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Antibioitc resistance threats
'So far lt looks like there are very few, If any, limits to how far a resistance gone can spread'
FARMERS should stop using antibiotics as growth promoters, say researchers in the US. They have uncovered evidence of a new route by which dangerous antibiotic resistance genes can spread. There is already strong evidence that feeding animals antibiotics can lead to the emergence of resistant strains of gut bacteria such, as salmonella, which can then be passed on to people in food or through direct contact with animals. Now microbiologist Rustam Aminov of the University of Illinois at Urbana-Champaign and his colleagues have discovered that bacteria in the soil and groundwater beneath farms seem to be acquiring tetracycline resistance genes ftom bacteria originating in pigs' guts. Once transferred, the resistance genes can persist in the hardier soil and water-borne bacteria and could be passed on to potentially dangerous bacteria in the environment, or in humans who drink the water. "This is very important. [The study] is the first of its kind to demonstrate this kind of broad ecological presence of tetracycline resistance genes," says Stuart Levy, director of the Center for Adaptation Genetics and Drug Resistance at Tufts University in Boston. "And this is just tetracycline. Add all the other drugs that might be there, and then I think it further supports the notion that we should be prudent in how we use antibiotics in animals and people." VVhile the European Union has banned the use as growth promoters of most antibiotics that are used in human medicine, farmers in the US still routinely add antibiotics such as tetracycline, penicillin and streptomycin to livestock feed to promote animal growth. Nearly 70 per cent of all antibiotics produced in the US are fed to animals as growth promoters, according to the Union of Concemed Scientists, a non-profit organisation based in Cambridge, Massachusetts. To study the environmental effect of these antibiotics around two swine farms that use tetracycline as a growth promoter, Aminov's team analysed samples from farm-waste lagoons and from groundwater reservoirs beneath the lagoons. They found that bacteria in the soil and groundwater carried tetracycline resistance genes, or tet genes, that were almost identical to those in bacteria living in the pigs' guts. This strongly suggests that the bacteria ftom the pigs are transferring their genes to the ones outside, says Aminov.
PIG OF A PROBLEM: bugs from pigs' gut can pass on resistance genes to soil bacteria
'People at both sites are drinking this groundwater without any treatment. This may be a new way of increasing the local' concentration of antibiotic resistance genes and circulating them between animals, humans and the environment,' he says. And as groundwater accounts for a substantial part of the public water supply in the US, the problem could be widespread.
Abigail Salyers, also at the University of Illinois, agrees. She and her colleagues recently showed that bacteria passing through human intestines exchange genes with the resident bacteria. They found that 80 per cent of the strains of a major bacterial species found in the colons of people in the late 1990s carried tetracycline resistance genes, compared with 30 per cent before 1970. Together, the studies suggest that antibiotic resistance genes are being transferred from the environment into our bodies, she says. "What we are seeing here is that if a resistance gene gets out into the bacterial population in nature, it's like letting the genie out of the bottle. So far it looks like there are very few, if any, limits to how far a resistance gene can spread," she says. Anit Ananthaswamy More at: Applied & Environmental Microbiology (vol 6T, p 1494)
Reap whid you sow Farming will cause more damage to the planet than global warming
MODERN agriculture is set to become as bad for the planet's health as global warming, a team of leading environmental scientists has warned. They list rainforest destruction, nitrogen pollution and the spread of diseases such as foot and mouth and BSE among the growing threats from agriculture. 'The environmental effects of agriculture are on a trajectory soon to rival those of climate change," says David Tilman, an ecologist at the University of Minnesota. Tilman and nine other ecologists in the US forecast that over the next 50 years growing populations and an increasing demand for meat will mean the world needs new farmland covering an area the size of the US. Meeting this demand will probably see the destruction of "the vast majority of the rainforests and savannah grasslands in Latin America and central Africa, which harbour a large portion of the Earth's biological diversity", Tilman told New Scientist. Animal diseases will be increasingly likely to spread to humans, the report warns. 'Livestock are well-known to be a major source of human disease, and current animal production methods may well be favouring the evolution of new human pathogens," says Tilman. The increased density of livestock also makes epidemics more likely. Foot and mouth disease in Europe is the latest manifestation of this phenomenon, says Tilman. "Epidemic losses of livestock are not a one-time fluke, but rather a predictable outcome of high animal densities and low genetic diversity." The study also predicts a pandemic of farm pollution, including fertilisers and pesticides, that could upset ecosystems worldwide. Human activities already release as much nitrogen and phosphorus into the environment as natural sources. Much of this comes from farms, including unused fertiliser and livestock sewage. It causes eutrophication-the over-fertilisation of soils and water that results in the growth of toxic algae in lakes and coastal waters, poisoned fisheries, nitrate-contaminated drinking water and acidified soils.
