B00B7H7M2E EBOK, page 25
Continue travelling and we’ll come to the thinnest layer of the Galaxy. If the Galaxy were a chocolate-mint wafer, the main disc would be the chocolate and the inner disc we’re now entering would be the mint cream layer. This is a disc of gas and dust, only 500 light years thick, and it is the Galaxy’s nursery. It houses the youngest stars and is the birthplace of new stars. The resemblance to the chocolate-mint wafer is not exact. This thin disc of gas doesn’t end out where the main disc layers end (as the filling of a chocolate-mint wafer does) but instead stretches out beyond that to a distance more than a third again as far from the centre of the Galaxy. At its extremes the gas disc bends like the rim of a hat. At one side of the Galaxy the edge of the hat brim curves upwards. On the other side of the Galaxy it curves downwards and then, further out, back upwards.
Continuing our journey out the other side of the Galaxy disc we find the same layers in reverse. Compared to its diameter along its plane (looking at it edge-on), the Galaxy is very thin indeed. A chocolate-mint wafer is too thick to be an accurate comparison, not large enough in diameter in relation to its thickness. The Galaxy has more the proportions of a gramophone record.
One thing astronomers now know – to follow up on an earlier attempt to map the Galaxy – is that Harlow Shapley was right to conclude that globular clusters outline the extent of the Galactic halo, though there are also some of them well beyond it. The Omega Centauri cluster, a globular cluster that Ptolemy catalogued (though not as a cluster), that Halley recognized as a cluster, and about which Herschel spoke in superlatives – ‘richest . . . largest . . . truly astonishing . . . whose stars are literally innumerable’ – does indeed turn out to be extraordinary. It is the brightest, largest and most massive cluster in the Galaxy, with tens of millions of stars. There are some 100 billion stars in the whole Milky Way Galaxy, many of them congregated in the central bulge, the bright yolk of the fried egg. If our journey had taken us directly through the bulge, we might have seen, or even ended up in, a massive black hole that is suspected to lurk at the very heart of all that brightness.
The Milky Way, which is now behind us as we move further afield, is only one among many billions of galaxies, not all of which are the same size and shape. They range from about 10 million to 10 trillion times the mass of the Sun, but that includes the extremes. A typical galaxy (the Milky Way is one of them) is midsize. We first pass among the other members of the ‘Local Group’ of galaxies. This group measures about five million light years across and is rather flattened in shape.
The designation, ‘group’, is part of the terminology astronomers use to describe the structure hierarchy of the universe, though this ‘hierarchy’ is not rigid, and thinking of the universe this way ignores the rich diversity of its structure. There are, moving to larger and larger scales, first galaxies, then groups, clusters, clouds, superclusters and supercluster complexes or ‘walls’.
‘Groups’ typically include three to six conspicuous galaxies and a number of smaller, dimmer ones. The Local Group is no exception. The Andromeda galaxy is its dominant spiral, with an estimated 400 billion stars. The Milky Way ranks second, and there is a smaller but still impressive spiral called M33. These three giants are all that an observer in, for instance, the Virgo Cluster would be able to see of our Local Group, but lesser galaxies in the group outnumber them ten to one, and there are probably others not yet observed because they’re hidden from Earth by dust clouds within the Milky Way. Andromeda has two small companions, M32 and NGC205, both elliptical galaxies. The Milky Way holds court with a retinue made up of its two best-known companions, the Magellanic Clouds, which are irregular galaxies, and three other satellite galaxies. Two of these, the Carina galaxy and the Sextans galaxy, are dwarf galaxies that are spherical in shape. The Sextans dwarf, discovered in 1990, is a little further away than the Magellanic Clouds and the total luminosity of its stars combined is only about 100,000 times the luminosity of the Sun – less than some single stars in the Milky Way. A more recently discovered satellite galaxy, the Sagittarius galaxy, is the closest neighbouring galaxy found so far. First recognized in 1993, it’s a dwarf that looks likely to be cannibalized by the Milky Way. It has already lost some of its outer stars to the Galaxy’s gravitational pull.
The distance to the Andromeda galaxy is fairly well established at some two and a quarter million light years, but there is still dispute about the distance to M33, the third largest Local Group spiral. When Edwin Hubble first measured it in the 1920s, he estimated that M33 was about as far away as the Andromeda galaxy. Allan Sandage, however, has reinterpreted Hubble’s data on the Cepheids used for that measurement, employing more modern techniques, and has concluded that M33 is more like three million light years from us, well beyond the Andromeda galaxy. Astronomers who have studied the same Cepheids at infrared wavelengths disagree with Sandage. They estimate that the two galaxies, though further away than Hubble estimated, are, as he thought, both about the same distance from us.
