38457440
Description
Flashcards by Lucy Clements, updated more than 1 year ago
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Created by Lucy Clements
over 1 year ago
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| Question | Answer |
| Velocity of a galaxy equation | V=H_0*r + v_pec |
| Define standard candles | Objects that are assumed to always have the same absolute brightness, so can use their apparent brightness to calculate their distance |
| How to measure distance of nearby objects? | Use parallax: apparent movement of an object due to earths orbit |
| What are cepheid variables? | Stars with period related to luminosity so can be used as standard candles |
| How to measure present day hubble parameter? | Need speeds of galaxies at different distances, so need absolute brightness of standard candles. Use a distance ladder: a chain of different standard candles to measure distance of distant objects |
| Simple estimate of universe age in a matter dominated universe | |
| Accurate calculation of universe age | |
| Why is an open matter-dominated universe older than a flat matter-dominated universe? | There is less matter to slow the expansion down, so must have started longer ago to be as slow as the expansion rate today |
| Why is an flat cosmological constant universe older than a flat matter-dominated universe? | It increases the expansion rate at late times, so for present day expansion rate to be fixed it must be older |
| What is the general observational limit on age of the universe? | The universe must be older than the objects in it. So we can place lower limits on age by measuring age of objects. |
| Describe techniques to measure age of the universe | In each case add time after big bang (about 1Gyr for white dwarfs+global clusters) |
| What is the Robertson Walker metric? | A fundamental quantity in general relativity that describes the geometry of space-time, giving distance between two points |
| What is ds, dtheta, dphi, for light rays travelling radially? | Light rays are null geodesics so ds^2=0 Travelling radially means dtheta=dphi=0 |
| Equation for red-shift of a wavelength | |
| What is the cosmological horizon distance? | The maximum distance light has travelled since the big bang at t=0 |
| Equation for cosmological horizon distance in flat universe? | |
| What is the cosmological event horizon? | Radius within which signals emitted at time t can be observed by time tmax |
| Proper distance | The length of the spatial geodesic at time t With dtheta=dpsi=0 |
| Proper distance in a flat universe | |
| Proper distance in a closed universe | |
| Proper distance in an open universe | |
| Luminosity distance | The distance an object appears to have assuming the inverse square law holds |
| Luminosity distance in terms of z | |
| Application of luminosity distance | Used to measure cosmological parameters with standard candles |
| Angular diameter distance | The distance an object of known physical extent (i.e. a ‘standard ruler’) appears to be at assuming Euclidean geometry. In other words it is a measure of how large objects appear |
| Example of standard ruler | Baryon Acoustic Oscillations, a feature of clustering galaxies |
| Issue with angular diameter distance | In the real Universe we don’t get objects with the same physical extent existing at all z (i.e. galaxies aren’t the same at low and high redshift) |
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