Eutrophication will double or triple worldwide in the next 50 years, according to the study. A marine 'dead zone' the size of New Jersey that has formed in the Gulf of Mexico because of farm run-off down the Mississippi is a sign of things to come, it says.
In developed countries, more than a third of all the nitrogen applied to farms in fertiliser eventually emerges as animal waste that is discharged into the environment. To counter this, Tilman calls for farmers to install equipment to treat animal wastes from all intensive livestock units. 'These cattle and hog factories may contain 10,000 or more animals. They produce as much sewage as human cities,' he says. But unlike human cities they are not required to treat their sewage.'
The 'unprecedented" loss of biodiversity that Tilman predicts as a result of farming will not only rival the impact of climate change, it could also encourage it. in a separate paper in Nature, Tilman's Minnesota colleague Peter Reich shows that ecosystems that contain more species will be able to soak up more carbon dioxide as atmospheric levels of the gas increase.
Degraded ecosystems with fewer species will be less good at this. "A species-poor world,' Reich concludes, 'is likely also to be a hotter world.' Fred Pearce More at: Science lvot 292, p 281) and Nature(vol 410, p 8091
Dark Stars? Is the cosmos teeming with rogue planets vathout stars to call their own?
JUST a few years ago, we knew of no planets outside the nine in our own Solar System. Astronomers have since found scores of other planetary systems, but a new survey suggests that we shouldn't limit our search to the environs of other stars. Spanish researchers say space is peppered with free-floating planets that are not attached to any star. So many, in fact, that astronomers are having to rethink their theories about how stars and planets form.
Last year, a team led by Rafael Rebolo at the Astrophysics Institute of the Canary Islands first spotted planets floating light years from the nearest star (New Scientist, 14 October 2000, p 20). Now the astronomers have surveyed the number of objects of different sizes in the Sigma Orionis cluster.
In a sample of 64 low-mass objects-ranging in size from brown dwarf stars to jupitersized planets-they found that the smaller the mass, the more of them there were. If this relation continues down to planetary objects too small to see, there could be hundreds of isolated planets within 30 light years of the Sun, making them as numerous as Sun-like stars. Such large numbers pose a problem for the current theory of how stars and planets form. This says that dense clouds of dust and gas collapse under their own gravity to form stars, while planets form later in the disc of debris that surrounds a newly formed star. Simulations suggest that objects less than a tenth the mass of the Sun just don't have enough gravity to directly collapse a gas cloud. "It's almost impossible to make something like a planet this way,' says Rebolo. So theorists are scrambling for new ideas. Some think isolated planets may have formed around a star like a normal planet. "In time they can enter unstable orbits and be ejected," says Maria Rosa Zapatero Osorio of the University of California at Santa Barbara. But there's controversy over how likely this is to happen. Alan Boss of the Carnegie Institution in Washington DC has another idea. He says that if massive stars form first in a gas cloud, they could radiate strongly enough to ionise material in the remains of the cloud. Magnetic effects could then make the ionised material spread out, helping it to fragment into small clouds before the pieces have a chance to collapse. "You can make very small objects, down to one or two Jupiter masses," says Boss. But even this can't explain why some clouds turn into a mixture of huge stars and isolated planets, while others make a load of medium-sized stars. 'It depends on the detailed dynamics of the system, and that's hard to predict,' says Boss. Eugenie Samuel, 'particles are supposed to have been formed along with normal matter in the furnace of the big bang. They are invisible because they don't feel all the forces that ordinary matter does, and light just ignores them. Astrophysicists usually assume that these dark matter particles are "cold"-that is, fairly heavy and slow moving, so they tend to clump together.