Groups like our Local Group have no particular structure or shape; in fact, they are also known as ‘irregular clusters’ and their galaxies are a hodgepodge of all types. However, that doesn’t mean their existence is a random occurrence, with all these galaxies just happening to be passing close to one another on their way to somewhere else. All the galaxies in the Local Group are bound together by mutual gravitational attraction, and all are orbiting a common centre of gravity. Andromeda and the Milky Way are, at the moment, approaching each other at a speed of 300 kilometres per second. Slipher’s discovery of the blue shift in Andromeda’s light was not an error. There could eventually be a head-on collision, but that is not something to write about on the walls of the underground yet. The event is still a few billion years in the future, and the merger of the two galaxies will take another several billion years after that to complete. In the end there will probably be, instead of two spiral galaxies, one huge elliptical galaxy. Then again, Andromeda and the Milky Way may only circle one another in a polite do-si-do and then move apart again. Which will it be? In 2005, NASA plans to launch the Space Interferometry Mission, a spacecraft carrying an array of telescopes capable of determining, among other things, the exact angle of Andromeda’s approach.
Obviously, relationships among galaxies are not always placid. Andromeda seems to have stripped M32 of a good number of its stars, while M32 has in turn caused distortion in the spiral structure of Andromeda. NGC205 is twisted by the pull of Andromeda. Most astronomers also believe that the Small Magellanic Cloud is being torn apart, probably by the gravity of the Milky Way Galaxy. There is a long, narrow ribbon of hydrogen gas, streaming half of the way around the sky and seeming to begin in the large pool of hydrogen that surrounds the two Magellanic Clouds. This may be gas from the ripped Small Cloud, left behind along its orbit around the Milky Way.
We would have to travel several million light years outside the Local Group to get to the nearest galaxies beyond it. But at this point we can learn more about the large-scale structure of the universe not by travelling further away from Earth, but by moving to larger and larger scales. Bear in mind the ‘hierarchy’ of the large-scale structure, but also be aware that the universe is not nested as neatly as Russian dolls, with galaxies within groups within clouds, within clusters, within superclusters, within supercluster complexes. There is far too much complication out there for such a simplistic picture to be anything but a distortion.
The Local Group is not far outside the border of the Coma-Sculptor cloud, a large cloud which in turn lies near the outer limits of the Virgo supercluster. The Virgo cluster, which is huge compared with the Local Group, is the giant heart of the Virgo supercluster and about 60 million light years from Earth. It is an enormous swarm of thousands of galaxies and a lot of hot gas – a ‘regular cluster’ because it doesn’t seem to have such a mix of galaxy types as are to be found in motley assortments like our Local Group. Instead, it contains more than 1,000 prominent galaxies centred on a pair of giant elliptical galaxies, with probably many more less prominent galaxies that astronomers haven’t yet detected, but very few spirals.
There’s little point in even trying to get a sense of the vastness described here. Probably we come closest when we are feeling most overwhelmed by our lack of ability to conceive of such size and how it compares with familiar distances. The box here gives approximate relative size scales in the universe as calculated in the mid-1990s, but in truth such numbers are beyond our human capacity to comprehend. Only for the sake of rough comparison, therefore, here are some figures having to do with the larger scales: a typical ‘group’ of galaxies is a few million light years across; the Local Group measures about five million light years; a cluster may be 10 to 20 million light years in diameter, a cloud some 30 million light years, a supercluster 100 to 200 million light years.
The question remains: Has astronomy discovered the ‘top’ of the hierarchy, or does it all go on and on to larger and larger structure? Will the voids turn out to be parts of systems of supervoids? Are the superclusters grouped in clusters of superclusters? Will we ever find a level at which the universe is isotropic and homogeneous? Is there such a level?
There are tentative answers to these questions. Pencil beam experts probing six billion light years of space (one three-billion-light-year-long beam pointing out each side of the Galaxy) have found no structure larger than the superbubbles. At extremely great distances, clusters, superclusters and voids seem to be spread more or less uniformly, with about an equal number in any direction you look. Perhaps it is here, in the ultimate sponge, that there is isotropy and homogeneity.
Astronomy has not been able to reveal an unbroken chronology leading from the era of the cosmic background radiation through to the present. There is a gap in the history of the universe like the lost years in the life of someone with amnesia. In terms of the structure of the universe there is one window (that provided by the cosmic background radiation) into the early universe about 300,000 years after the Big Bang . . . and, the next anyone is able to know, there are the voids and supercluster sheets and walls, much closer to the present in time and space. This is the same universe, but no one would guess that from appearances alone. Finding out what happened in the invisible stretch in between is one of the challenges facing the next generation of physicists and astronomers.
However, the one window to the time before that blackout does suggest homogeneity. Though the most interesting recent news about the cosmic microwave background has been the discovery of a minuscule lack of homogeneity, it is remarkably homogeneous, and it is a picture from further in the past than any other we have. However, it shows the universe only in its extreme youth. It’s like the smooth-skinned glamour shot that shows what a wrinkled dowager looked like in her late teens but gives us barely a hint of the ‘structure’ in that face today. It isn’t evidence that the universe now is isotropic and homogeneous.
