快猫短视频

Forget dark matter 鈥� a vast amount of normal matter visible in ancient gas clouds has gone AWOL. Now astronomers are finding clues to where it鈥檚 hiding

WHEN surveys our galaxy, she sees things that aren鈥檛 there. Atoms, specifically. Atoms that are present when she looks into deep space, to regions seen as they were just a billion or so years after the big bang, and which should still be in our cosmic neighbourhood today. Except they aren鈥檛. 鈥淲e lose them,鈥� says Grenier. 鈥淲e see all this atomic matter in the past, but not around us now.鈥�

Forget dark matter, dark energy or any other hypothetical substance postulated to plug gaping holes in the fabric of the universe. Here is a tangible scandal of cosmic bookkeeping right on our doorstep. When we tot up all the everyday atoms in our galaxy 鈥� the sort that make up its stars, planets and people 鈥� about half of what we expect to see is missing.

Grenier, an astrophysicist at CEA Saclay in Gif-sur-Yvette, France, intends to track it down. In the past few years, she and others have started to see some of the missing matter: hidden pockets of extremely cold matter all but invisible to conventional telescopes. Problem solved? Not a bit of it. The new entries in the cosmic ledger leave us a long way from balancing the books, and are raising questions of their own. They could cause a major recalculation of how stars live and die, how galaxies continue whirling round, and even how they come to be in the first place.

Counting the atoms in the cosmos might sound a pointlessly laborious task. It is anything but. The particles that make up atomic nuclei formed shortly after the big bang 13.7 billion years ago, and their numbers depend sensitively on what exactly went on back then. If we look at the far cosmos, seen as it was 10 to 12 billion years ago, we see the number of atoms our favoured theories of our cosmic origins predict, sitting quietly in clouds of hydrogen and helium gas. So we would expect to see the same numbers in our neck of the woods today.

In the intervening eons, the universe has grown and evolved. Those ancient clouds have slowly collapsed in on themselves to form stars and galaxies, and galaxies have grown through collisions and mergers to form ever-greater structures. The first stars have died in fiery explosions, seeding the cosmos with elements heavier than hydrogen and helium, forged in their nuclear furnaces. Those elements 鈥� carbon, oxygen, silicon and the like 鈥� have gone on to make further generations of stars, their attendant planets and, in at least one case, life.

鈥淎ll the matter in those distant, ancient clouds should be here in the cosmos today. But it isn鈥檛鈥�

The cosmos we see today, with its vast, featureless voids and occasional galactic jewels, is the product of these processes. Conventional matter is concentrated in galaxies, 90 per cent of it in brightly burning stars. A tiny fraction of the remaining 10 per cent is encased in planets, but most is spread out between the stars in a tenuous medium of hydrogen and helium gas, with just a smattering of heavier, solid dust particles. With a density of a few hundred atoms per cubic metre, this interstellar medium would count as a vacuum on Earth.

Yet neither the stars of the Milky Way nor what lies between them seem to hold nearly enough stuff. This is a problem completely independent from that of dark matter, a debate kicked off in the 1970s because studies of other galaxies鈥� rotations convinced astrophysicists that three-quarters of the matter in the cosmos was both invisible and fundamentally different from normal matter (see 鈥淚nstant Expert: Dark Matter鈥�, 快猫短视频, 5 February). Here it is just a case of there we see it, here we don鈥檛: if the distant clouds of the early cosmos become the galaxy clusters of the present, all the matter that is in one should be in the other. But it isn鈥檛.

Why do we think that? Because the hiding places are limited. Stars are hard to miss; optical telescopes will have spotted all but the very faintest around us. Though almost a vacuum, the interstellar medium is also far from invisible. Warmed by surrounding stars, the hydrogen it contains glows at radio wavelengths, allowing radio telescopes to map out its distribution across the sky.

In some patches of the galaxy, it is true, this warm hydrogen fog disappears. In these areas, the gas and dust mixture has begun to cool and contract, creating clouds in which the average density rises to a trillion atoms per cubic metre. Within these clouds, pairs of hydrogen atoms can combine to form molecules. In this process the dust grains act both as a matchmaker, providing surfaces on which atoms meet atoms, and as a guardian, deflecting ultraviolet rays from nearby stars that would otherwise break the molecules apart again. These areas are the nurseries of stars: if the molecular clouds clump together enough, they suddenly collapse under their own weight and the gas ignites to form new stars. Within the Milky Way, gas roughly equivalent to the mass of our sun is made into new stars in this way each year.

Unlike hydrogen atoms, cold hydrogen molecules do not readily emit radio or infrared waves, so this gas disappears from view. Carbon monoxide (CO), though, forms under exactly the same conditions, and does emit radio waves. So even though we cannot see the clouds of hydrogen molecules, the amount of CO we see makes us pretty sure we know how much hydrogen is there.

And so things remained until 2005, when a new tip-off came as to the whereabouts of the missing matter. Grenier and her team were reviewing observations made by NASA鈥檚 . This space telescope had collected data on sources of highly energetic gamma rays in our galaxy and beyond between 1991 and 2000, and its archive included a number of puzzling, unidentified gamma-ray sources spread throughout the Milky Way. The sources were persistent rather than flaring or varying over the period of the observations, and they were not associated with any exotic, distant objects known to emit gamma rays, such as supermassive black holes.

