B00B7H7M2E EBOK (35 page)

There is an interesting glitch when researchers measure distances to an object closer to the centre of the Galaxy than the radius at which the Sun orbits, a glitch that illustrates the sort of complications modern astronomers encounter. Within this radius, discovering a particular speed of something from its Doppler shift doesn’t give its distance. Instead, each speed indicates a choice between
two
possible distances. Along any line
of
sight, the speeds increase with distance from us until we reach a point that is the same distance away as the Galaxy centre. Beyond that the speeds decrease again until they read zero once more for an object that is following the same orbit as the Sun but is on the other side of the Galaxy. It’s necessary to look for additional clues to reveal whether something within the radius of the Sun’s orbit lies in the near distance or the far distance – in other words, which of the two possible distances indicated by this speed is the correct one.

Illustration 16 (
in the picture section
) is one of the best images of the Galaxy as a whole, showing the disc and central bulge. It came from the Cosmic Background Explorer (COBE) satellite. This is not a direct snapshot but a map compiled from observations made at infrared wavelengths, a part of the spectrum that is not obscured by dust clouds or more diffuse interstellar dust.

While some astronomers have been exploring and mapping the Galaxy, others have been studying the universe outside it. One question such map-makers puzzle about is, could Friedmann have been wrong? Modern cosmology still accepts his assumption that the universe looks the same to us in all directions, and that anywhere we were in the universe it would look the same to us in all directions. And yet that isn’t really the way it does look. Certainly not from Earth. In the sky from the southern hemisphere we see the Magellanic Clouds. We don’t see them from the northern hemisphere, though from there we see another distant smudge of light, the Andromeda galaxy. When we look at the Milky Way we’re looking along the plane of our Galaxy. We certainly see more stars there than in other parts of the sky. On this scale, anywhere we went in the universe we would likewise see visible matter distributed unevenly. Even on the much larger scale of a few hundred million light years, there is still structure including unimaginably large superclusters and voids 300 million light years wide. This is
not
evenly mixed raisin bread dough.

So if we agree with Friedmann that the universe is isotropic (
looking
the same in all directions) and homogeneous (with matter distributed evenly throughout space), we can’t be talking about it on these scales. To find that it is isotropic and homogeneous, we have to look at the universe on a much larger scale yet. How large? In truth, astronomers have not yet found the level at which the universe would all look alike, where we would not be able to tell one sample of it from another. That was one of the great surprises of the late 20th century.

The modern picture of the universe on the largest scales emerged in the 1980s and represented a dramatic change from the way it had been visualized before. Margaret Geller, John Huchra and Valerie de Lapparent, at the Harvard-Smithsonian Center for Astrophysics, decided it was worth following up on preliminary surveys that indicated there might be more structure in the universe than previously suspected. They proceeded to investigate by mapping the red shifts of a thousand galaxies across one strip of the northern sky. A strip becomes a wedge as we follow it further and further into space (
see Figure 7.4
), and this particular pioneering wedge now goes by the name of the Geller-Huchra Wedge.

When they began the project, no one including Huchra and Geller thought it likely to reveal anything particularly mind-boggling, and they were not even in any great hurry to interpret the data. When they did get around to it, jaws dropped. There was an eerie difference in this wedge of the universe from what nearly everyone had been expecting. Here was no homogeneous small-design-wallpaper pattern of galaxies with nothing to distinguish one portion of the picture from another. Instead there were huge voids with almost no galaxies in them, bordered by clusters of galaxies strung out like the lights of seaside towns and cities seen at night from a plane. There was a ‘Great Wall’ of galaxies, a billion light years long and tens of millions of light years thick. This was
structure
, to put it mildly! Huchra thought he and his colleagues must have made a mistake. Geller was more willing to relinquish old assumptions. As she quipped
in
an interview with science writer Timothy Ferris, ‘I have a strongly held scepticism about any strongly held beliefs, especially my own.’

Figure 7.4 The Geller-Huchra Wedge

Geller and Huchra proceeded to investigate further by mapping the red shifts in the wedges of space to either side of the original Geller-Huchra Wedge, using more sophisticated equipment. There had been no error. The awesome structure went on and on.

Other astronomers (including Alex Scalay, David Koo, Richard Kron, T.J. Broadhurst, Richard Ellis and Jeff Munn) have since probed the depths of space by means of ‘pencil beam’ surveys. They limit themselves not to a strip of sky but to an area about half the size of the full Moon. We’ve said that a strip becomes a wedge as we move deeper into space. Similarly a small circle becomes a cone (as in
Figure 1.4a
). A pencil beam survey produces a cone-shaped three-dimensional map that
keeps
getting larger as it reaches further distances. The results are plotted in a chart where increasing red shift is the horizontal axis and the number of galaxies discovered at various red shifts are shown as peaks. This method has also revealed the great voids, or ‘superbubbles’ as some call them. None have diameters more than twice the diameter of the previously mapped voids – which is significant when questioning whether there is larger structure yet.

