38468288
Beschreibung
Karteikarten von Lucy Clements, aktualisiert more than 1 year ago
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Erstellt von Lucy Clements
vor fast 2 Jahre
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| Frage | Antworten |
| Energy density equation | |
| Sketch black body distribution | |
| Peak and mean black-body radiation energy | |
| Photon to baryon ratio | |
| Present day temperature of photon distribution | T0 = 2.725 K |
| Alpha, the proportionality between total photon energy density and temperature (Equation and value) |
Image:
Alpha (binary/octet-stream)
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| Number density equation | |
| Baryon density parameter today (from Planck data) | |
| Mean baryon energy | |
| What is matter-radiation equality? | epoch at which matter and radiation densities are equal teq = 10^12 s |
| Photon energy density | |
| Matter density parameter today | Parameter m,o *h^2 = 0.14 |
| Why is neutrino density parameter is different from photon density parameter? | - Factor of 3 since there's 3 neutrino types - 7/8 since neutrinos are fermions but photons are bosons - (4/11)^4/3 from electron-positron annihilation producing photons not neutrinos |
| Temperature-time relation | |
| Temperature of matter-radiation equality | |
| Define nucleosynthesis | The synthesis of nuclei of the light elements when the universe was seconds old |
| Describe nucleosynthesis process | -While kT>1.3MeV, protons and neutrons in thermal equilibrium -Freeze-out at kT=0.8MeV, neutron-proton interconversion ceases, fixed n/p=0.2 -some of the neutrons decay between 0.8-->0.1MeV -nuclei form via nuclear fusion but high energy tail of photons destroy them -until kT=0.1MeV, Tnuc=10^10K at 1s when negligible high energy photons so nucleosynthesis can occur |
| describe how nucleosynthesis calculations can be used to measure baryon density parameter | - Compare calculated abundances of light elements with those observed - Abundances depend on baryon density parameter so can determine the present day value using these comparisons - Give baryon density parameter *h^2 between 0.016 and 0.024 |
| Describe how nucleosynthesis calculations can be used to find the number of species of light neutrinos | - Abundances of light elements depend on number of species of light neutrinos - Since this affects radiation density --> expansion rate --> Tnuc -Confirm 3 species |
| How are the abundances of light species observed? | 4He from the spectra of first generation, population III stars (correcting for 4He produced in the stars). • D from the absorption of quasar light by primordial gas clouds. •7Li from stellar spectra (correcting for production by cosmic rays and destruction in stars). |
| Define Cosmic Microwave Background Radiation | isotropic black-body radiation with temperature T0=2.725 K, produced when atoms form (recombination) and photons decouple at t=10^13s |
| Describe how the CMB forms | - Early universe: photons ionise any atoms so made up of nuclei and electrons (ionised plasma) - Cause photon scattering off charged particles, universe opaque - Universe cools and expands, recombination: photon energy drops so atoms can form - Universe becomes neutral, decoupling at 3000K: photons stop scattering, travel towards us unimpeded, universe transparent -See photons originating from 'surface of last scattering'=sphere 6000h^-1 - Photons temperature and wavelength redshifted as they travel towards us |
| How to CMB anisotropies arise? | Inflation in the early universe --> initial small overdensities --> temperature fluctuations in the CMB (anisotropies) - Temp fluctuations depend on density fluctuations evolution, depends on universe contents |
| How can anisotropies be used to measure universe geometry and total energy density? | - Temp fluctuations depend on density fluctuations evolution, depends on universe contents - Calculate angular power spectrum taking statistical average of coefficients of spherical harmonics that describe temp fluctuations - First peak gives size of universe at decoupling, so angle it subtends on the sky (and so multipole moment, l) depend on universe geometry - closed = bigger angles = smaller l at peak - open = smaller angles = bigger l - From geometry gives total energy density |
| What is this plot? Describe the 3 key areas | - Sachs-Wolf Plateau: low l, temp fluctuations from changes in gravitational potential - Acoustic (Doppler) Peaks: middle l, oscillations in photon fluid from gravity and pressure competition, from photon-electron interactions - Silk damping tail: high l, diffusion of photons during recombination damps temp fluctuations |
| Amplitude of anisotropies | |
| Value of Boltzmann constant needed for temp-time relation | k_B = 8.62 × 10−5 eV K−1 |
| What is the angular power spectrum? | square of the average temperature difference between directions separated by angle Θ Used for CMB anisotropies |
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