1	Astr 5460 Wed., Jan. 29, 2003 Today: Textbook Check Astro-ph (xxx.lanl.gov) WIRO TBA Assignment for Friday check Longair, Chapter 1 (History) Friday we’ll discuss Chapter 2 Note: This class will meet W&F, 5440 will be M&W
2	The Big Bang George Gamow (late 1940s) realized that running the expansion backwards means a hot, dense early universe that was radiation dominated, and nucleosynthesis was possible. With Alpher and Herman, predicted background radiation left over from this period, which would now have a temperature of about 5 K. Penzias and Wilson discover the background radiation in 1965 by accident, win Nobel prize.
3	Prediction from Big Bang Model: Abundance of the light elements Big Bang Nucleosynthesis T, r both high enough at start to fuse protons into heavier elements T, r both dropping quickly so only have time enough to fuse a certain amount. Simple models of expansion predict 24% abundance He 24% is the amount of He observed* Abundance of ²H, ³He, ⁷Li depends on r_{normal matter} Suggests r_{normal matter} is only 5% of r_critical But we need to also consider “dark matter” and its gravity
4	Prediction from Big Bang model: Cosmic Background Radiation Look out (and back in time) to place where H became neutral Beyond that the high density ionized H forms an opaque “wall” Originally ~3000 K blackbody radiation The material that emitted it was moving away from us at extreme speed That v produces extreme redshift (z=1000), so photons all appear much redder, so T appears cooler With red shift, get 2.7 K Planck blackbody Should be same in all directions
5	Cosmic Microwave Background Observations First detected by Wilson and Penzias in 1960’s Serendipitous detection – thought is was noise in their radio telescope but couldn’t find cause. Only later heard of theoretical predictions Best spectrum observed by COBE satellite Red curve is theoretical prediction 43 Observed data points plotted there error bars so small they are covered by curve. it is covered by curve. Isotropy also measured by COBE T varies by less than 0.01 K across sky Small “dipole” anisotropy seen Blue = 2.721 Red = 2.729 Caused by motion of Milky Way falling towards the Virgo supercluster.
6	Critical points with time running forward 10^-45 sec Quantum gravity? Physics not understood 10^-34 sec 10²⁶ K Nuclear strong force/electro weak force separate (inflation, matter/antimatter asymmetry) 10^-7 sec 10¹⁴K Protons, AntiprotonsÛphotons 10^-4 sec 10¹² K Number of protons frozen 4 sec 10¹⁰ K Number of electrons frozen 2 min Deuterium nuclei begins to survive 3 min 10⁹ K Helium nuclei begin to survive 30 min 10⁸ K T, r too low for more nuclear reactions (frozen number of D, He -- critical prediction) 300,000 yr 10⁴ K Neutral H atoms begin to survive (frozen number of photons – critical prediction) 1 billion yr Galaxies begin to form 13 billion yr Present time
7	Big Bang Essentials Hubble Expansion Black Body Background Radiation Light Element Abundances Age of oldest stars consistent with Ho age
8	Galaxy Formation Lots of theory here! Jeans (1902), gravitational collapse in a stationary medium, depends on sound speed and density Lifshitz (1946), general case including expanding medium, but collapse is not typically exponetial and structures grow very slowly – too slowly! Cannot start with infinitesimal perturbations. Zeldovich, Novikov, Peebles (1960s) used finite perturbations (1 part in 10000). Main test of all this is the cosmic microwave background radiation, since fluctations should leave imprints.
9	Galaxy Formation Thermal history of pregalactic gas can be worked out in detail (and we will do so!). Density fluctuations tied to temperature fluctuations, revealed finally by COBE, but small. Lots more details to go into here later in course. Two main ideas: top-down vs. bottom-up. Need for dark matter (hot or cold) became apparent – normal matter needs help to collapse into galaxies.
10	Very Early Universe Isotropy – the universe looks the same in all directions, again strictly true on large scales Small Baryon/Anti-baryon asymmetry Close to critical (Omega = 1) (will be HW) Initial fluctuations to seed structure growth
11	Very Early Universe Inflation (Guth, others, early 1980s) resolves some of these properties. Inflation posits an early exponential expansion of the universe that leaves the curvature flat (close to omega = 1) and takes regions in causal contact and moves them far beyond their local horizons (isotropy). May help form the fluctuations leading to galaxies.