According to the conventional theory, this clumpiness allowed cold dark matter to give birth to galaxies. Way back in what astronomers call the dark ages, when the Universe was a mere billion or so years old, cold dark matter gathered itself under gravity into giant blobs called haloes. These then attracted normal matter to form stars, turning into bona fide galaxies.
The problem is that in its simplest form, this theory says there should be an awful lot of little galaxies-the failed relics of galaxy formation that never managed to grow into giant elliptical galaxies or spirals.
Fruits of success Growing food the organic way may fatten up your wallet
ORGANIC food isn't just tastier and better for the environment, it also makes better business sense. That's the conclusion of scientists who have compared the economic efficiency and environmental impact of conventional, organic and "integrated' apple orchards.
John Reganold, a soil scientist at Washington State University in Pullman, and his colleagues ran a detailed comparison of factors such as soil quality, apple yield and environmental impact at the three types of orchard between 1994 and 1999. The organic orchard relied on manure as fertiliser and used natural methods to deal with parasites. The integrated farming system is not totally organic, but uses some organic methods to reduce the reliance on chemicals.
The team found the soil fared best in the organic plot. It held water better, and resisted degradation at the surface. Previous studies have shown that -the build-up of sulphur, commonly used as a natural fungicide, could be a long-term problem, but the team found no evidence of this. 'The incidence of pests was very low on all the plots," says Reganold. To their surprise, the researchers also found the organic orchard was more energy efficient than the conventional and integrated systems, requiring less tabour and less water per apple produced. It even made more money, mainly because organic apples command higher prices. It's not clear whether the study holds a lesson for farming in general. Some past attempts to compare conventional and organic farming methods have suggested that organic farming may be less economic and energy efficient than conventional methods. Some studies even suggested that it might be worse for the environment. But until now, "there've been almost no studies looking at the overall sustainability of both methods', says Reganold. Critics point out that if everyone farmed organically, organic food might not command a premium. "Why should consumers pay so much more for what is essentially a comparable product?" asks Dennis Avery of the Center for Global Food Issues in Churchville, Virginia, which campaigns on the environmental impact of agriculture. Reganold counters that consumers are getting more for their money. 'The organic apples were firmer and taste better, sweeter and less tart to a non-expert panel,' he says. .But Avery doubts organic farming will ever provide a large-scale solution. 'We're going to have 10 billion people to feed worldwide," he says. "I just don't see the relevance of this." Eugenie Samuel More at: Nature(vol 410, p 926)
The God in our Mind
EINSTEIN FELT IT. It's what draws people to church, prayer, meditation, sacred dance and other rituals. Chances are you've felt something like it too-in the mountains, by the sea, or perhaps while listening to a piece of music that's especially close to your heart. In fact, more than half of people report having had some sort of mystical or religious experience. For some, the experience is so intense it changes their life forever. But what is "it"? The presence of God? A glimpse of a higher plane of being? Or just the mystical equivalent of deja vu, a trick the brain sometimes plays on your conscious selp At some level, of course, all our thoughts and sensations-however unusual-must involve the brain. Indeed, experiments on the brain have led neuroscientists to suggest that the capacity for religion may somehow be hardwired into us. If so, why do people's religious experiences differ so profoundly, moving some so deeply while leaving others cold? Andrew Newberg, a neuroscientist at the University of Pennsylvania in Philadelphia, has been fascinated by the neurobiology of religion for more than a decade. He admits it's an awkward role for a scientist. 'I always get concerned that people will say I'm a religious person who's trying to prove that God exists, or I'm a cynic who's trying to prove that God doesn't exist," he says. "But we try to approach it without bias." Earlier this month he published a book, which lays out the most complete theory to date of how mystical or religious experiences can be generated in the brain. Together with the now deceased Eugene d'Aquili, a colleague from Penn, Newberg was keen to study the sensations that are unique to religious experiences but shared by people of all faiths. One of these is the sense of "oneness with the Universe" that enthralled Einstein. The other is the feeling of awe that accompanies such revelations and makes them stand out as more important, more highly charged, and in a way more real than our everyday lives.
