and a bias power P_bias. The temperature of the absorber is then T = T₀ + (P_signal + P_bias)/G

• The energy of an incoming photon is

determined by measuring the temperature increase it causes to the absorber.

Detection

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http://www.planck.fr/article227.html

http://bolo.berkeley.edu/bolometers/introduction.html

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Polarization

• Polarization in the CMB can be measured using a polarization

sensitive bolometer, with two layers of absorbers corresponding to perpendicular polarization directions

Detection

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http://www.planck.fr/article228.html

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Complications

• 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

Detection

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http://www.rssd.esa.int/SA/PLANCK/docs/Bluebook-ESA-SCI%282005%291_V2.pdf

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Case studies

• 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

Case studies

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COBE

• 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

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COsmic Background Explorer

Case studies

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COBE instruments

• 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.

19/34 Case studies

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COBE instruments

• 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

20/34 Case studies

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COBE instruments

• 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.

– The vibrating beam interrupter allows for continuous comparison between the sky and a cold zero-flux surface inside

the instrument

21/34 Case studies

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WMAP

• 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

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Wilkinson Microwave Anisotropy Probe

Case studies

http://map.gsfc.nasa.gov/m_ig.html

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WMAP instruments

• Basically the same idea as in COBE

23/34 Case studies

Credit: WMAP

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Results so far

• Anisotropy map after combining the different frequencies and thereby being able to

subtract the foreground radiation (our Galaxy)

• An example of a polarization map measured at 23 GHz.

Color indicates strength.

Most of the polarization comes from our Galaxy

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http://map.gsfc.nasa.gov/m_mm.html http://wmap.gsfc.nasa.gov/m_or.html

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Results so far (2)

_25/34Case studies

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Results so far (3)

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T

TE

E

B Average levels

for foreground model BB lensing signal

L. Page, et.al., ”Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis ”, ApJS, 170, (2007) 335

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The future: Planck

• Planned launch in July 31, 2008 + ε

• Will measure the anisotropies in the CMB with unpresedented sensitivity (ΔT/T ~ 2 · 10^-6) and angular resolution (5’)

The future: Planck

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http://www.rssd.esa.int/SA/PLANCK/docs/Bluebook-ESA-SCI%282005%291_V2.pdf http://astro.berkeley.edu/~mwhite/rosetta/node3.html#SECTION00030000000000000000

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Planck resolution

The future: Planck 28/34

Simulated skymaps

5°

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Planck resolution

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Instruments

• An off-axis telescope with diameter 1.5 m and two cryogenic instruments, LFI and HFI, shielded by

baffles

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Planck LFI

• An array of receivers based on so-called HEMT amplifiers, covering the frequency range 30-70 GHz and operating at 20 K

• All LFI channels can measure

polarization and intensity

The future: Planck

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Planck HFI

• An array of receivers based on bolometers, covering the frequency range 100-857 GHz and operating at 0.1 K

• Four channels can measure polarization

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Summary

• Accurate measurements of the Cosmic Microwave

Background can reveal a vast variety of properties about the universe, such as the composition, age and geometry

• To measure the tiny temperature variations, band filters,

interferometers, bolometers, transistors and diodes are used

• The field is highly active, with successful experiments and better ones coming up soon in the form of Planck

Summary

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References

^References_34/34

Mission homepages

COBE http://lambda.gsfc.nasa.gov/product/cobe/

WMAP http://wmap.gsfc.nasa.gov/

Planck http://www.rssd.esa.int/index.php?project=Planck Articles

W. Hu and M. J. White, "The Cosmic Symphony", Sci. Am., 290N2, (2004) 32

W.-M. Yao, et al., "Review of Particle Physics", J. Phys. G33, (2006) 1 C.L. Bennett, et al.,” First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Emission”, ApJS, 148, (2003) 97

G. Smoot, et al., ”COBE Differential Microwave Radiometers:

Instrument Design and Implementation”, ApJ 360, (1990) 685-695 N. W. Boggess, et al., ”The COBE Mission: Its Design and

Performance Two Years after launch”, ApJ 397, (1992) 420-429 L. Page, et.al., ”Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis ”, ApJS, 170, (2007) 335

”Planck: The Scientific Programme”, ESA-SCI(2005)1,

http://www.rssd.esa.int/SA/PLANCK/docs/Bluebook-ESA- SCI%282005%291_V2.pdf

Books

M. Lachièze-Rey & Edgard Gunzig, ”The Cosmological Background Radiation”, Cambridge University Press (1999)

C. H. Lineweaver et al., ”The Cosmic Microwave Background”, NATO ASI Series, Vol. 502, Kluwer Academic Publishers

Internet

http://bolo.berkeley.edu/bolometers/introduction.html http://www.planck.fr/article227.html

http://scienceworld.wolfram.com/physics/PlanckLaw.html http://lambda.gsfc.nasa.gov/links/experimental_sites.cfm http://astro.berkeley.edu/~mwhite/rosetta/