Universe Section

Question 1

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

Answer 1

For small velocities i.e. less than 0.1 times the speed of light relativistic effects are negligibly small.

Question 2

What is the evidence for dark matter?

Answer 2

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 together. Therefore there must be mass we cannot detect. This is dark matter.

Question 3

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

Answer 3

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

Question 4

What is the definition for The Big Bang?

Answer 4

The Universe was initially in a hot and very dense state and then expanded rapidly.

OR

All the matter in the universe was in an unimaginably small volume, almost a point, and rapidly expanded.

Question 5

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

Answer 5

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 6

What is Olber’s Paradox?

Answer 6

Olber said that if the universe was infinitely old and static that we should see stars in all directions. But the universe is not infinitely old 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 7

The colour of a star tells us about it’s surface temperature.This colour can be split into all the radiations being emitted producing a blackbody graph. What do the blackbody radiation graphs of stars tell us?

Answer 7

The temperature of stellar objects is related to the distribution of emitted radiation over a wide range of wavelengths. The hotter the object the shorter the peak wavelength emitted. A hotter star will emit more radiation per unit surface area per unit time than a cooler one. (If they are similar in mass, surface area etc)

Question 8

What is the evidence for dark energy?

Answer 8

Distant objects are accelerating away from us. There must be something to overcome the gravitational forces acting. The extra energy that these objects gain is called dark energy.

Question 9

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

Answer 9

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

Question 10

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 10

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 11

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

Answer 11

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 12

What does the Doppler effect apply to?

Answer 12

All waves

Question 13

What is the definition for time dilation?

Answer 13

Time dilation is the difference in a time interval as measured by an object moving relative to a stationary observer.

Question 14

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

Answer 14

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 15

What is Hubbles Law?

Answer 15

Hubble’s Law is the relationship between the recession velocity of a galaxy as it moves away from us and its distance.

Question 16

What is the definition for redshift?

Answer 16

Redshift of light - Light from stars or galaxies moving away from us is shifted to longer wavelengths and so the redder wavelengths of the spectrum.

Question 17

When will length contraction become apparent?

Answer 17

The stationary observer and a person or object moving at relativistic speeds experience length in the same way. Only when they compare the lengths they have experienced do they notice a difference. The person who is stationary will have experienced a shorter length in comparison.

Question 18

What happens to the source frequency during the Doppler Effect?

Answer 18

Nothing - it remains constant

Question 19

What evidence is there to support Big Bang Theory?

Answer 19

• Cosmic Microwave Background Radiation
• Explanation of Olber’s Paradox (Why we see a dark sky)
• Abundance of hydrogen and helium in the universe
• Redshift of galaxies

Question 20

When will time dilation become apparent?

Answer 20

The stationary observer and a person or object moving at relativistic speeds experience time passing in the same way. Only when they compare the times they have experienced do they notice a difference. The person who is stationary will have experienced a longer time in comparison.

Question 21

What is gravitational field strength?

Answer 21

The force due to gravity per unit mass (kg)

Question 22

Give one application for Hubbles Law

Answer 22

Calculating the age of the universe

Question 23

What assumption is made when calculating the Gravitational force?

Answer 23

The objects are spherical

Question 24

What can be said about the Gravitational force?

Answer 24

It is always attractive.

Question 25

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

Answer 25

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 26

What two principles underpin the theory of relativity?

Answer 26

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 27

Explain Redshift

Answer 27

Explanation Light from distant galaxies and stars can be observed on Earth. These galaxies and stars are moving away from us. So each light wave front 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. So the light is shifted to longer (more red) wavelengths. The frequency of the light emitted by the source does not change.

Question 28

Why does Redshift support Big Bang Theory?

Answer 28

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 29

What is the definition for length contraction?

Answer 29

Length contraction is the difference in length (in the direction of motion) of an object moving relative to a stationary observer. Length contraction only takes place in the direction that the object is travelling.