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.
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?
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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?).
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.
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 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
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Posted June 2009, edited September 2012.