But Newberg realised that rare, fleeting revelations would be almost impossible to study in the lab. It meant he had to ignore the one-off experiences that strike out of the blue and focus instead on meditation and prayer-sedate, but at least reproducible.
Through a colleague who practised Tibetan Buddhism, Newberg and d'Aquili managed to find eight skilled meditators who were willing to undergo brain imaging. The volunteers came to the lab one at a time, and a technician inserted an intravenous tube into one arm. Then the volunteer began to meditate as normal, focusing intently on a single image, usually a religious symbol. The goal was to feel their everyday sense of self begin
to dissolve, so that they became one with the image. "It feels like a loss of boundary," says Michael Baime, one of the meditators and also a researcher in the study. 'It's as if the film of your life broke and you were seeing the light that allowed the film to be projected."
Hidden in the next room, Newberg and d'Aquili waited. When the meditator felt the sense of oneness developing-usually after about an hour-they would tug on a string. This signalled the researchers to inject a radioactive tracer through the intravenous line.
Within minutes the tracer bound fast to the brain in greater amounts where the blood flow, and hence brain activity, had been higher. Later a scanner would measure the distribution of the tracer to yield a snapshot of brain ' activity at the time of binding. The technique, called Single Photon Emission Computed Tomography, or SPECT, allowed the subjects to meditate in the relative peace of the lab rather than the claustrophobic whirr of a scanner. Once the tests were completed, Newberg and d'Aquili compared the activity of the subjects' brains during meditation with scans taken when they were simply at rest.
'DOES OUR RELIGIOUS SENSE OFFER ANY INSIGHT FOR UNDERSTANDING THE GRAIN OF THE UNIVERSE? THAT'S A HAUNTING QUESTION'
Perhaps unsurprisingly, the researchers found intense activity in the parts of the brain that regulate attention-a sign of the meditators' deep concentration. But they saw something else, too. During meditation, part of the parietal lobe, towards the top and rear of the brain, was much less active than when the volunteers were merely sitting still. With a thrill, Newberg and d'Aquili realised that this was the exact region of the brain where the distinction between self and other originates.
Broadly speaking, the left-hemisphere side of this region deals with the individual's sense of their own body image, while its right-hemisphere equivalent handles its context-the space and time inhabited by the self. Maybe, the researchers thought, as the meditators developed the feeling of oneness, they gradually cut these areas off from the usual touch and position signals that help create the body image.
'When you look at people in meditation, they really do turn off their sensations to the outside world. Sights and sounds don't disturb them any more. That may be why the parietal lobe gets no input," says Newberg. Deprived of their usual grist, these regions no longer function normally, and the person feels the boundary between self and other begin to dissolve. And as the spatial and temporal context also disappears, the person feels a sense of infinite space and eternity.
More recently, Newberg has repeated the experiment with Franciscan nuns in prayer. The nuns-whose prayer centres on words, rather than images-showed activation of the language areas of the brain. But they, too, shut down the same self regions of the brain that the meditators did as their sense of oneness reached its peak.
This sense of unity with the Universe isn't the only characteristic of intense religious experiences. They also carry a hefty emotional charge, a feeling of awe and deep significance. Neuroscientists generally agree that this sensation originates in a region of the brain distinct from the parietal lobe: the "emotional brain", or limbic system, lying deep within the temporal lobes on the sides of the brain.
