Academic Reading — Test 71

Reading passage

For much of the history of astronomy, scientists assumed that the matter they could observe directly — stars, planets, glowing clouds of gas — accounted for almost everything in the universe. During the twentieth century, however, a series of careful measurements forced a radical revision of this picture. When astronomers measured how quickly stars orbited within galaxies, and how galaxies themselves moved within large clusters, they discovered far more gravitational pull than the visible material could possibly produce. To explain the discrepancy, they proposed the existence of an unseen substance, now called dark matter. It emits no light and does not interact with ordinary radiation in any detectable way, which means it cannot be photographed by even the most powerful telescopes. Estimates suggest that this invisible component outweighs ordinary matter by roughly five to one, yet its precise nature remains one of the deepest puzzles in modern physics. The difficulty of studying something that cannot be seen might appear insurmountable, but astronomers have developed an ingenious indirect method. Their chief tool rests on a prediction made by Albert Einstein in his general theory of relativity, published in 1915. Einstein argued that mass does not merely attract other mass; it actually distorts the very fabric of space and time around it. A ray of light passing close to a massive object therefore does not travel in a perfectly straight line but follows a curved path, much as a marble would roll along a stretched rubber sheet that has been weighed down in the centre. This bending of light by gravity is known as gravitational lensing, because the massive object behaves rather like a lens in a pair of spectacles, redirecting and concentrating the light that passes through it. The consequences of lensing can be dramatic. When a distant galaxy lies almost directly behind a massive foreground cluster, its light is bent into long, curved streaks or even complete rings, and a single background object may appear several times over in different positions. Crucially, the degree of distortion depends only on the total mass of the foreground object, regardless of whether that mass shines or stays dark. By measuring precisely how much the background images have been stretched, displaced or multiplied, astronomers can calculate how much matter must be present to cause the observed bending. In many clusters this calculation reveals far more mass than the visible galaxies and hot gas can account for, providing some of the strongest evidence that dark matter is real rather than a mere accounting error. Lensing comes in more than one form, and each variety serves a different purpose. The spectacular arcs and multiple images described above are examples of strong lensing, which occurs only where the foreground mass is exceptionally concentrated. Far more common is weak lensing, in which the distortion of any individual background galaxy is so slight that it cannot be noticed on its own. To exploit this subtle effect, astronomers photograph thousands or even millions of faint galaxies and measure the average way their apparent shapes are stretched. Because the underlying galaxies are oriented randomly, any consistent stretching across a region of sky must be caused by intervening mass. Through careful statistical analysis, researchers can therefore reconstruct a map showing how dark matter is distributed, even where no light betrays its presence. These techniques have transformed cosmology from a largely descriptive science into a precise, quantitative one. One celebrated study examined a pair of colliding galaxy clusters in which the hot gas, slowed by the collision, had separated from the bulk of the mass detected through lensing. The mass had sailed onward almost undisturbed, exactly as expected if most of it consisted of dark matter that barely interacts with anything. Surveys covering large areas of the sky now use weak lensing to trace the cosmic web, the vast network of filaments and voids along which galaxies are strung. The shape of this web is sensitive to the total quantity of matter and to the way the universe has expanded over billions of years, so lensing maps allow cosmologists to test competing theories with growing confidence. For all its power, the method has limitations that researchers must treat with care. The signal is weak, the instruments must be calibrated to extraordinary accuracy, and the blurring effects of the atmosphere and of the telescope itself can mimic or mask the genuine distortion. A small systematic error in measuring galaxy shapes can translate into a large error in the inferred mass, so collaborations devote enormous effort to checking their procedures. Even so, gravitational lensing remains the most direct means available for charting matter that emits no light, and forthcoming telescopes, both on the ground and in orbit, promise maps of unprecedented detail. In the coming decades, astronomers hope that these maps will finally reveal not just where dark matter lies, but what it actually is.