Chapter 20 - Cosmology

Question 1

What is 1 astronomical unit?

Answer 1

It is the average distance of the earth from the sun.

Question 2

What is one light year?

Answer 2

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

Question 3

What is 1 parsec?

Answer 3

The parsec is defined as the distance at which a radius of one AU subtends an angle of one arcsecond.

Question 4

What is a stellar parallax?

Answer 4

A technique used to determine the distance to stars that are relativity close to the earth (less than 100pc) by comparing their apparent positions against distant stars at times 6 months apart.

Question 5

What is the Doppler effect?

Answer 5

The Doppler effect - the apparent change in the frequency of a wave caused by relative motions between the source of the wave and the observer

Question 6

How is the Doppler effect used in starlight?

Answer 6

The Doppler effect can be used to determine the relative velocity of a distant galaxy.
First the absorption spectrum is observed of a specific element is determined in the laboratory. The same spectrum is observed in light from a distant galaxy. Any difference in the observed wavelengths of absorption lines must be caused by the relative motion between the galaxy and earth.

Question 7

What is blue shift?

Answer 7

-if the galaxy is moving towards the earth the absorption lines will be blue-shifted - they move towards the blue end of the spectrum as the wavelength appears shorter

Question 8

What is red shift?

Answer 8

If the galaxy is moving away from the earth the absorption lines will be red shifted - they all move towards the red end of the spectrum, because the wavelength is stretched.

Question 9

What is the Doppler equation for electromagnetic waves?

Answer 9

Change is wavelength/wavelength = v/c

Question 10

What is Hubble law?

Answer 10

The recessional speed of a galaxy is almost directly proportional to its distance from earth.

Question 11

What is the cosmological principle?

Answer 11

The cosmological principle is the assumption that, when viewed on a large enough scale, the universe is Homogeneous and Isotropic , and laws of physics are universal.

Question 12

Explain in detail the cosmological principle?

Answer 12

• the laws of physics can be applied across the universe. It means theories and models tested her on earth can be applied to everything within the universe over all time and space.
• homogeneous means that matter is distributed uniformly across the Universe. For a very large volume the density of the universe is uniform.
• isotropic means that the universe looks the same in all direction to every observer. If follows that there is no centre or edge to the universe

Question 13

How does Hubble law support Big Bang?

Answer 13

It shows that the universe is expanding - the galaxies are receding from each other as space itself is expanding in all dimensions. It means if we run back time the universe would be much smaller, denser and hotter and would eventually reach a single point. It is the single point that according to the Big Bang expanded out to form the present day universe

Question 14

How does microwave background radiation support the Big Bang?

Answer 14

When the universe was young space was saturated with Hugh energy gamma photons. The expansion of the universe means that space itself was stretched , the expansion stretched the wavelength of these high energy photons so we now observe this radiation as microwaves.
The universe was extremely hot and dense at the start the expansion of space has reduced its temperature to around 2.7K. This temperature would correspond to 1mm - microwave region(Wienx considering it as a black body)

Question 15

Stats what is meant by the Big Bang? Describe how it explains the origin of microwave background radiation?

Answer 15

• creation of the universe from which space/time evolved
• at the start the universe was hot/infinitely dense
• saturated with gamma photons
• expansion of the universe led to cooling
• the wavelength of the gamma radiation stretched to microwaves by the expansion
• the temperature of the universe is 2.7K if we treat it as a black body radiation the peak wavelength would correspond to 1mm = microwave

Question 16

Explain the importance of the discovery of microwave background radiation?

Answer 16

• predicted by the Big Bang

- no other theory could account for the origin of this radiation