history & data - yale astronomy · = -270.3 c most accurate planck curve ever measured after...
TRANSCRIPT
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Prof. Frank van den BoschDept. of Physics & Astronomy
The Cosmic Microwave Background:Listening to the Symphony of the Big Bang
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History & Data
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The Story of Discovery
Jim Peebles (Princeton)
1965: Penzias & Wilson serendipitously discovered CMB while testing Bell Lab’s horn-antenna on Crawford Hill
at Holmdell, New Jersey. Were awarded 1978 Nobel Prize in Physics
Arno Penzias & Robert Wilson
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http://www.youtube.com/watch?v=PH54cp2ggFk&feature=related
Roughly 1 percent of the static on your TV is CMB!!!
Observing the CMB: do it yourself...
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TextText
2.7K = -454.5 F = -270.3 C
Most Accurate Planck Curve ever measured
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After removal of contribution of Milky Way
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The CMB Power Spectrum
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Observations in anExpanding Universe
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Light and other radiation reaches us in form of photons. Photons are characterized by a wavelength, which reflects their energy.Due to expansion of Universe, photons get redshifted (become less energetic).
Cosmological Redshift
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observerhorizon (=visible part of universe)
grid of space (expanding)
Shortly after the Big Bang
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Some time later...
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and again some time later....
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event
Shortly after Big Bang, an event happens 3x3 grid cells away.....
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Light from event is on its way....
light from event
horizon of observer
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Event is finally observed...
light from event
horizon of observer
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All events that happened simultaneous on red circle, are observed simultaneous
horizon of observer
event 1
event 2
event 3
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grid of space (expanding)
What we observe today of what happened 8 Billion yrs after Big Bang (z=1)
What we observe today of what happened 380.000 yrs after Big Bang (z=1100)
What we observe today of what happened at the Big Bang (z=infinity)
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What an observer at Earth would see of CMB 8 Billion yrs ago
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What an observer at Earth would see of CMB 8 Billion yrs ago
CMB is hotter (T=5.4K rather than 2.7K) & originated from closer to Earth
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Physics of the CMB
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There comes a time, when photons are so energetic thatthey destroy hydrogen atoms: H --> p + e
At earlier times, photons interact strongly with free electrons (Thomson scattering), while electrons have Coulomb interactions with free protons: photon-baryon fluid.
In photon-baryon fluid, mean free path of photons is tiny;photons are trapped, and can’t get `out of box’
Going back in time, the Universe becomes denser & hotter.
In present day Universe, mean free path of photons exceeds horizon; average photon traverses visible Universe without interaction (which is why we can see galaxies etc.)
Turning Back the Clock...
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Shortly after Big Bang, Universe consists of dark matter anda photon-baryon fluid; photons are trapped.
About 380.000 yrs after Big Bang, Universe has cooled to the pointwhere electrons & protons combined to form Hydrogen atoms: sudden increase in mean free path of photons --> CMB
Recombination
to observer
time
Recombination; the Origin of CMB Radiation
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At recombination T ~ 3000K. Because of expansion of Universe, photons have been redshifted to T ~ 3 K.
The CMB are photons from the last scattering surfaceat a redshift of z~1100, when the Universe recombined.
The Last Scattering Surface
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At recombination T ~ 3000K. Because of expansion of Universe, photons have been redshifted to T ~ 3 K.
The CMB are photons from the last scattering surfaceat a redshift of z~1100, when the Universe recombined.