It was when the team started comparing Compton鈥檚 gamma-ray maps with other cosmic maps that the penny began to drop. First, the areas of gamma-ray emissions corresponded perfectly with regions of infrared emissions seen by NASA鈥檚 and satellites that possibly indicated the presence of dust. Second, Compton鈥檚 gamma-ray sources neatly encircled areas that had a high abundance of CO.

Where there is dust, there is generally hydrogen. The team reasoned that cosmic rays 鈥� atomic nuclei whizzing through space at close to the speed of light 鈥� were striking dust grains as they fell into the molecular clouds, sparking gamma-ray emissions. 鈥淲e saw so many gamma rays and so much dust that these sources had to be associated with vast clouds of dark hydrogen gas,鈥� says Grenier.

In other words, it seems there is more to the interstellar medium than warm atomic fuzz with some cool molecular regions. Vast transition zones also exist, where hydrogen has cooled off enough to become invisible, but conditions are not yet right for telltale molecules of CO to form (see diagram). Judging by the size of the areas of gamma-ray emissions, these regions of dark gas double the mass of the CO-revealed clouds they encircle ().

Even this is only a start. Most of the interstellar medium is warm atomic fuzz, so doubling the amount of colder molecular hydrogen in our galaxy will affect the overall matter budget by a few per cent at most.

Yet even the small amounts of 鈥渄ark gas鈥� found so far could have serious consequences for our ideas about how galaxies tick. 鈥淭o understand star formation, we first have to understand the interstellar medium,鈥� says at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. Our models of the rate at which stars are born and die depend critically on factors such as temperature, pressure, density and turbulence in molecular clouds. 鈥淔inding that there is more mass in the clouds affects all these things,鈥� says Langer. 鈥淎nd all of that will affect star formation.鈥�

鈥淓ven small amounts of 鈥榙ark gas鈥� could have serious consequences for theories of how galaxies tick鈥�

At the moment, no one is exactly sure how, because no one knows exactly how much dark gas there is. While the latest space telescopes are scouring the sky for more suspicious looking areas 鈥� the European Space Agency鈥檚 at microwave wavelengths and NASA鈥檚 at gamma-ray wavelengths 鈥� Langer鈥檚 team is taking a more direct approach. They are tracing the distribution of another atomic species: ionised carbon, aka C+.

This ion, which emits infrared light, is a precursor to the formation of CO. It is produced when ultraviolet light strips an electron from a carbon atom, a process most likely to occur on the edges of molecular clouds before the ion bonds with oxygen and the CO sinks further in. Early results based on observations by the European Space Agency鈥檚 , published in October last year, are already indicating much stronger C+ emission from possible transition zones than was expected, with the implication of much more hidden hydrogen ().

While others look further afield for the still-missing matter (see 鈥淥n a WHIM and a prayer鈥�), Langer thinks more painstaking surveys will radically revise how much dark gas is trapped in the innards of the galaxy. 鈥淭his is just the beginning of this story,鈥� he says. 鈥淭here may be whole clouds out there in transition that we have not seen yet.鈥�

At least one astronomer goes further. Since the mid 1980s, of the Observatory of Geneva, Switzerland, has been investigating the consequences of there being much larger quantities of hidden gas in the Milky Way. The latest discoveries have emboldened him to claim that the hidden dark gas could do at least some of the stuff that dark matter was conjured up to do.

鈥淗idden dark gas could do at least some of the stuff that dark matter was conjured up to do鈥�

Disappearing dark matter

Dark matter鈥檚 existence was first postulated when it was discovered that stars towards the edge of galaxies, which should be moving slower than those further in, are in fact are orbiting their galactic centres at roughly the same speed. This 鈥渇lat rotation curve鈥� invites one of two conclusions: either there is more matter pulling on the outer stars than we can see, or the law of gravity, which dictates the rotational speed, is somehow modified over large distances. Impressed by the accuracy of most gravitational predictions, the majority of cosmologists plump for the first option.

This solution is not without its problems, however. Dark matter is thought to be found within a spherical 鈥渉alo鈥� surrounding a visible galaxy, but that distribution struggles to produce the right rotation curve. Pockets of dark molecular hydrogen gas, about the size of the solar system, scattered around the outer reaches of a galaxy would do a far better job, says Pfenniger. 鈥淚f we were to triple the gas content of the Milky Way, we could flatten the rotation curves,鈥� he says.

That might sound nice and simple, but there鈥檚 a catch: it brings us into conflict with our models of the big bang, which predict twice, not three times, the number of atoms we see. For this reason, Pfenniger thinks his model is unlikely to do away with dark matter entirely, and proposes a compromise: unseen atoms contribute around half of the rotation curve solution, with the rest still coming from dark matter.

A test of his idea should come from the , an array of radio telescopes high in the Chilean Andes due to be fully operational in 2012. It should be sensitive enough to map exactly the gas in a radius of 100 to 1000 light years around us, and provide evidence for the presence of any small, dense pockets of atomic gas that might have escaped our notice.

Pending such conclusive observations, the mystery of the missing matter remains. Will Grenier and her like soon be seeing a lot more stuff that isn鈥檛 there? It鈥檚 hard to tell. 鈥淲e are just at the beginning of this investigation,鈥� says Langer, with barely disguised glee.