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Hunting for dark energy with the WiggleZ

Mysterious dark energy is thought to make up a large part of our universe. But what is dark energy and how will the WiggleZ help us understand it?

Contents

Key text

Box 1: Doppler shift
Box 2: Where next?
Box 3: Team, task, technology Activities Activity 1: Understanding our expanding universe
Further reading
Useful sites
Glossary

Key text

Hunting for dark energy with the WiggleZ

This topic is sponsored by the University of Queensland and Swinburne University of Technology under an ARC Discovery Project grant.

You will get more from this topic if you have mastered the basics of electromagnetic radiation – these links will take you to an annotated list of sites with helpful background information.

Here is an astonishing thought.

Astronomers are telling us that nearly all of the universe in which we live is invisible. This invisible, ‘dark’ part of our universe doesn’t seem to emit any light or radiation. Only about five percent of the universe is in the form of glowing galaxies, stars, clouds of gas and dust, and planets. That five percent includes us.

According to an increasingly popular theory, most of the dark universe is made up of mysterious stuff called dark energy (most of the rest is dark matter). Australian astronomers are among the leaders in the race to understand it. In fact, they were at the forefront in producing evidence that it exists at all. Now, using advanced equipment attached to the Anglo-Australian Telescope at Siding Spring Mountain in New South Wales, they are in the midst of a project to measure it.

To show that astronomers have a sense of humour, they have called their project WiggleZ, which sounds like the name of the four guys in the coloured skivvies. There the similarity ends. The ‘Z’ represents a way of measuring how far away galaxies are, and the ‘wiggle’ the astronomers are looking for is the memento of sound waves that bounced around the early universe. Sounds bizarre? Read on.

The riddle of dark energy

To understand what the Australian astronomers are doing and why, we need to know a bit more about dark energy. Not that anyone knows very much about it.

Back in 1929, the American astronomer Edwin Hubble announced that most galaxies we can see are moving away from us and from each other, and that the further apart two galaxies are, the faster they are separating. Hubble’s interpretation of his findings was that the universe is expanding. His discovery was based on redshift of the light arriving from galaxies. That is, the light from galaxies moving away from us stretches towards the red end of the spectrum (Box 1: Doppler shift). The faster a galaxy is travelling, the greater the redshift. Hubble found that fainter (so presumably more distant) galaxies had greater redshifts and so were moving faster.

Using various measures to establish how far away the galaxies were, Hubble (and those that followed him) found that their redshift (and therefore their speed away from us) was always proportional to their distance. The ratio of the two became the famous ‘Hubble constant’ and represents the current expansion rate of the universe.

But now we have evidence that the Hubble constant is not really ‘constant’. In the late 1990s, two teams, one led by Australian astronomer Brian Schmidt, found by measuring the brightness and movement of a certain type of exploding star (supernova) that the universe was expanding at an accelerating rate.

The award-winning teams suggested that something is causing the expansion of the universe to speed up. And that ‘something’ could be dark energy, although not all astronomers would agree (Box 2: Where next?).

Related site: 2007 Gruber Cosmology Prize, the expanding universe.
Provides background information on the award for the discovery of the accelerating universe.
(Gruber Foundation)
http://www.gruberprizes.org/GruberPrizes/Cosmology_LaureateOther.php?id=56&awardid=42

Back in 1917, that extraordinary genius Albert Einstein had a vision of something like dark energy. He proposed an unseen influence counteracted the pull of gravity between all the matter in the universe, in order to stop the whole thing collapsing by his newly formed equations. In 1917 Einstein didn't realise the universe was in motion. He called it the ‘cosmological constant’ but later gave it up as a bad idea once Hubble's discovery of the expansion of the universe was made public 12 years later. However, many modern astronomers think he was on to something.

The WiggleZ project

Modern astronomy tells us that the depths of the universe hold uncountable numbers of galaxies, huge congregations of stars similar to the Milky Way galaxy in which our Sun resides. We know these are not scattered at random. In some parts of the cosmos galaxies crowd together in clusters and even superclusters. Elsewhere, the universe seems relatively empty of galaxies, creating voids which have been likened to soap bubbles.

The WiggleZ project will hunt for some pattern in the way these galactic structures are distributed. The team wants to give 200,000 galaxies a precise location in space, including how far away they are. To do this, they will use state-of-the-art technology at the Anglo-Australian Telescope (Box 3: Team, task, technology).