Last Scattering Surface is like photosphere of Sun; no information from before recombination can reach us
(except for neutrinos & gravitational waves)
The Last Scattering Surface
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The origin of the dipole in the CMB is thethe Doppler effect due to our peculiar motion
Origin of the Dipole
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The origin of the dipole in the CMB is thethe Doppler effect due to our peculiar motion
directio
n of m
otion
Origin of the Dipole
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The origin of the dipole in the CMB is thethe Doppler effect due to our peculiar motion
directio
n of m
otion
Peculiar motion made up of:
Total vector sum of 369 km/s
Motion of Earth around Sun (~30 km/s)Motion of Sun around MW center (~220 km/s)Motion of MW towards Virgo cluster (~300 km/s)
Origin of the Dipole
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Dark Matter Perturbation --> Potential well
Origin of Acoustic Peaks
Pressure of photon-baryon fluid resists gravity,giving rise to oscillations --> sound waves
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Dark Matter Perturbation --> Potential well
Origin of Acoustic Peaks
Pressure of photon-baryon fluid resists gravity,giving rise to oscillations --> sound waves
Compressing a gas heats it up, expanding a gas cools it down --> Temperature fluctuations
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Origin of Acoustic Peaks
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Red is ColdBlue is Hot
Compression results in higher temperatureRarefaction results in lower temperature
Compression in valley & rarefaction at hillfollowed by
compression at hill & rarefaction in valley
Origin of Acoustic Peaks
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Density perturbation field is a combination of perturbationswith different wavelengths (different `modes’)
Modes with smaller wavelengths oscillate faster.
Modes stop oscillating at recombination (phases are `frozen’)
Origin of Acoustic Peaks
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Shown is a single mode, and the locationsof six `photon-boxes’.
Space
Spac
e
Origin of Acoustic Peaks
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At recombination, photons decouple & start to free-stream; angular scale of fluctuations proportional to wavelength of mode.
Space
Spac
e
Origin of Acoustic Peaks
=observer
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At recombination, photons decouple & start to free-stream; angular scale of fluctuations proportional to wavelength of mode.
Space
Spac
e
Origin of Acoustic Peaks
=observer
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Origin of First Acoustic Peak
First peak due to mode that just reaches maximal compression in valley/rarefaction on hill top for first time
largescales
smallscales
!T/
T
time RecombinationBig Bang
At recombination
Sachs-Wolfe
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Origin of First Acoustic Trough
At troughs, temperature fluctuations are not zero dueto motions of photon-baryon fluid --> Doppler effect.
largescales
smallscales
!T/
T
time RecombinationBig Bang
At recombination
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Origin of Second Acoustic Peak
Second peak due to mode that just reaches maximal rarefaction in valley/compression on hill top for first time
largescales
smallscales
At recombination
!T/
T
time RecombinationBig Bang
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d
largescales
smallscales
dampingDiffusion Damping
Recombination is notinstantaneous; rather the lastscattering surface has finitethickness d
Consequently, fluctuations on scales smaller than are washed out causing damping on small angular scales.
d
observer
last scatteringsurface
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Lessons Learned
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Curvature of Universe can be probed usinglarge-scale triangles
The Curvature of the Universe
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The Curvature of the Universe
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One such triangle comes from angular scaleof first acoustic peak: !" ! vsoundtrec
The Curvature of the Universe
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One such triangle comes from angular scaleof first acoustic peak: !" ! vsoundtrec
RESULT: Universe is flat
The Curvature of the Universe
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The Baryon-to-Dark Matter Ratio
Ratio of odd to even acoustic peaksholds information regarding ratioof baryons to dark matter
Increasing density of baryons causesstronger compression in valleys (due toself-gravity of baryons), and less compression on hill tops.
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The Baryon-to-Dark Matter Ratio
Ratio of odd to even acoustic peaksholds information regarding ratioof baryons to dark matter
Increasing density of baryons causesstronger compression in valleys (due toself-gravity of baryons), and less compression on hill tops.
RESULT: density in dark matter about 6 times
higher than that in baryons
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“Geometry” “Energy Density”
Flat geometry implies that Universe has critical density
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“Geometry” “Energy Density”
Flat geometry implies that Universe has critical density
Summing densities due to dark matter and baryonic matter only accounts for ~27% of “critical density”
Dark Energy makes up ~73% of Universe
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“Geometry” “Energy Density”
Flat geometry implies that Universe has critical density
Summing densities due to dark matter and baryonic matter only accounts for ~27% of “critical density”
Dark Energy makes up ~73% of Universe
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SUMMARY
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CREDITS: Many of the illustrations and movieshave been taken from the excellentwebsite created by Prof. Wayne Huhttp://background.uchicago.edu/
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