Spider flies to catch an elusive wave

A giant white balloon with a precious cargo earlier this month crash-landed in a remote corner of snow-covered Antarctica – at 76°21.91 S, 87°27.14 W, to be precise.

Inside cylindrical pressure vessels attached to the balloon’s payload, researchers may find answers to their questions about the early universe.

The Spider experiment – short for the Suborbital Polarimeter for Inflation, Dust and the Epoch of Reionisation experiment – sailed through the cold blue skies of Antarctica for 16 days about 35km above the surface of the Earth, searching for elusive gravitational waves in the ancient signals from the early universe.

Cynthia Chiang, a senior lecturer in the University of KwaZulu-Natal’s Astrophysics and Cosmology Research Unit, has been involved in the project since 2009, when she was a postdoctoral candidate at Princeton University, although she did “a couple of small things for Spider when [she] was a grad student [at the California University of Technology (CalTech)]. Through her, South Africa is involved in this international experiment. “We’re trying to find the signature from inflation … [which] generates gravitational waves,” she explains.

It all starts with the Big Bang, in which – according to the theory – all the matter and energy in the universe exploded out of a tiny pinprick in space about 13.8-billion years ago. Scientists have detected the remnants of this cataclysmic eruption in the almost uniform thermal cosmic microwave background.

Big Bang
When the universe exploded, everything we can now see was pure, hot energy and it was “dark” because the soup of particles – such as protons and electrons – was so dense that even light was trapped inside this morass. But about 400 000 years after the Big Bang, it became cool enough for these particles to congeal into atoms, and the photons could escape. This  light has been travelling billions of years to reach us on Earth. But it is no longer the energetic light we get from our own sun; it has been stretched into microwaves, which is why it is called cosmic microwave background (CMB) radiation – an image of what the ancient universe looked like.

“Although the standard Big Bang model works pretty well, there are a few outstanding problems that it fails to explain (such as the universe’s horizon and flatness, among others) … to get around these problems, the theorists came up with the idea of inflation, in which the universe experienced a short, violent ‘growth spurt’ immediately after the Big Bang,” Chiang explains. 

But how do you test this? “Our answer … is to search for a CMB polarisation signature that’s imprinted by gravitational waves produced during inflation,” she says, adding that these waves theoretically produce a “minuscule amount of a ‘swirling’ pattern’, known as B-mode polarisation” on the CMB.

In 1916, in his theory of general relativity, Albert Einstein predicted the existence of gravitational waves, distortions in space and time that – rather than the force of “gravity” – explain the dances of planets, stars and galaxies. These waves would prove the “inflation” hypothesis.

Spider researchers
So, the Spider researchers – from many collaborating institutions, namely Princeton University, the University of Toronto, Case Western Reserve University, Caltech and Nasa’s Jet Propulsion Laboratory, and the University of British Columbia – are not just looking for the faint CMB signals, but also distortions in these signals, known as polarisation, that could be proof of these gravitational waves.

“One of the biggest things to worry about when measuring the faint signal is that it is so small,” Chiang says. “That’s what makes Spider so powerful: there are six telescopes on board, all bundled together.”

Bicep2, the second-generation Background Imaging of Cosmic Extragalactic Polarisation experiment, was conceived by the same team that developed Spider, and made headlines last year when researchers claimed to have detected these elusive gravitational waves. But these findings were later thought to have been flawed as a result of an unanticipated quantity of Milky Way dust in the part of the sky they were observing.

Asked about the Bicep2 results, Chiang says: “The experiment convincingly detected something, but it’s still unclear exactly what that something is … we need to wait for more measurements of dust emissions at different frequencies and from different experiments.”

By being mounted in the balloon’s gondola, the Spider experiment avoids many sources of “noise”, which are signals that interfere with the telescopes’ observations.

“We’re also going above the atmosphere,” she says. “Water is a source of noise, so we needed to get above the weather and the water vapour. Ballooning at such a high altitude is like a cheap satellite.”

Hard drives
But “we didn’t have enough bandwidth to stream [the data] back in real time. Most of the data is still strapped to the [gondola] on the hard drives.”

Jamie Bock, head of the Spider receiver team at Caltech and the Jet Propulsion Laboratory, said in Caltech’s researcher newsletter that the experiment seems to have performed well, but “of course, we won’t know everything until we get the full data back as part of the instrument recovery”.

Jeff Filippini, an assistant professor at the University of Illinois, Urbana-Champaign, and a member of the Spider team, says the landing site was near a few outlying stations in West Antarctica, thousands of kilometres from where it was launched at the United States’ McMurdo Station. “We are negotiating plans for recovering the data disks and payload. We are all looking forward to poring over the data.”

Chiang says the experiment, which is funded by Nasa, the David and Lucile Packard Foundation, the Gordon and Betty Moore Foundation, the Canadian Space Agency and Canada’s Natural Sciences and Engineering Research Council, is scheduled to fly one more time. “It depends. It takes time to refurbish and [make it ready to] fly.”

Chiang says: “I’m so excited it all worked. It’s been a long time coming.”

In 2005, she was the person who hand-carried the proposal to Nasa, she says. Asked how she was chosen, Chiang laughs. “I got into my lab that morning, and saw an email with a subject line: ‘Who wants to be a hero?’ and ran.”

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