The inspiration for this search comes from images we have of the universe when it was very young, only a few hundred thousand years old. Images of the baby cosmos have been put together from measurements of the cosmic microwave background (CMB), first detected in the 1960s. The WiggleZ team wants to compare these pictures with a much more recent snapshot.

Related site: The WiggleZ of the universe
A ten minute podcast describing the WiggleZ survey to measure dark energy.
(Swinburne University of Technology, Australia)
http://il.cc.swin.edu.au/ilectures/ilectures.lasso?ut=268&id=4516

The CMB is a whisper of radiation that reaches detectors on Earth from all over the sky. When the CMB was set loose, only 400,000 years after the Big Bang, the universe was very hot and dense and the CMB took the form of light similar to that produced by a light bulb. Now, more than 13 billion years later, the universe has expanded and cooled so much that the CMB has mostly been reduced to a gentle wash of microwaves, not unlike those that power your microwave oven.

The CMB is almost the same whichever direction we look; almost but not quite. Pictures constructed from satellite data show very subtle variations in the strength of the microwave signal. These hot spots and cold spots, which differ in temperature by only millionths of a degree, can be interpreted as very slight differences in the crowding together of matter in the young universe. Hot spots had slightly more matter than average; cold spots a bit less.

These temperature differences seem to have a certain regularity, with peaks and troughs recurring in a detectable rhythm. The popular explanation for these fluctuations is that they come from a sort of sound wave that echoed around the early universe. Back then, matter had not yet formed into atoms. Instead we had a plasma of protons, neutrons and related particles, collectively called baryons. The sound waves left their imprint in the distribution of this early matter. So we can talk about ‘acoustic baryon waves’ or more informally ‘baryonic wiggles’.

The ‘hot’ and ‘cold’ spots in the cosmic microwave
background correspond to today’s galaxies.
(Credit: NASA/WMAP Science Team)

The Australian researchers are hunting for these wiggles today, or at least the imprint they left. It seems that over billions of years, as the universe grew larger and colder, and as stars and galaxies began to form, gravity pulled material towards and away from those early peaks and troughs of matter, sharpening the differences between them. In this way, they became the template on which the modern universe is formed, with the hot spots becoming the seeds of super-clusters of galaxies and the cold spots giving rise to relative voids. The WiggleZ team wants to find rhythms in the distribution of galaxies today, patterns which they can compare with those in the CMB pictures. Here is the key point. The difference between the patterns now and then should depend very much on the way dark energy has affected the expansion of the universe. Taking measurements of the spread of galaxies and voids at different ages of the universe will, they hope, give a time-line of varying cosmic expansion, and effectively measure the amount of dark energy.

Hunting for dark energy with the WiggleZ

Box 1 | Doppler shift

Spectroscopy is a vital part of an astronomer’s toolbox. By studying the spectrum or the wavelengths of light from an object in space, astronomers can get a range of information. For example, the change in position of lines in the spectrum from a star can tell astronomers how far away it is, whether it is moving towards or away from us and how fast it is moving.

The Doppler Effect

Listening to a siren or a racing car speeding past it sounds higher in pitch the closer it gets to you and lower as it moves away. Pitch is related to the frequency of sound waves, so the approaching sound seems to have a higher frequency (and shorter wavelength), the receding sound a lower frequency (and longer wavelength). This is called the Doppler Effect, where waves, in this case sound waves, change in frequency and wavelength as the source moves towards or away from you. There is no actual change in sound; the racing car isn’t making a different noise. It just sounds different due to the car’s movement relative to you.

Doppler shift

This apparent change in wavelength can also be observed for electromagnetic radiation, for example, visible light. So if a star is moving towards Earth, it appears to emit light that is shorter in wavelength compared to a source of light that isn’t moving. Because shorter wavelengths correspond to a shift towards the blue end of the spectrum, this is called blueshift. In contrast, the light from a star moving away from us seems to shift towards longer wavelengths. As this is towards the red end of the spectrum, astronomers call it redshift. At the large distances measured by the WiggleZ project, redshift is due to the expansion of the universe rather than a Doppler shift, although they both have the same effect on the spectra.

Lines in the spectrum from an object moving away are redshifted

The degree of shift can also give astronomers information about how fast the object is moving relative to us. So a faster object has a greater shift in wavelength.

