Cosmology

Question 1

The astronomical unit(AU)

Answer 1

The average distance from the Earth to the sun
1.50 x 10^11 m (not given in formula book)

Question 2

The light year

Answer 2

The distance travelled by light in a vacuum in a time of one year
9.46 x10^15 m

Question 3

Parsec

Answer 3

The distance at which a radius of 1 AU subtends an angle of 1 arcsecond (where 1 arsecond = 1 degree/3600)

Question 4

Parallax angle

Answer 4

p =1/d where d is measured in parsecs and p is measured in arcseconds

Question 5

Stellar paralax

Answer 5

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
This can be used to find the distance of stars closer than 100pc

Question 6

The doppler effect

Answer 6

When a wave source moves relative to an observer, the frequency and wavelength of the waves received by the observer change
If the wave source is moving away from the observer, the received wavelength will be longer and the frequency lower
If the wave source is moving toward the observer, the received wavelength will be shorter and the frequency higher

Question 7

Red shift

Answer 7

Galaxy is moving away from Earth, so absorption spectrum received will be shifted towards the red end

Question 8

Blue shift

Answer 8

Galaxy is moving towards the Earth (E.g. andromeda) , so absorption spectrum is shifted to blue end

Question 9

Doppler shift equation

Answer 9

Δf/f = Δλ/λ = v/c

Question 10

Hubble’s law

Answer 10

The recessional speed v of a galaxy is directly proportional to its distance from Earth
v=H0d (H0 is Hubble constant)
Comes from observation of increasing redshift as distance from Earth increases

Question 11

Hubble constant units

Answer 11

SI units: m^-1
Cosmology units: kilometres per second per megaparsec

Question 12

The cosmological principle

Answer 12

When viewed on a large enough scale, the universe is homogeneous and isotropic
Homogeneous - matter is distributed uniformly across the universe. For a vert large volume, the density of the universe is uniform
Isotropic - the universe looks the same in all directions to every observer. There is no center or edge.

Question 13

The big bang

Answer 13

At some point in the past, all the matter in the universe was contained in a singularity
This is considered the beginning of the universe and its has been expanding ever since

Question 14

Evidence for the big bang

Answer 14

Cosmic Microwave Background Radiation(CMBR) - when the universe was young, space was saturated with high-energy gamma photons. The expansion of the universe means space was stretched and this stretched the wavelength. We now observe these waves as microwaves
Hubble’s law - Hubble’s law shows that the universe is expanding. It follows that it was, at one point, a singularity

Question 15

Age of the universe

Answer 15

t= 1/H0

Question 16

Evolution of the universe

Answer 16

1) The big bang - time and space are created. The universe is infinitely hot and dense. Forces are unified
2) The universe expands rapidly - there is no matter, there are only high-energy gamma photons
3) Quarks/antiquarks and leptons are produced - this took place around 10^-4 seconds. Quarks combine to form hadrons
4)Protons and neutrons fuse together to form deuterium and helium nuclei
5) Stars from - nuclear fusion takes place and heavier elements are formed

Question 17

Dark energy

Answer 17

Modern estimates of expansion suggest the universe is accelerating. Energy fuelling this is dark energy
68% of the universe is dark energy

Question 18

Dark mass

Answer 18

There is a difference between observed and expected doppler effects from different galaxies
The observations can be explained if the mass is not concentrated at the centre of the galaxy. The mass that makes this possible is dark mass