As most cosmologists would have it, the universe began in a big bang almost 13.8 billion years ago. With it was born space, time and a fiery soup of exotic sub-atomic particles.
In the first fraction of a second, many scientists believe, the super-hot universe - with a temperature of 10,000 trillion trillion degrees - inflated in size by 100 trillion trillion. The expansion continues today but at a far more leisurely pace.
When it was one second old, the now-inflated universe contained matter, radiation and the four basic forces of nature. By the end of the third minute, the expanding cosmos had cooled enough for the first three elements - hydrogen, helium and lithium - to start forming.
It seems like an awfully long time ago, and it was. However, the aftermath of the moment of creation is still with us today in the form of something called cosmic microwave background radiation. This afterglow of the big bang, dating back 380,000 years after time zero, provides a wealth of information about the early universe - if only scientists can get their hands on it.
Now the European Space Agency's Planck satellite has revealed, with unprecedented accuracy, the minute temperature fluctuations in this primordial background radiation. In doing so, the state-of-the-art satellite has shed light on several key facts - from the age of the universe (now believed to be 13.799 billion years old) to the time the first stars formed (more than 500 million years after the big bang - much later than previously thought).
In particular, Planck is helping scientists find evidence for the bizarre epoch of cosmic inflation.
Theory has it that this exponential growth spurt would have greatly increased the size of minuscule quantum mechanical ripples, or perturbations, in the fabric of space and time, known as gravitational waves. These highly elusive waves, in turn, would have left an imprint in the form of curly, pinwheel-like patterns in the polarisation, or twisting pattern, of the earliest radiation emitted in the universe.
Scientists' hopes were raised last year when a team of scientists reported having found evidence for gravitational waves using a super-sensitive radio telescope called BICEP2, short for Background Imaging of Cosmic Extragalactic Polarisation 2, at the South Pole.
At the time, the stronger-than-predicted swirls were widely believed to have arisen from the first gravitational waves, predicted almost a century ago by Albert Einstein's theory of general relativity.
The latest Planck results suggest this was most unlikely. "As was widely predicted, the results contradict the BICEP2 claims," says ICRAR-Curtin University astrophysicist Rob Soria.
"The Planck findings suggest that our galaxy, the Milky Way, is dustier than we ever thought, meaning that the issue of dust mimicking the signal from the gravitational waves is more of an issue than the BICEP2 team assumed," explains Swinburne University astrophysicist Alan Duffy.
"The latest combined analysis by both the Planck and BICEP2 teams - using all the data available - shows that all the swirling patterns in the cosmic microwave background is entirely consistent with dust," Dr Duffy adds. "In other words, there was no detection of gravitational waves and hence no confirmation of the incredible theory of inflation."
Monash University physicist Paul Lasky agrees. "Scientifically, the combined analysis leaves us in an interesting position," Dr Lasky notes. "From a theoretical perspective, we have sound reasons to believe inflation occurred. Yet, from an experimental perspective, we have no evidence either way."
In no way, he emphasises, does this rule out inflation: "The new, combined observations provide an interesting constraint on the model."
Dr Lasky adds: "The BICEP2 team made mistakes in the way this was handled. Not necessarily in their treatment of the peer review process, but in the fanfare associated with the initial media release."
One positive outcome, he says, is that the public has had a chance to watch the scientific process in detail; results require thorough, independent analysis including many cross-checks before they become broadly accepted by the scientific community."
Cosmologists will "be even more diligent now when analysing the faint signals emitted a few hundred thousand years after the big bang", says Sydney University astrophysicist Krzysztof Bolejko.
A few other experiments are currently also searching for the signatures of gravitational radiation in the cosmic microwave background. These include the successor to BICEP2, called BICEP3, and the POLARBEAR project, located in the Atacama Desert of northern Chile.
"Hopefully, with these new instruments, we shall finally find traces of gravitational waves produced in the early moments of our universe," Dr Bolejko says.