Related sites

The Doppler Effect (CSIRO, Australia)
http://outreach.atnf.csiro.au/education/senior/astrophysics/spectra_info.html#specinfodoppler

Spectra and what scientists can learn from them (NASA, USA)
http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html

Spectroscopy – a vital tool (Nova: Science in the news, Australian Academy of Science)
http://www.science.org.au/nova/114/114box02.htm

Hunting for dark energy with the WiggleZ

Box 2 | Where next?

he WiggleZ program is not the only current effort to measure dark energy. Nor is dark energy the only way to explain an accelerating universe. This is frontier science, which is one of the reasons it is so exciting. There are a number of other possibilities that are being explored.

It may be that the space around us is relatively empty of matter compared to the average across the universe. With less matter there would be less gravity to slow the flight of galaxies. So the expansion we observe would be faster, with no need for dark energy.

Fainter than expected supernovae could appear dim for reasons that have nothing to do with dark energy. For example, our measurements of supernovae may be affected by the patchy distribution of matter between us and them.

But evidence for the existence of dark energy seems to be growing. There are other research teams hunting for dark energy in much the same way as the WiggleZ group. But the case to support its existence will be stronger if evidence comes from other techniques. And it does.

The cosmic microwave background images on their own, give some clues that dark energy exists and how much there is. In addition, some studies have shown that galaxies are clumping together under gravity more slowly that expected, suggesting dark energy is acting against gravity. Perhaps in the future, physicists will come up with further evidence for dark energy through experiments with particle accelerators.

Related sites

Dark energy a furphy, says new paper (Catalyst, 21 December 2007, Australian Broadcasting Corporation)
http://www.abc.net.au/science/articles/2007/12/21/2124258.htm?site=catalyst&topic=latest

Hunting for dark energy with the WiggleZ

Box 3 | Team, task, technology

The WiggleZ team draws together researchers from a number of universities in Australia as well as from the Anglo-Australian Telescope (where the observations are being made) and from the California Institute of Technology (Caltech). Caltech operates the GALEX space-based telescope which helped to choose the target galaxies for the WiggleZ team.

The WiggleZ project builds on the achievements of the earlier Two Degree Field (2dF) Galaxy Redshift Survey. This survey revealed the imprint of sound waves of the early universe in the way galaxies are now laid out across the universe in a ripple-like pattern. The WiggleZ team was formed in 2004 with the challenge of making the first precision measurements of this evidence for baryonic wiggles at high redshift. By comparing the observed pattern of galaxies now with the patterns in the early universe (from the cosmic microwave background) a measurement of dark energy will be made that is independent of other studies.

When complete in 2010, the WiggleZ survey will have mapped 200,000 chosen galaxies, spread across an area of the sky totalling 1000 square degrees. This enormous task will require around 200 nights of observation at the Anglo-Australian Telescope.

The key tool of the WiggleZ team is the AAOmega spectrograph. This is one of the world’s most complex astronomical instruments. Within the field of view of the telescope, a robot arm positions up to 400 individual optic fibres, each one aligned precisely to collect light from just one galaxy. Analysis of the light from each galaxy reveals its redshift, which provides its speed and (given the Hubble constant) how far away it is.

Related sites

WiggleZ Dark Energy Survey (Australia)

Anglo-Australian Observatory

The 2dF Galaxy Redshift Survey (Australia)

Sloan Digital Sky Survey: mapping the universe

Activities | Hunting for dark energy with the WiggleZ
1. Understanding our expanding universe

Other activities

Imagine the universe (National Aeronautics and Space Administration, USA)

2006 CosmicTimes – provides a ‘newsletter’ on dark energy and a series of five related class activities. In Measuring dark energy students simulate the discovery of dark energy using a Hubble diagram. In Century timeline students create a timeline of our understanding of the universe up to the discovery of dark energy.
http://cosmictimes.gsfc.nasa.gov/2006/2006.html

Teachers’ Domain (USA)

Gravity and the expanding universe – provides a short video describing the history of our understanding of the expansion of the universe including the discovery of dark energy. Class discussion questions are included.
http://www.teachersdomain.org/resource/phy03.sci.ess.eiu.expand/

Cosmic questions: our place in space and time (Harvard-Smithsonian Centre for Astrophysics, USA)

Educator’s guide – includes a series of activities including Modelling the expanding universe in which students create one-dimensional and two-dimensional models of the expansion of the universe. In Evidence for the expanding universe students use spectra of galaxies to measure their speed as they move away from us.
http://cas.sdss.org/dr7/en/proj/basic/universe/

Sky Server (Sloan Digital Sky Survey, USA)

The universe – students learn how big the universe is, and how scientists know it is expanding. They then make a Hubble Diagram.
http://cas.sdss.org/dr7/en/proj/basic/universe/

NOVA Online

Moving targets – students learn through an engaging interactive tutorial how the speed of stellar objects is measured using the Doppler effect.
http://www.pbs.org/wgbh/nova/universe/moving.html

Smithsonian Astrophysical Observatory

How fast do galaxies move? – students learn about spectroscopy and use data to measure the movement of galaxies. This activity can be completed online using the ‘virtual spectroscope’ or as a classroom activity.
http://www.cfa.harvard.edu/seuforum/galSpeed/

Activity 1 | Hunting for dark energy with the WiggleZ
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.

