Cosmology That part of astronomy which deals with the nature of the universe as a whole

Olber’s Paradox (1826) Why is the night sky dark?

Olber’s Paradox (1926) Why is the night sky dark?

Olber’s Paradox Why is the night sky dark? The universe has an edge The stars and galaxies ‘run out’ at some distance from us The universe has a beginning...but the universe would be brighter earlier on.... Light is redshifted (‘cooled’) by the expansion of the universe First suggested by Edgar Alllen Poe in 1848

Hubble’s Law Galaxies are moving away from us at a speed proportional to their distance V = H × d H is called Hubble’s constant

Best estimate of H so far puts the age of the universe at 13.6 billion years Hubble’s Law

The Big Bang Hypothesis: throughout the history of the universe, the galaxies have been moving away from each other. - There was a time in the past where the universe was extremely dense and hot: the universe today evolved from this initial state, which we refer to as the ‘Big Bang’ - The Big Bang does not occur in one place in the universe – it occured everywhere

The Big Bang Prediction 1: Until about 300,000 years after the big bang, the universe was hotter than the surface of the Sun (5800K) - As the universe cooled below this temperature, it should have released black body radiation (a spectrum like the one we get from the surface of the Sun) everywhere in the universe - As the universe aged, this radiation will have been redshifted - This radiation should still be here today, everywhere we look at microwave frequencies - Calculations show that it should correspond to a temperature of 2.73 K (-270 degrees celsius).

Penzias and Wilson (1960s, won 1978 Nobel Prize)

COBE (1989)

The Big Bang Prediction 2: Using what we know about nuclear physics, we can predict what elements were created during the first thousands of years after the Big Bang, and in what proportion

Initially, universe is a plasma

… a cosmic ‘soup’ of charged particles

PROTON NEUTRON ELECTRON

Universe much cooler, atoms start to form….. Hydrogen, Helium, normal ‘stuff’ 300,000 Years Later…..

Universe is now a GAS

HYDROGEN HELIUM

75% H 25% He 0.01% Deuterium (Hydrogen with one neutron) % Lithium, Beryllium How do we observe the amounts of different elements in the early Universe?

Look-Back Time The further away we look, the further back in time we are seeing

Globular clusters

WMAP and Structure Formation (2002)

GRAVITY!

Gravitational Collapse We started with a uniform gas Some regions were slightly denser (hotter) The denser regions had larger gravity and started to pull other particles towards them The gaseous Universe began to look ‘clumpy’

Histo ry (Figure from Wayne Hu) (Figure from WMAP team)

Histo ry

CMB Foreground-cleaned WMAP map from Tegmark, de Oliveira-Costa & Hamilton, astro-ph/

Boo m zoom Cosmic strings Open universe Inflation with  Inflation without  Using CMB blobs as a standardizable ruler: Guth & Kaiser 2005 (Science) + WMAP3 3

Histo ry CMB Foreground-cleaned WMAP map from Tegmark, de Oliveira-Costa & Hamilton, astro-ph/

Boo m zoom z = 1000

Boo m zoom z = 2.4 Mathis, Lemson, Springel, Kauffmann, White & Dekel 2001

Boo m zoom Mathis, Lemson, Springel, Kauffmann, White & Dekel 2001 z = 0

Mass of the Milky Way From Kepler’s third law, mass of galaxy is about 400,000,000,000 M Sun Dark Matter: Primary evidence

We seem to be missing about 90% of the mass of the galaxy! - Most of it cannot be emitting or absorbing light - Astronomers name it Dark Matter

Dark Energy and the Shape of the Universe Type Ia supernovae always have the same brightness – if we know that brightness, We can accurately measure the distance to them

Dark Energy 73% Dark Matter 23% “Normal Matter” 4% The Shape of the universe depends on what is in it Currently, it appears that the universe is close to flat