The limbic system is a part of the brain that dates from way back in our evolution. Its function nowadays is to monitor our experiences and label especially significant events, such as the sight of your child's face, with emotional tags to say 'this is important'. During an intense religious experience, researchers believe that the limbic system becomes unusually active, tagging everything with special significance.
This could explain why people who have had such experiences find them so difficult to describe to others. 'The contents of the experience-the visual components, the sensory components-are just the same as everyone experiences all the time," says Jeffrey Saver, a neurologist at the University of California, Los Angeles. "Instead, the temporo-limbic system is stamping these moments as being intensely important to the individual, as being characterised by great joy and harmony. When the experience is reported to someone else, only the contents and the sense that it's different can be communicated. The visceral sensation can't.'
Plenty of evidence supports the idea that the limbic system is important in religious experiences. Most famously, people who suffer epileptic seizures restricted to the limbic system, or the temporal lobes in general, sometimes report having profound experiences during their seizures. "This is similar to people undergoing religious conversion, who have a sense of seeing through their hollow selves or superficial reality to a deeper reality,' says Saver. As a result, he says, epileptics have historically tended to be the people with the great mystical experiences.
The Russian novelist Fyodor Dostoevsky, for example, wrote of 'touching God' during epileptic seizures. Other religious figures from the past who may have been epileptic include St Paul, Joan of Arc, St Theresa of Avila and Emanuel Swedenborg, the 18th-century founder of the New Jerusalem Church.
Similarly, neurosurgeons who stimulate the limbic system during open-brain surgery say their patients occasionally report experiencing religious sensations. And Alzheimer's disease, which is often marked by a loss of religious interest, tends to cripple the limbic system early on, says Saver.
'EPILEPTICS HAVE HISTORICALLY TENDED TO BE THE PEOPLE WITH THE GREAT MYSTICAL EXPERIENCES. DOSTOEVSKY WROTE OF "TOUCHING GOD" DURING SEIZURES'
The richness that limbic stimulation brings to experience may explain why religions rely so heavily on ritual, claims Newberg. The deliberate, stylised motions of ceremony differentiate them from everyday actions, he says, and help the brain flag them as significant. music, too, can affect the limbic system, Japanese researchers reported in 1997, driving it towards either arousal or serene bliss. Chanting or ritual movements may do the same. Meditation has also been shown to induce both arousal and relaxation, often at the same time. 'Sometimes people refer to it as an active bliss," says Newberg. That marriage of opposites, he thinks, adds to the intensity of the experience.
Even if these feelings of oneness and awe fall short of the personal experiences of God that many people report, anyone who still doubts the brain's ability to generate religious experiences need only visit neuroscientist Michael Persinger at Laurentian University in the bleak nickel-mining town of Sudbury, Ontario. He claims almost anyone can meet God, just by wearing his special helmet. For several years, Persinger has been using a technique called transcranial magnetic stimulation to induce all sorts of surreal experiences in ordinary people (New Scientist, 19 November 1994, p 29). Through trial and error and a bit of educated guesswork, he's found that a weak magnetic field-1 microtesla, which is roughly.that generated by a computer monitor-rotating anticlockwise in a complex pattern about the temporal lobes will cause four out of five people to feel a spectral presence in the room with them.
What people make of that presence depends on their own biases and beliefs. If a loved one has recently died, they may feel that person has returned to see them. Religious types often identify the presence as God. "This is all in the laboratory, so you can imagine what would happen if the person is alone in their bed at night or in a church, where the context is so important," he says. Persinger has donned the helmet himself and felt the presence, though he says the richness of the experience is diminished because he knows what's going on. Not everyone accepts that Persinger's apparitions could equal what religious devotees experience. 'That is quite detached from anything that's a genuine religious experience, in the same way that psychoactive drugs can affect mood, but not in a legitimate way," says Julian Shindler, a spokesman for the Chief Rabbi's office in London. 'It's not the genuine article, somehow."