Dusty detail
"Planck has shown that the BICEP2 results are in doubt because the researchers did not remove all the foreground noise from dust in our own galaxy, the Milky Way, which is superimposed on the distant signal," says Dr Soria.
"The scientists removed too little noise and thought the signal was all from the distant universe. They did not have a good map of the foreground noise to use in the subtraction," Dr Soria explains. Without the noise map, he notes, it is hard to know how much real signal there is.
Dr Duffy said that extraordinary claims require extraordinary evidence.
When attempting to uncover primordial gravitational waves, the signal has to pass through the gas and dust of our galaxy. This obscures the picture, he says, and can mimic the signal of the gravitational waves themselves. "Trying to detect these waves is akin to taking a picture during a sandstorm. Yet, with a detailed map of the dust in our galaxy, we can do just that."
Dr Duffy is confident that future images of the cosmic microwave background will show that gravitational waves are present at some level. "But these waves will almost certainly be weaker that the initial claim, which was surprisingly large."
To declare the hunt for gravitational waves is finally over, scientists need to detect them directly, Dr Duffy explains. "That is, we must measure the stretching and straining of space as the waves pass through special detectors." A direct detection of this kind would count as proof positive of the primordial waves.
"For a direct measurement of gravitational waves, we shall need to wait a few more years for an experiment using the Laser Interferometer Gravitational-wave Observatory," Dr Lasky says.
Such technology, the scientists believe, is all but guaranteed to find gravitational waves arising from major galactic events near the Milky Way.
View detectors
Gravitational waves are ripples in the gravitational field. They are analogous to light waves, which are ripples in the electromagnetic field. Both classes of waves propagate through space at the speed of light, namely 300,000 kilometres a second.
Gravitational-wave detectors rely on two laser beams fired at right angles to each other. They begin in-phase, which means they are oscillating at the same point. They then travel along a vacuum tube, bounce off a mirror at the end of a track, about one kilometre long, and meet back at the start.
"Without gravitational waves they would arrive back with the same oscillation - again, in-phase," Dr Duffy says. "But gravitational waves passing through will compress the length of one track relative to the other by much less than the size of an atom. Thus the laser beams would have travelled a different distance and be slightly out of phase."
This subtle pattern reveals the type of gravitational waves that passed through, and therefore the nature of the objects that formed the waves, be they exploding stars or colliding black holes.
Future caution
"Science is based around careful measurements and testing of theories against those measurements - with intense battles between scientists and teams to uncover the truths of our world," Dr Duffy notes. "Usually this process is hidden from the general public.
"Hopefully people will be comforted to know just how serious and diligent scientists are when they settle on a final answer - as they have done now - even if it isn't as exciting as a detection."
Dr Soria is less sanguine. "This sort of thing will keep happening more and more in science: big discovery claims that get quietly retracted a year later. It works: ironically, everyone is a winner. The BICEP2 people are winners, because even if their discovery claim was wrong, they now have in their CVs lots of papers with thousands of citations (essential for getting jobs), they won awards, they got tenured positions, fellowships and grant money to continue their research and test their claims.
"Because the BICEP2 claims multiplied the public interest and anticipation for their results, the Planck scientists also benefited a lot from the controversy.
"The only loser is the credibility of science: in the long run, non-scientists will get tired of hearing scientists rushing to claim discoveries that are, in fact, far from proved, and will downgrade the credibility of scientists," Dr Soria laments.
Links
Learn more about the physics of waves in chapter 7 (page 224) of Physics 11, Enhanced, Heinemann, third edition (Pearson, 2011)
Investigate some of the essential mathematics behind the latest Planck findings in Mathematics 10-10A Student Book, by Coffey et al (Pearson)
And delve into some more detailed mathematics in: Essential Advanced General Mathematics, third edition (Cambridge University Press, 2006)
Follow Alan Duffy and on Twitter
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