Update 6 July 2010: The Planck Telescope launched in May 2009. Researchers released the first full sky map of the cosmic microwave background on 5 July 2010.
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WE ARE poised to peer further back in time than ever before. Next week, cosmology’s biggest experiment in nearly a decade is due to blast into space. The will enable us to find out what happened just fractions of a second after the big bang, when the universe is thought to have blown up to cosmic proportions from a speck of space-time.
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The probe, which is fuelled and ready for launch in French Guiana, will examine in exquisite detail the cosmic microwave background (CMB), the relic radiation of the big bang. It is “like a surgical instrument”, says .
The CMB was released when the universe was about 380,000 years old. The expanding cosmos had cooled enough for free electrons and nuclei to combine to form neutral atoms, mainly hydrogen. Photons, which until then had been continually scattered by the free electrons, were suddenly able to zip away unhindered, and it is this radiation – since stretched to microwave wavelengths by the universe’s expansion – that makes up the CMB. It is all around us, and constitutes about 1 per cent of the “noise” on untuned analogue TV screens.
Radio telescopes have studied the CMB since it was discovered in 1965 – perhaps the most prominent in recent years being NASA’s , which launched in 2001 and is still collecting data. WMAP has measured variations in the temperature of the CMB as small as a few microkelvin (see “Sharpening up”). These so-called anisotropies are believed to be due to inflation, a process thought to have occurred just 10-34 seconds after the big bang, during which a speck about 10-20 times the size of a proton expanded to a mind-boggling size in a flash.
During inflation, quantum fluctuations in space-time were extended to cosmological scales: by the time the CMB was released, these fluctuations had led to variations in the distribution of matter across the universe. Denser regions of the universe produced CMB photons slightly colder than average, and vice versa.
Planck will create the sharpest possible map of all the CMB’s anisotropies, and will arguably provide the final word on their distribution (see “computer simulation, above right”). “It is the Everest excuse – we are going to get everything because it’s there,” says cosmologist .
By measuring these temperature variations accurately, cosmologists can calculate parameters such as the curvature of space-time, and the contribution of dark energy, dark matter and normal matter to the distribution of mass and energy in the universe. Planck will slash the uncertainties in the values of these parameters to less than 1 per cent. “In terms of the information that is available to do cosmology, Planck is about 15 times better than WMAP,” says , Planck’s project scientist at ESA’s offices in Noordwijk, the Netherlands.
“Planck will enable cosmologists to calculate parameters such as the curvature of space-time”
Anisotropies alone are not considered proof that inflation occurred, but Planck might just provide the “smoking gun”: detection of an imprint of gravitational waves – ripples in space-time predicted to have been caused by inflation. At the time the CMB was released, these waves would have stretched space-time in places and squashed it elsewhere. This would have polarised the so-called “B-mode” of the CMB photons – an aspect of their electromagnetic properties – in a very specific pattern. Planck has been designed to spot this (see “Looking back for signs of inflation”). “There is a chance that it is at a level where we can detect it,” says Tauber.
If Planck sees this signal, it will not only reveal that inflation actually occurred, it will also help answer other key questions. When exactly did inflation begin? How long did it last?
Cosmologists also want to know the “energy scale”, or energy density, of the universe during inflation. The higher the energy scale, the greater the amplitude of the gravitational waves, and the stronger the B-mode polarisation of the CMB photons should be. If Planck sees this polarisation, it means the waves would have been relatively strong and that the energy scale during inflation would have been high. “If Planck discovers gravitational waves, it’ll bring to the fore all of these [high-energy] models,” says Linde. He is also looking forward to Planck proving or disproving troubling WMAP observations (see “‘Axis of evil’ and other horrors”).
There is also the tantalising possibility that Planck will provide support to some scenarios involving string theory. The theory argues that our universe is just one of 10500 universes that make up the “multiverse”. When inflation is combined with string theory, the simplest models predict that the curvature of our universe, instead of being absolutely flat, will be ever-so-slightly curved. Planck will discern the curvature of space-time with enough precision to support or rule out such ideas.
Many will be watching nervously as Planck launches on 14 May, on its way to a solar orbit. Due to its high price tag, it’s unlikely that the mission will be rebuilt if something goes wrong. But if all goes well, a year from now Planck will have amassed 300 billion measurements of the sky, whereas WMAP would have accumulated 200 billion in nine years. As George Smoot, who won the Nobel prize in 2006 for his work on one of the probe’s predecessors, the COBE satellite, once said: “Planck is the future of looking back to the past.”
See a gallery of the most important telescopes in history
‘Axis of evil’ and other horrors
For all that it’s done for cosmology, NASA’s Wilkinson Microwave Anisotropy Probe has also thrown up some unwelcome surprises which Planck may resolve.
WMAP saw patterns of hot and cold spots in the cosmic microwave background (CMB) that are not randomly distributed as expected. Instead, they seem to be aligned along what João Magueijo and his team at Imperial College London (ICL) dubbed the “axis of evil”.
Cosmologists are divided over whether the effect is real or an artefact of WMAP’s instruments. If real, we may need to revise our notions of the universe’s shape: the observed pattern could mean it is longer in one direction than another. This could mean revising models of inflation – the period of expansion just after the big bang – which posits an isotropic universe that is the same in all directions. “If we see [the axis of evil] with Planck, then we will know that it is not an instrumental effect,” says ICL’s Andrew Jaffe.
WMAP has dealt yet another challenge to the simplest models of inflation. They predict that the amplitude of temperature variations in the sky should follow a bell-shaped curve known as a Gaussian distribution. But WMAP’s data has shown the distribution to be non-Gaussian at levels much larger than those permitted by these simple models. This is a very sensitive measurement, however – essentially looking for variations within the variations in temperature of the CMB – and so, again, opinion is divided over whether the effect is real or due to WMAP’s instruments.
“If [Planck] clarifies the issue with non-Gaussianity, that alone will be tremendously important,” says Stanford University’s Andrei Linde.
It is just as likely that the probe will throw up a few thorny problems of its own.
