ASTRONOMERS are gaining new insights into the nature of the Universe by opening up more regions of the electromagnetic spectrum to observation. Optical astronomy, using visible light, was the first to be developed, for obvious reasons — our eyes use optical light. Both light and radio are examples of electromagnetic waves, and both these kinds of radiation happen to pass through the atmosphere of the Earth almost unimpeded. So it was natural that radio should have been the first of the “new” astronomies to be developed, in the 1950s.
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But between radio wavelengths and those of visible light lies a part of the spectrum known as the infrared, from about 0.8 micrometers wavelength to a thousand micrometers (1 millimetre). Infrared radiation is invisible to our eyes, but you can feel it — the warmth you feel if you hold your hand near a hot radiator is infrared radiation. Unfortunately for astronomers, infrared is strongly absorbed in the atmosphere of our planet. In addition, any warm object produces infrared radiation. The atmosphere itself, and the telescopes and instruments that are used to study the sky, all radiate infrared themselves, and this can overwhelm the detectors. So infrared astronomy developed much more slowly than radio astronomy. It requires telescopes on very high mountains, above most of the Earth’s atmosphere, and detectors cooled to very low temperatures (typically, -200 oC or less), to reduce the background infrared “noise”. So one of the main infrared observatories in the world is on top of Mauna Kea, on the island of Hawaii, at an altitude of 4,200 meters.
Another approach is to launch detectors into space, where they are free from interference and can be kept cool. The IRAS satellite (from InfraRed Astronomy Satellite), involving Britain, the Netherlands and the United States, flew into orbit in November 1983, and produced the first map of the entire sky at infrared wavelengths.
These new windows on the Universe are providing fresh information about the cosmos for two reasons. First, infrared radiation is produced by relatively cool objects, with temperatures below about 3,000 K (the “absolute zero” of temperature, 0 K, is -273 oC). Slightly hotter objects, such as the filament in a light bulb or the surface of a star like the Sun (at about 6,000 K) radiate mainly visible light, and hotter objects still radiate X-rays. So infrared detectors pick out cool clouds of gas and dust in space, clouds which have been warmed by the shorter-wavelength radiation from the stars and re-radiate energy in the infrared.
The second useful feature of infrared radiation is that it is not absorbed by dust in space as strongly as ordinary optical light is. The shorter wavelength radiation has, the more easily it is absorbed and scattered. This is why sunsets on Earth are red — white light from the Sun is a mixture of all the colours of the rainbow, corresponding to different wavelengths. The short-wavelength blue light gets scattered around the Earth’s atmosphere, giving us blue skies. But longer wavelength red light penetrates through the dusty atmosphere, giving red sunsets. Infrared radiation penetrates dust even more easily. There is a lot of dust in our Milky Way Galaxy, between the stars. Infrared telescopes can peer through this dust to look at interesting objects, such as the activity at the centre of the Milky Way, that are blocked from view in optical light (PIC if possible). The information provided by infrared detectors can then be used to produce “false colour” images — pictures of what the Universe would look like if we had infrared vision.
But infrared instruments also tell astronomers a lot about objects closer to home. The planets themselves are studied at infrared wavelengths, and infrared telescopes took spectacular pictures of Halley’s comet in 1985. These helped astronomers to measure the size of dust grains in the cloud of cometary material, and work out how rapidly dust was boiling away from the central nucleus (R454/116NAS060).
Among the most intriguing objects probed by infrared detectors are huge clouds of gas and dust which are the birthplaces of stars. One of these stellar maternity wards is near the famous nebula in the constellation Orion. Hot, young stars, perhaps only a million years old, are forming at the centres of these clouds, but cannot be seen because their light is blocked by the clouds themselves. The clouds warm up, and radiate strongly in the infrared, although in ordinary photographs they appear black.
Individual newborn stars have also been captured photographically using detectors sensitive in the infrared. The images show that these stars are surrounded by clouds of dust, swirling around in the gravitational grip of the newborn star itself. Dust rings like this are probably precursors to the formation of planets.
All of the stars we see in the sky are part of the Milky Way Galaxy, a disk of billions of stars, mixed with gas and dust, swirling around in a spiral pattern, like cream stirred into black coffee. Regions of star formation within the Milky Way show up as bright dots to infrared detectors, regions where grains of dust mainly made of carbon are radiating away heat from collapsing star systems.
