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Physics Year 13 > Chapter 20 - Cosmology (The Big Bang) > Flashcards

Chapter 20 - Cosmology (The Big Bang) Flashcards

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1
Q

define the astronomical unit and give its distance in metres

A

the astronomical unit is the average distance from the earth to the sun

1 AU = 1.50 x 10^11 m

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2
Q

define the light year and give its distance in metres

A

“the distance that light travels in a vacuum in 1 year”

1 ly = 9.46 x 10^15 m

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3
Q

what are arcminutes and arcseconds

A

1 degree = 60 arcminutes
1 arcminute = 60 arcseconds
1 arcsecond = 1/3600 degrees

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4
Q

define the parsec

A

The parsec (pc) is defined as the distance at which a radius of 1AU subtends an angle of 1 arcsecond

(think R.A. triangle with adjacent 1pc, opposite 1AU and angle 1 arcsecond)

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5
Q

what is the value of 1 parsec in metres and light years

A

1 pc = 3.1 x 10^16 m

1 pc = 3.26 ly

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6
Q

what approximation can we make when using parsecs and why

A

angle (in arcseconds) = 1/d
where d is in parsecs
- we can use this because of the small angle approximation for tan(theta)

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7
Q

what is stellar parallax

A
  • stellar parallax is a technique used to determine the distance to stars that are relatively close (<100pc) from earth
  • it uses the apparent shift of closer stars against a backdrop of more distant stars as the earth orbits the sun (and the stars are viewed from different positions)
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8
Q

what is the stellar parallax equation

A

d = 1/p

where
d = distance in parsecs
p = angle in arcseconds

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9
Q

define the Doppler effect

A

whenever a wave source moves relative to an observer, the frequency and wavelength of the waves received by the observer change compared to what would be observed without relative motion

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10
Q

in what ways does wavelength change for a stationary/moving object and an observer

A
  • for a stationary object the wavelength observed by a stationary observer is the same at any point around the object
  • for a moving object and a stationary observer, the wavelength is shorter when the object is moving towards the observer because the waves are compressed and longer when the object is moving away from the observer because the waves are stretched
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11
Q

how can we use the Doppler effect to analyse starlight/galaxy light

A
  • we can analyse the shifts in frequency of the absorption lines on a spectrum from a distant galaxy to calculate a relative speed
  • an element’s spectrum is found in the lab
  • a spectrum is analysed from a distant galaxy (we look at the same pattern of lines so we know it’s not a different element)
  • if the same spectrum is found but is shifted then we know there is relative motion
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12
Q

what are the two things that can happen for the light from a distant galaxy that is moving

A
  • if the galaxy is moving towards earth then the
    wavelengths of light from the galaxy are compressed, the wavelength becomes shorter and the absorption lines move to the blue end of the spectrum – this is blue-shift
  • if the galaxy is moving away from earth then the wavelengths of light from the galaxy are stretched, the wavelength becomes longer and the absorption lines move to the red end of the spectrum – this is red-shift
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13
Q

what is the equation we can use for EM waves and red/blue shift

A

Delta(lambda) / lambda = delta(f) / f = v/c

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14
Q

define Hubble’s law

A

“the recessional speed, v, of a galaxy is almost directly proportional to its distance, d, from earth, v direct prop to d”

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15
Q

what are the two key features of Hubble’s law and what is the evidence for them

A

1) Hubble confirmed that the light from the vast majority of distant galaxies was red-shifted, thus they must be moving away from the earth
2) generally the further the galaxy from earth, the greater the red-shift of its absorption spectrum so the faster it’s moving away from earth

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16
Q

what does a graph of recessional speed against distance from earth give

A

almost a straight line because they are almost directly proportional

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17
Q

what is the equation for Hubble’s law and which units can we use

A

V = Ho d

V = velocity/speed = kms^-1 or ms^-1
d = distance = Mpc or m
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18
Q

what is the Hubble constant (in two forms) and give its units

A

67.80 +- 0.77 kms^-1 Mpc^-1

or

2.2 x 10^-18 s^-1

19
Q

what is the best way to think about the way that the universe is expanding

A
  • it is not that the galaxies themselves are moving away from each other but that space itself is expanding and stretching
  • this explains why points further away have greater relative motions
20
Q

what is a definition for the cosmological principle

A

“The cosmological principle is that when viewed on a large enough scale the universe is homogeneous and isotropic”

21
Q

what is meant by the cosmological principle (3 things)

A

1) The laws of physics are universal / they are the same anywhere in the universe
2) the universe is homogeneous, meaning that matter is distributed uniformly and for a very large volume the density is uniform
3) the universe is isotropic, meaning that it looks the same in all directions and to every observer, there is no centre or edge to the universe

22
Q

what is the main principle of the big bang theory

A

it suggests that at some point in the past all of the matter in the universe was contained in a single point, an infinitely small and dense singularity