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.

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).

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

Use Hubble’s data below to estimate H₀.

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.

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.

How is the WiggleZ Survey using the CMB to find out about dark energy?

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

Useful sites | Hunting for dark energy with the WiggleZ

Dark energy (HubbleSite, USA)

A clear and engaging website discussing dark energy and its discovery.
http://hubblesite.org/hubble_discoveries/dark_energy/

Dark energy (Universe Forum, Harvard Smithsonian Centre for Astrophysics, USA)

Provides clear information on evidence for and ideas about the nature of dark energy.
http://www.cfa.harvard.edu/seuforum/darkenergylanding.htm

Swinburne University of Technology (Australia)

WiggleZ Dark Energy Survey
Provides brief information on the researchers involved and the science behind the WiggleZ survey to detect the effects of dark energy.
http://wigglez.swin.edu.au/site/

The WiggleZ of the universe
A ten minute podcast describing the WiggleZ survey to measure dark energy.
http://il.cc.swin.edu.au/ilectures/ilectures.lasso?ut=268&id=4516

Australian Broadcasting Corporation

Australian scientists measure dark energy wiggles (The World Today, 8 May 2009)
Describes efforts to understand dark energy through the WiggleZ program.
http://www.abc.net.au/news/stories/2009/05/08/2565200.htm
The hunt for dark energy (In Depth, 2 April 2009)
Clearly explains the WiggleZ project to measure dark energy.
http://www.abc.net.au/science/articles/2009/04/02/2533693.htm?site=science/indepthfeature&topic=latest

Inflation model for the universe (The Science Show, 7 March 2009)
Transcript of an interview discussing inflation of the universe and dark energy.
http://www.abc.net.au/rn/scienceshow/stories/2009/2506008.htm

Dark energy pushing universe apart (News in Science, 17 December 2008)
Describes X-ray measurements of galaxy clusters confirming that dark energy has slowed down their growth.
http://www.abc.net.au/science/articles/2008/12/17/2448721.htm?site=catalyst&topic=latest

Dark energy a furphy, says new paper (Catalyst, 21 December 2007)
Provides an alternative explanation to dark energy for the perceived acceleration of the universe.
http://www.abc.net.au/science/articles/2007/12/21/2124258.htm?site=catalyst&topic=latest

Dark energy (Catalyst, 13 April 2006)
Transcript of a story on dark energy, including discussions with Australian researchers using supernovae to measure the effect of dark energy.
http://www.abc.net.au/catalyst/stories/s1615519.htm

The High-Z SN search (Australian National University)

Explains the important High-Z Supernova Search, led by Australian astronomer Brian Schmidt, which discovered dark energy in 1998. By using supernovae to measure the expansion of the universe, Schmidt’s team discovered it was accelerating.
http://msowww.anu.edu.au/~brian/PUBLIC/public.html

The cosmic yardstick — Sloan Digital Sky Survey astronomers measure role of dark matter, dark energy and gravity in the distribution of galaxies (Sloan Digital Sky Survey, USA)

Describes a survey that measured the effect of sound waves from the early universe on galaxy distribution, providing information on the properties of dark energy.
http://www.sdss.org/news/releases/20050111.yardstick.html

The universe from beginning to end (Australian Academy of Science)

Transcript of a lecture by Professor Brian Schmidt in which he discusses the accelerating expansion of the universe and evidence for dark energy.
http://www.science.org.au/sats2004/schmidt.htm

Glossary | Hunting for dark energy with the WiggleZ

baryon. Refers to all atomic matter, and other heavy subatomic particles that are made from three quarks.

baryonic wiggles. The imprint of sound waves in the early universe on the distribution of matter forming a series of peaks and troughs.

cluster. (astronomy). A group of galaxies held together by gravity. A galaxy cluster can contain from tens to several thousand galaxies.

cosmic microwave background (CMB). Background radiation left over from the Big Bang. The radiation has expanded in wavelength over time so that it is now mostly in the microwave region of the electromagnetic spectrum (but can also be detected at radio and infrared wavelengths). The CMB is at an almost constant intensity throughout the universe and is often used as evidence that the Big Bang occurred.

electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Electromagnetic radiation can also be described by its frequency, or how many peaks and troughs per second are reaching an observer. Frequency is inversely related to wavelength. The energy of light, if it is visable to humans, affects the colour it appears. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Electromagnetic radiation (Back to Basics, Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA).

frequency. A measure of how frequently a wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).