Infrared studies of the very heart of the Milky Way, the centre of our Galaxy, show a scene of violent activity, hidden from the view of optical telescopes by the intervening dust. This may be the location of a black hole millions of times more massive than our Sun. Some of the most astonishing discoveries made by infrared astronomy, though, come from far beyond our Milky Way. In the depths of the Universe, whole galaxies, each containing billions of stars, can be seen as single objects, evolving, interacting and interfering with one another. When two galaxies pass close enough to one another to be distorted by mutual tidal forces, this triggers a wave of star formation, heating the clouds within the galaxies and making them radiate in the infrared.
One famous example is the galaxy M82, which seems in optical photographs to be undergoing an enormous explosion. Infrared studies show that what is actually happening is a wave of star formation triggered by tides from its neighbour, M81. In other cases, strangely distorted blobs of light in ordinary photographs are revealed in the infrared as colliding galaxies, with two separate cores in the process of merging.
The infrared sky shows the Universe to be a more violent place than it appears in optical light. Infrared detectors probe deeper into the heart of the Milky Way and stellar maternity wards, and show whole galaxies tearing each other apart and swallowing each other up. And this is just the beginning of a new era of investigation that really began just ten years ago, with the launch of IRAS.
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Dark Matter and the Universe
There is more to the Universe than meets the eye. Studies of the way stars and galaxies move through space show that they are being tugged by large amounts of unseen “dark matter”; in addition, the favoured cosmological theories of the origin of the Universe (ADD REF to IS Big Bang 2) suggest that there ought to be much more dark stuff than the matter we see in the form of stars and galaxies.
The amount of dark matter needed to explain the behaviour of the Universe increases as we look further out into the Universe. In the early 1930s, the Dutch astronomer Jan Oort was one of the first to realise that dark stuff is needed to explain the behaviour of stars in our immediate neighbourhood. At that time, astronomers established that the stars of the Milky Way Galaxy are each in orbit around a centre quite distant from the Sun. Our Solar System lies about two-thirds of the way out from the centre of this swirling system, in the galactic suburbs. Stars in our neighbourhood can be studied in some detail, and these studies show that individual stars do not move precisely in a single plane, but wobble up and down as they orbit around the Galaxy. Of course, Oort could not watch an individual star moving up and down in this way. These changes take thousands or millions of years. But the overall distribution of stars above and below the plane of the Galaxy, and measurements of their speeds (determined by the Doppler effect) enabled him to conclude that the visible stars themselves contribute only one-third of the gravitational force holding the stars in place.
Since then, about as much mass as there is in visible stars has been identified as cold clouds of gas and dust spread between the stars, but that still makes up, together with the stars themselves, only two- thirds of the gravitational mass required to explain the local dynamics of the Galaxy.
The unseen dark matter can be measured in terms of a number called the mass-to-light ratio, M/L. This is defined to be 1 for our Sun — one solar mass of matter, in the form of a star, produces one solar luminosity of light. Oort’s figures tell us that in our neighbourhood M/L is about 3. But the number is bigger for the Galaxy as a whole. In the 1980s, spectroscopic techniques became good enough to measure details of the rotation of spiral galaxies, systems like our own Milky Way Galaxy. The visible part of a spiral galaxy consists of a central bulge of stars surrounded by a thin disk of stellar material — the proportions are roughly those of the yolk and white of a fried egg. If such a galaxy happens to be oriented “edge on” to us on the sky, it is possible to measure the speed with which different parts of the disc are rotating by placing a narrow slit across the image of the disc at different places, and measuring the Doppler shift in the spectrum at different distances out from the central bulge. More recently, the technique has been extended further out from the centres of some galaxies, using radio astronomy techniques to measure the velocities of clouds of hydrogen gas, still part of the disk.
When astronomers plot the velocities of the stars and clouds orbiting the disk of a distant galaxy at different distances from its nucleus, they obtain a rotation curve that is usually very symmetrical. The stars at a certain distance from the centre on one side of the galaxy are moving towards us at the same speed that stars on the other side of the galaxy, at the same distance from the centre, are moving away from us (these measurements have the overall red shift caused by the expansion of the Universe subtracted out). This was no surprise. But astronomers were surprised to find that outside the innermost regions of a spiral galaxy, on either side of the nucleus, the speed with which the stars are moving is the same all the way across the disc. In astronomers’ jargon, the rotation curves are extremely flat.
