IT IS one of the biggest telescopes on the planet, yet it looks remarkably small against the vast Antarctic landscape. In this blindingly white icy world where the December sun never sets, it is hard to judge distances. The true size of the South Pole Telescope’s 10-metre-wide dish only becomes apparent when our small tracked vehicle pulls up next to the building that houses it.
No one considers walking here from the US just a few hundred metres away. While a wind chill of almost -40 °C is considered balmy by residents at the station, my fingers grow numb soon after I take off my gloves to snap some pictures. And although I’m wearing goggles that cover half my face, wind-drawn tears freeze onto my eyelids.
“While a wind chill of -40 °C is considered balmy by residents of the South Pole station, my tears freeze”
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Senior scientist of the University of Chicago doesn’t seem to mind the extreme cold and remoteness of this barren place at the bottom of the world. After all, the station has its own music room, bar, even sauna – probably the hottest place on the continent. Still, Benson’s enthusiasm mainly stems from his work. He and his colleagues are hot on the trail of a revolutionary breakthrough in cosmology. Using a sensitive camera installed at the telescope just over a year ago, they hope to shed light on the first trillionth of a trillionth of a trillionth of a second after the birth of the universe. If that requires some sacrifices, so be it. “The South Pole is one of the best places on Earth to do this kind of research,” Benson says, as he wipes ice crystals from his beard.
The newborn universe was incredibly dense, insanely hot and filled with energetic radiation. As the universe expanded and cooled, the radiation’s energy became diluted and its wavelength was stretched until, almost 14 billion years later, nothing is left but an all-pervading glow of microwaves. Studying this cosmic microwave background (CMB), often dubbed the “afterglow of creation”, is the best way for cosmologists to decipher the infancy and subsequent evolution of the cosmos. For instance, minute temperature variations in the CMB, which were first discovered by space missions in the 1990s, revealed the existence of over-dense and under-dense patches of primordial matter that grew into the galaxy clusters and voids we see in the universe today.
The was built six years ago to study the CMB in detail. However, while CMB observations from the South Pole Telescope and others can give us an excellent picture of the universe a mere 380,000 years after the big bang, they can’t go any further back. At earlier times, space was filled with a seething plasma of charged particles that constantly absorbed and re-emitted photons, meaning that light couldn’t escape. Only when temperatures eventually dropped low enough for these particles to combine into neutral atoms could radiation, and therefore light, propagate freely through the universe. So we may have a baby photo of the universe from the moment the cosmos became transparent, but we haven’t captured the instant of birth.
It’s a pity, says Benson, for theory tells us that exciting things happened in these very first fleeting moments. According to the hypothesis of cosmic inflation, the universe started to expand exponentially when it was only about 10-36 seconds old, driven by a mysterious vacuum energy with negative pressure. In a tiny fraction of a second, the observable universe expanded from a size smaller than an atom to roughly the size of a grapefruit. Luckily for us, inflation came to a halt when the cosmic clock marked 10-33 seconds or so, and a more sedate form of expansion took over, allowing the subsequent formation of galaxies, stars and planets.
Inflation is a popular idea, supported by quantum physics and, to some extent, by evidence from missions such as ESA’s and NASA’s . It solves a number of nagging problems in cosmology. For example, it explains density variations in the early universe as “blow-ups” of tiny quantum fluctuations. It may even be related to the strange dark energy that appears to be accelerating cosmic expansion today.
Penetrating the darkness
Even so, frustratingly little is known about the physics of inflation. Many different models have been proposed, and astronomers aren’t even entirely sure that it really happened. Without being able to look back all the way to an inflationary epoch, it seems impossible to tell which model, if any, is right.
But there might be a way. Over the past decade, cosmologists started to realise that the sudden ending of inflation must have sent shudders through space-time known as gravitational waves, whose existence was predicted by Einstein’s general theory of relativity. Unlike radiation, these primordial gravitational waves could travel through the hot early universe, so their frequency and power tells us about the state of the universe at the time inflation ceased.
Primordial gravitational waves are too faint to be picked up by some experiments, such as the twin Laser Interferometer Gravitational Observatories in Hanford, Washington, and Livingston, Louisiana, which were built to spot the ripples through space-time sent out by colliding black holes or neutron stars. Yet such waves should still leave a telltale pattern in the cosmic microwave background. Detecting and characterising that pattern might allow us to distinguish between various models of inflation.
That’s why Benson is so excited about his team’s new camera, the South Pole Telescope Polarimeter (SPTpol). It is designed to make detailed measurements of the polarisation of the CMB radiation. In the same way that sunlight is polarised when it reflects from a lake or a road, the CMB radiation is polarised as it scatters off electrons on its journey through the universe. Gravitational waves are predicted to subtly change the polarisation pattern. As they ripple through space-time, they shift the electrons in a distinctive way and so leave their hallmark in the CMB (see diagram).
