Before the brightest stars and globular clusters galaxies fade below our limits of detection, and well before the hydrogen clouds can no longer be measured we come face to face with one of the most amazing properties of our world: the Universe is expanding

The space between galaxies is increasing. Smaller parts of the Universe may cling together but at increasing distances they fail to stop the outward motion; the Universe is proceeding on without them! If we cannot hold onto our neighbouring galaxies by gravity, the space between us and them will continue to increase. All galaxies around us are seen to be rushing away from us and we from them. Indeed when we measure the velocities of galaxies around us. the Universe is seen to be thinning out in all directions.

Bit by bit we are being left behind. The further we look out into space the greater volume we encompass. Greater distances mean greater velocities of expansion of one side relative to the other, so distant galaxies move away from us more quickly than near ones. However the rate of expansion is so small that we must go to cosmic distances before the general expansion is measurable. In the absence of any other forces, the change over the distance between the sun and the nearest star would only be a few centimeters per second and the cumulative
effect over a year would be to increase the distance by only one part in 30000000000

The Combined light of millions upon millions of stars that from individual galaxies is detectable long after the individual stars fail to be distinguishable in a telescope. Similarly, the composite spectrum of galaxies can be detected over vast distances. Bright and dark lines that appear in the spectra of galaxies provide our link to perhaps the most powerful distance indicator we can hope for at extragalactic distances. Lines in the spectra of distant galaxies show a systematic displacement in frequency when com¬pared to reference lines observed in the laboratory. This change can be simply explained by a relative motion between us and the galaxies, a Doppler shift. Since the spectral lines are always found to shift towards the red end of the spectrum, we come to speak of the motions of galaxies in terms of their redshifts. For all intents and purposes the redshift and the expansion of the Universe are one and the same.

The general expansion continues out as far as we can see. But can it just continue forever ? With distance, light dims and so our last sight is of the galaxies themselves, reduced to fading points of light. Our last chance now comes to learn about the distant void. Here we assume that all brightest galaxies of a given type are of the same intrinsic luminosity. With this assumption, the apparent luminosities of galaxies at the limit of detection become another measure of distance. We now have two independent measurements to the edge of the visible Universe (redshift and luminosity) and with them we have the makings of a grand experiment: to guess the fate of the Universe.

The distances to which we can see represent travel times, at the speed of light, which rival the age of the Universe as an entity in itself. On a simple view, the expansion could not have been going on for more than some 20000000000 years in the past. Running the expansion backwards further than that would pass the time when all matter in the observable Universe was at a single point, the big bang or the beginning of the Universe.

The Universe, too, is a product of its past and so its fate will some day be judged by observations of its history up to now. That judgement is not in. The distant galaxies were younger when they shone with the light that reaches us today. Their images are from the past. The galaxies were different, but so too was the Universe around each galaxy. Comparing apparent brightnesses of galaxies with their redshifts may be telling us about the evolution of the Universe, and it may equally well be telling us about the evolution of galaxies. More likely it is telling us about both. As always, an inconsistency between our two distance indicators is a signal for change and a chance for discovery.

When we look at galaxies far in the past, it is like estimating the distance to a forest so remote in space and time that the trees whose heights we measure have long since died and their offspring have evolved a million times. Are these, indeed, the distant ancestors of more familiar sights? Our estimate of distances, based on unknown properties, must be very insecure. Each theory of evolution and each reasoned guess as to their environ¬ment carries factors of potential error. And, perhaps, these things we see became extinct and correspond to no object in our present experience!

In our race to see the extremes of the Universe, to predict its past, and to speculate on its future, astronomers have encountered galaxies of amazing variety: beautifully-formed spirals in massive clusters or adrift alone in space; radio galaxies, exploding systems alive with motion, radiation and activity; elliptical galaxies, slowly dying where star formation ended a billion years ago, and finally the quasars, points of light with redshifts indicating that they lived with fierce intensity in the most distant past. Our ter¬restrial laws of physics appear to be sufficient to explain most of what we want to see.

Every generation of astronomy has worked in this mixed aura of awe and confidence. We slowly change our views, with reluctance at first, which turns to fervour when we must. And so heresy can turn to dogma, dogma to history. Often in the past we may have missed the forest for the trees, but now what secrets await us in the sky and in the worlds beyond the forest ?

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