This was a surprise because astronomers had assumed that the greatest amount of mass in a spiral galaxy was concentrated in the bright central nucleus, where there are many stars. If that were the case, then stars further out from the nucleus should be moving more slowly in their orbits, in exactly he same way that the outer planets of our Solar System (where most mass is concentrated in the Sun, at the centre) move more slowly in their orbits than the inner planets do. The most simple way to explain the flatness of the rotation curves is if there is a great deal of dark matter spread around each spiral galaxy in a huge unseen halo. If this halo is roughly spherical, then as it rotates it will drag the visible, bright stars around with it in just the way we see. In other words, most of the mass of a spiral galaxy like our own is not associated with the bright stars of the nucleus (or even those of the disc), and the mass-to-light ratio is about 5. Moving up from the scale of individual galaxies, the next level of structure in the Universe is provided by clusters of galaxies, anything from a few galaxies to many hundreds of galaxies held together in a swarm through mutual gravitational attraction. The speed with which each galaxy in a cluster is moving can be inferred from the Doppler effect (once again, the overall cosmological redshift, in this case for the cluster, has to be subtracted out), and the amount of mass in each galaxy can be estimated from its brightness, if we assume that the mass-to-light ratio is about 1.
The first person to make these kinds of studies was the Swiss astronomer Fritz Zwicky. At about the same time that Oort was discovering evidence of dark matter close to home in the Universe, Zwicky began to find evidence of dark matter on a much more impressive scale. If the galaxies in a cluster really are held together in an association by gravity, while the cluster as a whole moves through space like a swarm of bees, then the velocities of individual galaxies in the cluster must be less than the escape velocity from the cluster. But when Zwicky used the Doppler technique to measure the velocities of galaxies in one group, the Coma cluster, he found that they were moving much too rapidly, relative to one another, to be held together by the gravitational pull of all the stars in all the galaxies of the cluster.
It looked as if the flying galaxies ought to have moved apart, dissolving the cluster, long ago when the Universe was young. And he found the same thing when he looked at other clusters — they were all moving apart much too fast to be held together by the gravity of the matter we can see.
Although the evidence that clusters of galaxies contain large amounts of dark matter, with mass-to-light ratios as high as 300, continued to mount, for decades few astronomers worried much about this problem. In the 1930s, the concept of the expanding Universe, and even the fact that the Universe extended far beyond our Milky Way Galaxy, were new ideas, and the possible existence of dark matter seemed a minor puzzle compared with developing an overall picture of the origin and evolution of the Universe, and, indeed, of the galaxies themselves. It was only in the 1960s that the Big Bang model began to become established as the standard model of the Universe, and only after the Big Bang model was established that proper attention began to be paid to the detail of finding an explanation for the dynamic behaviour of galaxies in clusters.
One of the early triumphs of the Big Bang theory was that it seemed to explain how much matter there ought to be in the Universe, and the predictions seemed to match the amount of matter we can see. The famous cosmic microwave background radiation, discovered in 1964, was interpreted as a leftover relic from the fireball in which the Universe was born, and used to calibrate conditions in the fireball. With this calibration, the standard Big Bang model predicted that in the Big Bang primordial hydrogen should have been processed into helium in just the right amount to explain why the oldest stars are made up of about 25 per cent helium and 75 per cent hydrogen.
But the same calculation also limits the overall amount of matter there can be in the form of hydrogen, helium, and the rest of th familiar chemical elements (so-called “baryonic matter”). In order to match the conditions under which helium was manufactured in the Big Bang fireball to the abundances of hydrogen and helium seen in the Universe today, cosmologists also had to specify the overall density of baryons in the Universe. Assuming that all the matter in the Universe is made of baryons (the same sort of stuff that we and everything else on Earth is made of, and all the bright stars are made of), this density converts into a mass-to-light ratio for the Universe as a whole — and the number comes out as less than 100. In other words, at the very outside there could be as much as 100 times as much matter in the form of clouds of dust and gas, and so on, as we see in the form of bright stars. At the beginning of the 1980s, this was starting to cause concern among astronomers. As telescopes and observing techniques improved, the observational evidence was mounting that in the case of clusters of galaxies, M/L is at least 300. But a now well-established and highly successful Big Bang theory says that M/L for all the baryons in the Universe must be comfortably less than 100. A great deal of extra mass seemed to be needed — and it also seemed clear that it could not be in the form of baryons.