Whatever their validity, Persinger's experiments show that mystical experiences consist of not only what we perceive, but also how we interpret it. "We fit it into a niche, a pigeonhole," says Persinger. "The label that is then used to categorise the experience will influence how the person remembers it. And that will happen within a few seconds." There's a third aspect, too: the reinforcement that humans, as social animals, get from sharing religious rituals with others. "Religion is all three of those, and all three are hardwired into the brain," says Persinger. "We are hardwired to have experiences from time to time that give us a sense of a presence, and as primates we're hardwired to categorise our experiences. And we crave social interaction and spatial proximity with others that are the same. What's not hardwired is the content. If you have a God experience and the belief is that you have to kill someone who doesn't believe as you do, you can see why the content from the culture is the really dangerous part."
So where does all this leave us? For whatever reason-natural or supernatural-our big, powerful brains clearly allow a novel sort of experience that we call religion. But it's difficult to say much more than that. 'In a sense, biology evolving has discovered something new about the Universe," says Charles Harper, executive director of the Templeton Foundation, a private institution that explores the interaction between religion and science. 'Almost all cultures have this religious sense," he says. "Does that offer any insight for understanding the grain of the Universe? That's a haunting question."
Sceptics of religion are quick to claim that the brain's hardwiring proves that God has no real existence, that it's all in the brain. "The real common denominator here is brain activity, not anything else," says Ron Barrier, a spokesman for American Atheists based in Cranford, New Jersey. "There is nothing to indicate that this is externally imposed or that you are somehow tapping into a divine entity." But Newberg isn't so sure. "We can't say they're wrong," he says. "On the other hand, if you're a religious person, it makes sense that the brain can do this, because if there is a God, it makes sense to design the brain so that we can have some sort of interaction. And we can't say that's wrong, either. The problem is that all of our experiences are equal, in that they are all in the brain. Our experience of reality, our experience of science, our mystical experiences are all in the brain."
In fact, he goes on, practically the only way we can judge the reality of an experience is by how real it feels: "You can have a dream and it feels real at the time, but you wake up and it no longer feels as real. The problem is, when people have a mystical experience, they think that is more real than baseline reality-even when they come back to baseline reality. That turns everything around." To Newberg, it means that reductionist science, powerful as it is, has its limitations. Religious experts agree. "You could say Shakespeare's sonnets are nothing but a combination of pencil lead and cellulose," says Harper. "But you could also say this is the outflgw of a great soul, and that would also be true.' He says there are different levels of explanation which are each true at their own level, but which don't offer a comprehensive explanation.
just as physicists cannot fully understand the electron as either a particle or a wave, but only as both at once, says Newberg, so we need both science and a more subjective, spiritual understanding in order to grasp the frul nature of reality. El
Further reading: Why God Won't Go Away by Andrew Newberg, Eugene d'Aquili and Vince Rause (Ballantine Books, 2001) "The neural substrates of religious experience" by Jeffrey Saver and John Rabin, The Journal of Neuropsychiatry, voi 9, p 498 (1997) "Experimental induction of the 'sensed presence' in normat subjects and an exceptional subject" by C. M. Cook and Michael Persinger, Perceptual ond Motor Skills, vot 85, p 683 (1997)
THE IDEA THAT INVISIBLE GALAXIES HAUNT THE UNIVERSE HAS GOT ASTRONOMERS PEERING INTO THE DARKNESS. BUT WHAT THEY FIND MAY DISMAY THEM, SAYS STUART CLARK
GHOSTS gather in the shadows. just outside the cosy circle of our own galaxy-which is lit by the fires of a hundred billion stars-is a host of wraith-like galaxies made of almost nothing but exotic invisible matter. Instead of shining like celestial beacons across the Universe, they are virtually indistinguishable from the blackness of space. 'Dark galaxies might outnumber normal galaxies by a hundred to one," says Neil Trentham of Cambridge University. Trentham, like most astronomers, believes dark galaxies must be out there somewhere. He is attempting the almost impossible task of trying to see these sinister clouds of darkness. But other astronomers warn that the search is a waste of time. Like ghosts, dark galaxies may be a figment of the imagination. If so, our theories of how galaxies form are wrong, and we may have to change our ideas about what makes up most of the matter in the Universe, or rewrite the story of the Universe's first moments.
