Lecture 21 : The Cosmic Background Radiationbolatto/teaching/ASTR340_Fall19/lect21-a.pdf · Cosmic Background Radiation ªPenzias & Wilson had serendipitously discovered radiation

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Lecture 21 : The Cosmic Background Radiation

ªMapping of the CBR with COBEªThe results from WMAP &

Planckª Scale dependence of

fluctuations in CBRª Implications for cosmological

parametersReading: Ch. 14 of text © Sidney Harris

Post article about discrepancy in Ho “in context”

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More recent

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After HST “Key Project”

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Quiz

ªThe relative abundances of 4He, 3He, D, and 7Li tell us that

A. Ωmatter ~ 0.30B. Ωbaryons ~ 0.05C. Ωdark_matter ~ 0.25D. Ωdon’t_matter ~ 0.00

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Quiz

ªThe relative abundances of 4He, 3He, D, and 7Li tell us that

A. Ωmatter ~ 0.30B. Ωbaryons ~ 0.05C. Ωdark_matter ~ 0.25D. Ωdon’t_matter ~ 0.00

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Quiz

ªThe SN 1a “standard candles” results tells us that

A. We live in a universe with Ωmatter ~ 0.30B. We live in a universe with k = -1C. We live in a universe that is

acceleratingD. Most galaxies have black holes

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Quiz

ªThe SN 1a “standard candles” results tells us that

A. We live in a universe with Ωmatter ~ 0.30B. We live in a universe with k = -1C. We live in a universe that is

acceleratingD. Most galaxies have black holes

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11/11/19 9Penzias & Wilson (Bell Labs)

THE OBSERVATIONAL DISCOVERY OF THE COSMIC MICROWAVE BACKGROUND

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ªArno Penzias & Robert Wilson (1964-1965)ªPlanning to study radio emissions from our Galaxy

using new, sensitive antenna built at Bell LabsªNeeded to characterize and eliminate all sources

of noiseªCould not get rid of or identify a certain noise

sourceªcharacteristic “antenna temperature” of about 3 K.

ªTests showed that the noise was coming from space, and was approximately the same in all direction -- origin must be cosmic, not Galactic

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Cosmic Background Radiation

ª Penzias & Wilson had serendipitously discovered radiation left over from the early universe!

ª In fact, another group at Princeton U. had been looking for background radiation at the time of the discovery at Bell LabsªRecall that Gamow et al had predicted relic

radiation from hot early universeªJames Peebles independently rediscovered this

ideaªPeebles’s colleagues Dicke, Roll, & Wilkinson

were searching for CBR using receiver they built

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Origin and evolution of CBRª Recall that early Universe was hotª Matter and radiation remained coupled until about

400,000 years after big bang, at z = 1100ª After recombination/decoupling, radiation has been

free-streamingª When we observe the CBR, we are looking at

“surface of last scattering”ª Similar to looking at glowing cloudbank from a distanceª Observed properties of CBR directly trace conditions at z =

1100! ª “cloudbank” is a sphere surrounding us, at radius

corresponding to distance light travels over nearly 14 billion years (in expanding Universe)

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ª Each photon of relic radiation has lost energy as the universe expandedª E=hc/lª l(t)/l(t0)=R(t)/R0=1/(1+z)

ª Relic radiation photons are spread over increasingly large volumeª Vol(t)/Vol(t0)=(R(t)/R0)3=1/(1+z)3

ª Overall, radiation energy density has decreased proportional to R(t)-4 or (1+z)4

ª Radiation temperature changes with time (or redshift), with ª T(t)/T(t0)=R0/R(t)=(1+z)ª Temperature at surface of last scattering would have been

~3000K

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ªOverall shape of radiation spectrum is expected to be unchanged over cosmic time ªShould follow “blackbody” shape ªpeak wavelength in spectrum would vary

proportional to R(t) or (1+z)-1

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ª Relic radiation at present is expected to peak at microwave/radio frequencies, with 411 photons/cubic cm in total

ª Theory generally consistent with ground-based measurements, but atmosphere is opaque over large part of range where CBR is expectedª Need observations from above atmosphereª Through late 1980s, observations were from balloons,

rockets

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THE COSMIC BACKGROUND EXPLORER (COBE)

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2006 Nobel prize in Physics!ªAwarded to John Mather (GSFC & UMD) and

George Smoot (UC Berkeley)“…for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation”

See: http://nobelprize.org/nobel_prizes/physics/laureates/2006/

Note: COBE team involved >1000 scientists and engineers

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ªThe COBE missionªBuilt by NASA-Goddard Space Flight CenterªLaunched Nov. 1989ªPurpose was to survey infra-red and microwave

emission across the whole sky.ªPrimary goals:

ªto characterize the spectrum of the CBRªto measure any variations in the CBR with direction

ªHad a number of instruments on it:ªFIRAS (Far Infra-Red Absolute Spectrophotometer)ªDMR (Differential Microwave Radiometer)ªDIRBE (Diffuse InfraRed Background Experiment)

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ªInstruments measuring at different wavelengths “see” different structures…

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WMAP movie of the sky at different wavelengths

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Our Galaxy observed by the DIRBE instrument on COBE

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ªHigh-frequency DIRBE image of our Galaxyª“Near infra-red” (slightly longer

wavelengths then we can see by eye)ªCan see…ªDisk of our GalaxyªBulge of our GalaxyªDust in our galaxy (which makes the

background stars look red)

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ª Low frequency DIRBE imageªFar infrared (substantially longer wavelengths

than we can see with our eyes)ªLower resolution image than at high frequency ªAlso see Galaxy (not as symmetric)… we’re seeing

active regions.ªAlso see a “band” of light across the image (light

blue)… ªThis is the Zodiacal light: emission of IR by dust

in our solar system.