Then, the Big Bang theory itself underwent a dramatic transformation with the advent of an idea dubbed “inflation”, which describes the very earliest era of the Universe, which produced the fireball in which hydrogen was processed into helium. Inflation resolved some remaining puzzles about the Big Bang model, and is now part of the standard model itself. But while removing some old cosmological puzzles, it makes one firm prediction, concerning the density of the Universe. It says that there should be enough matter in the Universe to keep all the galaxies and clusters of galaxies in a gravitational grip, in much the same way that clusters of galaxies are held together by gravity. In that case, there must be so much matter in the Universe that the overall mass-to-light ratio is about 1000, at least three times greater even than the figure for clusters of galaxies. There must be at least ten times, perhaps a hundred times, more matter in the Universe than can be explained by all the baryonic matter produced in the Big Bang fireball. What is it? And where is it? Physicists already knew of one other kind of particle that might fit the bill. For the purposes of these calculations, electrons are included with baryonic matter — the mass of an electron is only half of one thousandth that of a proton and there are the same number of electrons in the Universe as there are protons, so they make only a minor contribution to the density. But there is another kind of particle which exists in vast quantities. These are the neutrinos, which participate in nuclear reactions that involve the weak force (ADD REF to IS Forces of Nature). It is a firm prediction of the standard Big Bang model that there should be about as many neutrinos in the Universe as there are photons — about a billion (109) times as many neutrinos as there are baryons.
Until the 1980s, neutrinos had traditionally been thought to have exactly zero mass, like photons. Some particle physics theories require this, but others allow the possibility that neutrinos might have a very small mass. With so many neutrinos in the Universe, if each one had a mass of a few tens of electron Volts (less than one ten- thousandth of the mass of an electron) they would together provide all of the dark matter required to hold the Universe together. Experiments to measure directly the masses of neutrinos are difficult and have so far proved inconclusive, only setting upper limits on the mass. So the best test of whether the dark matter is in the form of neutrinos is to analyse the distribution of galaxies in the Universe and determine whether this matches the pattern that would be produced if neutrinos have mass.
The situation is complicated, however, because there is another possible type of candidate for the dark matter. Particle physicists seeking to find a unified theory of physics have suggested that there may be one or more varieties of particle present in the Universe which have never been detected in the laboratory. It is intriguing that this suggestion that there might be more stuff in the Universe than we have ever seen was made independently of the astronomers’ discovery that there is more to the Universe than meets the eye. Scientists operating on the largest scales and on the smallest scales both require “new” forms of matter.
Different variations on the particle physics theme suggest different candidates for the extra particle(s), but some would have masses comparable to that of the proton, but be so reluctant to interact with everyday matter (except by gravity) that they have not yet been detected. These hypothetical particles are sometimes called WIMPs, for “weakly interacting massive particles”.
The generic name for such particles is “cold dark matter” (CDM). The terminology “cold” refers to the fact that they have relatively large mass and therefore emerge from the Big Bang travelling much more slowly than the speed of light. By comparison, neutrinos have very little mass (if any) and emerge from the Big Bang travelling at very high speeds, close to the speed of light. They are therefore known as “hot dark matter” (HDM). Over the past ten years or so, one of the major challenges for astronomers has been to determine whether the pattern of galaxies on the sky more closely resembles he pattern associated with CDM or the pattern associated with HDM.
The key difference is the influence of the two kinds of dark matter in the early universe, just after the Big Bang fireball, when stars and galaxies start to form. Hot dark matter particles would sweep everything before them, keeping the universe smooth and homogeneous until they slowed down and began to allow irregularities to grow. Because the distribution of matter on smaller scales would have been smoothed out by then, the first structures to form would be on the scale of superclusters of galaxies, shaped like huge sheets and filaments, which broke up to make galaxies and stars — a “top down” scenario.
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In a universe dominated bo cold dark matter, however, structure begins to form on smaller scales, very soon after the Big Bang. Clumps of dark matter attract baryonic matter, like water flowing into a pothole, and structure builds from the bottom up, with stars and galaxies clumping together to make superclusters and filaments. Both theoretical calculations and computer simulations help to indicate what kind of clumpiness would be seen in a universe dominated by hot dark matter, and what kind of clumpiness we would expect in a Universe dominated by cold dark matter. A universe dominated by hot neutrinos is predicted to have a rather simple structure, like the cells of a honeycomb (though not so regular), in which bright galaxies form only in well-defined sheets and not at all in the voids. The CDM universe is more messy and complicated, with a richer structure that looks more like the real Universe. Sheets and filaments do form, but they intertwine in a complicated way, and the voids are not completely empty.