Galaxies are thought to have formed from the sea of gas left behind by the big bang. If some patches of gas were slightly denser than others, their gravity would have pulled in surrounding material. But with the gravity of ordinary gas alone, this process would have taken far too longgalaxies would still be forming even today. So cosmologists have been forced to assume that there is also a lot of invisible matter in the Universe, outweighing the normal matter many times. This 'dark matter' can also explain how galaxies spin so fast without breaking apart. A large spherical halo of dark matter surrounding each galaxy could provide enough gravity to balance the spin, gluing the galaxy together. Physicists' attempts to merge the fundamental forces of nature have thrown up any number of candidates for the stuff of dark matter (New Scientist, 16 January 1999, p 24), called weakly interacting massive particles, or WIMPS. These hypothetical exotic majestic spirals like the Milky Way.
Astronomers already know of two species of galactic minnow. The small round galaxies called dwarf spheroidals and their untidier cousins, the dwarf irregulars, both weigh in at about ten million times the mass of the Sun, or only one ten thousandth that of the Milky Way. But there are far too few of them to agree with the theoretical models of cold dark matter, which predict ten or a hundred times as many.
To save the theory, astrophysicists assume that these small galaxies do exist-only they're invisible. Somehow, most of the smaller dark-matter haloes must have been unable to form stars. "The smallest dwarf galaxies.could be the very rare, one-in-ahundred cases which, for some reason, do form stars," says Trentham.
So what stops stars forming in the remainder? Here, dark-galaxy pundits split into two camps. Either something stops the gas entering the dark matter halo, or it falls in and then is somehow prevented from making stars.
Trentham believes that dark galaxies failed to attract any normal matter because they missed out on a feeding frenzy during the dark ages, when gas was plentiful. 'To produce normal galaxies, dark matter haloes grow in the early Universe, pulling in more dark matter and gas," he says. And according to his simulations, still unpublished, the smaller haloes would be celestial late developers. "Imagine a small dark halo growing five to ten billion years after the big bang. The gas between the galaxies has been heated up by the light from stars, and it is now moving so fast that it cannot be pulled in.'
Others are not convinced. Frazer Pearce of Durham University believes that even long after the dark ages, there was plenty of cold gas around to be captured. Astronomers see traces of dark gas clouds throughout intergalactic space, revealed because they absorb some of the light from the distant celestial beacons known as quasars. And these clouds are relatively cool. "The gas is at 20,000 K," says Pearce. And that's too cool to escape even a modest gravitational pull. "Even when you have gas at tens of millions of kelvin, you can't keep it out of haloes forever." Pearce thinks that these intergalactic cloudS are already sitting inside their own very small haloes of dark matter. So what stops them from forming stars? The tiniest galaxies might suffer a kind of boom-bust economy, he suggests. When the small dark haloes form, they attract some gas and stars form. The most massive of these burn quickly and explode as supernovae after just a few million years. That heats the remaining gas and flings it in all directionS. A few supernovae could make the gas hot enough to eject it back into intergalactic space. 'We are hoping that supernovae will blow these clouds to pieces and stop stars forming," says Pearce. The hot, scattered gas will eventually radiate its excess energy, slow down and get pulled in again by the halo, beginning another boom-bust cycle. Each short period of intense star formation, lasting a few tens of millions of years, would be followed by a dormant period of up to a billion years.
