Chapter 20 - Cosmology

Question 1

What are the units for distances (in order of increasing distances)

Answer 1

Astronomical units, light years, parsec

Question 2

Value of au

Answer 2

1.5 x 10^11

Question 3

Explain au

Answer 3

The average distance from Earth to the Sun

Question 4

Explain light years

Answer 4

The distance travelled by light in a vacuum in a time of 1 year

Question 5

Value of 1 light year

Answer 5

9.46 x 10^15

Question 6

When is light years used

Answer 6

Only when expressing distances to stars or other galaxies

Question 7

Explain 1 parsec

Answer 7

Generally, 1 parsec = the distance at which a radius of 1 AU sub tends an angles of 1 arcsecond

Question 8

What is 1 degree

Answer 8

60 arcminutes

Question 9

What is 1 arc minute

Answer 9

60 arc seconds

Question 10

Value of 1 pc

Answer 10

3.1 x 10^16

Question 11

As the pc increases…

Answer 11

The angle @ S will decrease.

If the point S is at a distance d parsec, the angle subtended becomes 1/d arcsecond

Question 12

What is stellar parallax

Answer 12

The apparent shift in the position of a relatively close star against the background of more distant stars as the Earth orbits the Sun.

Question 13

How can you use stellar parallax

Answer 13

A technique used to determine the distance to stars that are close to Earth, at a distance of less than 100 pc.

Question 14

Explanation for using stellar parallax

Answer 14

When in the figure, when earth is at position A, the star can be seen at position 1. Six months later, when earth is at position B, the star can be seen at position 2. You can use measurements to determine the parallax angle (in arcsecond)
D( pc) = 1 /p

Question 15

When can you use stellar parallax

Answer 15

Only for stars that are less than 100pc away from Earth. Otherwise, as pc increases too much, the the parallax angle decreases too much and accurate measurements can’t be used,

Question 16

Doppler shift in starlight

Answer 16

By look at the absorption lines in spectra form stars.
Firstly, you need to create those absorptions in the labs.
Then, it is observed from stars and if there is any difference, this is because of the relative motion between the galaxy and Earth.

Question 17

What is blue shift

Answer 17

Moving towards Earth, the wavelength appears shorter.

Implying that some stars are moving towards us.

Question 18

What is red shift

Answer 18

Moving away from Earth, longer wavelengths.

Implying that the most distant stars are receding from us.

Question 19

What is the Doppler equation

Answer 19

Change in wavelength / wavelength = change in frequency / frequency = velocity/ speed of light.

Question 20

When can the Doppler equation be used

Answer 20

Only for galaxies with speed far less than the speed of light.

Question 21

When is au usually used?

Answer 21

Express the distance between the Sun and other planets.

Question 22

Define Doppler Shift

Answer 22

The apparent shift in the observed frequency of waves due to the relative motion between the observer and source.

Question 23

Doppler Shift- applied to spectra- important points

Answer 23

1. Each dark spectral line of an absorption spectrum are all shifted the same % for 1 star.
2. The greater the % shift, the faster that star is receding away or moving towards us.

Question 24

Observations of Edwin Hubble

Answer 24

1. Light from most galaxies was red shifted, and so most galaxies are receding away from us.
2. The further away a galaxy, the greater the red-shift. So, the faster it is moving away. Therefore, the Universe is expanding, and this gave rise to the Big Bang.

Question 25

Hubble’s Law

Answer 25

The recessional speed, v of a galaxy is almost directly proportional to its distance, d from the Earth.

v ≈ H0 d
H0 = Hubble’s Constant.

Question 26

The Cosmological Principle

Answer 26

When viewed on a large enough scale, the Universe is homogenous and isotropic, AND the laws of Physics are universal.

Question 27

Define homogenous

Answer 27

Matter is distributed uniformly across the Universe.

The density of the universe is uniform and the same galaxies can be seen everywhere.

Question 28

Define isotropic

Answer 28

The Universe looks the same in every direction, to every observer.
Therefore, there is no centre to the Universe.

Question 29

What is the Big Bang Theory

Answer 29

14 Billions years ago, all the matter and energy of the universe was contained in an infinitely dense and hot point in Space and Time.
The Universe then rapidly started expanding outwards from this point and has been expanding ever since.

Question 30

Evidence for the Big Bang

Answer 30

1. Hubble’s Law:
   - Most galaxies are receding from us​
   - If The Cosmological Principle holds true, implies that at some point in the past, all galaxies/matter must have been contained at the same point in space.​
2. CMBR
   - Microwaves that uniformly fill the entire universe.
   - The BBT explains this by stating that this radiation began as high energy gamma radiation at a single point, at the start of the universe, which has since red-shifted as the universe has expanded to lower energy microwaves.

Question 31

Proof of dark matter

Answer 31

The rate of rotation w.r.t the distance from the centre of a galaxy is governed by the mass of the galaxy and the gravitational force of attraction towards the centre of the galaxy.
It has been shown that galaxies rotate much faster than the observed mass should allow. This extra mass is called dark matter and it acts at the extremities.

Question 32

What limits our understanding of the Universe?

Answer 32

That the Laws of Physics may break down on such large masses.

Question 33

What happens in the first milliseconds after the Big Bang?

Answer 33

Temperature decreases.
Universe expands.
Energy is converted to mass.