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ªDIRBE measures strong signal at shorter wavelengths (0.0001cm-0.01cm=1-100µm) than peak emission expected from CBR

ªBy extrapolating this signal to longer wavelengths with same pattern, can help remove any “foreground” of galactic emission from CBR signal at 0.01-1cm (=100 µm -0.01 m) observed with FIRAS

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The spectrum of the CMB (FIRAS)

QuizªWhy is the peak of the CMB at

wavelengths of ~ 0.2 cm?A. Because the Big Bang was coldB. Because the photons have cooled

getting to usC. Because the expansion of the universe

lengthen the wavelengthsD. Because of gravitational redshifting

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QuizªWhy is the peak of the CMB at

wavelengths of ~ 0.2 cm?A. Because the Big Bang was coldB. Because the photons have cooled

getting to usC. Because the expansion of the universe

lengthen the wavelengthsD. Because of gravitational redshifting

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ªSpectrum has precisely the shape predicted by the theory of blackbody radiation (radiation in a thermal equilibrium)!ªShape of “blackbody” spectrum, within

0.03%ªCharacteristic temperature of 2.725K,

within ±0.002K

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Variations in CBRª CBR is expected to be nearly isotropic (same in all directions),

since universe had nearly uniform conditions at epoch of decoupling

ª But universe could not have been exactly uniform then…ª if it had been perfectly uniform, there would be no

galaxies, stars, planets today!ª There must have been small fluctuations in density and

temperature of both matter and radiationª Small fluctuations early on were “seeds” of structure that

would grow laterª If universe was not exactly uniform at surface of last

scattering, there should be small variations in CBR…ª This is what the DMR instrument on COBE sought to measure

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DMR map of the microwave sky

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Raw DMR mapªMap of the microwave sky (frequency of

50GHz, or wavelength of 0.6cm )…ªThis is the CBR over the whole sky!ªThe map is very uniform.ªMeans that the CBR is extremely isotropic

Supports the idea that the Universe is isotropic(one of the basic cosmological principles).

ª In fact, if we measure the universe to be isotropic, and we’re not located at a special place in the Universe, we can also deduce that the Universe is homogeneous!

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Subtract off average level…

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ªWhat causes this “dipole” pattern of redshift and blueshift in CBR?

ª Class discussion!

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Motion of Earth through spaceªDipole pattern is caused by doppler shift of

radiation frequencies because Earth moves relative to mean “cosmic rest frame” of CBR

ª “Blue” direction (higher intensity/energy of CBR) is direction of Earth’s motion

ª “Red” direction is opposite directionªTotal velocity is 390 km/sªComponents:

ªEarth orbits Sun at 30 km/sªSun orbits Galactic center at 220 km/sªMilky Way Galaxy moves inside Local GroupªLocal group is falling into Virgo clusterªVirgo Cluster may have systematic motion

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After removing mean and “dipole”

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Removing galactic “foreground”

Different dependence of galactic dust emission and CBR on frequency can be used to separate the two…

ª Short-wavelength IR from DIRBE is used as spatial “template”

ª scale amplitude for dust emission in DIRBE map to long wavelengths

ª Subtract off from long-wavelength DMR map…

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Resulting map with galactic “foreground” removed

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CBR fluctuations from COBEª “Cleaned” map shows a random pattern of

fluctuations in the CBRª These correspond to temperature differences of 30

millionths of a Kelvin; i.e. fractional perturbations are about 3 parts in 105

ª What are these fluctuations?ª The early universe was very close to being perfectly

homogeneousª But, there were small deviations from homogeneity… some

regions were a tiny bit colder and some were a tiny bit hotter.

ª When matter and radiation decoupled, this pattern of fluctuations was preserved into the radiation field.

ª We see this now as fluctuations in the CBR.

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Fluctuations as “seeds” of structure

ªWhy are the fluctuations important?ªBefore decoupling, fluctuations in the radiation

field also meant fluctuations in the mass densityªAfter decoupling, these small fluctuations in

density can get amplified (slightly dense regions get denser and denser due to gravity).

ªThese growing fluctuations eventually collapse to give galaxies and galaxy clusters.

ªSo, by studying these fluctuations, we are looking at the “seeds” that grow to become galaxies, stars, planets, people, dogs, cats etc.

Quiz

ªWhy does the CMB have a “dipole”?A. Because a side of the Big Bang was

hotterB. Because of the CMB anisotropiesC. Because the galaxy is rotatingD. Because we are moving respect to the

CMB

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Quiz

ªWhy does the CMB have a “dipole”?A. Because a side of the Big Bang was

hotterB. Because of the CMB anisotropiesC. Because the galaxy is rotatingD. Because we are moving respect to the

CMB

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Wilkinson Microwave Anisotropy Probe (WMAP)

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WMAPªNASA mission to map out the fluctuations in

the CMB in fine detail…ªDetermined fine detail of the CMB fluctuations that

depends upon the curvature of space (k) and W.ªCharacterized the seeds for structure formationªMuch higher spatial resolution (0.2°) than COBE (7°)

ª Launched 2001, operated until very recently

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The WMAP satellite in orbit

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WMAP measurements of the microwave background radiation

Images: NASA WMAP project

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WMAP’s map of the CMB

The microwave light captured in this picture is from 379,000 years after the Big Bang, over 13 billion years ago!

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Cosmic lumps: the power spectrum