38488741
Description
Flashcards by Lucy Clements, updated more than 1 year ago
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Created by Lucy Clements
almost 2 years ago
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| Question | Answer |
| Speed of galaxy rotation curves | G is gravitational constant M(<r) is mass within the orbit r is the orbit radius |
| How do galaxy rotation curves provide evidence for dark matter? | For planets, constant M(<r) so object is outside mass distribution means v is proportional to r^-1/2. But, for spiral galaxies, measurement of doppler shift of Hydrogen 21cm line shows at large r, v becomes constant. So M(<r) is proportional to r, suggesting luminous galaxy is surrounded by invisible dark matter halo |
| Equation for M(<r) in terms of density | |
| What galaxy cluster observations indicate dark matter? | 1) Total mass from virial theorem (more mass than expected without dark matter) 2) Baryon fraction from x-ray gas (only 0.1 so must be some other matter too) 3) Mass distribution from gravitational lensing (since mass bends space which bends light from back galaxies) |
| How does virial theorem support dark matter? | Virial theorem: 2T+V=0, where T=kinetic, V=potential energy 2T+V=<v^2>M-GM^2/R Hence M=R<v^2>/G Velocity measured from speeds from doppler effect, radius estimated from projected positions of galaxies. More mass than we'd expect - mass density parameter=0.3 |
| How does baryon fraction from x-ray gas support dark matter? | Baryon fraction=baryon mass/total mass Equivalent to baryon/matter density parameters 1st term: x-ray spectra, 2nd term: x-ray surface brightness Result baryon fraction=0.1 |
| How does mass distribution from gravitational lensing support dark matter? | Mass bends space --> light from distant background galaxies bends Strong lensing=multiple images of background galaxy Positions+intensities of images gives mass distribution |
| explain how the clustering of galaxies constrains the neutrino density parameter | - galaxy clustering depends on initial density fluctuations and how these grow - galaxy red-shift surveys (Baryon acoustic oscillations) have therefore measured galaxy clusters to find matter density and baryon fraction - HDM free streams and erases density fluctuations on small scales, so measurement of galaxy clusters limits them - Limits hot, relativistic, light dark matter = light neutrinos to 0.1 - Limits total mass of light neutrinos to <1eV - So most of dark matter is cold + non-relativistic |
| Define WIMPs | weakly interacting massive particle produced in Big Bang with roughly right density to be dark matter, expected to exist in extensions of the standard model of particle physics |
| Explain why WIMPs are produced with right density | - Early times: high temp, WIMPs created by collisions so in thermal equilibrium as also destroyed by annihilation with themselves - Constant comoving density, n - Universe cools: too low energy to produce massive WIMPs, only being destroyed, n decreases rapidly - Freeze out: n becomes so low no more annihilation since weakly interacting - Again n constant now = required present day WIMP density |
| Describe how WIMPs can be detected | - Produced at Large Hadron Collider - Detected directly via elastic scattering off nuclei - Detected indirectly via annihilation products in high density regions |
| Define Axions and how they can be detected | Dark matter candidate, very light, extremely weakly interacting particles. Mass constrained by cooling stars + supernovae observations. Detected by conversion to photons in magnetic field |
| Define primordial black holes and how they can be detected | Form in early Universe from collapse of density fluctuations and will be non-baryonic if formed before nucleosynthesis Amount determined by microlensing of stars: temporary brightness during gravitational lensing when massive object crosses line of sight Also disrupt the galactic disk |
| Deceleration parameter for flat universe containing matter and cosmological constant | |
| What are type 1a supernovae? What do the observations tell us? | Occur when mass accretion from binary companion causes explosion of a white dwarf. Standardisable candles: correlation between maximum absolute brightness and rate at which they fade so used to measure luminosity distance Observed: distant supernovae are dimmer than expected so universe is accelerating so cosmological density parameter>1/3 |
| What is the cosmological constant problem? | Expected value of cosmological constant from particle physics is ~10120 times bigger than observed |
| What is the fluid description? | cosmological constant can be described as fluid with equation of state with w=-1, so negative pressure (from fluid and friedmann, noting density is constant with time) |
| What is quintessence? | model of dark energy with different (and possibly time varying, rather than cosmological constant) equation of state |
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