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

Black-Body radiation curve at 2.7 K peak wavelength ~ 1 mm

Cosmic Microwave Background Radiation (CMBR)

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 causethe 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 )

• 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

And Curvature of the Universe

• Density determines shape of the Universe

Flat (matter + energy density c)

Closed (spherical) Open (hyperbolic)

• Visible matter + energy (0.05) + dark matter

(0.25) , dark energy (0.7), i.e. m~ 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 causethe observed acceleration ?

Densities of Visible Matter, Dark Matter, and Dark Energy

Flat Euclidean - Triangle 180o

Matter-Energy densityand the “shape” of theUniverse

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

mR3

radR4

Matter and Energy Density Dominated Expansion

• Primordial radiation dominated Universe• As the Universe expands: V ~ R3

• 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 firststars and galaxiesIonizedneutral atomsto ion-plasma at about 500million years afterBig Bang orat z ~ 10

Lyman-alpha clouds: Red-shifted light absorption by neutralHydrogen 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 cosmicHorizon at any given epoch in the history of the Universe; light from objects outside will not have reached us.