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A2 Physics > 3.9.3 - Cosmology > Flashcards

3.9.3 - Cosmology Flashcards

1
Q

What is changed during the Doppler effect?

A

The wavelength and frequency of the sound heard.

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2
Q

As something moves away how is the sound heard?

A

Lower-pitched, waves spread out.

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3
Q

As something moves towards you how is the sound heard?

A

Higher-pitched, waves are bunched together.

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4
Q

What does the sound depend on with the Doppler effect?

A

The velocity of the object.

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5
Q

What does the Doppler effect occur in?

A

All waves, including electromagnetic radiation.

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6
Q

What happens when a light source moves away from us?

A

The wavelength of the light reaching us becomes longer and the frequencies become lower. This shifts the light that we receive towards the red end of the EM spectrum and is called a red shift.

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7
Q

How us the light we receive from a star moving away from us different to actual light emitted?

A

It is towards the red end of the EM spectrum and so is redder.

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8
Q

What happens when a light source moves towards us?

A

The light undergoes blue shift, the star looks bluer than it actually is.

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9
Q

If a star is red shifted what will the value of z be?

A

Positive.

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10
Q

If a star is blue shifted what will the value of z be?

A

Negative.

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11
Q

What is cosmological red shift and what does it show?

A

All distant galaxies show red shift and so are moving away from us. This shows space is actually expanding and the light waves are being stretched along with it.

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12
Q

What are the cosmological principles?

A

On a large scale the universe is homogenous (every part is the same as every other part) and isotropic (everything looks the same in every direction).

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13
Q

What is the recessional velocity?

A

The amount of red shift given by a galaxy.

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14
Q

What did Hubble realise?

A

The speed that galaxies moved away from us at depended in how far away they were.

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15
Q

In v = Hd what is v in?

A

kms-1

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16
Q

In v = Hd what is d in?

A

Mpc

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17
Q

What is the Big Bang theory?

A

The universe started off very hot and very dense and has been expanding ever since.

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18
Q

How is red shift evidence for the Big Bang theory?

A

The spectra from all galaxies show red shift, so they are moving apart.

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19
Q

How can you estimate the age of the universe?

A

t = d/v = 1/H

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20
Q

Why is there a limit on the observable size of the universe?

A

Because we can only see light that has had time to travel to us.

21
Q

Why is the observable size of the universe just an estimate?

A

The rate of expansion of the universe hasn’t been constant.

22
Q

What is dark energy and what does it explain?

A

All mass in the universe is attracted together by gravity, which would suggest the rate of expansion show be slowing down, however it is accelerating. This acceleration is being explained by dark energy - a type of energy that fills the whole of space.

23
Q

What does the Big Bang model predict?

A

Lots of EM radiation was produced in the very early universe.

24
Q

What has happened to the radiation produced in the Big Bang?

A

Because the universe has expanded, the wavelengths of this cosmic background radiation are in the microwave region - so is known as cosmic microwave background radiation.

25
Q

What does CMBR show?

A

The Doppler effect, it agrees with the cosmological principle and has a perfect black body spectrum.

26
Q

How does the Big Bang theory explain the large abundance of helium in the universe?

A

The early universe was very hot, so must gave been hot enough for hydrogen to fuse.

27
Q

What are spectroscopic binary stars?

A

2 stars that orbit each other, and are too far away to be resolved with telescopes.

28
Q

What is an eclipsing binary system?

A

One whose orbital plane lies almost in our line of sight, so the stars eclipse each other as they orbit.

29
Q

How can orbital period be calculated from an eclipsing binary system?

A

By observing absorption lines in the spectrum - as stars orbit each other, the separation of the lines increases from zero to a maximum and then back to zero in half a period.

30
Q

How can a graph of apparent magnitude and time be used in a binary star system?

A

As the stars eclipse each other, the apparent magnitude drops because some of the light is blocked out. It drops more as the dimmer star passes in front of the brighter star.

31
Q

What are quasars?

A

The most distant objects seems and shoot out jets of material - very radioactive sources.

32
Q

What do quasars produce?

A

A continuous spectrum of the Balmer series of hydrogen but red-shifted enormously.

33
Q

What does the huge red shift of quasars suggest?

A

They’re a huge distance away.

34
Q

Quasars are very powerful and so what is it suggested that they are?

A

Very powerful galactic nuclei centred around a huge black hole

35
Q

What is an active galaxy?

A

One in which the black hole at the centre ejects huge amounts of material from their nucleus.

36
Q

What is the black hole in an active galactic nucleus surrounded by?

A

A doughnut shaped mass of whirling gas falling into it, which emits matter and radiation.

37
Q

What do the magnetic fields produce in an active galactic nucleus?

A

Jets of matter and radiation streaming out from the poles.

38
Q

What are exoplanets?

A

Any planet not in our solar system.

39
Q

Why are exoplanets difficulty to find?

A
  • They’re orbiting stars much brighter than them, so the bright light from the stars they’re orbiting drowns out any light from the exoplanet.
  • They’re too small to distinguish from nearby stars.
  • Only a few of the largest and hottest exoplanets that are furthest away from their stars can be seen directly using specially built telescopes.
40
Q

What two ways can exoplanets be detected?

A

Doppler shift, the transit method.

41
Q

What else is the Doppler shift method called?

A

The radial velocity method.

42
Q

What does the Doppler shift method do?

A

Measures how much the emissions from stars have been red or blue shifted.

43
Q

Why does the Doppler shift method work?

A

An exoplanet orbiting a star has a small effect on the star’s orbit. It causes tiny variations in the stars orbit as the star and exoplanet are actually orbiting around the centre of mass between them - but as the star is so much bigger than the exoplanet, the centre is much close to the centre of the star. This wobble causes tiny red and blue shift’s in the star’s emission which can be detected.

44
Q

What is the problem with the Doppler shift method?

A

The movement needs to be aligned with the observer’s line of sight - if the planet orbits the star perpendicular to the line of sight then there won’t be any detectable shift in the light from the star.

45
Q

What can be calculated using the Doppler shift method?

A

The minimum mass of the exoplanet.

46
Q

What does the transit method measure?

A

The change in apparent magnitude as an exoplanet travels in front of a star.

47
Q

Why does the transit method work?

A

As an exoplanet orbits a star, if it passes in front of the star then it blocks out some of the star’s light, causing a change in apparent magnitude. This can be viewed as a dip in the star’s light curve.

48
Q

How and what can be calculated with the transit method?

A

The radius of the exoplanet and the amount that the star is dimmed depends on the size of the orbiting planet.

49
Q

What are the limitations of the transit method?

A

Even if aligned wit the observer, the transit is likely to last only a tiny fraction of its whole orbital period - so transits can be few and far between, making them easy to miss. Therefore you can only use this method to confirm already observed exoplanets.

A2 Physics (25 decks)

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