Cosmology

Question 1

what is the Doppler affect

Answer 1

The Doppler effect has an effect on electromagnetic waves (e.g. light) from
stars and galaxies. For a source moving away from the observer there is an
apparent increase in the wavelength, Δλ, given by:

Δλ/λ = v/ c (on formula sheet)
can also work with frequency

Question 2

what shift do sources of electromagnetic radiation get when moving away/towards us

Answer 2

For a source moving away from the observer the wavelength increases
moving the lines towards the red end of the visible spectrum. This is a red shift. For
a source moving towards the observer there would be a blue shift.
(Only distant galaxies are moving away from us- closer galaxies and stars in the
Milky Way move towards us due the force of gravity)

Question 3

What is the ratio of change in wavelength to wavelength called(Δλ/λ)

Answer 3

(Δf/ f) is called the red shift, z, when it is applied to the
movement of stars or galaxies so z = v/c. (z has no units). (on formula sheet)

Question 4

what are binary stars

Answer 4

Many stars were formed in pairs called Binary stars, which are two stars
which circulate around each other about a common axis.

Question 5

What equations are needed to derive keplers 3rd law ?

Answer 5

F=mv^2/r F=GMm/r^2 v=2πr/T

Question 6

What equations are needed to find the mass of a binary star

Answer 6

T^2=4π^2r^3/GM (re arrange for m)
Δλ/λ = v/ c (Δλ,λ read from graph)
v = 2πr/T

Question 7

What are exoplanets ?

Answer 7

Exoplanets are planets which are not in our solar system

Question 8

Why are exoplanets hard to detect

Answer 8

They are difficult to detect because they do not give out much light and so the
light from the star which they are orbiting drowns out this reflected light.
They are also so small that most telescopes do not have a good enough resolving
power to distinguish them from the star which they are orbiting.

Question 9

What are the methods of detecting exo planets

Answer 9

transit method
Detection of exoplanets from the variation in Doppler Shift- the
radial velocity method.

Question 10

What is the transit method

Answer 10

When an exoplanet passes in front of a star there will be a small dip in the
brightness of the light coming from the star as some light is blocked. This change
can be seen by a dip in the light curve for a star.

The time it takes for the star to go from max to min brightness can be used to
measure the size of the planet. The can also be estimated in the size of the dip from
max to min, which depends on the relative size of the planet compared to the star.
The problem with this method is that it is unlikely that an exoplanet will pass in front
of a star in the line of sight of the observer. In addition, the time that the exoplanet is
blocking the star for can be a very small fraction of the time period of the planet,
meaning that it is hard to spot these drops in brightness.
This method can only be used to confirm exoplanets detected by other methods.
Even then, if there is no dip in brightness, it does not mean that there is no
exoplanet- it could just be that that the exoplanet does not pass between the star
and the observer.

Question 11

What is the radial velocity method

Answer 11

When an exoplanet orbits a star, the star and the exoplanet orbit about their
centre of mass. As the star is much more massive than the planet, the centre of
mass will be much closer to the star. The star will “wobble” about this centre of mass.
When it moves towards an observer, the light from the star will be slightly blue
shifted. It will be red shifted when the star moves away. If these Doppler shifts are
detected in the light from a star it suggests the presence of an exoplanet and the
minimum mass of the exoplanet can be calculated from the shifts in wavelength.

Question 12

What is hubbles law

Answer 12

In 1929 Edwin Hubble found that this red shift was proportional to the distance of
the galaxy from our own. This is known as Hubble’s Law. The further away a
galaxy is the faster it is receding from us; if it is double the distance it is receding at
double the rate.

v = HoD

It was found that all distant galaxies in all directions appeared to exhibit a red
shift and so are moving away from us.

Question 13

what is a standard candle

Answer 13

A standard candle is an astronomical object which has a known luminosity (type 1 a supernova sephids)

Question 14

What is the formula for intensity (must learn)

Answer 14

I = L/4πD²

Question 15

What is the greatest absolute magnitude of a type 1a supernova

Answer 15

The greatest absolute magnitude, M, of a type 1a supernova is thought to
always be approximately -19.6

Question 16

what does the redshift in the light from distant galaxies suggest?

Answer 16

The redshift in the light from distant galaxies suggests that galaxies in all
directions are moving away from us. This suggests a picture of a uniformly
expanding universe. The observation of the expanding universe has lead to the Big
Bang model of the universe.

Question 17

how to calculate the age of the univers

Answer 17

to = 1/Ho Ho must be in s^-1

The universe is thought to be about 15 billion years old. This makes the
assumptions that the Hubble constant has been constant over the life of the
universe, whereas theory and measurement suggests it has not been.

Question 18

What is the big bang thoery

Answer 18

The Big Bang model: The universe started off as an infinitely hot, infinitely
dense point (a singularity) and has been expanding (and cooling) ever since.

Question 19

Evidence for the big bang

Answer 19

The fact that the spectra from distant galaxies in all directions are red shifted
suggests that the universe is expanding and so must have been much denser in the
past.
The observation of a spectrum of cosmic microwave background radiation, which is
the remnants of electromagnetic radiation from the early universe, agrees with Big
Bang Theory.
The theories of particle physics, when applied to the expanding universe, correctly
predict the percentage of helium formed in the early universe.

Question 20

what are quasars

Answer 20

Quasars have a very large redshift in their optical hydrogen Balmer lines
which means that they are very distant. They are the most distant measureable
object that can be observed. They are very powerful radio source and smaller than a
galaxy.
Quasars are thought to be supermassive black holes which eject huge amounts
of material, at the centre of galaxies. Not all black holes do this- those that do are
called active galactic nuclei.

Question 21

what is the evidence for dark matter ?

Answer 21

The centripetal force caused by the gravitational attraction of stars in the outer part
of a galaxy and the stars in the centre of the galaxy is not large enough to keep the
outer stars rotating about the centre of the galaxy at the speed it does.
When light travelling through a galaxy is bent by gravitational lensing, the amount it
bends shows that there must be more mass in the galaxy than can be accounted for
by the mass of the stars which can be seen.

Question 22

evidence for dark energy ?

Answer 22

The distance of distant galaxies from earth can be calculated from the redshift using
Hubble’s Law. These galaxies look fainter than they should for the distance
calculated from the redshift, suggesting that they are farther away than expected and
that the universe has expanded more than expected, suggesting accelerated
expansion.