20 : Cosmology

Question 1

Astronomical unit (AU)

Answer 1

the average distance from the earth to the sun =
1.5 x 10^11m
Often used to express the average distance between the sun and other planets in the solar system

Question 2

The light-year (ly)

Answer 2

Distance travelled by light in a vacuum in a time of one year
- 9.46 x 10^15m
Used to express distances to stars or other galaxies

Question 3

The parsec (pc)

Answer 3

• 60 arcminutes in 1°
• 60 arcseconds in each arcminute
• 1arcsecond = (1/3600) °
  The distance at which a radius of one Au subtends an angle of one arcsecond
• 1 AU / 1pc = tan (1arcsecond)
• 1pc = 3.1x10^16m

Question 4

Stellar parallax

Answer 4

A technique used to determine the distance to stars that are relatively close to earth (less than 100pc)
- the apparent shift in the position of a relatively close star against a backdrop of much more distant stars as the earth orbits the sun
- d=1/p where distance is the from star and o is the parallax angle measured in arcseconds

Question 5

Doppler shift

Answer 5

• a wave sources moves relative to an observer, the frequency and wavelength of the waves received by the observer change compare to observed without relative motion

Question 6

Doppler shift in starlight

Answer 6

• The effect can be used to determine the relative velocity of a distance galaxy
• the absorption spectrum of a specific element is determined in the laboratory. The same spectrum is observed in light from a distance galaxy. Any difference in observed wavelengths of the absorption lines must be caused by the relative motion between galaxy and the earth

Question 7

Analysing Doppler shift

Answer 7

• galaxy is moving towards the earth the absorption lines will be blue-shifted they move towards the blue end of spectrum as wavelengths appear shorter
  Galaxy moving away from the earth the absorption lines will be red shifted they all move towards red end of spectrum because wavelength appears stretched

Question 8

Doppler equation

Answer 8

∆ λ /λ = ∆f/f = v/c
- shows that the faster the source moves the greater change in observed wavelength or frequency

Question 9

Humbles law

Answer 9

When analysing Doppler shift Hubble found
- he confirmed earlier observations that the light from the vast majority of galaxies was red-shifted, had a relative velocity away from the earth
- in general the further away the galaxy was the greater the observed red shift and so the faster the galaxy was moving
Hubble law:
The recessional speed v of a galaxy is almost directly proportional to its distance d from the earth

Question 10

Hubble constant

Answer 10

V=H.d
Constant of proportionality which is the gradient of speed - distance graph

Question 11

Expanding universe

Answer 11

Key evidence for big bang theory
- the light from nearly all galaxies are red-shifted
- the fabric of displace and time is expanding in all directions
- not just galaxies moving away from each other but actual space expanding
- as a result any point in the universe is moving away from every other point in the universe the further the points are away from each other
- from our position on earth this explains why light from more distant galaxies is more red-shifted indicating that they are moving faster than those nearer to us

Question 12

Cosmological principle

Answer 12

The assumption that when viewed on a large enough scale universe is homogenous and isotopic and laws of physics is universal
- homogenous - matter is distributed uniformly across the universe. For a very large volume the entity of the universe is uniform this means the same type of structures (galaxies) are seen everywhere
- isotropic - the universe looks the same in all directions to every observer. It follows that there is no centre or edge to the universe
- laws of physics - the rounds and models tested on the earth apply everywhere else within universe over all time and space

Question 13

Supporting big bang theory

Answer 13

• Hubbles law
• microwave background radiation

Question 14

Microwave background radiation

Answer 14

• When universe was young and extremely hot space was saturated with high-energy gamma photons the expansion of the universe means that space itself was stretched over time. This expansion stretched the wavelength of high-energy photons so now em radiation is seen as microwaves
• the universe was extremely dense and hot when it was young expansion over of billions years has reduced temperature to 2.7 K
• universe can be treated as a black-body radiator

Question 15

The age of the universe

Answer 15

Estimate age by assume it was uniformly expanded (it has not - expansion is accelerating)
- hubbles law shows galaxies receding from each other which means in the past they would of been next to eachother
- galaxy at distance d is moving away At constant speed v then a time t=d/v must have elapsed
Age of universe = 1/ Hubble constant

Question 16

the Big Bang

Answer 16

• Time and space created
• universe is infinitely dense and hot

Question 17

10^ - 35 s after big bang

Answer 17

• Expands rapidly ( incredible acceleration = inflation )
• no matter in universe
• Full of EM radiation inform of high-energy gamma protons
• temp = 10^ 28 k

Question 18

10^ - 6 s after big bang

Answer 18

• First fundamental particles (quarks) gain mess through a mechanism

Question 19

10^ - 3 s after big bang

Answer 19

• Quarks combine to form first hadrons such as protons and neutrons
• most of mass in universe created in first second through pair production

Question 20

1s after big bang

Answer 20

• Creation of matter stops after is once temp drops to 10^ 9 k

Question 21

100s after big bang

Answer 21

Protons and neutrons fuse together to form helium nuclei - expansion of universe is so rapid that no heavier elements are created
- 25% of matter is helium nuclei

Question 22

380000 years after big bang

Answer 22

Universe cools enough for first atoms to form
- the nuclei capture electrons - the em radiation from this stage is what is detected as microwave background radiation

Question 23

30 million years after big bang

Answer 23

• First star appears
• through nuclear fusion in stars the first heavy elements begin to form

Question 24

200 million years aster big bang

Answer 24

Our galaxy begins to seem as gravitational forces pull clouds of hydrogen and existing stars

Question 25

9 billion bears after big bang

Answer 25

- The solar system forms from the nebula left by supernova of a larger star after the sun forms remaining material forms the earth and other planets

Question 26

10 billion bears after big bang

Answer 26

Primitive life on Earth begins

Question 27

13.7 billion years after big bang

Answer 27

- 2.7K - 200000 years ago first modern humans evolve

Question 28

Accelerating universe

Answer 28

- Observed supernovas which was less intense than predicted - the only possible conclusion was that expansion of the universe was accelerating - the acceleration needed a source of energy which was not been detected ' dark energy'

Question 29

Dark energy

Answer 29

- A hypothetical form of energy that permeates an space