But the simplest versions of the CDM model cannot account for all of the details of the distribution of galaxies across the sky. Some additional influence is needed to account for the structure of the real Universe. Arguments about what this additional influence might be have included the suggestion that the nature of gravity might have to be modified, or that ripples produced by gravitational radiation (ADD REF to IS General Relativity) could have played a part in determining the distribution of baryons in the early Universe. But the simplest resolution of this puzzle comes from analysis of the ripples in the cosmic microwave background radiation, detected by the COBE satellite.
The pattern of ripples detected by COBE was imprinted on the background radiation some 300,000 years after the Big Bang, at the time when the radiation last interacted with matter. At that time, the Universe had cooled to about 6,000 K (roughly the temperature at the surface of the Sun today), and electrons could combine with nuclei, for the first time, to make stable, electrically neutral atoms, which do not interact significantly with the background radiation. This pattern of ripples extends over much larger scales than even the largest supercluster filaments traced out by looking at the patterns of galaxies on the sky. But the geometrical structure of the pattern of ripples is the same on all scales (if you look at half the sky, you get the same kind of pattern as for the whole sky, or for a quarter of the sky, and so on). Furthermore, it is the same kind of pattern as the pattern made by the bright galaxies. So it seems reasonable to suppose that this scale- invariant pattern is typical of the way matter is (and was) distributed throughout the Universe.
In order to make exactly this kind of pattern in the distribution of mass across the sky, you need a mixture of about two-thirds cold dark matter, one-third hot dark matter and just a smear (perhaps 1 per cent of the total mass) of ordinary atomic (baryonic) matter. In such a “mixed dark matter” scenario, the CDM provides the clumps on which galaxies and clusters of galaxies grow, while the HDM fills some of the space between the clumps, smoothing the overall density of the Universe and reducing the contrast between the clumps and the voids. Atomic matter — the bright stuff of stars and galaxies — feels the gravitational influence of both kinds of dark matter, and so the foamy distribution of galaxies we see today represents the averaged out influence of waves made up of hot and cold dark matter (see Box One). In a possibly slightly over-optimistic assessment of these data, taking all the observations at face value, one group of researchers, at Queen Mary and Westfield College, has given very precise figures for the mixture of materials. They suggest that the Universe is made of 69 per cent CDM, 30 per cent HDM and 1 per cent baryonic matter. And they even calculate the required mass for the HDM neutrinos — 7.5 eV. This is 0.0014 per cent of the mass of an electron, and comfortably below the upper limit of 20 eV so far set by experiments.
An experiment which confirmed that the mass of the neutrino is around 7-8 eV could, therefore, be taken as evidence from the laboratory that we really do know what the Universe is made of. It would mean that the mass of the lightest particle (apart from those with zero mass) had been predicted from measurements of the entire Universe.
Experiments are also underway to detect CDM particles. Because the theories do not say how many of these particles there should be in the Universe, the range of possible masses for each particle is quite large; but, once again, detection of anything in the right sort of range would be strong vindication of the mixed dark matter model of the Universe. One way or the other, the next step forward in understanding what most of the Universe is made of is likely to be taken in laboratories here on Earth.
The Visible “stuff”
The pattern of the distribution of visible stuff across the Universe shows bright galaxies distributed in filamentary chains and sheets, with voids containing very little bright stuff. In simple terms, this pattern can be explained if the average distribution of matter across the Universe (baryonic matter and dark matter) varies slightly from place to place, like the long smooth swell of an ocean. If galaxies then form only from exceptionally high peaks in the initial density distribution (short-wavelength ripples on top of the swell), they will be strongly concentrated in the crests rather than the troughs of the long-wave perturbations. The distribution of bright stuff in galaxies will be more “clumpy” than the overall distribution of mass.
MACHOs are no match for WIMPs Some of the dark matter needed to explain the dynamics of spiral galaxies may be in the form of “massive astronomical compact halo objects”, or MACHOs (the name was deliberately chosen as a counter to the term WIMP). These could be either very dim stars (brown dwarfs) each with about the same mass as Jupiter, or black holes each with a mass up to a million times that of our Sun. But although MACHOs could account for the invisible halos needed to explain how galaxies like the Milky Way rotate, they are themselves made from baryons produced in the Big Bang, and so cannot provide the much larger amount of dark matter needed to explain the overall structure of the Universe. Although they are dark, in the context of the cosmological discussions they are part of the 1 per cent of the Universe made of ordinary atomic stuff.
