C20 - Cosmology (The Big Bang)

Question 1

What is an arc minute?

Answer 1

1/60 th of a degree.
A minute of arc.

1° = 60 arcminutes

Question 2

What’s an arc second?

Answer 2

1/3600 th of degree.
A second of arc.

1 arc minute = 60 arc seconds

Question 3

What is an astronomical unit, AU?

Answer 3

A measurement of distance from the Earth to the Sun

Question 4

What’s a light year, ly?

Answer 4

The distance travelled by light in a vacuum in one year.

“The distance a photon if light would travel through space in one year”.

Question 5

What’s a parsec, pc?

Answer 5

The distance at which a radius of one AU subtends an angle of one arcsecond.

“The distance of an object that would have a parallax of 1 arcsec when observed from Earth.”

Question 6

How are distance and the parallax angle related?

Answer 6

d = 1/p

Where d is in metres and p is the parallax angle in arcseconds.

Question 7

What’s stellar parallax?

Answer 7

A technique used to determine the distance to stars that are relatively close to the Earth.

Parallax is the apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun.

Question 8

What’s parallax?

Answer 8

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

Question 9

How is stellar parallax used to determine distances?

Answer 9

As the Earth orbits the Sun, when the Earth is in position A, the parallax angle of a nearby star is determined.

6 months later (when Earth is at its opposite end of orbit) another parallax angle of the star is determined (against the backdrop of distant stars).

These are then used to calculate the distance:
d = 1/p

Question 10

What’s the Doppler effect?

Answer 10

The change in wavelength and frequency of waves when a wave source moves relative to an observer.

Question 11

How can absorption spectra be used to analyse the Doppler effect and motion of galaxies from star light?

Answer 11

Any difference in wavelength of absorption lines must be caused by the relative motion between the galaxy and Earth. (Red shift analysed)

If moving towards the Earth, it will appear blue shifted.
If moving away from the Earth, it will be red shifted.

Question 12

How can the Doppler effect in starlight be analysed mathematically?

Answer 12

Using the Doppler equation:

Δ λ/λ = Δf/f = v/c

Question 13

How is red shift calculated? (z)

Answer 13

Red shift = z = Δ λ/λ

Question 14

What is Hubble’s law?

Answer 14

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

vd = (Hubble’s) constant

Question 15

What 2 key observations were made when Hubble analysed the Doppler shift?

Answer 15

1) majority of galaxies were red-shifted.

2) the further the galaxy was, the greater the red shift and so the faster the galaxy was moving.

Question 16

What is the expanding universe model?

Answer 16

A model explains that space and time is expanding in all directions.

Question 17

How is Hubble’s law’s graph plotted?

Answer 17

Distance along the x axis (in Mpc - mega parsecs)

Recessional speed along y axis (in km/s)

Straight line with positive gradient (= to Ho)

Question 18

What is Hubble’s law’s graph’s gradient equal to?

Answer 18

Hubble’s constant - Ho

Question 19

What’s the cosmological principle?

Answer 19

The assumption that, when viewed in a large enough scale, the universe is homogeneous, isotropic and the laws of physics are universal.

Homogeneous - matter is distributed uniformly across the universe (uniform density).

Isotropic - the universe looks the same in all directions to every observer. There’s no edge or centre.

Question 20

What evidence supports the Big Bang theory?

Answer 20

Cosmic microwave background radiation

Red shift

Abundance of lighter elements

Hubble’s law

Question 21

How was microwave background radiation detected?

Answer 21

When scientists attempted to detect signals from objects in space, they detected a uniform microwave signal which they couldn’t account for.

Question 22

How can the existence of CMBR be explained (2 ways)?

Answer 22

1) When the universe was young and very hot, space was saturated with high E gamma photons.
The expansion of the universe stretched the wavelengths of these high E photons so now it’s observed as microwaves.

2) The universe was extremely hot and dense when young. Expansion reduced the temperature to 2.7K. The universe may be treated as a black body radiator - at this temperature the peak wavelength would correspond to the microwave region.

Question 23

How can the age of the universe be determined?

Answer 23

Age of universe = t = 1/Ho

Question 24

What’s dark energy?

Answer 24

A hypothetical form of energy that fills all of space and would explain the accelerating expansion of the universe.

Question 25

What’s dark matter?

Answer 25

A hypothetical form of matter spread throughout the galaxy that neither emits no absorbs light - it could explain the differences between predicted and observed velocities of stars in galaxies.

Question 26

What’s a black body?

Answer 26

An idealised object that absorbs all the electromagnetic radiation incident on it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature.

Question 27

What happened after the Big Bang? (As T decreases - time after…)

Answer 27

Big Bang - time and space created. Infinitely dense and hot.

10^-35s: rapid expansion and acceleration (known as inflation). There’s no matter, only high E gamma photons.

10^-6s: fundamental particles gain mass

10^-3s: quakes combine to form hadrons.

1s: creation of matter stops
100s: proteins and neutrons fuse to form deuterium and He nuclei.

380,000 yrs: universe cools for first atoms to form. Nuclei capture electrons. EM radiation at this stage is later detected as CMBR.

30mil yrs: first stars appear, thus producing elements heavier than Li.

200mil yrs: Milky Way forms due to gravitational forces.

9bil yrs: solar system forms from the nebula of a supernova of a larger star. Sun formed > Earth formed > primitive life begins

13.7bil yrs (now): modern humans evolve

Question 28

What is the universe made up of?

Answer 28

5% matter

27% dark matter

68% dark energy

Question 29

What can be a cause of the acceleration of the universe?

Answer 29

Dark energy.

A supernova was observed but light detected was less than predictions, suggesting the universe is accelerating.