To the dark side

ANCIENT Greek philosophers thought the world was composed of four elements: earth, air, fire and water. To explain the heavens, though, many saw a need for something more—quintessence (quinta essentia in Latin), a fifth element. Quintessence was part of the universe, but lay out of humanity’s grasp.

These days, physicists face a similar problem. The material around them, made up of atoms, seems unable to explain what most of the rest of the cosmos is made of. Energy and matter are two sides of the same coin, so a list of universal ingredients must measure both.

The latest research has it that some form of unseen energy, of unknown nature, makes up about two-thirds of that total “energy density”. Of the stuff with mass, which thus counts as matter, inscrutable “dark matter” constitutes another quarter of the universe’s contents. What is left is what humanity perceives as normal matter. But it is far from normal. Every atom of Earth, every star dotting the heavenly vault, every bit of dust between and beyond: together they account for less than 5% of the universe’s recipe.

The idea that there might be more to the universe than the familiar atomic matter of stars, planets and people began to take shape in the early 1930s, when Fritz Zwicky, a Swiss astronomer, noticed something odd about the behaviour of galaxies. Those within clusters, he observed, move far faster than they should, based on the sizes of their gravitational fields, as inferred from their visible matter. Jan Oort, a Dutchman, had noticed the same thing about stars nearer Earth. Something, and a lot of it, was generating gravity without producing or reflecting light. It was Zwicky who dubbed it dark matter. At that time he estimated it outweighed the familiar kind by ten to one.

To start with, people thought dark matter was just normal matter in the guise of things that did not shine—orphan planets, perhaps. But close study of the cosmic microwave background (CMB), an all-suffusing bath of radiation left over from the Big Bang, put limits on the amount of such normal matter that the Big Bang could have created. These limits are too low to explain the apparent preponderance of dark matter if it is simply atoms that are not lit up.

These days, “cold dark matter”, as it is now known, is thought to be made of something which does not interact much with ordinary matter except via the force of gravity. One class of weakly interacting particle, neutrinos, is well known, but these are too light to balance the ledger (see chart). To distinguish what they seek from neutrinos, researchers refer to the components of cold dark matter as weakly interacting massive particles, or WIMPs.

A problem of considerable gravity

In the 1980s, as simulations based on cold dark matter lined up better and better with astronomers’ detailed maps of galaxy clusters, it seemed that the two types of matter, dark and visible, accounted for all of the stuff in the universe. In 1990, however, a survey of 2m galaxies suggested that they did not. Though matter was drawing things together, something else was pushing them apart—and that something else was so powerful that it formed much of the universe.

The story of what that something might be began in 1915, when Albert Einstein was putting the finishing touches to his general theory of relativity. Einstein’s problem was that his theory predicted a universe whose size could vary—in contrast to the idea then widely held, not least by himself, that it was of a fixed extent.

The answer to what such a stretchy universe might mean lay in observations made a few years earlier by Vesto Slipher, an American astronomer who had been measuring the speeds and distances from Earth of far-flung galaxies. What he found was that galaxies more distant from Earth were moving away faster. Edwin Hubble, another American, came to the same conclusion in 1929.

One way to explain these findings was that the fabric of space itself was expanding. Einstein, when faced with the uncomfortable idea of a universe that changed its size, had fudged his equations with a “cosmological constant”, denoted by the Greek letter lambda, whose value could be set to keep the universe static. Slipher’s and Hubble’s results caused him to throw this out. His equations predicting an expanding universe had been right.

After the survey of 1990, though, there were murmurs that lambda needed dusting off in order to fix things—not by holding them steady, but by pushing them apart. And the murmurs continued until 1999, when two groups of astronomers who were studying the farthest (and therefore the oldest) visible supernovae found that the speed-with-distance relation first seen by Slipher and Hubble was different in the young universe. It appeared that, after an enormously rapid, but short burst of expansion at the beginning, known as inflation, the expansion of space had settled down to something slower than it is now. It then started speeding up again (see chart). Not only was the universe getting bigger all the time, it was getting bigger, faster.

Soon after this discovery, Michael Turner, a cosmologist, gave the cause a name: dark energy. Its effects were rounded up and accounted for in the revived cosmological constant. These days, “lambda-CDM”, which pairs the constant with cold dark matter, is the gold-standard theory in cosmology.

The best test bed for lambda-CDM is the CMB, which a succession of space telescopes have examined it in ever-greater detail. Rather than being uniform across the sky, it is riddled with ripples and hotspots. These deviations from the average are tiny—a few hundred-thousandths of a degree—but they are telling.

