Oh, For a Theory of Everything

Even 100 years after Einstein conjured up his famous equation the nature of most of the universe still eludes us

By Stephen Battersby

Discovering dark matter a few decades ago and dark energy a few years ago have thrown physics into a turmoil.
If gravity behaves differently over long distances then physicists will have to rewrite Einstein's general theory of relativity.

When physicist Stephen Hawking starting writing his bestseller A Brief History of Time, he was warned that including just a single equation would halve the sales of his book. Despite the financial implications, Hawking felt compelled to include one, E = mc², underlining the iconic status of Albert Einstein's famous formulation.

The equation, published exactly 100 years ago, has come to symbolise the upheavals of early 20th-century physics. Einstein's theories of relativity, along with quantum physics, changed our ideas of space and time, cause and effect, and spawned theories for everything from the big bang to black holes. For decades, relativity and quantum mechanics have provided the foundation for modern physics.

Now this foundation is cracking. Enigmatic discoveries of dark matter a few decades ago and of dark energy a few years ago have thrown physics into turmoil. Nearly 96% of the universe is made of these two mysterious elements, and today's best physical theories cannot explain them.

Neither can the theories explain the powerful force that cosmologists think inflated the universe when it was only a fraction of a second old. Even the biggest of "big physics" experiments – such as the Large Hadron Collider, which will start smashing protons head-on sometime in 2007 to hunt for exotic new particles may be unable to shed light on these cosmological quandaries.

So, in their quest to find answers to the deepest questions of physics – and maybe even discover a 21st-century version of E = mc² – many researchers are turning to a new generation of cosmological experiments. It's a sure sign that cosmology has come of age.

The most striking examples of these experiments include remarkable space-based and terrestrial telescopes to probe for dark energy, a sophisticated hunt for dark matter deep inside abandoned mines, and the attempt to study the very first instants of the universe using the microwave equivalent of Polaroid sunglasses. With luck, these instruments will take us closer to the elusive final theory that unifies all the forces of nature, something that Einstein himself spent 30 years pursuing in vain (NewScientist, 30 April, p 30).

The latest crisis in physics was precipitated by the discovery of dark energy in 1998. Two teams of astronomers independently discovered that distant stellar explosions called type la supernovae are dimmer than expected. Their only explanation for the observation was that the expansion of the universe is accelerating, dragging galaxies apart ever faster. The unknown cause of this acceleration was named dark energy, and it makes up 73% of the energy and matter in the universe.

"Dark energy cuts to the heart of physics," says Steven Kahn, deputy director of the Kavli Institute for Particle Astrophysics and Cosmology in Stanford, California.

Quantum theory even suggests a source for this energy: an all pervasive field known as the quantum vacuum. This field is though to be a melee of short-lived particles that pop in and out of existence, and might endow space with some net energy.

That is because relativity and quantum physics have both hinted at something akin to dark energy. According to Einstein's general relativity, if space has an inherent energy then this would produce a repulsive force that would counteract gravity and could explain the acceleration seen by the cosmologists.

But when physicists calculate the value of this vacuum energy using the standard model of particle physics, the answer works out at more than a hundred orders of magnitude too large. That would produce a repulsive force strong enough to rip all matter apart in an instant. There are speculative particle theories in which the vacuum energy works out to be zero, but that's no good either. To produce the observed acceleration, dark energy must have a density of about one nanojoule per cubic metre.

So to better understand the problem, NASA and the US Department of Energy are planning to fund the Joint Dark Energy Mission. The main aim for JDEM is to spot thousands of type 1a supernovae and use them to measure how the expansion of the universe has varied over time.

Knowing this rate will tell cosmologists whether dark energy really is some sort of constant vacuum energy, or a varying energy field called quintessence. A constant vacuum energy would cause the acceleration to take off more swiftly than would a quintessence field.

JDEM will be complemented by a revolutionary ground-based instrument designed to sneak up on dark energy from another direction. The large synoptic survey telescope (LSST) will monitor the whole sky in depth with its massive mirror, and its detailed images will reveal how galaxy clusters and superclusters have grown through the action of gravity. LSST's survey of the growth of clusters could reveal the strength of dark energy, which inhibits clustering.

What's more, the LSST could reveal whether gravity becomes repulsive at long range, as some think it does. If gravity indeed behaves differently over long distances, it would limit the size of superclusters of galaxies, which the LSST should be able to measure.

