Cosmology

Question 1

What is the cosmological principal?

Answer 1

The universe is isotopic and homogeneous.

It obeys the laws of physics

Question 2

Explain how stellar parallax is used to determine the distance of stars.

Answer 2

• Parallax - the apparent shift of a star against a background of distance stars.
• caused by the Earth’s orbit around the sun.
• parallax angle of 1 arcsecond when displacement of Earth is 1AU corresponds to distance 1 pc.

Question 3

Define Parallax

Answer 3

The apparent shift in position of a star against a backdrop of distant stars

Question 4

What equation gives the parallax angle?

Answer 4

d=1÷p

Question 5

Define the Doppler Effect

Answer 5

The change in wavelength when a wave source moves relative to the observer

Question 6

Give the equations for the Doppler effect

Answer 6

Δλ/λ≈Δf/f≈v/c

Question 7

How does Hubbles Law link to the Doppler effect?

Answer 7

• the change in λ from distance galaxies showed a red shift of the galaxy. -the red shift shows the Galaxy is moving further away from Earth.
• the greater the change in λ the greater the recessional speed of the Galaxy.

Question 8

State Hubbles law

Answer 8

The recessional speed of a galaxy is almost directly proportional to its distance from Earth.
v≈H₀d

Question 9

What are the two main things that provide evidence for the Big Bang?

Answer 9

• Hubbles law

- Microwave background radiation

Question 10

How does Microwave background radiation provide evidence for the Big Bang? (in terms of high energy photons)

Answer 10

• Space was saturated with high energy gamma photons.
• As the universe expanded the wavelength of the gamma photons were stretched out.
• This EMR is observed as microwaves which has constant intensity in all directions.

Question 11

How does Microwave background radiation show evidence for the Big Bang? (In terms of temperature)

Answer 11

• As the universe expanded it cooled and is now 2.7K.
• modelling the universe as a black body, the peak λ at this temperature corresponds to 1mm
• this is in the microwave spectrum

Question 12

What are the two units of Hubbles constant?

Answer 12

S^-1

Kms^-1 Mpc^-1

Question 13

State the thing that explains the acceleration of expansion of the universe

Answer 13

Dark Matter

Question 14

How is Hubbles constant determined?

Answer 14

The gradient of a recessional speed x distance graph.

Question 15

Explain how Hubbles constant provides evidence for the Big Bang

Answer 15

• almost all observed galaxies have been redshifted
• galaxies further away are moving at higher speeds
• this shows the the universe is expanding in all directions

Question 16

What percentage of Dark Matter makes up the universe?

Answer 16

68%

Question 17

Define dark matter?

Answer 17

A hypothetical form of energy that is causing the universe to accelerate its expansion

Question 18

Describe the evolution of the universe after the Big Bang

Answer 18

• the Universe is hot and dense
• the universe expands rapidly and is full of high energy gamma photons
• universe cools down and matter created by pair production.
• quarks and leptons are formed then hadrons
• deuterium and helium nuclei form
• atoms formed
• galaxies formed by strong gravitational attraction
• stars create solar systems

Question 19

What is the value of 1 AU

Answer 19

1.5x10¹¹ m

Question 20

Why is hydrogen used for emmision spectral lines and not other elements?

Answer 20

• Hydrogen is the most common element in stars and has most intense spectral lines
• Intensity of other elements may be too low to be measured accurately.

Question 21

Explain how the emission spectrum of a distant star may be equivalent to the spectrum of a star further away

Answer 21

• Spectrum will be less intense

- The galaxy is moving faster so greater wavelength (greater red shift)