Academic Reading — Test 113

Reading passage

For centuries, astronomers could only estimate the mass of a distant object by measuring the light it emitted. A brighter star was generally assumed to be heavier, and a faint galaxy was thought to be modest in size. This approach, however, rests on a flawed assumption: that everything which exerts a gravitational pull also shines. The discovery of dark matter, an invisible substance that neither emits nor absorbs light, overturned this comfortable picture. Today, the most reliable way to weigh the largest structures in the universe relies not on the light those structures produce, but on the way they bend the light of objects lying far behind them. The principle behind this technique was anticipated by Albert Einstein. His general theory of relativity, published in 1915, proposed that mass distorts the fabric of space and time, and that light travelling through this distorted region follows a curved path rather than a straight line. A sufficiently massive body therefore acts rather like a lens, deflecting and focusing the light that passes near it. Einstein himself doubted that the effect would ever be observed in practice, believing the alignments required would be too rare. He was proved wrong. Astronomers now routinely detect this phenomenon, which is known as gravitational lensing, and they exploit it as a precise weighing instrument. Galaxy clusters are the ideal subjects for such measurements. Containing hundreds or even thousands of individual galaxies, together with vast clouds of hot gas and enormous quantities of dark matter, a single cluster may hold a mass equivalent to a million billion suns. When such a colossal object sits between the Earth and a more distant galaxy, its gravity bends the light from that background galaxy as the light travels towards us. The result is that the background galaxy appears distorted: its image may be stretched into a thin, curved streak, duplicated into several separate images, or, in the rarest and most striking cases, smeared into a complete ring of light encircling the cluster. This last configuration is called an Einstein ring, in honour of the man whose equations predicted it. The degree of distortion is not random; it depends directly on the amount of mass doing the bending. A heavier cluster warps space more sharply and therefore produces more pronounced arcs and larger separations between duplicated images. By carefully measuring the shape, position and brightness of the distorted background images, astronomers can work backwards to calculate how much mass must be present to produce exactly that pattern. Crucially, this calculation captures the total mass of the cluster, including the dark matter that no telescope can see directly. For this reason, lensing has become the gold standard against which other, more indirect methods of estimating cluster mass are checked. Two broad varieties of the phenomenon are recognised. Strong lensing, which produces the dramatic arcs and rings described above, occurs only when the alignment between observer, cluster and background galaxy is nearly perfect, and it probes the dense central regions of a cluster. Weak lensing is far more common but far subtler. Here the background galaxies are not obviously distorted; instead, their shapes are altered by a tiny, almost imperceptible amount. No single galaxy reveals anything useful, because galaxies come in many natural shapes to begin with. The trick is statistical: by averaging the apparent shapes of thousands or even millions of faint background galaxies, astronomers can detect a faint, systematic stretching that betrays the gravitational influence of intervening matter spread across wide regions of sky. The implications of these techniques extend well beyond the weighing of individual clusters. Because lensing is sensitive to all mass, regardless of whether it emits light, it offers one of the few ways to map the distribution of dark matter across the cosmos. By comparing such maps with the locations of visible galaxies, researchers can test competing theories about how the universe evolved from its smooth early state into the clumpy structure we observe today. Future surveys, conducted with telescopes designed to image enormous patches of sky in fine detail, are expected to chart the shapes of billions of galaxies. From this immense catalogue, scientists hope to refine their estimate of how much matter the universe contains and to understand the mysterious force that appears to be driving its accelerating expansion. What began as an obscure prediction, dismissed even by its author as a curiosity, has thus become one of the most powerful tools in modern astronomy.