Chapter 20 - Cosmology Flashcards Preview

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Flashcards in Chapter 20 - Cosmology Deck (33)
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1
Q

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

A

Astronomical units, light years, parsec

2
Q

Value of au

A

1.5 x 10^11

3
Q

Explain au

A

The average distance from Earth to the Sun

4
Q

Explain light years

A

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

5
Q

Value of 1 light year

A

9.46 x 10^15

6
Q

When is light years used

A

Only when expressing distances to stars or other galaxies

7
Q

Explain 1 parsec

A

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

8
Q

What is 1 degree

A

60 arcminutes

9
Q

What is 1 arc minute

A

60 arc seconds

10
Q

Value of 1 pc

A

3.1 x 10^16

11
Q

As the pc increases…

A

The angle @ S will decrease.

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

12
Q

What is stellar parallax

A

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

13
Q

How can you use stellar parallax

A

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

14
Q

Explanation for using stellar parallax

A

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

15
Q

When can you use stellar parallax

A

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,

16
Q

Doppler shift in starlight

A

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.

17
Q

What is blue shift

A

Moving towards Earth, the wavelength appears shorter.

Implying that some stars are moving towards us.

18
Q

What is red shift

A

Moving away from Earth, longer wavelengths.

Implying that the most distant stars are receding from us.

19
Q

What is the Doppler equation

A

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

20
Q

When can the Doppler equation be used

A

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

21
Q

When is au usually used?

A

Express the distance between the Sun and other planets.

22
Q

Define Doppler Shift

A

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

23
Q

Doppler Shift- applied to spectra- important points

A
  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.
24
Q

Observations of Edwin Hubble

A
  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.
25
Q

Hubble’s Law

A

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.

26
Q

The Cosmological Principle

A

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

27
Q

Define homogenous

A

Matter is distributed uniformly across the Universe.

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

28
Q

Define isotropic

A

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

29
Q

What is the Big Bang Theory

A

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.

30
Q

Evidence for the Big Bang

A
  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.
31
Q

Proof of dark matter

A

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.

32
Q

What limits our understanding of the Universe?

A

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

33
Q

What happens in the first milliseconds after the Big Bang?

A

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