No human being, and no universe, can be captured adequately in a still photograph. The question of what is out there can’t be considered meaningfully without asking, as well, how what is out there is moving. What movement does the Galaxy have, for instance, in relation to the rest of the universe besides Andromeda? One way to find out is to measure its motion against the cosmic microwave background radiation, because this radiation comes from a distance far beyond the remotest galaxies. The reasoning is that as the Galaxy moves through the cosmic background radiation, the radiation will measure warmer in ‘front’ of the Galaxy (in the direction towards which it’s moving) and cooler ‘behind’. If the temperature of the radiation reads the same in all directions, then the Galaxy isn’t moving. George Smoot and colleagues measured these temperature differences in the early 1970s from high-altitude U2 planes. They found that the Milky Way Galaxy is moving at a rate of about 600 kilometres per second in the direction of the Virgo cluster. However, the Virgo cluster meanwhile is moving in the other direction and the distance between us is increasing – though not as rapidly as it would were the expansion of the universe the only movement at work here.
The motion of other galaxies besides our own is more difficult to plot, but one of the more intriguing results of this effort has been the discovery that several hundred galaxies, the Milky Way among them, are sidling off in a direction and at a speed that the expansion of the universe does not explain. The Great Attractor was the name Alan Dressler, one of those who first calculated this movement, gave to the huge theoretical mass concentration that must be drawing them, but it was difficult to identify any obvious culprit whose gravitational pull was to blame. The identity of the Great Attractor remained for a time one of the baffling enigmas of science, and the mystery has not yet been completely solved. However, in early 1996 Renee Kraan-Korteweg, at the Observatory of Paris-Meudon, and her colleagues reported sighting a massive galaxy cluster that appears to be located just about where the Great Attractor ought to be. Because dust in the Milky Way blocks much of this cluster’s light, scientists had not realized before how wide and massive it is.
Does the universe as a whole move? Of course the universe is expanding, but what about other types of motion? For instance, does the universe rotate? Is it moving through some larger environment? The reply comes in the form of another question: In relation to what could the universe as a whole be said to rotate or not to rotate, or to move or not to move? We have come almost full circle to questions that date back to antiquity. Some of them have become meaningless. Others seem likely to remain forever unanswered. But some of the answers that our ancestors, even those with minds like Kepler and Newton, probably didn’t think could ever be known by human beings are now, according to modern astronomers, astrophysicists and theoretical physicists, almost within our reach.
CHAPTER 8
The Quest for Omega
1930–1999
When I started, cosmology was very much like philosophy. There was very little chance of measuring something precisely. It’s now turning into high precision science.
Alexander Szalay
HOW OLD IS the universe? What is its future? Much of the work going on in state-of-the-art physics and astrophysics at the turn of the twenty-first century focused on these two fundamental questions. In order to answer them, researchers wanted to know the mass density of the universe – the elusive ‘omega’.
The mass density of the universe means the amount of matter there is per cubic metre, averaged throughout the observable universe. Obviously this matter is unevenly distributed, at least on scales normally accessible to us. One way to find out the average density would be to add up all the matter in the universe and then divide by the number of cubic metres in the universe. On the face of it, that would appear to be a ludicrously difficult undertaking.
Never underestimate modern astrophysicists. It is possible to arrive at some estimates. In one method, called ‘representative sampling’, researchers divide the sky into sections of equal size, count the number of galaxies in a section, then multiply the count from that section by the total number of sections. Combined with knowledge about the masses of galaxies, this procedure gives a rough estimate of the total mass of the universe. Studies like the Hubble Deep Field make such sampling increasingly substantive.
Another way to try to find the average mass density is to study the way the universe appears to be working – how fast it’s expanding, whether the expansion is speeding up or slowing down, how gravity seems to be affecting different parts of the universe, what other forces besides gravity come into play, and how the contents of the universe have evolved over time. This sounds considerably more difficult than counting galaxies and multiplying by sections. It is extremely complicated. Nevertheless, theorists have a formula that they believe shows how the mass density of the universe is related to such questions, an equation that allows them to weigh the answers one against the other. It is the so-called ‘equation for omega’.
The equation shows how the mass density, omega, affects the future of the universe. If omega turns out to be more than one (meaning that there is more than an average throughout space of one hydrogen atom per 10 cubic metres), the universe will eventually stop expanding and contract. That would be a ‘closed’ universe. If omega is less than one (less than an average throughout space of one hydrogen atom per 10 cubic metres), the universe will expand forever. That would be an ‘open’ universe. If omega is precisely one, then the universe is at the ‘critical density’ that will allow it to expand at precisely the right rate to avoid recollapse, eternally slowing down its expansion but never completely ceasing to expand. That would be a ‘flat’ universe – the type of universe inflation theory predicts. (See box below.)
Why should there be such a tight connection between the mass density of the universe and the fate of the universe? First, ‘mass’ is the measure of how much matter there is – in a planet or star or galaxy or, in this case, in the universe as a whole. Every particle of matter in the universe is attracting every other by means of gravitational attraction. How greatly objects are influenced by one another’s gravitational attraction depends on how far apart they are. The closer they are the more they ‘feel’ one another’s pull. So when it comes to the question of whether or not the universe will eventually contract or whether it will keep expanding, much hangs on how densely or thinly the matter in the universe is spread out. In fact, the mass density very possibly does dictate the fate of the universe.
