Cosmology Flashcards
Describe the Doppler effect
Where the wavelengths of light waves from a star moving towards earth are shorter, and the wavelengths of light from a star moving away from earth are longer, than they would be if the star was stationary.
Explain red shift
Waves are emitted and detected in the opposite direction to the motion of the source. The waves detected are more spaced out so have a longer wavelength so the absorption lines in the star’s spectrum are shifted towards the red end where wavelength is larger.
Explain blue shift
Waves are emitted and detected in the same direction to the motion of the source. The waves detected are less spaced out so have a shorter wavelength so the absorption lines in the star’s spectrum are shifted towards the blue end where wavelength is shorter.
What is Doppler shift (z) and which direction is the star moving in the formula booklet?
The fractional change in wavelength or frequency. Towards observer.
What is a star’s radial speed?
The component of its velocity parallel to the line between earth and the star.
When are the maximum and minimum observed wavelengths for the spectrum from a star in a binary system?
Only works if orbiting in the same plane as the line from earth to the star. Maximum wavelength when star is moving away from earth (λ+Δλ). Minimum when star is moving towards earth (λ-Δλ).
What are two stars that can’t be resolved called?
A spectroscopic binary.
How do spectroscopic binary stars work?
Each spectral line splits into two after the stars cross the line of sight, then merge back into a single spectral line when they move towards the line of sight. The stars orbit a common centre of mass. They have the same period. The heavier star has a smaller orbit and moves slower and has a lower change in wavelength.
What are the units in Hubble’s law?
v is speed at which galaxy is receding from us in km/s
d is distance to galaxy in Mpc
How to calculate the age of the universe
The speed of a galaxy increases by 65km/s or 3.26Mly for every Mpc. A galaxy travelling at the speed of light would be the edge of the universe at a distance of 300,000/65 x 3.26 Mly. Light has to travel at 15000 million years to go that far. Or you can do 1/H where H is in s^-1.
How was the speed of recession of distant galaxies calculated?
Using the red shift of light from the galaxy.
Evidence for Big Bang: CMBR
The Big Bang Theory predicted that radiation was created in the Bang and has been travelling through the universe ever since. As the universe has expanded, its mean temperature has decreased and is now about 2.7K. The spectrum of radiation from space has a peak in the microwave region and matches the theoretical spectrum of thermal radiation from an object of 2.7K. It can be interpreted as the radiation left over from the Big Bang. The photons have been stretched to longer wavelengths and lower eneegies. Because the radiation was detected from all directions in the universe and varies little in intensity it must be universal.
Evidence for Big Bang: relative abundance of H and He
The mass of other elements in negligible. Stars and galaxies contain about 3 times as much H as He. Therefore protons:neutrons = 7:1. This is because rest energy of neutron is slightly more than proton so when universe cooled enough to allow quarks in 3s to form baryons, protons formed more readily. Precise calculations using the exact difference in rest energies also yield a ratio of 7:1. Big Bang theory predicted fusion of H occured when universe young to form He. The fusion stopped as the universe expanded and cooled.
How did the theory of dark energy arise?
Use different techniques to measure distances to very distant type 1a supernovae. One used red shift and Hubble’s law, other used apparent and known absolute magnitude. The magnitude method gave a distance further than its red shift suggested. To reach this distance, the supernova must have been accelerating so the expansion of the universe has been accelerating. No known force could cause this so a previously unknown force must be releasing energy called dark energy.
Properties of quasars
Has very powerful light output (comparable to galaxies). Relatively small in size (not much bigger than a solar system). Large red shift indicating it is 5k and 10k ly away.
