Lecture #24: Plan Cosmology Expansion of the Universe Olber s Paradox Birth of our Universe
Reminder: Redshifts and the Expansion of the Universe Early 20 th century astronomers noted: Spectra from most galaxies are shifted towards red wavelengths Shift is largest for dimmer, more distant galaxies
Reminder: The Hubble Law (1929) V = H d V: recession velocity d: distance to galaxy H: Hubble Constant. (~70 km/sec per Mpc) First clue that the Universe is expanding!
Expansion of the Universe Hubble Law applies to any observer in any galaxy No matter where you are, an expanding Universe will give this appearance!
1-D Analogy of Expanding Universe: Buttons on a rubber band
3-D Analogy of Expanding Universe: Raisins in a rising raisin bread dough!
The Meaning of Redshift Light waves are stretched by expansion redshift! Redshift = z = Δλ / λ where Δλ = change in wavelength λ = original wavelength of the photon
Another Consequence of Hubble Law: The Universe had a beginning! Age ~ 1 / H ~ 14 billion years (actually 13.80 ± 0.02 b.y.) Big Bang at t = 0
Olber s Paradox The night sky is dark!!!
Olber s Paradox
Solutions to Olber s Paradox 1. Age of the Universe is finite Spatial extent of visible universe (within the cosmic horizon) is finite 2. Age of stars is also finite Visible Universe is not filled with star light 3. Universe is expanding Light from very distant galaxies is highly redshifted
Light from the Big Bang Every time we look at the night sky, we are looking back in time It takes more than 4 years for light from the closest star to arrive It takes billions of years for light from the most distant galaxies to arrive Can we see light from the Big Bang? Almost!
The Last Scattering Epoch The early Universe is opaque: High temperatures All matter is ionized (electrons are not attached to nuclei) Photons are absorbed At t > 400,000 years, the Universe is cool enough for electrons and nuclei to recombine, allowing light to travel freely Now the Universe is transparent!
Light from the Early Universe What is the expected light from the Universe 400,000 years after the Big Bang? Hint: the Universe is dense and T ~ 3000 K Universe radiates like a blackbody at T ~ 3000 K Wien s Law applies: λ m T = 3 10 6 λ m (400,000 years ago) ~ 1000 nm (near-infrared) But as the Universe expands λ m (stretches) T
Light from the Early Universe Today (after a factor 1000x expansion) λ m ~ 1 mm = 10 6 nm T ~ 3 K Cosmic Microwave Background (Penzias, Wilson 1965; Nobel Prize)
Clumpiness in the CMB Fluctuations: ΔT / T ~ few 10-6 (COBE Satellite: Smoot, Mather 1992; Nobel Prize)
Clumpiness in the CMB Fluctuations: ΔT / T ~ few 10-6 (WMAP Satellite 2001 2010)
Clumpiness in the CMB Fluctuations: ΔT / T ~ few 10-6 (Planck Satellite 2009 13 )
Clumpiness in the CMB These fluctuations are the seeds for proto-galaxies, proto-clusters, & proto-superclusters! - ΔT Δ(density) - Fluctuations grow with time due to gravity (WMAP Satellite 2001 10)
A Timeline of the Universe
A Timeline of the Universe
Radiation, Matter and Antimatter Early Universe was very hot!!!
Radiation, Matter and Antimatter Early Universe was very hot!!! Blackbody radiation has enough energy to create matter and antimatter (E = Mc 2 ) For a proton-antiproton pair to form, T > 10 13 K! Matter and antimatter annihilate on contact, releasing energy (= Mc 2 ) Predominance of matter today 10-9 asymmetry between matter & antimatter in the past!
Question #13 Give one of the three solutions to Olber s paradox ( why is the night sky dark? )