Chapter 20 - Cosmology

Question 1

Astronomical unit (AU)

Answer 1

Average distance from Earth to Sun. Used to express distance between Sun and other planets.

Question 2

Light year (ly)

Answer 2

Distance travelled by light in a vacuum in one year. Used for distances to stars/other galaxies.

Question 3

Parsec (pc)

Answer 3

Distance at which a radius of 1AU subtends an angle of 1 arcsecond.

Question 4

Stellar parallax

Answer 4

Used to determine distance to stars close to Earth.

Apparent shift in position of a relatively close star against background of distance stars, as Earth orbits Sun.

Question 5

Cosmological principle

Answer 5

Universe is homogenous, isotropic and laws of physics are universal.

Question 6

What is the absorption spectra of a galaxy moving away from Earth like?

Answer 6

Absorption spectra will be red-shifted i.e wavelengths will become longer.

Question 7

What does the Doppler equation show?

Answer 7

The faster the source, the greater the observed Δλ/Δf.

Question 8

Hubble’s law

Answer 8

v ∝ d

v is the recessional speed of a galaxy, d is the distance from Earth.

Question 9

What is the model of the expanding Universe supported by?

Answer 9

Distant galaxies are more red-shifted, indicating that they are moving faster than the closer galaxies.

Therefore, fabric of space and time is expanding.

Question 10

Big Bang theory

Answer 10

The Universe was a very hot, dense singularity which then expanded.

Question 11

Supporting evidence for the Big Bang theory?

Answer 11

Hubble’s law.

Microwave background radiation.

Question 12

Microwave background radiation

Answer 12

• When the Universe was young, space was saturated with high-energy gamma photons.
• Expansion and cooling of Universe stretched λ of photons, which are now seen as microwaves.
• Temperature of Universe is 2.7K.

Question 13

Evolution of the Universe

Answer 13

• Began as very hot, dense singularity.
• Expansion and cooling.
• Creation of matter e.g. quarks.
• More matter than antimatter.
• Quarks combine to form hadrons.
• Protons and neutrons combine to form H and He nuclei.
• Stars form and heavier elements made through nuclear fusion.

Question 14

Accelerating universe

Answer 14

Light from supernova was less intense than predicted so expansion must be accelerating.
This must be fuelled by ‘dark energy’.

Question 15

Dark matter

Answer 15

Expected that velocity of starts would decrease as distance from centre of galaxy increases, but this is not true.

Therefore mass of galaxy is not concentrated in the centre so there must be another type of matter spread through the galaxy.