Fictitious Forces
PEOPLE who argue that forces such as centrifugal force are mathematical fictions and should not be allowed to sully the pages of respectable journals like New Scientist may be getting themselves in a spin for no good reason. A Brazilian physicist claims that the so-called fictitious forces have every right to be treated on the same footing as other forces, and that by giving these forces respectability physics teachers could help their students to develop a better understanding of the way the Universe works.
The usual approach is to say that the only “real” forces are the ones associated with interactions such as gravity and electromagnetism. The sideways push that you feel in a car cornering at high speed is produced only by the non-uniform motion of the car, not by any interaction of this kind, and is therefore dubbed a fictitious fyrce. But where does centrifugal force come from? What is the non- uniform motion of the car being measured relative to? Isaac Newton argued that there must be some God-given “absolute space”, which defines the absolute standard of rest in the Universe. He noted that the distant “fixed stars” provide a good approximation to this unique standard of rest, but left it at that.
Later thinkers developed the idea, now known as Mach’s Principle, that the fixed stars (we would now refer to the distant galaxies) actually define the standard of rest. The resistance to motion (inertia) we feel when we try to push an object is a result, on this picture, of trying to make it accelerate relative to the average distribution of all the matter in the Universe. Similarly, centrifugal forces are caused by rotation relative to the distant galaxies, and in principle you could produce centrifugal forces by standing still and making the Universe rotate about you.
No wonder Arden Zylbersztajn, who discusses the implications in the European Journal of Physics (vol 15 p 1) describes the situation as “a conceptual minefield”. But as he points out, if inertia depends on motion relative to the distant galaxies, so we can see where it comes from, why shouldn’t the inertial forces be real?
Zylbersztajn, who works at the Federal University of Santa Catarina, in Florianopolis, Brazil, suggests that a good way to teach students about forces such as gravity is to replace Newton’s first and second laws by a new rule which says that the resulting force acting on any object is always zero. This strange suggestion makes sense when he presents an example, in terms of a satellite orbiting the Earth in free fall.
From the point of view of the satellite, the satellite is at rest, and prevented from falling by a “fictitious” outward centrifugal force, which balances the pull of gravity. The net force is zero. But from a Machian point of view, the centrifugal force results from a real interaction with the rest of the Universe.
It is, says Zylbersztajn, “easy to foresee a student being failed in a public examination because he or she considered as real the centrifugal force acting on a satellite”. But, he continues, this is a perfectly acceptable, if non-traditional, perspective and “there is nothing sacred about tradition”.
Asking Alice for the Missing Mass
THE POSSIBILITY that the dark matter needed by cosmologists to explain the structure of the Universe may be in the form of “mirror” particles, occupying a shadowy but undectable world alongside our own, has been revived by Hardy Hodges, of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Massachusetts. This form of dark matter would be composed of baryonic material (protons and neutrons) just like that of everyday matter, but with a reversal of left-right asymmetry. With one set of baryons left handed and the other right handed, the laws of physics say that they would never interact with one another except through gravity.
It is, of course, those laws of physics themselves that are “reflected” in the mirror world, not the physical shape of the particles. The idea dates back to the 1950s, when the existence of a left-right asymmetry in those laws was first recognised. It was later proved that reversing the symmetry in a magic mirror would indeed eliminate all contact between the particles except through gravity.
Science fiction writers loved the idea, and developed stories in which two planets, one composed of right handed matter and the other of left handed matter, co-existed in the same space, interpenetrating one another without either being affected. Particle physicists were no less imaginative, coming up with the notion of an “Alice string”, a linear defect in space with the curious property that a left handed particle circling the string would be turned into a right handed particle, and vice versa.
But these early ideas only suggested that the mirror particles might exist, not that they must exist. In the 1980s, interest in the mirror world reached a peak when it was discovered that one promising version of a unified “theory of everything”, called superstring theory, automatically required that every type of particle in our world should be mirrored by an equivalent type of particle with opposite parity. In this version of the story, the “mirror” particles were dubbed “shadow matter”, partly because the name “mirror matter” is sometimes used for antimatter, which is quite a different phenomenon. Cosmologists were intrigued by all these suggestions, because their observations show that there is a lot more matter in the Universe than we can see in the form of bright stars and galaxies. At least ten times as much dark stuff is also present in the Universe, holding everything together gravitationally but otherwise not revealing its presence at all. Shadow matter would be ideal for the job — if you could make enough of it in the Big Bang.
But there was the rub. Even superstring theory only said that each type of particle must be mirrored in the shadow world. Just one shadow proton, for example, would be sufficient to do the job, as far as the laws of physics were concerned, of “mirroring” all the protons in our world.
