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OCR Physics A-level > 20 : Cosmology > Flashcards

20 : Cosmology Flashcards

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1
Q

Astronomical unit (AU)

A

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

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2
Q

The light-year (ly)

A

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

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3
Q

The parsec (pc)

A
  • 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
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4
Q

Stellar parallax

A

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

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5
Q

Doppler shift

A
  • 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
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6
Q

Doppler shift in starlight

A
  • 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
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7
Q

Analysing Doppler shift

A
  • 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
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8
Q

Doppler equation

A

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

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9
Q

Humbles law

A

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

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10
Q

Hubble constant

A

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

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11
Q

Expanding universe

A

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

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12
Q

Cosmological principle

A

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

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13
Q

Supporting big bang theory

A
  • Hubbles law
  • microwave background radiation
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14
Q

Microwave background radiation

A
  • 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
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15
Q

The age of the universe

A

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

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16
Q

the Big Bang

A
  • Time and space created
  • universe is infinitely dense and hot
17
Q

10^ - 35 s after big bang

A
  • Expands rapidly ( incredible acceleration = inflation )
  • no matter in universe
  • Full of EM radiation inform of high-energy gamma protons
  • temp = 10^ 28 k
18
Q

10^ - 6 s after big bang

A
  • First fundamental particles (quarks) gain mess through a mechanism
19
Q

10^ - 3 s after big bang

A
  • Quarks combine to form first hadrons such as protons and neutrons
  • most of mass in universe created in first second through pair production
20
Q

1s after big bang

A
  • Creation of matter stops after is once temp drops to 10^ 9 k
21
Q

100s after big bang

A

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

22
Q

380000 years after big bang

A

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

23
Q

30 million years after big bang

A
  • First star appears
  • through nuclear fusion in stars the first heavy elements begin to form
24
Q

200 million years aster big bang

A

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

25
Q

9 billion bears after big bang

A
  • 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
26
Q

10 billion bears after big bang

A

Primitive life on Earth begins

27
Q

13.7 billion years after big bang

A
  • 2.7K
  • 200000 years ago first modern humans evolve
28
Q

Accelerating universe

A
  • 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’
29
Q

Dark energy

A
  • A hypothetical form of energy that permeates an space

OCR Physics A-level (28 decks)

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