Booklet 1B - Cosmology (Factual)

Question 1

What causes a projectile to follow a curved path?

Answer 1

The projectile has a constant horizontal velocity and a constant vertical acceleration due to the gravitational field strength.

Question 2

Explain how a satellite remains in orbit.

Answer 2

The satellite has a constant horizontal velocity and a constant vertical acceleration due to the gravitational field strength. This causes it to follow a curved path like all projectiles. This curved path matches the curve of the planet.

Question 3

Define Gravitational Field Strength

Answer 3

Gravitational Field Strength is the weight per unit mass (kg)

Question 4

What can be said about the direction of the Gravitational force?

Answer 4

It is always attractive.

Question 5

In the Gravitational Force equation what is the distance, r?

Answer 5

The distance, r, between the two objects is the distance between their centres (of mass).

Question 6

What assumption is made when calculating the Gravitational force? (This is a reasonable assumption for near-spherical objects like planets)

Answer 6

The mass of an object is concentrated at its centre.

Question 7

What two principles underpin the theory of relativity?

Answer 7

1. When two observers are moving at constant speeds relative to one another, they will observe the same laws of physics.
2. The speed of light (in a vacuum) is the same for all observers.

Question 8

What is the definition of time dilation?

Answer 8

Time dilation is the difference in a time interval as measured by two observers in reference frames which are moving relative to each other.

Time intervals in this case are measured to be longer for an observer measuring events in a reference frame moving relative to them (referred to as t’).

Question 9

What is the definition of length contraction?

Answer 9

Length contraction is the difference in length (in the direction of motion) as measured by two observers in reference frames which are moving relative to each other.

Lengths in this case are measured to be shorter for an observer measuring events in a reference frame moving relative to them (referred to as l’).

Question 10

At what speeds would you have to take relativistic effects into account?

Answer 10

Relativistic effects need to be considered for speeds which are more than 0.1 times the speed of light. For smaller speeds they are negligibly small.

Question 11

What is meant by the Doppler effect?

Answer 11

The observed change in frequency when any wave source is moving relative to an observer.

Question 12

For Doppler Effect calculations you use the following formula.

Describe when you should use the + and when you should use the - in the equation.

Answer 12

Sound source moving towards observer USE - as less time for waves to reach observer.

Sound source moving away from observer USE + as more time for waves to reach observer.

Question 13

Explain what a stationary observer would notice if a car with a siren on it was moving towards them.

Answer 13

Each wave front is emitted closer to the observer than the previous wave. So each wave front reaches the observer in less time than the previous wave. This causes the number of wave fronts arriving at the observer per second to increase. So the frequency the observer experiences is increased.

Question 14

Explain what a stationary observer would notice if a car with a siren on it was moving away from them.

Answer 14

Each wave front is emitted further away from the observer than the previous wave. So each wave front reaches the observer in a longer time than the previous wave. This causes the number of wave fronts arriving at the observer per second to decrease. So the frequency the observer experiences is decreased.

Question 15

Why can’t we use the Doppler equation for Electromagnetic waves?

Answer 15

Because electromagnetic waves travel at the speed of light and therefore relativistic effects have to be taken into account.

Question 16

What is the definition of redshift?

Answer 16

Light from stars or galaxies which are moving away from us is shifted to longer wavelengths (the redder wavelengths for the visible spectrum).

Question 17

Explain the cause of Redshift

Answer 17

Most galaxies are moving away from us, so each wavefront of light observed on Earth is emitted from slightly further away than the last. This causes the number of wave fronts arriving per second at the Earth to decrease. This is an apparent decrease in frequency, which results in an increase in the wavelength of the light we see on Earth.

The frequency of the light emitted by the source does not change.

Question 18

What is Hubbles Law?

Answer 18

Hubble’s Law states that the speed of recession of a galaxy (the speed it moves away from us) is directly proportional to its distance away from us.

Question 19

Explain how Hubble’s Law can estimate the age of universe.

Answer 19

Hubble’s observations show galaxies move away from each other and Earth in all directions. So the universe must be expanding. So if you could go back in time the galaxies etc. would be closer together, so if you go back to the point when all the galaxies were at one point, this would be the beginning of the universe. t=1/H₀

Question 20

What is the definition for The Big Bang?

Answer 20

The Big Bang marks the start of the universe, when everything was in an unimaginably small volume (almost a point) and rapidly expanded.

Question 21

What happens to the temperature of the universe as it expands?

Answer 21

The temperature of the universe decreases (cools) as it expands?

Question 22

What evidence is there to support Big Bang Theory?

Answer 22

• Expanding Universe (Hubble’s Law)
• Cosmic Microwave Background Radiation
• Olber’s Paradox (Why we see a dark sky)
• Abundance of Hydrogen and Helium in the Universe

Question 23

How do the blackbody radiation graphs of stars of different temperature compare?

Answer 23

A hotter star will have a shorter peak wavelength. A hotter star will also emit more radiation per unit surface area per unit time across all wavelengths than a cooler one (if they are similar in mass).

Question 24

What evidence is there that Cosmic Microwave Background Radiation supports Big Bang Theory?

Answer 24

• It is very uniform throughout the Universe.
• It shows the characteristics of blackbody radiation.
• It corresponds to a temperature of approx. 3K (2·74K), exactly matching theoretical models of the cooling of the universe due to expansion.
• CMBR is thought to be the “afterglow” of the Big Bang, cooled to a faint signal in the microwave region.

Question 25

What is Olber’s Paradox?

Answer 25

Olber said that if the universe was infinitely big and old and static that we should see stars in all directions and the sky would not be mostly dark. But the universe is finite in time as it had a beginning (so light from distant stars may not have had time to reach us yet) and the universe is expanding (galaxies are moving away from us so the light may be redshifted into the infra-red part of the spectrum so we will not see the light).

Question 26

Why is the abundance of helium and hydrogen in the universe evidece for the Big Bang?

Answer 26

Big Bang Theory says that matter started as a very simple form that became more complex as time passed. The simplest elements would have formed first and existed in larger quantities than heavier elements, which would not exist until stars are born. These first elements were hydrogen and helium. The fact that there is an abundance of these elements in the universe supports the theory.

Question 27

Why does Redshift support Big Bang Theory?

Answer 27

Redshift occurs because galaxies are moving away from us. This is the continued expansion from the Big Bang. If the galaxies were not moving away the light would not experience redshift.

Question 28

What does the eventual fate of the universe depend on?

Answer 28

The mass-energy density in the universe. This affects the size of the gravitational forces which will control whether the universe will be open (expand forever) or closed (reach a maximum size and start to contract).

Question 29

Why do we not currently know the eventual fate of the universe?

Answer 29

Our measurements of mass-energy density have such large uncertainties that we cannot be sure whether the universe is open or closed.

Question 30

What is the evidence for dark matter?

Answer 30

The mass of a galaxy can be estimated by the orbital speed of the stars within it. From our estimations there is not enough mass to keep the galaxy spinning at the speed it does. Therefore there must be mass we cannot detect. This is called dark matter.

Question 31

What is the evidence for dark energy?

Answer 31

Distant objects are accelerating away from us. There must be something we have yet to discover to cause this acceleration and overcome the attractive gravitational forces acting. The extra energy that these objects gain is called dark energy.