Cosmology

Question 1

What is the doppler effect?

Answer 1

The change in observed frequency when a source of waves is moving towards or away from an observer.

Question 2

What happens when the source moves towards the observer?

Answer 2

Wavelength decreases and frequency increases. Observer sees light blue shifted.

Question 3

What happens when source moves away from observer?

Answer 3

Wavelength increases and frequency decreases. Observer sees the light red-shifted.

Question 4

What happens to a star’s absorption lines as it moves towards and away from the Earth?

Answer 4

They are shifted when compared to the same lines measured in a laboratory.

Question 5

When is the doppler equation valid?

Answer 5

When v is much less than c

Question 6

When is relative velocity positive?

Answer 6

When 2 objects are approaching each other.

Question 7

What is a binary star system?

Answer 7

2 companion stars orbiting a common centre of mass.

Question 8

What are the three types of binaries?

Answer 8

visual binaries, eclipsing binaries and spectroscopic binaries.

Question 9

When is the deeper dip in apparent magnitude observed in an eclipsing binary?

Answer 9

When the cooler star passes in front of the hotter star

Question 10

What is red-shift formula? And why is z positive for red shifting?

Answer 10

z = -v/c. Recession velocity is taken to be negative.

Question 11

What are quasars?

Answer 11

Very luminous objects whose spectra shows broad absorption lines and high red shifts. Therefore they are thought to be more distant objects in the universe.

Question 12

What does Hubble’s law prove? What is the Hubble’s law’s assumption?

Answer 12

Distance of galaxies is proportional to amount of red-shift. Universe is expanding since galaxies further away from us are moving away from us faster than the galaxies closer to us. Assumption is that the rate of expansion of the universe is constant.

Question 13

What units is distance measured in in Hubble’s formula?

Answer 13

MPc.

Question 14

What units is velocity measured in in Hubble’s formula?

Answer 14

Kms-1.

Question 15

How can we calculate the age of our universe?

Answer 15

t=d/v so t = d/Hd. since v =Hd. T = 1/H.

Question 16

Why was the theory of dark energy put forward?

Answer 16

Recent observations of distant type 1A supernovae disagree with Hubble’s law and contradict idea that rate of expansion is constant, suggesting rate of expansion of universe is accelerating, which suggests that the universe if older than predicted by Hubble’s law. Dark energy idea put forward to try and explain it. There is an overall repulsive force throughout the universe which causes empty space to expand.

Question 17

What are the two main pieces of evidence for the big bang?

Answer 17

First piece of evidence comes from Hubble’s relationship and red-shifting of distant galaxies. Secondly, cosmological microwave background radiation which consists of high energy EM radiation produced from big bang can still be observed today, but redshifted down to the microwave region.

Question 18

What evidence backs CMBR theory.

Answer 18

The radiation observed in universe follows a black body radiation curve with a peak in the microwave region at 2.7k, which can be interpreted as the left over heat from the big bang. It is the radiation released when the universe cooled sufficiently for matter and radiation to ‘decouple.

Question 19

What is the final piece of evidence supporting the big bang?

Answer 19

There was a very brief period of fusion that occurred when the universe was young. The HBB model predicted that H:He would be produced in a 3:1 ratio during very early and brief stage of nuclear fusion. This matches the ratio we have today.

Question 20

What is an exoplanet?

Answer 20

A planet that orbits a star other than the sun.

Question 21

What are the 3 types of observing exoplanets?

Answer 21

Direct observation, radial velocity, transit method

Question 22

What are the difficulties of direct observation?

Answer 22

The planet and the star are often too close to be resolved. Planets are much fainter than the star and often only reflect the light of the star which they orbit. For direct observation exoplanets should be: very large, far from the star and very hot.

Question 23

What is the radial velocity method?

Answer 23

The periodic variation in the doppler shift in the star’s spectral lines can be measured due to the star’s wobble when a planet orbits the star. This wobble is due to the gravitational pull it experiences from the planet.

Question 24

What are the difficulties of radial velocity method?

Answer 24

Small exoplanets require a very sensitive spectrometer to detect them. Some exoplanets may have a very long year, so long periods of detection may be required.

Question 25

What is the transit method?

Answer 25

Detecting a dimming in the star’s brightness as an exoplanet moves across it. A light curve can be sketched and the fractional change in magnitude gives the ratio of the star size to the planet size. Length of time between transits gives the length of the year.

Question 26

Difficulties of transit method?

Answer 26

Small planets only cause a small dip in the light curve. Some exoplanets can have a very long year, so long periods of observation are required.