Cosmology

Question 1

What is the equation for doppler shift?

Answer 1

Δf/f = Δλ/λ = v/c
Δf/λ = change in frequency/wavelength (Hz)
f/λ = reference frequency / reference wavelength (frequency/ wavelength of the source)
Δv = relative velocity of the source (- the observer’s velocity)
c = speed of light

Question 2

What is redshift?

Answer 2

The wavelength of light is stretched, so that the light is seen as ‘shifted’ towards the red part of the spectrum.
z = Δλ/λ = - v/c
(z, the measure of redshift, is positive if a star is moving away from Earth)

Question 3

What is an eclipsing binary star system?

Answer 3

Where 2 stars orbit around a common centre of mass with their orbital plane in the Earth’s line of sight.

Question 4

What does the light intensity curve of orbiting eclipsing binary stars look like?

Answer 4

1- intensity is at full brightness to begin with
2 - then big star blocks little star (so all its light it blocked) (primary minima)
3 - back to normal
4 - little star blocks big star so there is less of a dip in intensity (secondary minima)
5 - back to normal

Question 5

What does the Hubble diagram look like?

Answer 5

recession velocity on y axis and distance on x axis.
They are directly proportional - straight line diagram.

Question 6

What is Hubble’s law? (written)

Answer 6

The recessional velocity of a galaxy is proportional to its distance from Earth

Question 7

What is Hubble’s law? (equation)

Answer 7

v = Hd
v = recessional velocity of an object (km/s)
H = Hubble constant (km/sMp/c)
d = distance between the object and the Earth (Mpc)

Question 8

What does Hubble’s law and diagram show us?

Answer 8

That the furthest galaxies are moving away faster.

Question 9

What is the Hubble constant?

Answer 9

H = 67.4 +/- 0.5 (km/s Mp/c)
in data sheet = 65

Question 10

How can Hubble’s constant be used to find the age of the universe?

Answer 10

Hubble’s constant tells us the speed of an object at any distance, and since the distance between all t=objects in the universe at beginning of expansion was 0, the time in this equation must be the age of the universe. so 1/H = estimate of the universe

Question 11

How old is the universe?

Answer 11

13.7 billion years old

Question 12

What is the big bang theory?

Answer 12

The most renowned theory for the beginning of the universe.
- All matter at t=0 was in one hot, dense singularity and since then has been expanding and cooling

Question 13

What are 3 key pieces of evidence for the big bang theory?

Answer 13

1 - Galactic redshift & Hubble’s law = evidence for expansion of universe
2 - CMBR - evidence that the Universe had a hot beginning
3 - relative abundance of hydrogen and helium = evidence the Universe as once far hotter and denser than it is now.

Question 14

How does galactic redshift provide evidence for the expansion of the universe?

Answer 14

1 - observations show that distance galaxies are all moving away from us = the stretch of the wavelengths due to the expansion of the universe
2 - Galaxies further away moving away faster - v ∝ d = means at some point in the past galaxies must have been at the same point.

Question 15

What is cosmic microwave background radiation?

Answer 15

The radiation detected in all parts of the Universe. (in the microwave region of the electromagnetic spectrum)

Question 16

How does CMBR provide evidence for a hot beginning?

Answer 16

• Theory predicts the existence of a uniform black body radiation that peaks in the microwave region - extremely uniform, meaning the universe was initially much smaller
• CMBR is consistent with radiation that has been redshifted overtime - used to be producing gamma rays (when universe was high energy and hot), wavelengths stretched overtime
• radiation left over from big bang

Question 17

How does the relative abundance of hydrogen and helium prove that the universe was originally extremely hot?

Answer 17

• Theory predicts hydrogen fused into helium nuclei in early universe (as it was so hot)
• 3:1 ratio of hydrogen to helium

Question 18

What is the process of nucleosynthesis of hydrogen and helium?

Answer 18

• Once protons and neutrons could exist freely in the universe, neutrons would decay into protons (so hydrogen to helium ratio is 3:1 for this reason)
• the universe would have been hot enough to fuse into helium nuclei (free neutrons became stable when bound to helium nuclei

Question 19

What is the Hubble Tension problem?

Answer 19

The present measure of the expansion rate significantly exceeds the prediction.

Question 20

What is dark energy?

Answer 20

An unknown source that is said to have possibly sped up the rate of expansion.

Question 21

Why is dark energy a controversial solution to the Hubble Tension problem?

Answer 21

• It cannot be detected directly but there is evidence for its existence
• Nothing is known of its nature and origins
• No mechanisms can currently explain how it drives the accelerated expansion

Question 22

What are quasars?

Answer 22

Extremely luminous star-like sources of radiation with very high redshifts.

Question 23

How were quasars discovered and when?

Answer 23

IN THE 1960s
- strong radio emissions
- High luminosities
- Extremely large redshifts
- Small size

Question 24

What is the formation of a quasar?

Answer 24

• They are supermassive black holes surrounded by an accretion disc of matter
• They are found at the centre of extremely distant galaxies
• When they become active (when mater falls into it), they become quasars

Question 25

What is the structure of a quasar?

Answer 25

A supermassive black hole surrounded by an accretion disc and emits jets of radiation at the poles.

Question 26

What are the most distant measurable objects?

Answer 26

quasars?

Question 27

Why are quasars strong emitters of all wavelengths?

Answer 27

• Gravitational potential energy of infalling matter is transferred to electromagnetic radiation - so quasars aren’t just strong radio emitters

Question 28

How do we know that quasars are some of the most distant measurable objects?

Answer 28

• They have extremely large redshifts
• This allows astronomers to see very far back into the early universe so it was not long after the big bang

Question 29

What is an exoplanet?

Answer 29

A planet found outside out Solar System, in orbit around another star

Question 30

What are 2 reasons why are exoplanets difficult to detect directly?

Answer 30

1- Light from host star is much brighter than
the reflected light from the planet
2 - they subtend extremely small angles compared to the resolution of telescopes

Question 31

What are 3 methods to indirectly detect exoplanets?

Answer 31

• transit method
• radial velocity method
• Direct observation

Question 32

How does the transit method work?

Answer 32

• Look at the points where the light from the star dips, this could be when the planet passes in front of the star and obscures some of the light

Question 33

What does the transit method also reveal about an exoplanet?

Answer 33

• It’s size and orbital period

Question 34

What are 2 limitations to the transit method?

Answer 34

• The accuracy can be reduced if the Earth, planet and star are not aligned in the same plane
• Only planets with short orbital periods can be detected

Question 35

How does the radial velocity method work?

Answer 35

Detects very small changes in the star’s speed caused by gravitational pull of orbiting planets. This creates a periodic shift in the star’s spectral lines.

Question 36

What is the main limitation of the radial velocity method?

Answer 36

Low-mass or Earth-like planets do not cause so much ‘wobble’ as high-mass planets since they have a greater gravitational pull on the star.

Question 37

What is the equation for intensity?

Answer 37

power/area

Question 38

What is the surface area if a sphere?

Answer 38

4πr²