23
Q

what are the two main pieces of evidence to support the big bang theory

A

1) Hubble’s Law

2) Microwave Background Radiation (CMB)

24
Q

how does Hubble’s Law provide evidence for the big bang theory

A
  • it shows that the universe is expanding, galaxies’ redshift shows they are receding from each other as space itself is expanding
  • this implies that at some point in the past the universe must have been infinitely small and dense
  • in itself an expanding universe can also be explained by other theories
25
Q

how can cosmic microwave background radiation provide evidence for the big bang theory, describe CMB/its key features

A
  • Cosmic microwave background radiation
  • uniform intensity
  • temperature approx 3K
  • the big bang theory predicted CMB
  • no other theory could explain it
26
Q

how can we explain CMB in terms of stretching wavelength

A
  • when the universe was very young and hot, it was saturated with high energy gamma photons
  • as the universe expanded, space itself expanded
  • this increased the wavelength of the photons moving them into the microwave region of the EM spectrum
27
Q

what is the black-body radiation way to explain CMB

A
  • when the universe was young, it was extremely hot and dense, as it expanded, it cooled
  • now it has a temperature of around 2.7K
  • if we model the universe as a black body then this would correspond to a (lambda(max)) of around 1mm
  • this lies in the microwave part of the EM spectrum
28
Q

what can we do using Hubble’s law to roughly determine the age of the universe, what is the issue with doing this

A
  • we can model the universe’s expansion as being constant and uniform
  • this is a poor assumption given that the rate of expansion is increasing
as
v = Ho d
v/d = Ho
d/v = 1/Ho
t = 1/Ho

this gives a rough age for the universe of around 14 billion years

29
Q

describe the state of the universe at the big bang

A
  • time and space are created
  • the universe is a singularity - it is infinitely dense and hot#
  • all forces are unified
30
Q

describe the state of the universe at t = 10^-35 seconds

A
  • the universe expands rapidly including incredible acceleration - inflation
  • temperature = 10^28K
  • no matter, only high energy gamma photons
31
Q

describe the state of the universe at t = 10^-6 seconds

A
  • the first fundamental particles in the form of quarks and leptons form and gain mass
32
Q

describe the state of the universe at t = 10^-3 seconds

A
  • the quarks combine to form the first hadrons e.g. protons, neutrons
  • most of the mass in the universe created in first second through pair production
33
Q

describe the state of the universe at t = 1 second

A
  • creation of matter stops as temperature drops to 10^9K
34
Q

describe the state of the universe at t = 100 seconds

A
  • protons and neutrons fuse to form deuterium and helium nuclei
  • no heavier elements form
35
Q

describe the state of the universe at t = 380,000 years

A
  • universe cools enough for first atoms to form
  • nuclei capture electrons
  • EM radiation has stretched sufficiently for it to be microwave and detectable
36
Q

describe the state of the universe at t = 30 million years

A
  • first stars appear

- through nuclear fusion first heavier elements appear

37
Q

describe what happens to the temperature of the universe as it expands

A

temperature decreases

38
Q

what was the discovery of dark energy/how was it discovered

A
  • discovered in 1999 by analysing the characteristic light from distant type Ia supernovae
  • light was less intense than predicted
  • only explanation was universe’s expansion is accelerating
  • therefore there must be more energy than we can view
  • this is dark energy
39
Q

what was the discovery of dark matter/what is the evidence for it

A
  • Doppler shift from stars in galaxies showed their rotational velocity was not as predicted as you move further from the centre of the galaxy
  • it is predicted that as you move further from the centre of the galaxy, rotational velocity drops (given that most of the mass is in the centre as is observed)
  • But this was not observed, rotational velocity remained higher for longer than predicted
  • the only explanation was that there’s more matter than has been observed, so it must be dark matter
40
Q

what are some other marking points about the development of the universe following the big bang

A
  • forces were unified at the big bang
  • more matter than antimatter created
  • expansion led to cooling
  • universe now 2.7K
41
Q

describe the three possibilities for the future of the universe

A 
open = expands for all time, occurs where density < critical density
closed = expands then contracts, occurs where mass is greater than predicted, density > critical density
flat = expands to a limit, occurs where density = critical density
42
Q

describe Olbers’ paradox and how Hubble explains it

A
  • Olbers’ paradox is that the universe is infinite in age and size so all lines of sight should end in a star so the night sky should be bright, not dark
  • but Hubble explains why it is dark
  • in an expanding universe the light may undergo redshift to outside of the visible light region
  • or the age of the universe is finite and the light from the stars has not reached us yet
43
Q

explain how the universe became transparent to EM radiation

A
  • matter-radiation equilibrium/ big soup of plasma prevents the passage of EM radiation
  • atoms formed
  • this allowed EM radiation to pass
44
Q

what is the conversion between Ho in kms^-1 Mpc^-1 to Ho in s^-1

A

divide by 3.09x10^19

Physics Year 13 (15 decks)

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