Hubble constant. By observing distant galaxies Edwin Hubble showed a relationship between a galaxy’s speed away from us (recessional velocity, v) and its distance (d).

H_o=v/d

The Hubble constant, H₀, corresponds to the current rate of expansion of the universe.

neutron. A particle having no charge that is a constituent of an atom. It has a mass similar to a proton.

optical fibre. A glass or plastic thread that acts as a guide for lightwaves. Light entering one end of the fibre is trapped and travels down its length as it is reflected internally off the walls. For more information see Communicating with light – fibre optics (Australian Academy of Science).

particle accelerator. A machine, such as the Large Hadron Collider, that accelerates charged particles to high speeds. Electric fields are used to speed up the particles and magnetic fields are used to guide them. Accelerators can be in a ring shape or in a straight line and are used to study subatomic particles.
For more information see LHC the guide (CERN).

plasma. A gas containing free-moving charged particles (ions and electrons). Because the numbers of positive and negative particles are equal, plasma is electrically neutral. Plasma forms when a gas is raised to such a high temperature that the atoms separate into electrons and ions. It is the fourth state of matter because its properties are different from those of solids, liquids, and other gases. Stars such as our Sun are mostly made of plasma.

proton. A particle with positive electric charge of the opposite sign to an electron. Protons are present in the nucleus of all atoms. The proton is the same as a hydrogen ion or the nucleus of a hydrogen atom.

quark. A basic particle that makes up protons, neutrons and other baryons.

redshift. The shift in the spectrum from an object towards longer wavelengths or the red end of the spectrum. Spectral lines from the object are compared to those from sources of light on Earth. Redshift occurs when an object such as a star moves away from the observer or when light travels through the expanding universe.

spectrograph (spectrometer/spectroscope). An instrument used to analyse the spectrum from a source of radiation such as a star. Electromagnetic radiation from the source is spread out into its constituent wavelengths (like a rainbow).

spectroscopy. The technique of detecting and analysing the spectrum of an object to get information on its chemical and physical nature (eg, temperature, motion). Using a spectroscope the radiation or light from an object is dispersed into its different colours or wavelengths (like a rainbow). The position of emission and absorption lines in the spectrum provides information on what chemicals are present. For example, emission at a wavelength of 21 centimetres in the radio range of the spectrum (hydrogen also emits light in the visible region). corresponds to hydrogen. Large telescopes have spectroscopes to measure the properties of astronomical objects.

spectrum. Plural spectra. The distribution of electromagnetic radiation when it is dispersed (eg, the dispersal of visible light into a rainbow). Astronomers gain different information about astronomical objects by examining their spectra from different parts of the electromagnetic spectrum (eg, visible light, radio waves, X-rays).

square degrees. While degrees are used to measure angles in a circle, square degrees are used to measure angles in a sphere. For example, from Earth the Moon has a diameter of approximately 0.5°, but the whole Moon covers around 0.20 deg². For more information see Steradian (Math is fun).

supercluster. A very large grouping of galaxies, or a group of clusters of galaxies. Galaxy superclusters are believed to be the largest structures in the universe with tens of thousands of galaxies covering over one hundred million light-years.

wavelength. The distance between two adjacent wave crests. Visible light and X-rays are both electromagnetic waves and differ from each other only in the length of the wave. The wavelength of visible light ranges from 400 to 700 nanometres while the wavelength of X-rays ranges from about 0.01 to 10 nanometres. For more information see Electromagnetic radiation (Back to Basics, Australian Academy of Science).

External sites are not endorsed by the Australian Academy of Science.

Posted June 2009

The Australian Foundation for Science is a supporter of Nova.

This topic is sponsored by the University of Queensland and Swinburne University of Technology under an ARC Discovery Project grant.

Hunting for dark energy with the WiggleZ

Key text

Click here for a pdf version of the timeline. Click timeline headings for further information.

Further reading | Hunting for dark energy with the WiggleZ

Useful sites | Hunting for dark energy with the WiggleZ