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Flashcards in Cosmology Deck (26)
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1
Q

What is the doppler effect?

A

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

2
Q

What happens when the source moves towards the observer?

A

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

3
Q

What happens when source moves away from observer?

A

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

4
Q

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

A

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

5
Q

When is the doppler equation valid?

A

When v is much less than c

6
Q

When is relative velocity positive?

A

When 2 objects are approaching each other.

7
Q

What is a binary star system?

A

2 companion stars orbiting a common centre of mass.

8
Q

What are the three types of binaries?

A

visual binaries, eclipsing binaries and spectroscopic binaries.

9
Q

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

A

When the cooler star passes in front of the hotter star

10
Q

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

A

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

11
Q

What are quasars?

A

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.

12
Q

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

A

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.

13
Q

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

A

MPc.

14
Q

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

A

Kms-1.

15
Q

How can we calculate the age of our universe?

A

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

16
Q

Why was the theory of dark energy put forward?

A

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.

17
Q

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

A

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.

18
Q

What evidence backs CMBR theory.

A

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.

19
Q

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

A

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.

20
Q

What is an exoplanet?

A

A planet that orbits a star other than the sun.

21
Q

What are the 3 types of observing exoplanets?

A

Direct observation, radial velocity, transit method

22
Q

What are the difficulties of direct observation?

A

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.

23
Q

What is the radial velocity method?

A

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.

24
Q

What are the difficulties of radial velocity method?

A

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.

25
Q

What is the transit method?

A

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.

26
Q

Difficulties of transit method?

A

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.