Booklet 1B - Cosmology (Factual) Flashcards
Describe an experiment to measure the acceleration of a falling object.
- Setup the apparatus as shown.
- Measure the displacement the ball falls using a metre stick and the time for it to fall use the timer. The initial speed will be zero.
- Use the equation s = ut + 1/2at2 to solve for a, the acceleration due to gravity.
What causes a projectile to follow a curved path?
The horizontal and vertical components of a projectile are independent of each other. The projectile has a constant horizontal velocity and a constant vertical acceleration due to the gravitational field strength.
Explain how a satellite remains in orbit.
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.
Define Gravitational Field Strength
Gravitational Field Strength is the weight per unit mass (kg)
What can be said about the direction of the Gravitational force?
It is always attractive.
In the Gravitational Force equation what is the distance, r?
The distance, r, between the two objects is the distance between their centres (of mass).
What assumption is made when calculating the Gravitational force? (This is a reasonable assumption for near-spherical objects like planets)
The mass of an object is concentrated at its centre.
What two principles underpin the theory of relativity?
- When two observers are moving at constant speeds relative to one another, they will observe the same laws of physics in their own reference frame.
- The speed of light (in a vacuum) is the same for all observers.
What is the definition of time dilation?
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’).
What is the definition of length contraction?
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’).
At what speeds would you have to take relativistic effects into account?
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.
What is meant by the Doppler effect?
The observed change in frequency when any wave source is moving relative to an observer.
For Doppler Effect calculations you use the following formula.
Describe when you should use the + and when you should use the - in the equation.
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.
Explain what a stationary observer would notice if a car with a siren on it was moving towards them.
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.
Explain what a stationary observer would notice if a car with a siren on it was moving away from them.
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.
Why can’t we use the Doppler equation for Electromagnetic waves?
Because electromagnetic waves travel at the speed of light and therefore relativistic effects have to be taken into account.
What is the definition of redshift?
Light from stars or galaxies which are moving away from us is shifted to longer wavelengths (the redder wavelengths for the visible spectrum).
Explain the cause of Redshift
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.
What is the Hubble-Lemaitre Law?
The Hubble-Lemaitre 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.
Explain how the Hubble-Lemaitre Law can estimate the age of universe.
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
What is the definition for The Big Bang?
The Big Bang marks the start of the universe, when everything was in an unimaginably small volume (almost a point) and rapidly expanded.
What happens to the temperature of the universe as it expands?
The temperature of the universe decreases (cools) as it expands?
What evidence is there to support Big Bang Theory?
- The Large number of galaxies showing redshift rather than blueshift
- Cosmic Microwave Background Radiation
- The darkness of the sky (Olber’s Paradox)
- Abundance of Hydrogen and Helium in the Universe
How do the blackbody radiation graphs of stars of different temperature compare?
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).
What evidence is there that Cosmic Microwave Background Radiation supports Big Bang Theory?
- 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.
What is Olber’s Paradox?
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).
Why is the abundance of helium and hydrogen in the universe evidece for the Big Bang?
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.
Why does Redshift support Big Bang Theory?
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.
What does the eventual fate of the universe depend on?
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).
Why do we not currently know the eventual fate of the universe?
Our measurements of mass-energy density have such large uncertainties that we cannot be sure whether the universe is open or closed.
What is the evidence for dark matter?
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.
What is the evidence for dark energy?
The accelerating rate of the expansion of the universe. There must be something we have yet to discover to cause this acceleration and overcome the attractive gravitational forces acting.
