Post-16: The Expanding Universe
Pre- and Post‐Workshop Information
Pre‐Workshop:
The following information is designed to prepare students for the workshop and
is linked to the curriculum (below). It is not essential reading however teachers
may find it useful to discuss these concepts beforehand with the class.
Post‐Workshop:
Summary questions are provided for the students to answer in class or for
homework as a way of cementing the material covered in the workshop.
Extension activities to the workshop are available to carry out as part of a lesson
or to be given out as a mini‐project. These can be found here:
http://www.nmm.ac.uk/schools/post-16-programmes-at-the-royal-observatory
Glossary
Acceleration
Dark matter
Electromagnetic
spectrum
Emission
Frequency
Galaxy
Nebula
Radiation
Stellar
Universe
Gradient
Red shift
Vacuum
Gravity
Speed
Velocity
Wavelength
Waves
Curriculum Links
Post‐16: AQA Physics: Unit 5A 1.3–4; Edexcel Physics: P2.3 41, P2.3 47, P2.5 68‐69,
P5.6 129, P5.6 132‐133, P5.6 135; OCR Physics: G482 2.4.3, G482 2.5.4, G485 5.5.1
k‐n, G485 5.5.2 a‐h
Pre‐Workshop Information for Teachers
In 1923 Edwin Hubble looked at galaxies (which he thought were nebulae or
clouds of bright gas) through a 100” reflector (telescope with a mirror) on Mt
Wilson in the US. He took a photo of Andromeda (which can be seen on a clear
night with the naked eye). Hubble grouped these galaxies according to similar
characteristics. The Galaxy Zoo project (www.galaxyzoo.org) is an example of
citizen science whereby the public have an opportunity to contribute to scientific
research. The project involves classifying millions of galaxies. Knowing the
morphology of a galaxy tells us something about its evolution and gives us an
insight into the structure and history of the Universe
Hubble analysed the light from very distant galaxies and found that their spectra
were all redshifted. This Doppler effect is best described by taking the example of
a police car with its siren on, moving at high speed towards an observer. The
crests of the sound wave will be closer together as the source moves towards the
observer and therefore the frequency of the sound wave will be higher. As the
police car moves away from the observer, the crests of the sound wave are spaced
further apart and the frequency is lower. The same thing happens with light. If a
star is moving slightly closer to us, the light is shifted to a slightly higher
frequency. When the star moves slightly further away, the light is shifted to a
slightly lower frequency. Hubble’s results showed distant galaxies are receding
from us. This was a huge discovery as previously scientists thought the Universe
was static. Hubble’s results of an expanding Universe then led to the Big Bang
theory for the origin of the Universe.
Hubble showed there was a positive linear relationship between the velocity and
distance of distant galaxies i.e. more distant galaxies are receding faster. The
gradient of this graph is called H0 (Hubble’s constant), this gives the time since
the Big Bang i.e. the age of the universe. The distance to a galaxy depends on its
luminosity (total power output) and the intensity received at the telescope
detector – luminosity per unit area. Velocity can be calculated from the Doppler
shift – how much spectral lines are shifted from their rest frame wavelength.
The most accurate value for the Hubble constant has been determined using a
different technique to Hubble. A satellite called the Wilkinson Microwave
Anisotropy Probe (WMAP) has been measuring temperature fluctuations as small
as 0.0002 K in the left‐over radiation from the Big Bang (called the cosmic
microwave background, CMB) since 2001. This is radiation that has cooled and
expanded with space thus entering the microwave (long wavelength) region of the
electromagnetic spectrum. Ripples in the CMB indicate the initial conditions for
the formation of galaxies and reveal the shape and fate of the Universe. WMAP
has measured the age of the Universe to be 13.7 billion years.
The European successor to WMAP, a satellite called Planck is currently mapping
the sky using radio receivers operating at very low temperatures. They will reveal
anisotropies (temperature differences) in the CMB to a resolution of 1 microkelvin
and will determine a more precise value for H0.
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Post‐Workshop Questions
1) What is the difference between velocity and speed?
2) On a distance‐time graph what does the slope (gradient) represent? What can you
say about the object’s motion if the slope gets steeper?
3) If an object moves a distance of 2 km in 3 minutes, what speed is it moving at?
4) Calculate the distance in km that light travels in one year (speed of light, c = 3 x
108 m/s).
5) Calculate the amount of energy transferred from a 60 W light bulb that is switched
on for 5 minutes. Use the formula: energy (Joules) = power (Watts) x time (seconds).
6) What factors interfere with observations of the night sky?
7) Why is it not currently possible to measure accurate distances to stars and
galaxies?
8) Define intensity and explain why it decreases with distance from the source.
9) A star is 15 parsecs away. What is its distance in light‐years?
(1 parsec = 3.0857 X 1013 km)
10) What is the Universe made of?
11) Outline the supporting evidence for the Big Bang.
12) Write down Hubble’s law. What are the units of Hubble’s constant (H0)?
13) Which other physical properties related to velocity and distance could you use
instead to demonstrate Hubble’s law?
14) Which types of objects are best to measure for the Hubble diagram?
15) Why is the ultimate fate of the Universe difficult to predict?
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Post‐Workshop Questions: Answers
1) Velocity is speed (distance travelled over time) with direction.
2) The gradient represents the speed (distance ÷ time). A steeper slope shows the
object has a greater speed.
3) Speed = distance ÷ time. Speed of object = 2000 m ÷ 180 sec = 11.1 m/s
4) Distance = speed x time = (3 x 105 km/s) x (60 x 60 x 24 x 365.25 secs) = 9.47 x
1012 km
5) Energy (Joules) = power (Watts) x time (seconds). Energy = 60 x 300 sec = 18
000 J
6) The atmosphere distorts light reaching us from space, light pollution reduces
our ability to observe faint objects, weather (clouds) affects our view of the night
sky.
7) Limits of current technology. We have to make lots of assumptions for distant
galaxies e.g. assuming all galaxies in a cluster are at the same distance from us
and the brightest galaxies have the same brightness.
8) Intensity = luminosity ÷ 4π x distance2. Intensity drops as 1/r2 as the energy of
a galaxy travels outwards in a sphere (surface area 4πr2) of increasing radius
moving at the speed of light.
9) From question 4, 1 light‐year (ly) = 9.47 x 1012 km. Star is 15 x 3.09 x 1013 =
4.64 x 1014 km. Distance in ly = 4.64 x 1014 km ÷ 9.47 x 1012 km = 49 ly.
10) Energy, luminous matter (stars, rock, dust and gas), dark matter and dark
energy.
11) Hubble found distant galaxies were moving away from us indicating the
Universe is expanding and must have originated from a single point. Other
evidence comes from the discovery of relic radiation from the Big Bang, the
cosmic microwave background which has a low energy (2.7 K) and an
overabundance of helium that must have been produced in the early hot
Universe.
12) v = H0r, where v is the velocity of the receding galaxy, r is the distance from
Earth and H0 is the Hubble constant, units km/s/Mpc.
13) Redshift can be used instead of velocity (redshift, z = Δλ/λ) and absolute
magnitude can replace distance.
14) Distant galaxies and quasars are good to look at; quasars are incredibly
luminous and are the most distant measurable objects having the largest
redshifts.
15) The best measurements come from WMAP which studied small ripples in the
cosmic microwave background. Our understanding of the Universe depends on
the resolution and sensitivity of our instruments and being able to observe it
however 96% of the Universe consists of dark energy and dark matter which can
only be detected by its interaction via gravity.
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