Cosmology

Question 1

How can we potentially tell if a galaxy is rotating?

Answer 1

One side is redshifted

Other side is blueshifted

Question 2

If a galaxy does not have one side redshifted and one side blueshifted

could it still be rotating?

Answer 2

Yes, if the rotation isn’t along the line between the galaxy and Earth

Question 3

How do you calculate the recessional velocity of a galaxy given its redshift?

Answer 3

v = zc

Question 4

Is a redshift velocity the true velocity of a star?

Answer 4

No, the star could have a v_|

Question 5

For this binary star system

1. When is redshift of star A maximum?
2. When is redshift = 0?
3. When is blueshift maximum?

Answer 5

Question 6

What is Hubble’s law?

Answer 6

A galaxy’s recessional velocity is proportional to its distance from Earth

v ∝ d

Question 7

How can you use this graph to calculate the age of the Universe?

Answer 7

(Reciprocal of the gradient)

Question 8

Why does this graph not mean Earth is at the centre of the Universe?

Answer 8

This graph would be produced if you took measurements from any galaxy

All galaxies are moving away from every other galaxy

Question 9

What is the radial velocity method?

Answer 9

Small redshift of star caused by exoplanet

Used to identify exoplanets

Question 10

What is the transit method?

Answer 10

Regular dip in brightness of star due to exoplanet passing in front

Question 11

What is an exoplanet?

Answer 11

A planet in another solar system

Question 12

Why are exoplanets so difficult to detect?

Answer 12

Exoplanets don’t emit light, they only reflect

And it is a tiny amount compared to the star

Question 13

What are the key stages of the Big Bang Theory?

Answer 13

1 Photons produce particles of matter & antimatter from a vacuum

2 An excess of matter over antimatter occurs

3 Nuclear fusion occurs

4 Nuclear fusion stops

5 Atoms form

6 Stars & galaxies form

7 Nuclear Fusion occurs in stars

8 Life starts

Question 14

What is the main evidence in support of the big bang?

Explain what it is

Answer 14

Cosmic Microwave Background

Radiation leftover from when the Universe formed neutral atoms

(redshifted to microwaves as Universe has expanded)

Question 15

What is a Quasar?

Answer 15

Very luminous galaxy nucleus where mass is spiraling into the supermassive blackhole at the center

Question 16

What is required for a quasar to form?

Answer 16

An accretion disk (gas, dust and matter) close to the supermassive black hole at the center

Question 17

Why can’t most quasars be detected?

Answer 17

Only detected if Earth is in line with radiation jets

Question 18

What is redshift?

Answer 18

Absorption line wavelengths of galaxy with recessional velocity are increased (shifted to red end)

Bigger velocity → Bigger redshift

Question 19

Why are radiowaves detected from a Quasar?

Answer 19

Emitted radiation is massively redshifted to radiowaves

Question 20

What does the large redshift of Quasars tell us about Quasars?

Answer 20

They are some of the most distant objects in the Universe

Question 21

What is the Schwarzschild Radius for an object?

Answer 21

What it’s radius would have to be shrunk to for it to become a black hole
(where escape velocity > speed of light)

Question 22

How do Type 1A Supernovae Form?

Answer 22

• In a binary star System
• One star has become a White Dwarf
• But has absorbed enough mass from other so M > 1.4xMSun

Question 23

What are Type 1A Supernovae useful for?

Answer 23

One of the brightest ‘Standard Candles’ (known Absolute Magnitude)
Used to measure distances to furthest galaxies

Question 24

What do the redshifts of the most distant Type 1A Supernovae tell us?

Answer 24

The redshifts are greater than expected
So Universe’s expansion is accelerating
Dark energy causes this acceleration

Question 25

In the equation for **doppler shift** what do the different **λ** represent?

Answer 25

**λ₀** → Wavelength if observer was stationary **∆λ** → observed **λ** - **λ₀**

Question 26

What is **blueshift**?

Answer 26

**Absorption line wavelengths** of galaxy **with negative recessional velocity** are **decreased** (shifted to violet end) Bigger velocity → Bigger blueshift

Question 27

What is the Absolute Magnitude curve for a **Type 1A Supernovae**?

Answer 27

Question 28

What is a **standard candle**?

Answer 28

An astronomical source with a **known absolute Magnitude/Luminosity** (eg all Type 1a supernovae have peak M=-19.3)

Question 29

How are **standard candles** used?

Answer 29

Measuring the distance to the galaxy in which they reside 1. Known M (absolute magnitude) 2. m (apparent magnitude) measured with telescope 3. Run through m-M equation

Question 30

What are **Gamma Ray Bursts?**

Answer 30

Huge amounts of gamma radiation emitted when a star supernovas to form a neutron star or black hole

Question 31

What are **pulsars?**

Answer 31

A neutron star that rotates extremly fast (up to 600 times per second) emitting EM radiation across a wide range of wavelengths from its poles?

Question 32

What is the other main piece of evidence in support of the **big bang theory?** (In addition to the CMB)

Answer 32

In the Universe, hydrogen and helium abundance by mass is currently 73% and 25%. Fusion from stars only contributes towards 2-3%. Must have been a time when the Universe was smaller and hotter and fusion could occur everywhere in space.

Question 33

Why does the Cosmic Microwave Background support the Big Bang Theory?

Answer 33

Gamma radiation produced when neutral atoms formed should be leftover but have redshifted (as space exapnds) to microwave radiation. The small fluctuations in CMB correspond to the galaxies seen today.

Question 34

What is the black body temperature of the CMB

Answer 34

2.725K (From Wien's displacement law using peak wavelength)