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When theorists looked closely at the possibilities, it seemed at first very difficult to make the right amount of shadow matter in the very early Universe. The process of inflation, which is thought to have expanded the Universe exponentially from a quantum seed to macroscopic size in the first split-second of its existence, would, it seemed, favour one form of matter over the other. Getting the two kinds in balance, even as closely as a ratio of 1:10, looked impossible. Hodges new analysis suggests, however, that it is possible to achieve a near balance between the abundances of ordinary and shadow matter at the end of the epoch of inflation (Physical Review D, vol 47 p 456). Significant “dilution” of one of the worlds is only one of three possibilites. Another variation on the theme sets the ratio very close to 1 (which is also ruled out by observations, unless there is some other form of dark matter as well), and the third possobility allows the ratio to be between 1 and about 10. This could work very well to provide some or all of the required dark matter.
Hodges argues that the “mirror baryons” will form stars, “Jupiters” and other objects which could be floating around in galaxies like our own, and could soon be detected by their gravational influence. It is even possible that some of those Alice-through-the-Looking-Glass stars may be orbited by shadow planets, on which shadow cosmologists are even now discussing the possibility that their entire Universe containsd another parity-reversed world embedded within it. Time, perhaps, for the science fiction writers to take up the theme again.
Considering the Universe as a Hole
DOES our Universe exist on the inside of a single magnetic monopole produced by cosmic inflation? According to Andrei Linde, one of the founding fathers of inflation, it is at least possible, and may be likely. And in a delicious touch of irony, Linde, who works at Stanford University, made this outrageous claim in a lecture at a workshop on the Birth of the Universe held recently in Rome, where the view of Creation is usually rather different.
Inflation is the idea that very early in its life the visible Universe went through a brief period of exponential growth, doubling in size hundreds of times in much less than a second. This model is well supported by the pattern of the cosmic microwave background radiation revealed by the COBE satellite and other instruments. But one of the reasons why theorists came up with the idea in the first place was precisely to get rid of magnetic monopoles — strange particles carrying isolated north or south magnetic fields, predicted by many Grand Unified Theories of physics but never found in nature. Standard models of inflation solve the “monopole problem” by arguing that the seed from which our entire visible Universe grew was a quantum fluctuation so small that it only contained one monopole. That monopole is still out there, somewhere in the Universe, but it is highly unlikely that it will ever pass our way.
But Linde has discovered that, according to theory, the conditions that create inflation persist inside a magnetic monopole even after inflation has halted in the Universe at large. Such a monopole would look like a magnetically charged black hole, connecting our Universe through a wormhole in spacetime to another region of inflating spacetime. Within this region of inflation, quantum processes can produce monopole-antimonopole pairs, which then separate exponentially rapidly as a result of the inflation. Inflation then stops, leaving an expanding Universe rather like our own which may contain one or two monopoles, within each of which there are more regions of inflating spacetime.
The result is a never-ending fractal structure, with inflating universes embedded inside each other and connected through the magnetic monopole wormholes. Our Universe may be inside a monopole which is inside another universe which is inside another monopole, and so on indefinitely. What Linde calls “the continuous creation of exponentially expanding space” means that “monopoles by themselves can solve the monopole problem”. Although it seems bizarre, the idea is, he stresses, “so simple that it certainly deserves further investigation”.
“New physics” needed to explain solar neutrinos
THERE is no way to explain the deficiency of neutrinos coming from the Sun by adjusting astrophysical models of how the Sun works, according to two independent studies published in Physical Review Letters and Physical Review. This means that the solution to the solar neutrino problem must lie in a better understanding of the way neutrinos work – – that is, on “new physics”.
The problem us that a combination of astrophysics, telling us about the temperature, pressure and so on inside the Sun, and particle physics, telling us how many neutrinos should be produced by reactions going on under those conditions, predicts a higher flux of neutrinos at the Earth than has been observed. The long-running chlorine based detector in the Homestake gold mine in South Dakota finds only one third of the expected number of neutrinos. Other detectors find slightly more, but still less than theory predicts.
For several years, there has been a lively debate about whether the astrophysical models or the particle physics theories are wrong. Now, a consensus supporting the astrophysics and pointing the finger at particle physics is emerging.
In one study, Martin White, Lawrence Krauss and Evalyn Gates, of Yale University, have developed a particularly neat way of comparing the observational data with theoretical predictions. They present the data in a graphical form which clearly highlights the differences between the various possibilities (Physical Review Letters, vol 70 p 375).
