Cosmology

Question 1

what is the cosmological principle

Answer 1

the universe is homogenous and isotropic

Question 2

define homogoneous

Answer 2

uniform distribution of matter

Question 3

define isotropic

Answer 3

physical property which is measured to be the same in all directions

Question 4

what is 1AU (definition and value)

Answer 4

astronomical unit - average distance from the earth to the sun

1.5 x 10^11 m

Question 5

define a light year

Answer 5

distance travelled by light in a vacuum in a year

Question 6

define the parsec

Answer 6

the distance at which a radius of 1AU subtends an angle of one arcsecond

Question 7

what is 1 degree in arcseconds

Answer 7

1 degree = 60 arcminutes = 3600 arcseconds

Question 8

what is stellar parallax

Answer 8

a technique used to measure the distance of stars relatively close to us

Question 9

what is parallax

Answer 9

the apparent shift in the position of a relatively close star against a backdrop of more distant stars

Question 10

define the doppler effect

Answer 10

the apparent change in wavelength due to relative motion to an observer

Question 11

what is red shifting

Answer 11

when a star/galaxy is moving away from an observer, the spectra lines will red shift (increased wavelength)

Question 12

what is hubble’s law

Answer 12

recessional velocity of a galaxy is proportional to its distance from us

Question 13

where can hubble’s law be applied

Answer 13

stars/galaxies outside of our own galaxy

Question 14

why can hubble’s law not be used on very large or very short scales

Answer 14

short scale - gravitational attraction effects velocity
large scale - non-linear expansion rate may not be constant

Question 15

what is evidence of the big bang

Answer 15

• in every direction, the universe is red-shifting
• in every direction, the temperature is about 2.7K
• EM radiation started as gamma, but as universe expanded and cooled, wavelength stretched to microwave radiation ( λmaxT = constant)

Question 16

what happened up to 1 second after the big bang

Answer 16

-started off as a hot, dense singularity
- time and space created
- inflation occurs
- expansion leads to cooling
- energy pair production - matter and antimatter elementary particles
- more matter than antimatter
- quarks combine to form protons and neutrons

Question 17

what happened up to 100 seconds after the big bang

Answer 17

protons and neutrons fuse to form hydrogen nuclei and primordial helium

Question 18

what happened up to 380,000 years after the big bang

Answer 18

• atoms form as electrons cool down and combine with nuclei
• universe becomes transparent - photons free to travel as gamma radiation
• expansion of universe - gamma radiation now microwave radiation

Question 19

what happened up to 30 million years after the big bang

Answer 19

• first stars appear
• first heavier elements appear

Question 20

what happens up to 200 million years after the big bang

Answer 20

our galaxy forms as gravitational forces pull clouds of hydrogen and existing stars together

Question 21

what is dark matter and how was it discovered

Answer 21

• discovered that stars at the edge of galaxies are moving faster than they should given the distribution of stars
• this violates newton’s law
• we call this missing mass ‘dark matter’
• estimated 25% of the universe

Question 22

what is dark energy and how was its presence theorised

Answer 22

• everything has gravity, therefore expansion of the universe should slow over time, or at least remain constant
• however, the universe is expanding at an ever increasing rate
  scientists use dark energy to explain this, energy we do not know of which accelerates stars and galaxies
• estimated 75% of the universe