Lecture 5: Matter Dominated Universe: Overview of Cosmic...
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1 Lecture 5: Matter Dominated Universe: CMB Anisotropies and Large Scale Structure Today, matter is assembled into structures: filaments, clusters, galaxies, stars, etc. Galaxy formation is not completely understood. Main mechanism is gravitational instability: ρ then ρ now θ θ Overview of Cosmic History Looking Back in Time • Blah Surface of Last Scattering Before decoupling: matter and radiation tightly coupled. After: radiation propagates freely. The CMB retains an imprint of the initial conditions on the surface of last scattering .
Transcript of Lecture 5: Matter Dominated Universe: Overview of Cosmic...
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Lecture 5: Matter Dominated Universe: CMB Anisotropies
and Large Scale Structure
Today, matter is assembled into structures: filaments, clusters, galaxies, stars, etc. Galaxy formation is not completely understood. Main mechanism is gravitational instability:
ρthen
ρnow
θ
θ
Overview of Cosmic History
Looking Back in Time
• Blah
Surface of Last Scattering
Before decoupling: matter and radiation tightly coupled.
After: radiation
propagates freely. The CMB retains an
imprint of the initial conditions on the surface of last scattering.
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Almost perfect CMB isotropy --> almost uniform matter distribution at recombination
z = 1100 T ~ 3000 K t ~ 3x105 yr
Tiny CMB anisotropies.
The “ripples” in T --> ripples in density.
After decoupling, gravity amplifies these initial density ripples.
!
"TT
#10$5 #"%%
Three mechanisms give rise to anisotropies
• Sachs-Wolfe effect
• Doppler effect
• Re-ionization (Sunyaev-Zeldovich effect) !
"# ~ 1o!
"# ~ 10o
!
"# ~ 0.1o
Sachs-Wolfe effect Photons from an over-dense region must climb out of the potential well, losing energy --> longer wavelength . --> lower T.
last-scattering surface
!
"#TT~ #$$
!
"# > 0
Doppler effect Gas velocity on the last-scattering surface produces Doppler shifts.
!
"#TT
=#$$
=Vc
!
V > 0!
V < 0
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Re-ionisation (Sunyaev-Zeldovich effect)
Once stars form, their UV radiation re-ionises nearby gas. Once galaxy clusters form, gas falling in is shock-heated
to X-ray temperatures (~106-8 K). Free electrons liberated scatter CMB photons. We see CMB silhouettes of the hot gas.
Galaxy Clusters are filled with hot X-ray gas
optical (galaxies) X-ray (hot gas)
SZ effect: CMB silhouettes of galaxy cluster x-ray gas
Carlstrom et al., 2002.
Abell 2163
Mass distribution at
recombination. Velocity distribution at
recombination. Ionised gas in intervening
galaxy clusters.
Sachs-Wolfe effect
Doppler effect
Sunyaev-Zeldovich effect
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Integrated Sachs Wolfe Effect
• Similar to SW but not caused at CMB surface but during transit of photons to us.
• Affects very large scales
2004 Precision Cosmology
Energy Dark 04.073.0Matter Dark 04.027.0
baryons 004.0044.0flat 02.002.1
expanding 371
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±=!
±=!
±=!
±=
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ionrecombinat 11090yr 101379
onreionisati 20yr 10180
now yr 102.07.13
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=!=
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RR zt
zt
t
Planck • Launched by ESA in 2009.
2003 WMAP, 2004
Planck 2009+
Models
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WMAP (and Planck) measure cosmological parameters to exquisite accuracy.
Anisotropies are the starting point for galaxy formation!
Galaxy Formation Simulations • Numerical:
– Dark matter particles + gravity, fully numerical, cosmological
• Hydrodynamical: – Dark matter + gas + (SF), numerical, non-
cosmological • Semi-analytic:
– Add on to numerical simulations with prescriptions to assign galaxies to haloes
Large-Scale Structure formation Purely numerical dark matter simulations, e.g.,
Millennium Simulation, MS II, Aquarius (each at progressively lower mass resolution 107 – 104 Msolar
Typically ~109-10 particles randomly placed then adjusted to match large scale anisotropies.
Gravitational accelerations computed. Particle positions followed in time. Movies at: http://www.mpa-garching.mpg.de/aquarius Cosmologically representative and robust (reproduceable)
Large-Scale Structure formation Simulations on supercomputers. Typically ~1010 particles randomly placed then
adjusted to match large scale anisotropies. Gravitational accelerations computed. Particle positions followed in time.
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Mass resolution critical The Cosmic Web Large Scale Structure:
Like Soap Bubbles
Empty Voids
~50Mpc.
Galaxies are in 1. Walls between voids.
2. Filaments where walls intersect.
3. Clusters where filaments intersect.
Observations v
Simulations
>100Mpc excellent
agreement
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Hydrodynamical simulations Dark matter and gas simulated as particles. Stars-formed once certain density reached.
Working on including condition for self-gravitating
Semi-analytics Dark matter simulations form halos not galaxies
Assign galaxies and galaxy properties using prescriptions
Merger trees
Numerics -> Halo Merger Trees Hydro -> Gas infall
Semi-analytics ->Galaxies
Assign galaxies ASAP and try to follow development of galaxy
Properties, e.g., types, colours, SFRs etc.
Semi-Analytics Too much flexibility, almost anything can be fitted
