It seems mighty impressive and, if it came to fruition, it would be every bit as impressive as it sounds. A grand unified theory of physics, which is about as close as humans might get to an all-encompassing theory of everything, is possibly the most ambitious scientific undertaking in history.
Although research efforts are far from over, tantalising signs are now emerging that the long search may not have been in vain. In particular, recent claims that scientists had detected signs of gravitational waves, or ripples, in the fabric of space-time, have renewed hopes of finding a "complete theory" of the cosmos.
Compelling, albeit indirect, evidence for the elusive waves – analogous to tremors following the moment of creation, the Big Bang – was collected by a super-sensitive radio telescope called BICEP2, short for Background Imaging of Cosmic Extragalactic Polarisation 2, at the South Pole.
The telescope picked up the twisting pattern, or polarisation, of the earliest light emitted in the universe. Although they have yet to be verified, the stronger-than-predicted swirls are believed to have arisen from the first gravitational waves, predicted almost a century ago by Albert Einstein's theory of general relativity.
These space-time ripples are analogous to light waves being ripples in the electromagnetic field, says Swinburne University of Technology astrophysicist Alan Duffy. As with light, gravitational waves exhibit a range of wavelengths: longer ones produced during the early universe and shorter ones caused by the in-spiral of two black holes, for example.
"To declare the hunt for gravitational waves is finally over, we'd need to detect them directly," Dr Duffy says. "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.
Upcoming detectors include the advanced LIGO experiment, currently under construction and using a technique known as laser interferometry. Such technology, Dr Duffy believes, is all but guaranteed to find gravitational waves arising from major galactic events near our home galaxy, the Milky Way.
Detectors explained
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.
Inflation
In the first minuscule fraction of a second, scientists reckon, the young, super-hot universe – with a temperature of 10,000 trillion trillion degrees – experienced a phenomenal growth spurt, expanding 100 trillion trillion trillion trillion times and at a speed much faster than that of light. The expansion, known as cosmic inflation, continues today, although at a far more leisurely pace.
Inflation is an extra theory – a controversial hypothesis, some might say – that has been layered over the Big Bang to explain several key cosmic facts.
"In particular, inflation tells us why the universe is so smooth and flat," says Swinburne University astrophysicist Karl Glazebrook. "You may not think so, gazing around you, but on cosmic scales it is really uniform and the Big Bang was smooth to one part in 100,000 at its end."
Inflation, he says, explains the remarkable smoothness by having the universe expand early-on very quickly. "This smooths it out like crinkles in a balloon being blown up," he says.
The rapid expansion generates gravitational waves, which endure way past the age of inflation.
"The waves distort the light emitted by the super-hot gas of particles that filled the universe shortly after the Big Bang," says Curtin University astrophysicist Rob Soria. "We can still observe that light today, all around us, in the form of microwaves, with the initial distortions."
The theory of inflation can explain how the universe spontaneously gives rise to the vast structures around us today – from galaxies to clusters of galaxies containing thousands of Milky-Way-sized members.
Gravitational waves, Dr Duffy adds, should shed light on the nature of inflation – in particular, what particle species was involved.
"There's still the huge open question of whether we can understand the tiny fractions of a second before inflation, a goal I would never have thought feasible," Dr Duffy says. "Yet it now becomes the next obvious target."
M-theory
Once their existence has been confirmed by means of a direct detection, gravitational waves may also be used to confirm something called M-theory. This is the only real candidate for what British physicist Stephen Hawking has described as "a complete theory" of the universe, which he now believes may lie within reach. Among other things, this would aim to unite the ultra-small-scale world of quantum mechanics with the large-scale realm of relativity theory in what is called a quantum theory of gravity.
(By "theory", physicists refer not to an idea or hypothesis but to a rigorously established mathematical model that stands or falls by experimental test.)
In a nutshell, M-theory is a fundamental model of physics invoking no less than 11 dimensions. It's an extension of string theory in which particles, usually conceived of as tiny blobs of frozen energy, or even points, are held to be one-dimensional wiggling strings with length but neither height nor width, as such.
"Infinitely thin pieces of string," is how Professor Hawking describes it in The Grand Design (Bantam Press). The reason only some of these dimensions are experienced is that the others are imperceptibly curled in on themselves.
To gain a physicist's grasp of M-theory requires the mastery of such abstruse topics as wave-particle duality, imaginary time and an as-yet-unconfirmed phenomenon known as super-symmetry – which posits that every fundamental particle has a superheavy partner.
"The tiny vibrating strings are far too small to be tested by any conceivable experiment on Earth," Dr Duffy says. "Yet gravitational waves were once of a similar size to these strings – raising the prospect that they might still retain an imprint of this theory that we can test."
Multiverse
As if that were not enough, gravitational waves might also shed light on the enigmatic – and for some people wretchedly disturbing – prospect of a multiverse.
This refers to a multitude of universes, perhaps infinitely many, each having their own quirky parameters and dimensions. M-theory, in fact, permits at least 10 to the power 500 universes – namely 10 multiplied by itself 500 times.
The idea of the multiverse is decades old, reminds CSIRO physicist Kurt Liffman. "One model is that of the total universe undergoing infinite inflationary expansion, with small pockets having their expansion quenched by decay of the repulsive gravitational material. These pockets then undergo Big Bang expansion."
Under this science-fiction-like scenario, each pocket universe allegedly grows into its own separate cosmic realm, replete with its own set of fundamental physical laws.
"If gravitational waves can demonstrate that the inflation model of the early universe is correct, then this, in turn, implies that the multiverse model might also be correct," Dr Duffy says. "In that case, our universe would be but one of many embedded in the ever-expanding domain of a supra-universe."
Gravitational waves have a spectrum not unlike that of acoustic waves which we hear as specific sounds. He views this spectrum as the only practical way to learn about the early universe's extraordinarily high energies and temperatures because space was not transparent to any form of light during those early years.
"To test our physics at this level with CERN [home of the Large Hadron Collider straddling the Swiss-French border] would require a ring larger than the solar system," Dr Duffy says. "This is probably not practical given current funding – or indeed ever."
Publish or perish
Unusually for a supposedly momentous find, the discovery was published online on the BICEP2 webpage but was made public before being accepted for publication in a respected peer-reviewed journal.
"While this is not ideal, the results have undergone numerous internal reviews which help increase confidence in the findings," Dr Duffy says.
In principle, all scientific papers should go through the quality-control process of peer-review, Dr Liffman says.
"Yet there are exceptions. For example, it is quite reasonable for researchers to announce preliminary results at a scientific meeting and invite colleagues to critique their results. Authors can take note of the criticisms and so have a stronger paper, once it is submitted to peer-review."
Dr Soria's concerns go further. "The discovery was announced before a paper had even been submitted, let alone accepted," he says. "Personally, I am not happy about it. Scientists cannot use peer review as a magic formula to define 'official consensus' when it suits them and ignore or bypass it when it does not.
"By claiming a discovery via video conference, the BICEP2 people were essentially encouraging referees to accept their paper quickly – when it is finally submitted to a journal.
"If discoveries are announced via media conferences rather than peer-reviewed journals, there is a risk that the significance of new findings becomes a function of influence and money of an institution's media office," Dr Soria says. "I am not against using web review instead of peer review – that's the way of the future – but I dislike the idea of media conferences hyping findings before any review has taken place."
The peer-review process is critical to science, Dr Duffy says, "but so too is the repeatability of results. The true test will be if the Planck satellite, launched in 2009, confirms the BICEP2 results." Scientists are busy collecting and analysing the satellite's crucial data and are expected to release their final results later this year. Stay tuned.
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