Hunting for dark energy with the WiggleZ

Activity 1

Understanding our expanding universe

By measuring dark energy, the WiggleZ survey will help to explain why our universe seems to be expanding faster now than it did in the past. But we didn’t always know the universe was getting away from us. Even Einstein thought it stayed pretty much the same.

  1. In groups of 2-4 people copy or print the timeline below onto a piece of A3 paper. Use the information in the website Hunting for dark energy with the WiggleZ and the links in the timeline to find out how our understanding of the expansion of the universe has changed over the last century. Record your findings under the headings on your A3 sheet.
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Click here for a pdf version of the timeline. Click timeline headings for further information.

Hubble’s Law

Edwin Hubble found that galaxies were moving away at a velocity proportional to their distance from Earth. So, the further away, the faster the galaxy seemed to be travelling. He described their movement by the equation:

v=H0d

where v is the recession velocity (speed away from us), H0 is Hubble’s constant and d is the distance to the galaxy. Modern estimates of H0 are around 71 km/sec/Mpc (1 Mpc or Megaparsec = 3.25 million light years).

  1. If a star in a galaxy was measured to be moving away at 1,100 kms-1, use the above information to work out how far away the star is in light years?

The original estimate made by Hubble for H0 was somewhat larger than today’s 71 km/sec/Mpc. He estimated H0 by plotting the distance to galaxies (measured from their brightness) against their velocity (measured from redshift). The slope of the graph provided H0

  1. Use Hubble’s data below to estimate H0.

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  1. Why do you think Hubble’s H0 was so different from modern estimates?

In 1965 we started to understand the expansion of the universe better when evidence for the Big Bang turned up unexpectedly. Turn on your TV and you can see it for yourself. The fuzzy ‘noise’ you see when you’re not tuned into a channel is partly due to left over radiation from the Big Bang. Called the cosmic microwave background (CMB), this radiation is found throughout space.

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Cosmic microwave background
(Credit: NASA/WMAP Science Team)

Notice the patchy look of the CMB. These patches became the galaxy structures we see today and can be used to give us information about the evolution and expansion of our universe.

  1. How is the WiggleZ Survey using the CMB to find out about dark energ

  2. Do you think the WiggleZ project will prove that dark energy exists? Explain your answer.

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Posted June 2009.