In December 1995, the Wide Field and Planetary Camera 2 on the Hubble Space Telescope was used to make a different kind of survey. It took 342 exposures over 10 consecutive days of a very tiny speck of the sky,
the apparent size of the full Moon, near the handle of the Big Dipper. This is a region relatively uncluttered with nearby stars or galaxies. The camera took separate images through filters for ultraviolet, blue, red and infrared light. The resulting composite picture is what is known as the ‘Hubble Deep Field’, a narrow, deep ‘core sample’ of the sky, something like the core samples geologists take of the Earth’s crust. The Deep Field doesn’t reveal the ages of the galaxies it photographed or their distances. Galaxies at many different stages of the universe’s history are stacked against one another in the picture.

The ‘Field’ reached back some 10 billion years and captured an unprecedented view of young, never-before-observed galaxies, some of which are four billion times fainter than can be detected by the human eye. Only the Cosmic Background Explorer satellite, when it measured the wrinkles in the cosmic microwave background radiation, looked deeper into space and into the past than this. The Deep Field showed spirals, ellipticals and a rich assortment of other galaxy shapes and sizes in many stages of evolution. Even though the photograph gives no reliable measure of the distances of the galaxies in it, the great number of very faint galaxies led astronomers immediately to suspect that some of them may have formed when the universe was very young. Researchers were soon hard at work measuring
red
shifts. By the spring of 1997, their calculations had yielded distances for thousands of the faint galaxies as far out as those having a red shift of 1. A red shift of 1 indicates that the light now reaching Earth from that galaxy left the galaxy when the universe was no more than half its present age. Larger red-shift numbers mean the light originated even further back in time. A red shift of 3, for example, means the universe was between 12.5 and 25 per cent of its present age.

Studying the galaxies as far out as those having a red shift of 1, researchers interpreted what they found to mean that galaxies with spiral and elliptical shapes probably have relatively uneventful lives (as galaxy biographies go). It seems that spirals and ellipticals must not change much over billions of years, for the oldest don’t look markedly different from those nearby, and the number of them in the distant past was comparable to the number in the universe today. Experts hoping the Deep Field would reveal the formation process of spirals and ellipticals were disappointed. Evidently, to see that, they would have to push the search still further into the past.

Other types of galaxies in the Deep Field appear to have led more action-packed lives. Their irregular convoluted shapes suggest that galaxy collisions and mergers were far more common in the early universe than they are today, which makes sense, if things were so much more crowded together then. Researchers have also concluded from the Deep Field that the star formation rate in the universe has declined dramatically during the second half of its history.

For most of the galaxies found at the far limits of the Hubble Deep Field observations, it isn’t yet possible to use red-shift measurements to calculate distances, because the amount of light from these faint galaxies isn’t sufficient for even the largest telescopes to measure their red shifts. Astronomers have, however, worked out other techniques. One method is to use other objects as distance calibrators. Fortunately, some galaxies generate powerful emissions in the radio part of the spectrum,
detectable
at extremely great distances. Present understanding of these ‘radio galaxies’ has it that this emission comes from their active cores. Also fortunately, many of them are surrounded by other types of galaxies, and measuring the red shift of the radio galaxies allows astronomers to estimate how far away these whole clusters of galaxies are. Some are at red shifts as large as 2.3, and that means that the light reaching Earth today left them when the universe was less than 30 per cent of its present age.

State-of-the-art astronomy coordinates data from different kinds of telescopes – ground-based and space-based – observing in different parts of the spectrum. Some distant clusters of galaxies have been studied using the Hubble Telescope in conjunction with the most powerful ground-based telescopes, such as the Keck 10-metre telescope in Hawaii, and orbiting X-ray telescopes such as the German ROSAT X-ray Observatory. From this investigation astronomers have learned that some of the ‘young’ galaxy clusters were probably already extremely massive. Light from them shows that their stars were already mature when that light left them, so the galaxies must have formed much earlier.

Another approach to finding the distances of the faint galaxies in the Hubble Deep Field has been to study the light coming from quasars that are even further away, looking for evidence in the spectral lines that this light has encountered clouds of gas in halos around galaxies in its journey to Earth. Such detective work has nosed out galaxies at red shifts of 3 and even higher. Another helpful clue has been that all very remote galaxies have a distinctive ‘signature’ in their colour. Hydrogen, which is present both in galaxies and in the space between them, absorbs all ultraviolet light shorter than a certain wavelength. The upshot is that in the spectrum of light from the most distant galaxies there is a cut-off at that wavelength. Using filters, researchers find that a galaxy ‘disappears’ at wavelengths beyond that.

As of 1998, the most up-to-date maps of the universe continued to show clusters of galaxies lining up to form filaments enclosing vast voids. Clusters and superclusters are parts of supercluster complexes or ‘walls’ or ‘sheets’ up to a billion light years in length, enclosing even more enormous voids. Someone has commented that if we step back from this picture, we will see that the universe is a Swiss cheese. Richard Gott and colleagues at Princeton prefer a different mental image. Not a Swiss cheese, though that’s close. A sponge, they say, is better. All of the material in a sponge is joined together. So are all the holes. In the universe the rich clusters of galaxies are more likely to be found at junctures the equivalent of where the pieces of a sponge’s material come together. Other theorists think that galaxies, clusters and superclusters are like a glowing froth on an ocean of dark, invisible matter, similar to the froth that appears among ocean waves. Anyone who has swum in the sea has seen the voids, filaments and clusters of foam that form and reform continually on the swells.