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Dark Secrets New Scientist 21 Mar 98

THE mysterious dark matter thought to make up most of the Universe's mass could be made of particles 10 trillion times heavier than a proton, new research suggests. If so, detectors might one day trap dark matter in the form of an extremely heavy type of hydrogen. More than 90 per cent of the matter in the Universe is invisible, and can be detected only by its gravitational effects on stars and galaxies. Now theoretical work done by Edward Kolb of Fermilab, near Chicago, suggests that the dark matter sprang into being as superheavy particles moments after the Universe formed. The particles would have been created at the end of the inflationary epoch, the period of tremendous growth during the first split second of the Universe's existence. The inflationary epoch was characterised by huge negative pressure. Einstein's general theory of relativity recognises pressure as a source of gravity just like mass, so the huge pressure resulted in powerful gravity. "It was the sudden removal of this gravity, when the Universe switched to a more sedate expansion at the end of inflation, that could have produced superheavy particles," says Kolb. During the inflationary epoch, particles and their antiparticles were continually conjured out of nothing as energy was briefly borrowed from the gravitational field. However, when inflation ended, the sudden change in gravity pulled particles and their antiparticles farther apart, making it harder for them to meet and annihilate each other. A similar change in gravity, in space rather than time, causes a net production of particles in the vicinity of a black hole, a process known as Hawking radiation. Kolb's team says it is the strength of gravity at the end of inflation that determines the mass of superheavy particles. Although the strength is uncertain, they calculate that it should have been enough to create particles about 10 trillion times heavier than a proton. Kolb's team has also calculated the number that would have appeared, and they say it is "just right to account for the Universe's dark matter". If true, this could be bad news for experimental physicists. Relatively few particles would account for the dark matter, so the chances of detecting them would be small.

However, Kolb says there is no reason why such particles should not be electrically charged or feel the strong force, making them easier to find. A positively charged particle might even attract an electron to make a superheavy kind of hydrogen that would be detectable. It's a long shot, Kolb admits - superheavy particles are no more likely to make up dark matter than any of the other exotic candidates that have been suggested, such as "neutralinos" or "axions". "Our work is simply a reminder that we haven't yet exhausted all the possibilities," he says. His team has submitted the work to Physical Review D. Marcus Chown

Gravity and Anti-matter Scientific American 258 1988
Goldman, Hughes and Nieto (review)

The suggestion of the existence of supersymmetric partners to the graviton, the gravitino, graviphoton etc. led to proposals of additional weak forces moderating gravity. A similar class of theories called metric theories depending on space-time curvature allowed the graviton to decompose into such particles.

So far no evidence has been forthcoming for such modifications, which suggested that the components might act differently on mass and binding forces leading to different gravitational effects for matter and anti-matter. This should be taken into account in the current theory suggested below.

The Force of Darkness - generalized relativity theory

New Scientist 7 Mar 98

A MYSTERIOUS second type of gravity may help choreograph the motion of matter in the Universe-and help save it from the singularities nature abhors. Physicists in Britain say the force would be felt directly only by matter with a previously unsuspected "gravitational charge". "Such matter would interact via the new form of gravity in addition to the familiar form between massive bodies," says Robin Tucker of the University of Lancaster. The existence of gravitational charge would have profound implications for the early Universe and for black holes. "It could prevent the formation of a singularity, a point of infinite density, at the beginning of time and possibly in the heart of a black hole," says Tucker. Tucker and his colleagues had been revisiting Einstein's theory of gravity, the general theory of relativity. They knew that Einstein had made a certain assumption about gravity and space-time to simplify the equations. "If you relax this arbitrary assumption, a more general form of gravitation can be contemplated," says Tucker. One consequence of the general form is the existence of a new interaction coupled to a gravitational "charge" that may be carried by some types of matter. The charge comes in two kinds, similar to positive and negative electrical charges. The force would be carried by a particle that may be as heavy as a grain of sand, and would act over distances smaller than an atom. Normal matter does not carry this charge. But Tucker and his colleague Charles Wang speculate that it might be carried by the invisible dark matter which makes up most of the mass of the Universe. The force would have been felt in the early Universe, and in any other region of the Universe where matter is very dense. Tucker and Wang found that if the gravitational charge has one sign, it might balance the normal attraction of gravity with a repulsive force. This could avoid the problem of a singularity at the beginning of time and in black holes. Alternatively, if the gravitational charge on dark matter has the opposite sign, the normal attraction of gravity would be reinforced, producing a more pronounced singularity and affecting the expansion rate of the Universe. Either way, the presence of gravitational charge could have determined the kind of Universe we live in. Gravitational charge could also be having a more indirect effect on today's Universe. Although the new interaction has a very short range, it can nevertheless affect the shape of space-time and this would in turn affect the motion of normal matter. "If the Sun had a small gravitational charge, it would affect the motion of the planets," says Tucker. "We can therefore use our knowledge of planetary motion to set bounds on the amount of gravitational charge in our neighbourhood." His results will appear in next month's issue of Classical and Quantum Gravity. Marcus Chown

Could gravity explain quantum mechanics? (review) New Scientist 7 Mar 98

In an unusual Ph.D. thesis Mark Hadley has set out a theory which interprets particles as knotted regions of space-time in which there is a time-like loop as well as space contained within the particle. Wormholes in space-time conceived by Misner and Wheeler were traditionally only spatial distortions because of the causal paradox time loops would involve, but Hadley sees these properties as exactly those generating the known causality violations of quantum mechanics.

He notes that a particle appearing to interact with its own history in a causality violating way could precisely create the indefinition required to explain quantum uncertainty. He also notes that such an object would interact with both its past and its future too much as a jiggling rope connecting all tits boundary conditions in a way which would make specifying its motion from the initial conditions impossible. "Not surprisingly this changes everything" says Hadley. "For a quantumparticle there is another end, another unknown boundary condition in the future and not everything is determined". The non-commutation of quantum variables could also be explained by properties leaking into the particle's past.