But once again, there are dissenters. 'The idea that it is really easy to blow the gas out of galaxies is problematic," says George Lake of the University of Washington. "People have tried to do simulations, overestimating the effect of supernovae, and they still can't get the stuff flung out. I think the whole idea is misguided." Lake thinks dark galaxies are a mirage. "It is not that these dark galaxies lie below current detection limits, they are just not there. They do not exist at all." The only way to settle the argument is to look for these spectral galaxies. Astronomers have a few ideas about how to catch them (see "Ghost hunting"), but what if, after the trawling is over, they return with empty nets?
Warm dark matter
Pearce believes that the conventional theory of galaxy formation could be repaired. Instead of cold dark matter, the Universe may be filled with another, slightly different
strange substance. The lighter a particle is, the faster it is likely to be moving. And fast-moving particles are much less likely to clump together. Very lightweight particles such as neutrinos would be a kind of "hot dark matter". These zippy particles would be so resistant to clumping that they would tend to smooth out structures even on the scale of large galaxies-so they can't make up most of the dark matter in the Universe, or we wouldn't be here. Instead, Pearce thinks a dearth of little dark galaxies could be explained by "warm dark matter", of an intermediate mass and speed. Warm dark matter would happily form big clumps that make ordinary galaxies, but would move too fast to be captured by the weak gravity of small haloes. The mass of a warm dark matter particle would need to be around 10-33 kilogramsa millionth the mass of a proton. This presents no problem for particle physicists, who can tweak their speculative theories to produce particles of virtually any mass required. But Lake thinks this running repair is useless. He points out that the conventional theory of structure formation also predicts too many biggish objects, larger than galaxies but smaller than the most common kind of galaxy cluster, which contains hundreds of galaxies. It will take more than tinkering to repair this anomaly. Theorists may be able to use warm dark matter to wash out little galaxies, but it won't get rid of these heavier structures. Lake believes that there is something fundamentally wrong with our theories about the early Universe. For structures to form at all in the Universe, there must be some initial variations in the density of gas. Cosmologists think that these variations were created by quantum fluctuations in the first fraction of a second after the big bang, and then magnified by a process called inflation (New Scientist, 16 December 2000, p 26). But how big were these fluctuations? The usual assumption is that, like a ftactal pattern, the Universe was as lumpy on small scales as it was on large scales-a "scale-free fluctuation spectrum". This is a catch-all solution that Lake thinks is used to mask our almost total ignorance of the early Universe. There is no strong evidence for it, and yet it has become entrenched in cosmological orthodoxy. "The strange thing is that people now treat it as though it were a unique prediction of inflation.' Lake believes this is where the problem lies, because the assumption that there are no special scales doesn't fit the shortage of small galaxies and small clusters. Galaxies and galaxy clusters mark distinct peaks in the fluctuation spectrum, rather than being part of a smooth continuum of structures, he says. "It seems like we have some notes or harmonics in the Universe.' If he is right, then we can use this knowledge to work out just how the fluctuations formed. This could sound the death knell for dark matter. Powerful peaks in the fluctuation spectrum suggest bigger variations in gas density on the right scale to produce galaxies. And with a better head start, the gravity of ordinary gas would have been enough to make galaxies, obviating the need for dark matter.
That still leaves the problem of why some galaxies manage to spin so fast without falling apart, of course, but there may be an another explanation for that. Some researchers maintain that Newton's law of gravity might be slightly different on large scales. According to the theory of Modified Newtonian Dynamics, gravity pulls a little harder at large distances than conventional wisdom dictates, enabling it to hold fast-spinning galaxies together.
If Lake is right, astronomers will have to let the whole notion of dark matter slip away quietly into the night. The ghosts will have been banished for good.
Stuart Clark is an astronomy writer and Director of PubLic Astronomy Education at the University of Hertfordshire
Further reading: "Completely dark galaxies, their existence, properties, and strategies for finding them," by Neil Trentham and others, at http://xxx.Lant.gov/abs/astro-ph/0010545 The Bigger Bang, by James E. Lidsey, Cambridge University Press 12000)