Ast 207 F2009 Nov-09Ast 207 F2009Universe at 400,000yr—30 Nov• Hwk 9 & many hwk8 grades have not been recorded on angel.• Cosmic background radiation is a snapshot of U at 400,000 yr.– Radiation from the Big Bang separated from matter when universe became neutral (not ionized) at a=0.001.• What is in the snapshot?– Examine snapshot to a very fine accuracy (1part in 105)– History of MW & local group: motion of 300km/s– Fluctuations at an angular scale of 1°.– Growing clumps of mass ⇒mass density of the universe– Evidence of first starsTemperature of radiationfrom Big Bang,measured by WMAP satelliteAst 207 F2009What of the universe’s history can be observed?• Penzias & Wilson, 1965, “A measurement of the excess antenna temperature at 4080Mc/s,” ApJ 142, 419– “The excess temperature is … isotropic, unpolarized, and free from seasonal variation.”• Dicke, Peebles, Roll, & Wilkinson, 1965, “Cosmic Black-body Radiation,” ApJ 142, 414.– “Could the universe have been filled with black-body radiation from this possible high-temperature state?”• Time line of observable events (from Dicke’s paper)– Before 3min, nothing is observable, because universe is opaque.– Formation of helium, 2H, 3He, & 7Li at 3min.– Plasma recombines at 0.5Myr. Matter becomes un ionized. Universe becomes transparent.– Stars & galaxies form at 200Myr. (not in Dicke’s paper)Bob Dicke1916-1999Ast 207 F2009 Nov-09Ast 207 F2009Ionized/un-ionized gas• Ionization is the loss of an electron.H atom → p + e-• Recombination is when electron and nucleus combine.p + e-→ H atom• Ionization occurs if the temperature is hot enough.• Name one thing in this room that is/has ionized gas.• Fluorescent light•Air• Air in my lungs• Light scatters poorly off of electrons bound in an atom or molecule.• Light scatters readily off of free electrons.photonelectronAst 207 F2009• If the air were ionized, light would travel 500m before it is scattered. Light scatters nearly equally in all directions.• If the air were ionized, could you, sitting in Spartan Stadium, watch the Spartans beating Michigan? Could you see stars at night?•YY•YN•NY•NN• If the air were ionized, the air would be a fog.photonelectronAst 207 F2009 Nov-09Ast 207 F2009Matter and Radiation Decoupled• Early, hot universe– Hydrogen is ionized.• Protons and electrons are free– Universe is opaque.• Photons travel only short distances.• Scattered by free electrons.• Decoupling: p + e-→ H atom– T = 3000 K; universe 400,000 yr old.– Universe becomes transparent• Photons decouple from matter, continue in whatever direction they were moving.• After decoupling– Photons travel in same direction. Preserves information about the matter at decoupling.– In certain directions, matter & light were slightly more dense. Light preserves that information.photonelectronAst 207 F2009EarthTransparentUniverseOpaqueUniverseOpaqueUniverse13.7 billionlight yearsA Snapshot in TimeDenser hereRadiationhotterAst 207 F2009 Nov-09Ast 207 F2009Wilkinson Microwave Anisotropy Probe (WMAP) Satellite• Measure spatial variations in temperature of the CBR• Sensitivity is 0.000035K (a part in 100,000).• Anything in the instrument even 0.0001K warmer is fatal.• Symmetric design• Record temperature difference between left & right channels. Temperature difference is small.• Rotate entire instrument.• Instrumental problems do not change; radiation from the sky does change.Ast 207 F2009Wilkinson Microwave Anisotropy Probe (WMAP) SatelliteDave’s notebook 3/20/1991, Greg TuckerDave Wilkinson1935-2002, b. Hillsdale MIAst 207 F2009 Nov-09Ast 207 F200910 microwave eyes on right10 microwave eyes on leftAst 207 F2009Local Motion• WMAP: Temperature of CBR across whole sky• Notice Milky Way (pink)• Earth, sun, MW, & local group of galaxies move at 300 km/s with respect to whole universe.1. The earth, sun, and Milky Way are moving toward which direction? In which direction is wavelength compressed?adcb2.732 K2.724 KAst 207 F2009 Nov-09Ast 207 F2009Remove motion• Remove motion and show with increased contrast• Largest fluctuations are at an angular scale of 1º.Temperature fluctuations:Light & dark
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