Constant Chaos New Scientist 28 Mar 98

LIGHT from far-flung quasars is threatening to revolutionise physics. An international team of scientists say they may have found evidence from these distant beacons that a constant which determines the strength of the electromagnetic force may have been different earlier in the Universe. "It would certainly be a major development, making obsolete much of what we know about the Universe," says astronomer Mike Hawkins of the Royal Observatory in Edinburgh. The constant in question is known as the fine structure constant [e^2/hc], which depends on three other quantities: tile charge on the electron, Planck's constant and the speed of light. "If our results are correct, one or more of these constants must have caried over the history of the Universe," says Christopher Churchill in Pennsylvania State University. Working with Churchill and John Barrow at Sussex University, team leader John Webb and his colleagues at the University of New South Wales in Sydney used a new technique to measure the fine structure constant at various epochs of the Universe. They looked at "bites" taken out of quasar spectra by light-absorbing atoms in gas clouds between the quasars and Earth. The difference between the wavelengths absorbed by any two elements is sensitive to the value of the fine structure constant at the time of absorption in each cloud. The team measured wavelength differences to within a ten-thousandth of a nanometre at the 10-metre Keck telescope in Hawaii. The results showed that the fine structure constant was several parts in 100 000 smaller than today between a red shift of 1.0 and 1.4. The red shift is a measure of how much the Universe has expanded since the clouds emitted light, and is a rough guide to their age. The researchers have submitted the results to Physical Review Letters. If the variation is real, the implications are profound. Many theories attempting to unify gravity with the other forces, such as string theory, require extra dimensions of space-time. We don't experience them because they are "rolled up" smaller than an atom. In such theories, the constant that determines the strength of the electromagnetic force depends on how tightly the extra dimensions are rolled up. "If the fine structure has changed with time," says Churchill, "it could be evidence that the size of the rolled-up dimensions has changed." In the past, several scientists, among them British physicist Paul Dirac, have speculated that fundamental constants of physics evolve with time. According to Churchill, such changes might be coniiected with changes in the energy density of the vacuum. He points out, for instance, that the speed of light depends on the interaction between photons and the quantum vacuum which seethes with "virtual" particles popping in and out of existence. "if the energy density of the vacuum were greater in the past, the speed of light would be slightly different," he says. Differences in the fine strructure constant could also have affected element-building nuclear reactions in stars, which are extremely sensitive to such physical constants. "A change in the fine structure constant could change the rate at wliicil stars burn their fuel and so subtly alter the entire evolution of stars," savs Churchill. "It's hard to imagine the full ramifications of changes to the fine structure constant," comments Hawkins. He says that depending on how the constant changed with time, it might make some atoms unstable during certain epochs, or alter the wavelength of the cosmic microwave background-the radiation left over from the big bang fireball. "If the variation is true, it is extremely significant," adds Tom Kibble, a theoretical physicist at Imperial College, London. "But I would certainly want to look for other possible explanations of the data before accepting this one." Churchill admits the results could be a mirage due to small calibration errors: "We're currently working very hard to rule this out." Marcus Chown

Einstein in free fall NS 13 Jun 98 11

A FRESH clash between Einstein's general theory of relativity and quantum mechanics has come to light. A physicist in New Mexico claims that quantum mechanics predicts that particles on Earth are affected by massive objects millions of light years awav. If he is right, one of the basic asswnptions of Einstein's theory must be wrong. A central premise of general relativity is that you cannot tell the difference between being in free fall towards a massive object and being in no gravitational field at all. Someone sitting in a capsule which is failing towards a shell of matter would feel exactly the same as someone inside that shell, where the gravitational forces balance out to zero. Neither would feel themselxves pressing down on their seats. in other words, objects are indifferent to their gravitational "potential": how tightly bound they are to a gravitational body. "If you look at the foundations of general relativity, it's stronglv dependent on this notion of free fall," savs Dharam Ahluwalia, a physicist at Los Alamos National Laboratory. But Ahluwalia says that quantum mechanics may soon demolish the idea that objects cannot sense their potential. Quantum mechanics has already overturned a good deal of classical theory, such as the laws of electroma,netism. Before quantum theory, physicists thouaht that an electron shooting past an ideal solenoid-a tube which has maanetic fields on the inside but not on the outside-would be unaffected by the field.

But because of the "smeariness" of real electrons, they are affected by a field they shouldn't be able to "see". Ahluwalia put gravity into the Schr6dinger equation, which is normally used to describe the quantum behaviour of a particle in different electromagnetic potentials. He found there is a gravitational analogue of the solenoid effect: particles can "feel" their gravitational potential. In a forthcoming issue of Modern Physics Letters B, Ahluwalia says that this effect would influence the way neutrinos flip from one type to another. Scientists reported evidence for this behaviour last week (see p 25). Ahluwalia says neutrinos with mass would "feel" their gravitational potential, and one with a large potential at the centre of a shell would change from one neutrino "flavour" to another more slowly than one in free fall a large distance away. "Personally, I believe it must be true," says Samuel Wemer of the University of Missouri in Columbia, who is hoping to see similar effects at work in electrons at the centre of a tube filled with a tonne of mercury. "In principle, it could be observed." if confirmed, the new idea would imply there are tiny inaccuracies in some predictions of general relativity theory. It would also suggest that distant galaxies affect the properties of nearby particles by contributing enormous potential. The black hole at the centre of the Milky Way and the galaxies and dark matter that make up the "Great Attractor" which is pulling on our Galaxy would both change how quickly neutrinos oscillate near the Earth. Charles Seite