Ch 20 Cosmology

Question 1

What is the cosmological principle?

Answer 1

• the universe is isotropic and homogenous
• laws of physics are universal

Question 2

What does isotropic mean?

Answer 2

Universe is the same in all directions to every observer
Universe has no centre or edge

Question 3

What does homogenous mean

Answer 3

Matter is uniformly distributed
Universe has a uniform density

Question 4

What is the astronomical unit (AU)?

Answer 4

• average distance from the Earth to the Sun
• 1.5 x 10^11 m

Question 5

When is AU most often used?

Answer 5

To express the average distance from the Sun and other planets in the Solar System

Question 6

What is a light-year (ly)?

Answer 6

• distance travelled by light in a vacuum in a time of one year
• 9.46 x 10^15 m

Question 7

When are ly mainly used?

Answer 7

When expressing distances to stars or other galaxies

Question 8

What is an arcsecond?

Answer 8

1/3600 degree

Question 9

What is an arcminute?

Answer 9

1/60 degree

Question 10

What is a parsec (pc)?

Answer 10

The distance at which a radius of one AU subjects an angle of one arcsecond

3.1 x 10^16 m

Question 11

Formula for a pc?

Answer 11

1 pc = 1 AU / tan (1 arcsecond)

Question 12

Relationship between distance ‘d’ pc and angle subtended?

Answer 12

if point is at a distance ‘d’ parsec, angle subtended is 1/d arcsecond

Question 13

When do we use stellar paradox?

Answer 13

To determine distances of stars that are relatively close to the Earth, distances less than 100 pc

Question 14

What is parallax?

Answer 14

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.

Question 15

What is the parallax angle ‘p’?

Answer 15

Angle of stellar parallax

Question 16

Formula relating arcseconds with parsecs?

Answer 16

d (pc) = 1 / p (arcsecond)

Question 17

What is the Doppler effect?

Answer 17

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

Question 18

How can the Doppler effect be used to determine the relative velocity of a distant galaxy?

Answer 18

• Absorption spectrum of a specific element is determined in the laboratory
• Since the same spectrum is observed in light from a distant galaxy, we can compare their absorption spectrums.
• If the observed wavelengths of the absorption lines are different, it is caused by the relative motion between the galaxy and the Earth.

Question 19

Blue shift vs red shift?

Answer 19

Blue shift if the galaxy is moving towards the Earth, since the wavelength appears shorter
Red shift if the galaxy is moving away from the Earth, since the wavelength appears stretched

Question 20

What is the Doppler equation?

Answer 20

Δ λ / λ = Δ f / f = v / c

Question 21

Observations that led to Hubble’s law?

Answer 21

• light from the vast majority of galaxies was red-shifted
• the further the galaxy was the greater the observed red shift and so faster the galaxy was moving

Question 22

What is Hubble’s law?

Answer 22

The recessional speed v of a galaxy is almost directly proportional to its distance d from the Earth.
v ∝ d

Question 23

What is Hubble’s constant H₀?

Answer 23

2.2 x 10^-18 s^-1

Question 24

What does the Big Bang theory state?

Answer 24

At some moment in the past, all the matter in the Universe was once contained in a single point.

Question 25

Evidence for the Big Bang theory?

Answer 25

Hubble’s law - if we ran time backwards, space would be receding into a point Microwave background radiation

Question 26

Explanation for Microwave Background Radiation?

Answer 26

- when the universe was young and extremely hot, space was saturated with high-energy gamma photons - as Universe expanded, the wavelength of these photons stretched to become microwaves OR - Universe can be treated as a black-body radiator - at 2.7K (temperature of space), wavelengths would correspond to around 1mm, which is in the microwave region

Question 27

Formula for the age of the universe?

Answer 27

t ≈ 1 / H₀

Question 28

What is the estimated age of the universe?

Answer 28

~ 14 billion years

Question 29

What is dark energy?

Answer 29

- hypothetical form of energy that explains the acceleration of the universe - fills all of space and accelerates expansion of Universe - should make up for 68% of energy in Universe

Question 30

What is dark matter?

Answer 30

Theory for why the mass in the universe is spread out, instead of concentrated in the centre like galaxies are. Explanation as to why velocity of galaxy does not decrease the further from the centre of the Universe it is. Should make up 27% of the mass in the Universe. Does not interact with light therefore not seen in telescopes.

Question 31

The Big Bang (Series)

Answer 31

Time and space are created The Universe is a singularity - it is infinitely dense and hot

Question 32

10^-35 s

Answer 32

Universe expands rapidly, phase of incredible acceleration called inflation. No matter in Universe, full of electromagnetic radiation (high-energy gamma photons) Temperature is around 10^28 K

Question 33

10^-6 s

Answer 33

The first fundamental particles gain mass

Question 34

10 ^-3 s

Answer 34

Quarks combine to form the first hadrons, such as protons and neutrons. Most of the mass in the Universe was created within the first second

Question 35

1 s

Answer 35

Creation of matter stops, temperature falls to around 10^9 K

Question 36

100 s

Answer 36

Protons and neutrons fuse together to form deuterium and helium nuclei, around 25% of the matter in the Universe is helium nuclei.

Question 37

380 000 years

Answer 37

The universe cools enough for the first atoms to form. This stage’s electromagnetic radiation is now detected as Microwave Background Radiation.

Question 38

30 million years

Answer 38

The first stars appear. First heavy elements begin to form in these stars through nuclear fusion.

Question 39

200 million years

Answer 39

Milky Way forms as gravitational forces pull clouds of hydrogen gas and existing stars together

Question 40

9 billion years

Answer 40

The Solar System forms from the nebula left by the supernova of a larger star.

Question 41

11 billion years

Answer 41

Formation of the Earth

Question 42

12 billion years

Answer 42

Primitive life on Earth begins

Question 43

13.7 billion years

Answer 43

Modern humans evolve. Temperature of the universe is 2.7 K