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.
How were quasars discovered?
Astronomers found a very strong source of radio waves and its visible spectrum contained strong lines that couldn’t be explained. It was realised that these were due to a very large red shift so it was receding very fast very far away. Based on this distance it was found to be about 1000 times more luminous than the Milky Way.
Formation of a quasar
They are found in or near galaxies that are often distorted. These active galaxies are thought to have a supermassive black hole at the centre. Matter would be pulled in and become very hot when it compresses as it bears the event horizon. Overheating results in clouds of hot glowing gas being thrown back into space. A spinning supermassive black hole would emit jets of hot matter in opposite directions along its axis of rotation. We think quasars form from active supermassive black holes. When we observe a quasar we are looking back in time at an active supermassive black hole.
What are exoplanets?
Planets that orbit stars that are not our sun so are not in our solar system.
Why is direct imaging of exoplanets orbiting other stars difficult?
Because an exoplanet is much fainter than its parent star and is hidden by the parent star’s glare.
Describe the general procedure for the radial velocity method for detecting exoplanets
You observe the line spectrum of the light from a star to detect a periodic Doppler shift. A periodic shift in wavelength would occur if a star and planet were orbiting a common centre of mass. Each time a star moves around its orbit, each spectral line blue shifts when it’s moving towards earth and red shifts when it’s moving away.
How to calculate the time period of the orbital motion of the star for the radial velocity method.
It is the time period of the Doppler shift. E.g the time between successive shifts to minimum wavelength.
How to calculate the star’s speed of orbital motion in the radial velocity method.
Measure the maximum change in wavelength from the mean wavelength and use Doppler equation.
How to find radius of star’s orbit in radial velocity method
Use v=2πR/T. Only applies when planet orbits star in the same plane as the line of sight from earth to the star. Otherwise use v/cos. Angle is from orbital plane to line of sight. If angle not known, use v and you get a minimum value for R.
General procedure for the transit method for detecting exoplanets
You measure and record the intensity of light from a star. If it regularly dips, the likely cause is an exoplanet passing through the line of sight to the star because it will block out (absorb) light from the star.
Difficulties with transit method
Only works if exoplanet passes between the earth and the star at some point in its orbit. Planets with larger orbits are less likely to do this.
General light curve of an exoplanet transit
Relative intensity against time. Horizontal line, then dips to lower horizontal line, then rises to same horizontal line as before. Has broad dip.
How to estimate radius of exoplanet from transit method
Fractional drop of intensity at each dip equals ratio of area of exoplanet disc to area of star disc.
What does variation in regularity of transit times indicate?
The presence of other planets due to their gravitational effect on the known exoplanet.
