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 onarXiv, 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 nuhtty.” 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.”