Spotting that pattern is going to be tough – a bit like listening for the sound of a cricket during a rock concert. The weak polarisation signal from primordial gravitational waves is overwhelmed by a much stronger one from density fluctuations in the early universe. This strong signal was first detected in 2002 by a telescope called the Degree Angular Scale Interferometer, also at the South Pole. No one knows how hard it will be to detect the gravitational-wave polarisation pattern, says Benson. “It’s a subtle effect,” says the South Pole Telescope’s principal investigator John Carlstrom. So far, the best upper limits of gravitational-wave polarisation were obtained in 2006 and 2007 by yet another South Pole instrument, the Background Imaging of Cosmic Extragalactic Polarization () experiment.
High and dry
How come so many CMB telescopes are located at one of the most remote and inhospitable places on the planet? To observe cosmic microwave radiation, you need to be high and dry. Atmospheric water vapour absorbs microwaves – the same principle that makes a cup of water in your microwave oven hot. As a result, you can’t observe the CMB at sea level: there’s just too much water-laden atmosphere above your telescope. And even on a high mountaintop, you need really dry air. The South Pole is at an altitude of 2830 metres and the air is extremely dry, a fact evident to every visitor. At times I find it hard to breathe, and climbing a flight of stairs is a difficulty. By the end of the day, my lips feel like parchment.
In fact that’s nothing. The Array for Microwave Background Anisotropy, AMiBA, is some 3400 metres up on the slopes of Mauna Loa on Hawaii. Conditions are particularly good– for telescopes, at least – in the Atacama desert in Chile. Since 2012 the Polarbear experiment to measure the CMB polarisation has been housed at an altitude of 5200 metres near the summit of Cerro Toco. Later this year, it will be joined by the ACTPol camera at the nearby . This will provide the most sensitive measurement ever of the CMB polarisation, says of Cornell University in Ithaca, New York, who helped develop the detectors for both the South Pole and Atacama telescopes and who now works on the Atacama team.
Competition between the two teams is fierce, but friendly, says Niemack. And it won’t stop at the current generation of polarimeters. The SPTPol team is already building a new, upgraded instrument that will be ten times as sensitive as its predecessor. Meanwhile, Niemack and his collaborators are designing an advanced version of ACTPol. Sensitivity, angular resolution, frequency coverage and sky coverage all have a role in the hunt for the elusive polarisation fingerprint of inflation. “We don’t know the strength of the signal yet. This is exploratory science,” says of Princeton University.
With so much at stake, many teams are on the same treasure hunt. The Planck mission, for instance, has been mapping the CMB in unprecedented detail since its launch in 2009. In March this year, the team published the most detailed maps ever made of the CMB across the entire sky. They are still analysing measurements from the satellite’s polarimeters. “We plan to publish our first polarimetry data about a year from now,” says project scientist Space Research and Technology Centre in Noordwijk, the Netherlands. He hopes Planck will be first to detect the polarisation signal from inflationary gravitational waves.
But Planck’s detectors are not as sensitive as some of the ground-based instruments, and they are unable to observe the smallest-scale patterns. That gives other CMB polarisation experiments an opportunity to take the lead. And there are plenty of them.
Several are running at high-altitude locations. Others dangling under balloons have recently flown high above Antarctica, Australia and New Mexico. The BICEP-2 device has been running since 2009 on a small telescope at the South Pole, while the completed a over Antarctica in January.
Yet more experiments are being planned. “What I can say for sure is that there will be a lot of progress in the next few years,” says Spergel. He believes that confirming cosmic inflation through CMB polarisation will be deserving of a physics Nobel prize.
And who knows, the measurements may already have been collected – if not by SPTPol or Planck, then by BICEP-2 or EBEX. “We plan to publish preliminary results this year,” says Jamie Bock, who is on the BICEP-2 team, “but we’re still analysing three years of more powerful data.” Analysis involves calibration, understanding exactly how the instrument processes signals and noise, and investigating systematic errors. “It’s not easy,” says Bock, “you have to worry about everything.”
Bock claims that the sensitivity of BICEP-2 has got to “interesting levels”. But he won’t be drawn on whether the team has found any imprint of inflation yet: “I can’t say, and if I could, I couldn’t tell you.”
At the moment, the field is wide open. “We don’t know the level at which primordial gravitational waves produce CMB polarisation,” says Shaul Hanany who leads the EBEX team, “so we also don’t know who will be the first to detect the signal. But a detection might come within the next two years or so.” EBEX suffered a glitch in one of the motors used to point the 1.5-metre telescope during its flight, but the effect on the final results is yet to be determined.
Witnessing how scientists like Benson leave their comfortable homes for months on end to live a spartan life at the bottom of the world makes you realise how serious they are about the quest for a firm proof of inflation. There’s no guarantee of success. If the primordial waves are not strong enough, the telltale polarisation pattern “may never be discovered”, says Carlstrom. Which is not to say that inflation did not occur, he adds. “Based on polarisation measurements, you will never be able to refute inflation.”
Cosmologists aren’t disheartened by such a prospect. Even seeing nothing at a certain level will enable them to rule out a broad class of different inflationary models. “That’s progress,” says Spergel.
