Three Pillars of Big Bang Theory 1 Hubbles

Three Pillars of Big Bang Theory 1. Hubble’s Law Redshift of galaxies 2. 2. 73 K Cosmic Microwave Background Remnant radiation from the Big Bang 3. Primordial and fixed ratio of H to He 90% to 8% by number

The Big Bang • Hubble expansion law depends on the amount of matter and energy (both are equivalent!) in the Universe; more precisely, on the visible and dark matter and energy density • 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

Expansion NOT Explosion (No Bang!)

Space itself expands, Viz. distances between galaxies

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 Ho ~ 70 km/sec/Mpc • Expansion history of the Universe; ‘look-back’ time to the Big Bang: Age To= 1/Ho ~ 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

Densities of Visible Matter, Dark Matter, and Dark Energy

Only ~4% matter-energy is visibly detectable Rest is “Dark” Baryons: Protons, Neutrons Atoms

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

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!).

Matter-Energy density and the “shape” of the Universe Flat Euclidean - Triangle 180 o Matter + energy density just right to balance expansion

Expansion History with Different Matter/Energy Density

Data from supernovae in galaxies at various redshifts

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 one galaxy might remain, most likely a medium-sized Elliptical

The Local Group of Galaxies Andromeda (M 31 or NGC 188)

Local Group of Galaxies Around Milky Way

Hot Dark Matter (HDM), Cold Dark Matter (CDM) HDM: fast moving e. g. Ions, neutrinos, exotic particles CDM: slow motion e. g. failed stars, lonely planets, “brown dwarfs”

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

Tiny fluctuations grow with time into large-scale structure

CMB Anisotropy Due to Large-Scale Structure: Deviations at small angular scales

Matter and Energy Density Dominated Expansion • Primordial radiation dominated Universe • As the Universe expands: V ~ R 3 • Density = M/V • Matter density falls off as ~ M/R 3 • But energy density falls of as ~ E/R 4 • Photons redshift to lower energies as ~1/R • But unknown “dark energy” may trump both matter and radiation

Matter and Energy Densities vs. Age of the Universe rm ~ 1/R 3 rrad ~ 1/R 4

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