11/10/2010AST207, F2010 1Helium Production in Big Bang10 Nov• Homework 8 is on angel. Due noon on Mon, 15 Nov.• Homework 9 will be due Fri, 19 Nov at start of class. No late papers. Covered on Test 3 (22 Nov).– Long assignment. Start early.• A fossil is a remnant or trace of the past. What is a fossil from the Big Bang?– There are 7 protons for every neutron– The surface of the sun is 25% He and 75% H.• What does that fossil tell about the BB?Fossil from Burgess ShaleFossil from Big BangpHepppppppppppObjectives• What are the fossils (something that can be examined) from the universe at 3 min?• What did astronomers learn by examining the fossils?11/10/2010AST207, F2010 2“Collecting the Fossil”•4He, 7Li, 2H, & 3He are made in BB.– Lots of 4He– Trace amounts of 7Li, 2H, & 3He. Diagnostics.• Measure abundances with spectra of “primordial objects”– First stars in our galaxy , made before much of the material had been processed through stars.– Dwarf galaxies, where material is processed through stars very slowly.• Deuterium 2H has same spectra as hydrogen 1H but slightly shifted.– Abundance of 2H: Strength of 2H spectral line compared with 1H line.O’Meara, et al., 2001, ApJ 552, 718.400km/s=d wavelegnth = wl/750Results• Horizontal bars are measurements.• Lines are models for differing amounts of photons/(n+p)– The temperature of the radiation from the BB tells us the number of photons.1. How many 1H nuclei are there for every 2H nucleus according to the measurements?2. The model for photons/(n+p) = 1011is inconsistent with the measurements. The measured 4He is too ___.A. HighB. Low• Measurements are consistent with models for photons/(n+p) = 41010.photons/(n+p)10910101011Ned Wright’s Cosmology Notes11/10/2010AST207, F2010 3Examining the fossil, conclusions• Calculations, which contain U expanding and nuclear physics, yield abundances of 4He, 7Li, 2H, & 3He. The only free parameter is number density of n and p.• Measured and calculated abundances are consistent.– 7Li is slightly off• Understanding of BB (and nuclear physics) is confirmed.• Surprise: Most of neutrons and protons are not in stars. Lots in gas between galaxies. Location of about 50% is not known. Fossil from Burgess ShaleObjectives• Why did the abundance of neutrons change before the “fossil was laid down” and not afterwards?• #n/#p does not change when neutrons are in a stable nucleus. (Done on Mon)• How do free neutrons and protons change identity? How does the temperature of the radiation aff ect this process? (Now)11/10/2010AST207, F2010 4Changing free neutrons & protons• Neutrons were free before nuclei formed at 3min.• Proton changes into neutronp + e‐+ energy n + – Need 2MeV of energy• Neutron changes into proton– Positron must hit neutronn + e+ p + energy + – Happens spontaneously in 1000s (17min)n p + e‐+ energy + • 1electron‐Volt is the typical energy of a chemical reaction.• 1.5eV is the energy a battery gives to one electron.• 1eV = 1.61019J• 1MeV is the typical energy of a nuclear reaction.pn2 MeVnp2 MeVChanging free neutrons & protons• Case of equilibrium: There are many collisions between neutrons and positrons and between protons and neutrons.– This is the case when the density of electrons and positrons was high.– When density of electrons and positrons was lower, collisions became too infrequent to maintain equilibrium. Neutrons decayed into protons.• In equilibrium, neutrons change into protons and protons change into neutrons. Change occurs by rules of probability.• A collision of a neutron or a proton occurs. What is the result of the collision?Probability of neutron/Probability of proton/– E is energy it takes to change a p into a n (2MeV)– T is the temperature. kT is average energy available./(Prob. n)/(prob. p)0.01 0.990.1 0.910.3730.0510 0.00005np2 MeV11/10/2010AST207, F2010 5Changing free neutrons & protonsProbability of neutron/Probability of proton /– E is energy it takes to change a p into a n (2MeV)– T is the temperature. kT is average energy available.1. When the average available energy is much higher than energy needed to make a neutron, the probability of getting a neutron is ___ probability of getting a proton.A. About the same asB. Much less thanC. Much more than2. In this case, the number of n is ___ the number of protons.3. When the average available energy is much lower than energy needed to make a neutron, the probability of getting a neutron is ___ probability of getting a proton. (Same foils.)4. In this case, the number of n is ___ the number of protons.5. Case in problem 1 is when universe was ___ than case in #3.A. YoungerB. Older/(Prob. n)/(prob. p)0.01 0.990.1 0.910.3730.0510 0.00005np2 MeVp + e‐+ energy n + n + e+ p + energy + • When the temperature of the radiation was hot and the average energy of was much big ger than 2 MeV, neutrons could change into protons as easily as protons int o neutrons.• As universe cooled, n p occurs more often than p n, and p becomes more abundant than
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