The lambda-CDM model reduces the universe to an equation with six easy pieces. When these are balanced to line up theory and observations—such as staggeringly precise measurements of the CMB taken by a European space telescope, Planck—the equation elegantly accounts for a wide range of phenomena. The model’s predictions best match the Planck picture when the universal recipe includes 68.3% dark energy (a bit less than original estimates), 26.8% dark matter, 4.6% familiar matter and 0.3% neutrinos.

Untangling the web

It is all well and good knowing how much of these things there is, but what exactly are they? Relativity requires that, because dark matter has mass, it bends the paths of light rays. This can induce a phenomenon called gravitational lensing. Astronomers have therefore become expert at deciphering images of distant, normal-matter objects that have been distorted en route by dark matter. What they have found is a cosmic web. Vast lumps and long threads of dark matter are spread through the universe, dragging atomic matter, in the form of galaxies, along with them.

Three kinds of experiments aim to find the WIMPs of which these threads are presumed to be made. The most straightforward try to catch WIMPs directly. Weakly interacting is not non-interacting, so WIMPs should occasionally bump into atoms, transferring a smidgen of energy as they do so. About a dozen WIMP-hunting machines have been set up in laboratories deep underground, to try to measure the tiny traces of heat, light or sound that would result from such collisions.

Another way is to look for what dark matter leaves behind. Many theories hold that WIMPs are their own antiparticles—dark antimatter, if you like—and when matter and antimatter meet, they annihilate each other in a blaze of light. Since there is lots of dark matter, annihilations should be happening in lots of places, leading to spikes of light of a particular colour. Astronomers are on the lookout for such peaks in places where they suspect dark matter may be at its densest, such as the centres of galaxies, including the Milky Way.

Or you could just make WIMPs in a laboratory. This requires particle collisions of the highest achievable energies. And that, in turn, calls for the Large Hadron Collider, in Switzerland. There is some chance that tiny amounts of dark matter will be created there, in the soup of particles the machine’s users hope to stir up.

Dark energy, meanwhile, will be an even trickier beast to capture, for theorists think it does not manifest itself in any form of particle they might plausibly create or observe. It is presumed to be distributed evenly, so should permeate laboratories on Earth. But it is weak. Efforts to spot its influence on an old-time weight-on-a-wire gizmo called a torsion balance have turned up nothing. So have experiments using a fancy source of neutrons to check for gravitational weirdness over distances of microns. Pinning dark energy’s nature down, it seems, will happen in the same way as its discovery: by looking to the skies.

There are several ways to do this. The simplest is to measure ever more precisely the distances of supernovae and galaxies of various ages, and thus plot a more detailed graph of the interplay between dark energy’s push to expand the universe and matter’s pull to contract it.

More subtle approaches should be able to squeeze extra value out of those same data. Light and matter pushed one another to and fro in the early universe, creating pressure waves. As the early universe expanded, matter was scattered along these so-called baryon acoustic oscillations: more densely at the peaks of the waves and less so at the troughs. A plot of the oscillations’ imprints on the distribution of matter at various distances from Earth should reveal more about dark energy’s historical influence.

A third approach which may reveal that history is to focus on a weak variety of gravitational lensing. Light is bent not only by discrete chunks of mass in single galaxies, but also by the large-scale structure of all the stuff that surrounds galaxies. Better telescopes will be able to measure how dark energy has effected tiny changes on the lensing over time.

Nothing more constant than change

For now, though, there is a dearth of data to shore up or shoot down diverse conjectures, leaving a vast playground for theorists to mess around in. The most credible ideas involve a fundamental constant of physics—perhaps the cosmological constant itself—varying in time or space. So far, it appears that dark energy’s effects came into prominence only in the past 6 billion years or so. A time-varying cosmological constant would thus be an easy fix.

That is the idea behind a theory that a handful of physicists have cleverly called quintessence. But such adjustments all run into a whack-a-mole problem: fix the darknesses and your modified theory fails to account for some other phenomenon.

Banishing the darkness is a worthy goal. Indeed, the ultimate fate of the universe is at stake. The amount and nature of dark energy, and whether its properties vary with the age or size of the universe, will dictate which of three futures happens. In one, the universe expands calmly for ever. More dramatically, the expansion may speed up as the effects of dark energy overwhelm everything else. The universe then thins out of existence in a “Big Rip”. Or matter may eventually win out over dark energy—its gravity slowing and reversing the expansion, and drawing everything together again in a “Big Crunch”.

In an era when so much science seems understood, all this uncertainty might appear to be an embarrassment. Not so. Any physicist would tell you it is an opportunity, a frontier. After all, from his point of view, there remains 95% of the universe—a quintessential part, it might be said—yet to be discovered.

LATER IN THIS SERIES:
• What caused the Cambrian explosion?
• Why does time pass?
• What is consciousness?