20, Cosmology

Question 1

What is an astronomical unit?

Answer 1

The average Earth Sun distance (about 150 million km).

Question 2

What is a light-year?

Answer 2

The distance travelled by light in a vacuum in one year.

Question 3

What is a parsec?

Answer 3

The distance at which the the right angle triangle formed by an angle of 1 arc-second and opposite length of 1 AU has an adjacent length of 1 pc.

Question 4

How does parallax work?

Answer 4

For close objects, moving the position at which you observe something changes the position of the object relative to the background, by waiting half a year for the earth to orbit the sun the position of close stars changes against the stars that are further away, the angle between these two measurements (in arc-seconds / 2) is inversely proportional to the distance in parsecs.

Question 5

What does the equation p = 1/d show?

Answer 5

The parallax angle for a star = 1 / the distance to the star in parsecs.

Question 6

What is the Doppler effect?

Answer 6

The way in which waves are compressed or stretched as the wave source moves towards or away from the observer.

Question 7

How can the Doppler effect be used to find the recession velocity of stars and galaxies?

Answer 7

By observing the absorption lines you can directly measure the change in wavelength of the light emitted from the galaxy (as absorption lines can are specific to certain elements emitted at the same wavelength), this can be used with the Doppler equation to find the velocity.

Question 8

What is the equation Δλ/λ ≈ Δf/f ≈ v/c and what does it mean?

Answer 8

This is the Doppler equation. it shows the change in wavelength / original wavelength is approx equal to the change in frequency / original frequency which is approx equal to the velocity of the object / the velocity of the wave.

Question 9

What is Hubble’s law?

Answer 9

the recessional speed of a galaxy is directly proportional to its distance from the observer.

Question 10

What is the equation v = H₀ d?

Answer 10

Velocity = Hubble’s constant * distance from observer.

Question 11

What is the explanation for Hubble’s law?

Answer 11

That space is expanding such that all points are moving away from each other.

Question 12

What is the cosmological principle?

Answer 12

The assumption that on a large scale the Universe is homogeneous (same everywhere) and isotropic (same looking in all directions) and the laws of physics are universal.

Question 13

What are the two important pieces of evidence for the big bang.

Answer 13

Hubble’s law and the CMB.

Question 14

What are the two explanations for the CMB?

Answer 14

1:
Early universe was filled with gamma photons which were red-shifted over the expansion of space.
2:
The universe is a black body radiator with temperature ~ 2.7 K.

Question 15

How and why can the Hubble constant be used to predict the age of the universe?

Answer 15

Age of the universe ≈ 1 / H₀, if a galaxy at distance d is moving at constant speed v then a time d/v must have elapsed since it was at out position.

Question 16

How did the universe evolve over time?

Answer 16

Started as a singularity at the big bang.
Rapid expansion.
Quarks then hadrons form up until about 1s.
helium nuclei and deuterium form (100s).
first atoms at about 400,000 years.
Stars at 30 million years.
Milky way forms at 200 million years.
Solar system forms at 9 billion years.

Question 17

What is the evidence for dark energy?

Answer 17

It appears that the rate of expansion of the universe is increasing (H₀ is increasing) from measuring the intensity of specific standard candles.

Question 18

What is the evidence for dark matter?

Answer 18

Observations of the velocities of stars in galaxies do not match predictions based on the fact that we see more matter at the centre of galaxies so the velocities should decrease the further from the centre you are but the velocity is roughly constant.