If so, physicists will have to rewrite Einstein's greatest achievement, the general theory of relativity. It might also give them a clue about how to unify their theory of gravity with quantum mechanics, which has proved impossible so far, and remains the sternest test of any would-be "theory of everything".

As some cosmologists begin to probe dark energy, others are still trying to figure out the second main ingredient of the universe: dark matter. Back in the 1960S, astronomers discovered that galaxies are rotating so fast that they should fly apart. The popular explanation is that extra gravity from some invisible form of matter, contributing almost 10 times as much mass as the visible stars and gas, is keeping the galaxies intact.

The prime candidates for this dark matter are called weakly interacting massive particles, or WlMPs. They are predicted by a few speculative particle theories, including supersymmetry, which posits that every known particle has a heavier partner.

The hunt for WIMPs is taking place around the world, but the clear leader is an experiment called CDMSII, 700 metres under Minnesota in the Soudan mine. CDMSII is looking for WlMPs drifting through the Earth. So far, it has failed to find any, which has already ruled out many versions of supersymmetry.

Supersymmetry is in turn central to string theory, which says that everything in the universe is made of tiny strings whose different vibrational modes give rise to the different particles and their masses. Confirming some version of supersymmetry would be a big boost for string theorists, who have come closest to developing a theory of everything.

Other clues to such a theory might come from a study of what happened immediately after the big bang, when physicists believe an unknown energy field caused space to expand exponentially, during a fleeting epoch known as inflation.

At that time, ranging from approximately 10 -32 to 10 -14 seconds after the big bang, the universe was so hot that particles were routinely smashing into one another at energies higher than any man-made particle accelerator can achieve - so relics of that era could give cosmologists insights into a realm beyond known physics.

A relic from slightly later on is the cosmic microwave background (CMB) – a sea of radiation left over from when the universe was about 270,000 years old. Most cosmologists believe that the CMB also holds traces of what happened during inflation. The idea is that inflation should have generated violent gravitational waves, which would have distorted space and polarised the CMB in a swirling pattern.

In 2007 a European satellite called Planck will begin looking for this polarisation, and simpler ground-based instruments planned for the next few years should also join the hunt. Measurements of polarisation could tell physicists about the energy of the field that caused inflation – an invaluable clue in their search for a theory that unifies all forces.

Such precise measurements of the CMB might even reveal the structure of space and time, says Martin Bojowald of the Max Planck Institute for Gravitational Physics in Potsdam, Germany. He works on loop quantum gravity, another approach to a theory of everything, which postulates that the very fabric of space-time is made of tiny pieces, or quanta.

According to Bojowald's calculations, inflation could have magnified this graininess of space, leaving a mark on the CMB specifically a slight excess of small bumps in the background radiation. Planck and its companions might just spot this.

Along with Planck, CDMSII, JDEM and the LSST, a host of other projects are already running or being planned that will look into space for more clues to new physics. The internationally funded Gamma Ray Large Area Space Telescope (GLAST) might see, for example, a spike of gamma rays at a specific energy, which would be emitted if dark matter particles collide and annihilate.

GLAST, which will be launched in 2006, could also test one of Einstein's axioms that the speed of light and all other electromagnetic radiation is constant. Some theoreticians have suggested otherwise, arguing that high-energy radiation could actually travel more slowly. If so, high-energy gamma rays from distant events will hit GLAST a fraction later than low-energy rays from the same sources.

The challenges of these extraordinarily precise experiments have forged an alliance between cosmologists and particle physicists, who study such vastly different arenas. Cosmology's ever larger and more complex experiments are demanding the kind of skills that particle physicists already possess. For instance, the rate at which the Large Synoptic Survey Telescope will spew out data is unheard of in astronomy, but is nothing new for particle smashers.

And particle physicists realise that they need answers from cosmology to further their own work. "The high-energy physics community has been forced to take seriously that doing these observations is important for their field, and that's now coming to be accepted by funding agencies," Kahn says.

One convert is Bruce Winstein, a former high-energy particle physicist and now director of the Kavli Institute for Cosmological Physics in Chicago. He is leading a project to search for polarisation in the CMB. "I was excited about cosmology in terms of the fundamental science it was doing."

Cosmology, itself an offspring of Einstein's theories, might now help physicists to supersede him. If these ambitious new projects can glean enough clues from the cosmos, then before very long somebody might be writing down the new E = mc².

Source: NewScientist 24 September 2005