Cosmology Flashcards
What is the astronomical unit
Average distance from the Earth to the Sun
(1.5 x 10^11 m)
When is the astronomical unit most often used
To express the average distance between the Sun and other planets in the Solar System
What is the light year
The distance travelled by light in a vacuum in a time of one year
When is the light year often used
When expressing distances to stars or other galaxies
How many arcminutes are there in 1 degree
60
How many arc seconds in one arc minute
60 arcsecond
What is the parsec
Distance at which a radius of one AU subtends an angle of one arcsecond
How can you determine the value of 1 parsec in metres
tan(1 arcsecond) = 1 AU / 1 pc
What is stellar parallax
A technique used to determine the distance to stars that are relatively close to the Earth, at distances less than 100 pc
What is Parallax
Apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun
When can we calculate the Parallax angle
Using precise measurements
How to calculate the distance to the nearby star
d = 1 / p
Where p is measured in arcseconds
Limited so stars less than 100pc from the Earth
What is the doppler shift
Whenever a wave source moves relative to an observer, the frequency and wavelength of the waves received by the observer change compared with what would be observed without relative motion
What can the doppler effect be used to determine for a distant galaxy
The relative velocity
How can the doppler effect be used to determine the relative velocity of a distant galaxy
Absorption spectrum of a specific element is determined in the laboratory
Same spectrum is observed in light from a distant galaxy
Any difference in the observed wavelengths of the absorption line must be caused by the relative motion between the galaxy and the Earth
What happens to the absorption lines when the galaxy is moving towards the Earth
Absorption lines will be blue-shifted because the wavelength appears shorter
What happens to the absorption lines when the galaxy is moving away from the Earth
Absorption lines will be red-shifted because the wavelength appears streched
What happens to the absorption lines when the galaxy is moving away from the Earth
Absorption lines will be red-shifted because the wavelength appears stretched
Doppler Equation
( change in lambda / lambda ) = ( change in frequency / frequency) = ( relative velocity / speed of light )
Hubble’s Law
Recessional speed of a galaxy is almost directly proportional to its distance (d) from the earth
What should the graph of recessional speed against distance for all galaxies look like
The plotted data should produce a straight line through the origin - spread in the original data points suggests that Hubble’s law is valid - but there is a large uncertainty in the value for the gradient of the best-fit line
Hubble’s Constant
Gradient of the graph
v = (H (Hubble’s Constant)) x (d))
Units = s^-1 or km s^-1 Mpc^-1
What is the cosmological principle
When viewed on a large enough scale, the Universe is homogenous and isotropic, and the laws of physics are universal
What are the two key pieces of evidence for the Big Bang Theory
Hubble’s Law and Microwave Background Radiation
