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.
See a gallery of the most important telescopes in history
WE ARE poised to peer further back in time than ever before. Next week, cosmology鈥檚 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 鈥渓ike a surgical instrument鈥�, 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 鈥� since stretched to microwave wavelengths by the universe鈥檚 expansion 鈥� that makes up the CMB. It is all around us, and constitutes about 1 per cent of the 鈥渘oise鈥� on untuned analogue TV screens.
Radio telescopes have studied the CMB since it was discovered in 1965 鈥� perhaps the most prominent in recent years being NASA鈥檚 , 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 鈥淪harpening up鈥�). 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鈥檚 anisotropies, and will arguably provide the final word on their distribution (see 鈥渃omputer simulation, above right鈥�). 鈥淚t is the Everest excuse 鈥� we are going to get everything because it鈥檚 there,鈥� 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. 鈥淚n terms of the information that is available to do cosmology, Planck is about 15 times better than WMAP,鈥� says , Planck鈥檚 project scientist at ESA鈥檚 offices in Noordwijk, the Netherlands.
鈥淧lanck will enable cosmologists to calculate parameters such as the curvature of space-time鈥�
Anisotropies alone are not considered proof that inflation occurred, but Planck might just provide the 鈥渟moking gun鈥�: detection of an imprint of gravitational waves 鈥� 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 鈥淏-mode鈥� of the CMB photons 鈥� an aspect of their electromagnetic properties 鈥� in a very specific pattern. Planck has been designed to spot this (see 鈥淟ooking back for signs of inflation鈥�). 鈥淭here is a chance that it is at a level where we can detect it,鈥� 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 鈥渆nergy scale鈥�, 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. 鈥淚f Planck discovers gravitational waves, it鈥檒l bring to the fore all of these [high-energy] models,鈥� says Linde. He is also looking forward to Planck proving or disproving troubling WMAP observations (see 鈥溾�楢xis of evil鈥� and other horrors鈥�).
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 鈥渕ultiverse鈥�. 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鈥檚 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鈥檚 predecessors, the COBE satellite, once said: 鈥淧lanck is the future of looking back to the past.鈥�
See a gallery of the most important telescopes in history
鈥楢xis of evil鈥� and other horrors
For all that it鈥檚 done for cosmology, NASA鈥檚 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 Jo茫o Magueijo and his team at Imperial College London (ICL) dubbed the 鈥渁xis of evil鈥�.
Cosmologists are divided over whether the effect is real or an artefact of WMAP鈥檚 instruments. If real, we may need to revise our notions of the universe鈥檚 shape: the observed pattern could mean it is longer in one direction than another. This could mean revising models of inflation 鈥� the period of expansion just after the big bang 鈥� which posits an isotropic universe that is the same in all directions. 鈥淚f we see [the axis of evil] with Planck, then we will know that it is not an instrumental effect,鈥� says ICL鈥檚 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鈥檚 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 鈥� essentially looking for variations within the variations in temperature of the CMB 鈥� and so, again, opinion is divided over whether the effect is real or due to WMAP鈥檚 instruments.
鈥淚f [Planck] clarifies the issue with non-Gaussianity, that alone will be tremendously important,鈥� says Stanford University鈥檚 Andrei Linde.
It is just as likely that the probe will throw up a few thorny problems of its own.