The existing data,” they say, “argue against an astrophysical solution of the solar neutrino problem”. Essentially the same conclusion has been reached by John Bahcall, of the Institute for Advanced Study, in Princeton, and Hans Bethe, of Cornell University (Physical Review D, vol 47 p 1298). Their approach involves a so-called “Monte Carlo” simulation. In this technique, the standard values for parameters such as temperature and pressure are varied at random, within the range of a statistical normal distribution, in a series of models run on a computer. This should give a good idea of the range of possible models that could be consistent with known physics. But none of a thousand solar models investigated in this way in a full Monte Carlo simulation is consistent with the results of the Homestake or Kamiokande detectors.
In both cases, the researchers find a hint that the “suppression” of neutrinos is greater at lower energies, probed by the Homestake detector. But even when the solar models are, as Bahcall and Bethe put it, “fudged” to match the Kamiokande data, they still do not agree with the Homestake results. The bottom line is that “new physics is required beyond the standard electroweak theory if the existing solar neutrino experiments are correct”.
Do we live in the middle of the Universe?
Is it possible that we are living near the centre of the Universe? For centuries, the history of astronomy has seen humankind displaced from any special position. First the Earth was seen to revolve around the Sun, then the Sun was seen to be an insignificant member of the Milky Way Galaxy, then the Galaxy was seen to be an ordinary member of the cosmos. But now comes the suggestion that the “ordinary” place to find observers like us may be in the middle of a bubble in a much greater volume of expanding space.
The suggestion is the latest variation on the theme of inflation, the theory which says that during the first split second of its existence the Universe as we know it expanded from something smaller than a proton to the kind of size it has today. In fact, so-called “eternal inflation” says that there never was a singular beginning, and that the Universe around us is just one bubble among indefinitely many constantly appearing out of the superdense state (at the so-called Planck density), like bubbles appearing in a fizzy drink when the top is released.
The conventional version of inflation says that the process of rapid expansion forces spacetime in the bubbles to be flat, which means that the density of matter in each bubble should be indistinguishably close to the minimum value required to eventually bring the present, more sedate expansion to a halt (the “critical density”). But that suggestion, along with other cherished cosmological beliefs, has now been challenged by Andrei Linde (one of the founders of inflation theory, who now works at Stanford University), Dmitri Linde (of CalTech) and Arthur Mezhlumian (also of Stanford).
Linde and his colleagues point out (in a Stanford “preprint”, number SU ITP 94 39) that the Universe we live in is like a hole in a sea of superdense, exponentially expanding inflationary cosmic material, within which there are other holes. All kinds of bubble universes will exist, and it is possible to work out the statistical nature of their properties. In particular, the two Lindes and Mezhlumian have calculated the probability of finding yourself in a region of this super- Universe with a particular density — for example, the density of “our” Universe.
Because very dense regions blow up exponentially quickly (doubling in size every fraction of a second), it turns out that the volume of all regions of the super-Universe with twice any chosen density is 10 to the power of 10 million times greater than the volume of the super- Universe with the chosen density. For any chosen density, most of the matter at that density is near the middle of an expanding bubble, with a concentration of more dense material round the edge of the bubble. But even though some of the higher density material is round the edges of low density bubbles, there is even more (vastly more!) higher density material in the middle of higher density bubbles, and so on forever. The discovery of this fractal structure surprised the researchers so much that they confirmed it by four independent methods before venturing to announce it to their colleagues. Because the density distribution is non-uniform on the appropriate distance scales, it means that not only may we be living near the middle of a bubble universe, but that the density of the region of space we can see may be less than the critical density, compensated for by extra density beyond our field of view.
This is convenient, since observations by the Hubble Space Telescope suggested recently that cosmological models which require the critical density of matter may be in trouble. But there is more Those Hubble observations assume that the parameter which measures the rate at which the Universe is expanding, the Hubble Constant, really is a constant, the same everywhere in the observable Universe. If Linde’s team is right, however, the measured value of the “constant” may be different for galaxies at different distances from us, truly throwing the cat among the cosmological pigeons. We may seem to live in a low-density universe in which both the measured density and the value of the Hubble Constant will depend on which volume of the Universe these properties are measured over!
That would mean abandoning many cherished ideas about the Universe, but with the bonus that by measuring these strange properties we could hope to learn “something important about quantum cosmology and particle physics near the Planck density”.
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Cosmology for Beginners - Infrared Sky
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