Cosmology COPY Flashcards
What is the doppler effect?
Effects the electromagnetic waves from stars and glalaxies. For a source moving away from the observer there is an apparent increase in the wavelength
What is the doppler effect equation?
∆wavelength / wavelength if source was still = v/c
v = velocity of star
c = speed of light
When does the doppler effect equation work
As long as the velocity of the source is a lot less than the velocity of the wave
What is the doppler effect equation corrisponding to frequency?
∆f / f = v / c
What does the doppler effect say for waves moving towards the observer?
The wavelength would appear to decrease and the frequency to increads
What happens to the change in frequency, as effected by the doppler effect, if the observer sends waves put which then bounce off the moving object
The change in the frequency, ∆f, is doubled do:
∆f / f = 2v/c
What is red shift? Blue shift?
For a source moving away from the observer the wavelength increases moving the lines towards the red of the spectrum
For a source moving towards the observer there would be a blue shift
What is the red shift equal to?
The ratio of ∆wavelength / wavelength
v/c
What does ‘at a redshift of …’ mean?
The ratio of ∆wavelength / wavelength is …
What are binary stars?
Two stars which circulate around each other so=round each other about a common axis
What is the time periof of the motion of two binary stars?
1 on frount, the 2 then 2 again
Why do the dips in luminosity for binary stars vary?
The big dip is when the dimmer star blocks light from the brighter star and vise versa
When is the maximum intensity seen for a binary star system?
When both starts can be seen
How can the mass of a binary star using the Doppler Shift
Define exoplanets
Planets that are not in our solar system
Why are exoplanets difficult to resolve?
- They don’t give out much light and so the light from the star which they are orbiting drowns out this reflected light
- They are so small that most telescopes do not have a good enough resolving power to distinguish them from the star they are orbiting
Explain the ‘wobble’ from a star
- When an exoplanet orbits a star, the exoplanet and the star orbit about their centre of mass
- As the star is more massive than the planet, the centre of the mass will be mich closer to the star
- The star will ‘wobble’ about this centre of mass
- When the star wobbles towards towards an observer, the light will be slightly blue shifted. It will be red shifted when the star moves away.
- If doppler shifts are detected then it suggests the prescence of an exoplanetand the minimum mass can be detected
When will the ‘wobble’ for detecting exoplanets only work?
When the movement of the planet is alligned with the line of sight of the observer
Explain the detection of exoplanets from the transit method
- When an exoplanet passes in frount of a star there will be a small dip in intensity coming from the star as some light is blocked
- The time it takes for the star to go from max to min brightness can be used to measure the size of the planet
What are the problems with the transit method to detect exoplanets?
- It is unlikely that an exoplanet will pass in frount of a star in the line of sight of the observer
- The time that the exoplanet is blocking the star for can be very small fraction of the time period of the planet, meaning ti is hard to spot the drops
- A dip in brightness can be caused by other things and stars can have variable intensities.
Draw the light cruves for a big planet passing in frount of a star compared to a small planet
What distant galaxies experience red shift?
All distant galaxies in all directions and so are all moving away from us
What is Hubble’s Law?
Red shift is proportional to the distance of the galaxy from our own.
The further away a galaxy is, the faster it is moving away.
Draw a graph showing Hubble’s Law (numbers on axis not needed)
What is the Hubble’s Law equation and what are the units of all the components?
v = H0D
v = velocity in km/s
H0 = Hubble’s constant (on formula sheet)
D = distance to galaxy in Mpc
What is the equation that links intensity, luminosity/power and distance?
I = L/4πD2
What is a standard candle?
An astronomical object with a known luminosity
Draw the light curve from a type 1a supernova (only need to know x-axis)
What is the greatest magnitude of a type 1a supernova?
-19.6
What is the value of the Hubble’s constant?
65 kms/Mpc
Derive the formula to find the age of the universe using Hubble’s constant
How old is the universe?
15 billion years
What is assumed when calculating the age of the universe?
The Hubble’s constant has been constant over the life of the universe
What is the big bang model?
The universe started off as an infinitely hot, infinitely dense point and has been expanding ever since
What is the fraction of helium formed in the early universe?
25%
What is the temperature of the universe?
2.7 K
Draw the spectrum of CMBR
What is the evidence for the Big Bang Theory? (3)
- Spectra from different galaxies in all directions are red shifted. This suggests that the universe is expanding and so must have been denser in the past
- The observation od CMBR, which is the remnants of EM radiation from the early universe
- The theories of particle physics, when applied to the expanding universe, correctly predict the percentage of helium formed in the early universe
Give the key features of a quasar. (3)
- Very large red shift and so very different
- Very powerful radio source
- Supermassive black holes which eject huge amounts of material
What are active galactic nuclei?
Supermassive black holes which eject huge amounts of material (quasars)
What is the evidense for dark matter? (2)
- The centripetal force caused by the gravitational attraction of stars in the outer part of a galaxy is not large enough to keep the outer stars rotating about the centre of the galaxy at the speed it does
- When light travels through a galaxy it is bent by gravitational lensing, the amount it bends shows that there must be more mass in the galaxy that can be accounted for by the mass of the stars which can be seen
What effect does dark energy have?
- In addition to the mass we can see and dark matter, it causes space to be flat
- It effects the rate of expansionof the universe (accelerates it)
What is the evidence for dark energy?
The distance of distant galaxies from earth can be calculated from the redshift using Hubble’s Law.
These galaxies look lainted than they should for the distance calculated from the redshift, suggesting they are farther away than expected and that the universe has expanded more than expected.
This suggesting accelerated expansion.
