Transcript Document

Quiz 4 Distribution of Grades: No Curve
The Big Bang
• Hubble expansion law depends on the amount of
matter and energy (both are equivalent!) in the
Universe; more precisely, on the
matter and energy density (and ??)
• Define density parameter, and Critical Density
• Just after the BB the Universe must have been
extremely hot and dense; as it expands it cools
• Initially, radiation and matter are coupled together
in a hot, dense soup; Universe is opaque
• Later, atoms form and radiation can escape –
Recombination Epoch  Dark Ages
Background radiation and
temperature of the Universe
• Radiation from the Hot Big Bang must fill
the whole universe
• As the universe expands, the temperature
must decrease
• Must be able to detect this background
radiation – signature of the Big Bang
• Penzias and Wilson detected this Cosmic
Microwave Background Radiation (CMBR)
Discovery of Cosmic Microwave Background
Microwave antenna used by Penzias and Wilson to detect the CMBR
The Cosmic Background Explorer (COBE) Spacecraft
Cosmic Microwave Background
Radiation (CMBR)
Black-Body radiation curve at 2.7 K
peak wavelength ~ 1 mm
COBE Results for the CMBR: The Universe is a perfect blackbody
at a radiation temperature of 2.7 K
Hubble Parameter H_o and Redshifts
• Measure redshifts of spectra and calibrate by all
known steps using ‘standard candles’
• Distance to LMC is calibrated with Cepheid P-L
relation
• Best estimate of H_o = 67 +/- 8 km/sec/Mpc
• Expansion history of the Universe; ‘look-back’
time to the Big Bang: Age T_o = 1/H_o ~ 13-14 Gyr
• Cosmological Principle: Universe is uniform and
isotropic (same in every direction) on large-scales
(not locally !)
How rapid is the Expansion of the
Universe? Was it the same always?
The answer depends on the matter/energy density of the Universe,
which will slow the expansion due to gravity. But what could cause
the observed acceleration ?
The Cosmological Constant
• Einstein introduced an ‘arbitrary’ parameter, called the
“Cosmological Constant” into General Relativity to
obtain a ‘static’ universe (the Hubble expansion had not
been observed then) – Einstein’s ‘greatest blunder’ (as
he called it himself) ??
• The cosmological constant counteracts gravity
• Quantum effects in gravity – vacuum energy – could
also play the same role  Dark Energy ; density denoted
as (Capital Greek WL)
• Recent data suggest Einstein may have been right !
• But what is the shape of space-time in the universe ?
• It is determined by the path light rays would follow
• Universe: Space-time, Matter, Energy
• Very little matter-energy is observable
Critical matter-energy density balances expansion and gravitational collapse
Mass Density/Critical Density:
Density Parameter
Critical density is the density of matter required to just ‘close’
the Universe; if < 1 then Universe will go on expanding;
if >1, it will stop expanding and will contract back (the Big Crunch!).
Only ~4% matter-energy is visibly detectable
Rest is “Dark”
Baryons: Protons, Neutrons  Atoms
Matter and Energy
W And Curvature of the Universe
• Density determines shape of the Universe
W = 1  Flat (matter + energy density r = rc)
W > 1  Closed (spherical)
W < 1  Open (hyperbolic)
• Visible matter + energy (0.05) + dark matter
(0.25) , dark energy (0.7), i.e. W = Wm+ WL ~
0.3 + 0.7 = 1
How rapid is the Expansion of the
Universe? Was it the same always?
The answer depends on the matter/energy density of the Universe,
which will slow the expansion due to gravity. But what could cause
the observed acceleration ?
Densities of Visible Matter, Dark Matter, and Dark Energy
Matter-Energy density
and the “shape” of the
Universe
Flat  Euclidean
- Triangle 180o
Matter + energy density just right to balance expansion
Deceleration (acceleration) parameter q determines rate of expansion
Expansion History with Different Matter/Energy Density
Large-Scale structure of the Universe
• Galaxies group into Clusters
• Milky Way is part of the Local Group: 39 galaxies
out to ~ 1 Mpc
• Large-Scale Structure:
- Groups: 3 to 30 bright galaxies
- Clusters: > 30 (up to 1000’s) of bright galaxies;
often with many more dwarf galaxies,
1 – 10 Mpc across;
~ 3000 clusters known
- Superclusters: Clusters of Clusters
- Voids, filaments, & Walls
Large-Scale Structure:
Hubble Deep Field Survey
Galaxies, Clusters, Superclusters
Galactic Dynamics
• Nearest comparable cluster to the Local Group is the Virgo
Cluster at about ~ 18 Mpc, size ~ 2 Mpc, ~ 2500 galaxies
(mostly dwarfs), Mass ~ 100 trillion times M(Sun)
• Galaxies are large compared to distance between them;
most galaxies within a group are separated by only ~ 20
times their diameter (by comparison most stars are
separated by 10 million times the diameter)
• Tidal interactions, collisions, cannibalisation, splash
encounters, starbursts, mergers, etc.
• The MW and Andromeda are moving towards each other at
~120 Km/sec, and might have a close encounter in ~3-4
Gyr; tidal distortion and merger after 1-2 Gyr
• Eventually only one galaxy might remain, most likely a
medium-sized Elliptical
The Local Group of Galaxies
Andromeda (M31 or NGC 188)
Local Group of Galaxies Around Milky Way
Hot Dark Matter (HDM), Cold Dark Matter (CDM)
Large-Scale Structure
• How did matter distribute on a universal scale?
• How did the galaxies form and evolve?
• How do we detect imprints of early universe?
WMAP
• How do we determine large-scale structure?
Galaxy Redshift Surveys, e.g. SDSS (Sloan Digital Sky
Survey)
How did galaxies evolve?
• Baryon-to-photon ratio increases with time
• Quantum fluctuations lead to inhomogeneity in the
primordial radiation background
• Amplitude of fluctuations grows, manifest in
temperature variations or power spectrum
• Oscillations imprinted on the radiation background
• Observed in present-day CMB  PLANCK
Satellite
CMB Anisotropy Due to Large-Scale Structure:
Deviations at small angular scales
Matter and Energy Densities vs. Age and
Volume of the Universe
rm ~ 1/R3
rrad ~ 1/R4
Matter and Energy Density Dominated Expansion
• Primordial radiation dominated Universe
• As the Universe expands: V ~ R3
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Density = M/V
Matter density falls off as ~ M/R3
But energy density falls of as ~ E/R4
Photons redshift to lower energies as ~1/R
But “dark energy” may trump both
Recombination Epoch: Atom formation
and radiation-mattter decoupling
End of Dark Ages:
Reionization
• Dark Ages: Following atomic
recombination, radiation and matter
decoupled and radiation escaped leaving
material universe unobservable or dark
• Until the first stars lit up and formed
galaxies, at a redshift of about
Reionization:
Formation of first
stars and galaxies
Ionized
neutral atoms
to ion-plasma
at about 500
million years after
Big Bang or
at z ~ 10
Lyman-alpha clouds: Red-shifted light absorption by neutra
Hydrogen of light from distant galaxies up to Reionization
 (observed) = (1+ z)  (rest)
rest (Lyman alpha) emitted by distant galaxy is absorbed at
 > 1215 A by H-clouds at various redshifts, resulting in a
Lyman-alpha “forest” of lines at different obs > 1215 A.
“Hot spots” in the early universe as we see them today:
Indicators of the curvature of the Universe
Cosmic Horizon: Largest Visible
Distance at a Given Time
Partial solution to Olber’s paradox: we can only see out to the cosmic
Horizon at any given epoch in the history of the Universe; light from
objects outside will not have reached us.