Dark energy
A problem of cosmic proportions
Three experiments are starting to study dark energy, the most abundant stuff in the universe. But a theory has just been published purporting to show it does not exist
IN THE 1920s astronomers realised that the universe was running away from them. The farther off a galaxy was, the faster it retreated. Logically, this implied everything had once been in one place. That discovery, which led to the Big Bang theory, was the start of modern cosmology.
In 1998, however, a new generation of astronomers discovered that not only is the universe expanding, it is doing so at an ever faster clip. No one knows what is causing this accelerating expansion, but whatever it is has been given a name. It is known as dark energy, and even though its nature is mysterious, its effect is such that its quantity can be calculated. As far as can be determined, it makes up two-thirds of the mass (and therefore, E being equal to mc2, two-thirds of the energy) in the universe. It is thus, literally, a big deal. If you do not understand dark energy, you cannot truly understand reality.
Cosmologists are therefore keen to lighten their darkness about dark energy, and three new experiments—two based in Chile and the third in Hawaii—should help them do so. These experiments will look back almost to the beginning of the universe, and will measure the relationships between galaxies, and clusters of galaxies, in unprecedented detail. When they are done, though the nature of dark energy may remain unresolved, it should at least be clearer.
If, that is, it actually exists. For a core of cosmological refuseniks still do not believe in it. They do not deny the observations that led others to hypothesise dark energy, but they do deny the conclusion. For them, then, these experiments provide an opportunity to test alternative theories.
Darkness and dawn
The most advanced of the new experiments is the five-tonne, 570-megapixel Dark Energy Camera, which was installed last year at the Cerro Tololo Inter-American Observatory in Chile, 2,200 metres (7,200 feet) above sea level in the Atacama Desert. It is expected to open for business in a few weeks’ time, taking 400 one-gigabyte pictures of the sky each night, for 525 nights over five years.
This photographic marathon is part of the Dark Energy Survey (DES), a project led by Joshua Frieman of the University of Chicago. Dr Frieman’s plan is to scan an eighth of the sky, examining 100,000 galaxy clusters as he does so and measuring the distances to 300m individual galaxies within those clusters.
The reason for all this effort is that tracing the way the sizes and shapes of galactic clusters change over time allows each round of the battle between gravity and dark energy to be studied in detail. Gravity, which tends to slow down the expansion of the universe, causes clusters to become more compact. Dark energy, which tends to speed universal expansion up, causes clusters to spread out. The rate of contraction or expansion of clusters shows the relative strengths of the two forces. Dr Frieman and his colleagues cannot follow the changes in any given cluster since they see only a snapshot of its history. But looking at the differences between lots of clusters of various ages is the next best thing.
Previous observations have suggested that for more than half of the universe’s 13.7-billion-year life, gravity had the upper hand. Only about 6 billion years ago did dark energy overtake it. The DES hopes in particular to study the transitional period, by peering back as far as 10 billion years by the simple expedient of looking at clusters up to 10 billion light-years away.
The second of the new experiments, the Subaru Measurement of Images and Redshifts (SuMIRe), led by Hitoshi Murayama of the Kavli Institute for the Physics and Mathematics of the Universe, in Tokyo, is based on a mountain top in Hawaii. It will start collecting data next year, in a manner similar to the Dark Energy Camera, but better. Though it will look at only a tenth of the sky, rather than an eighth, it can see farther—13 billion light-years, rather than 10 billion. It also has more bells and whistles than the Dark Energy Camera; specifically, it has an integral spectrograph, for working out redshifts.
Redshifts are one of astronomy’s most important sources of information. They tell you how far away a galaxy is. They are caused by the Doppler effect, a phenomenon familiar on Earth as the change in pitch of a police-car or ambulance siren as the vehicle approaches and then recedes. Light, too, is subject to Doppler shifts, and the light from a receding object is thus redder (ie, of longer wavelength) than it otherwise would be. The faster the object is moving away, the redder it is. It was this that allowed those 1920s astronomers, led by Edwin Hubble, to work out that the universe is expanding. The Dark Energy Camera, which lacks a spectrograph, has to rely on other telescopes which do have them to make its redshift measurements for it. Having an integral spectrograph will thus give SuMIRe an advantage.
The third experiment, ACTPol (Atacama Cosmology Telescope Polarisation sensitive receiver), run by Lyman Page of Princeton University, is rather different. Instead of looking at light from galaxies, it will study microwaves from the cosmic microwave background (CMB). This was created around 380,000 years after the Big Bang, and thus preserves an imprint of what the early universe looked like.
ACTPol, too, is in Chile, on the peak of a mountain called Cerro Toco. Tests began on July 19th. Its purpose is to look at the CMB’s polarisation, any part of which will have been distorted in meaningful ways by the microwaves’ passage through intervening galaxies from their creation to their arrival on Earth. And from that, using a lot of statistical jiggery-pokery, a third estimate of the yo-yo effect of gravity and dark matter on galactic clusters should emerge.
If these three experiments work, and agree with one another, it will be a big step forward in understanding how the universe has evolved from an object smaller than an electron into the vastness seen today. Theoreticians will be able to plug the new data into their models of dark energy, and see what comes out. But others will be able to use the data too. And they may come to different conclusions.
Crazy enough to be correct?
Even as astronomers vie to explain the mystery of the expanding universe, some theorists are trying to explain it away. The most recent such attempt has just been published by Christof Wetterich, of the University of Heidelberg, in Germany. Not only does he not believe in dark energy, he does not believe the universe is expanding at all.
That, in the context of modern cosmology, is a pretty grave heresy. But Dr Wetterich’s latest paper, published on arXiv, an online repository, attempts to back it up.
In Dr Wetterich’s picture of the cosmos the redshift others attribute to expansion is, rather, the result of the universe putting on weight. If atoms weighed less in the past, he reasons, the light they emitted then would, in keeping with the laws of quantum mechanics, have been less energetic than the light they emit now. Since less energetic light has a longer wavelength, astronomers looking at it today would perceive it to be redshifted.
At first blush this sounds nuts. The idea that mass is constant is drilled into every budding high-school physicist. Abandoning it would hurt. But in exchange, Dr Wetterich’s proposal deals neatly with a big niggle in the Big Bang theory, namely coping with the point of infinite density at the beginning, called a singularity, which orthodox theories cannot explain.
Dr Wetterich’s model does not—yet—explain the shifts in the shapes of galactic clusters that the Dark Energy Camera, SuMIRe and ACTPol are seeking to clarify. But perhaps, one day, it could. Dr Wetterich is a well-respected physicist and his maths are not obviously wrong. Moreover, his theory does allow for a short period of rapid expansion, known as inflation, whose traces have already been seen in the CMB. Dr Wetterich, however, thinks this inflation did not happen just after the beginning of the universe (the consensus view), for he believes the universe had no beginning. Instead, a small static universe which had always existed turned into a large static one that always will exist—getting heavier and heavier as it does so.
There was thus no singularity.
Probably, this theory is wrong. As Cliff Burgess of Perimeter Institute, a Canadian theoretical-physics centre, puts it, “The dark energy business very easily degenerates into something like a crowd of people who are each claiming to be Napoleon while asserting that all the other pretenders are clearly nutty.” But theories last only as long as they do not conflict with the data, and when the new experiments have finished there will be a lot more data for them to conflict with, and thus reveal who the real Napoleon actually is. Perhaps, therefore, the last word should go to Niels Bohr, one of the founders of quantum theory. He once said to a colleague, Wolfgang Pauli, “We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct.”
