Academic Reading — Test 157

Reading passage

For most of human history, our knowledge of the heavens came entirely from the light we could see with our eyes. The development of radio astronomy in the twentieth century changed this profoundly, opening a window onto objects and processes that emit no visible light at all. The story began almost by accident in the early 1930s, when an American engineer named Karl Jansky was asked by his employer, a telephone company, to investigate the sources of static that interfered with transatlantic radio communication. After methodically recording signals for many months, Jansky identified a faint, persistent hiss that rose and fell on a daily cycle. He eventually concluded that it originated not from any terrestrial source but from the centre of our own galaxy, the Milky Way. His discovery is now regarded as the foundation of radio astronomy, although at the time it attracted surprisingly little interest from professional astronomers. The field remained a curiosity until after the Second World War, when wartime advances in radar technology gave scientists both the equipment and the expertise to build sensitive radio receivers. Researchers constructed ever larger dish-shaped antennas to collect and focus the extremely weak radio waves arriving from space. Because radio waves are far longer than waves of visible light, a single dish provides relatively poor resolution, meaning that fine detail is hard to distinguish. To overcome this limitation, astronomers learned to combine the signals from several widely separated antennas, a technique known as interferometry. By linking instruments many kilometres apart, they could achieve a level of detail that no single dish could match, and this approach remains central to the discipline today. The most celebrated achievement of radio astronomy came in 1965, and like Jansky's original finding, it owed a great deal to chance. Two researchers, Arno Penzias and Robert Wilson, were working at a laboratory in New Jersey with a large horn-shaped antenna originally built for satellite communication. As they prepared to study faint radio emissions from the galaxy, they were troubled by a persistent background noise that appeared to come from every direction in the sky, regardless of where they pointed the instrument. The signal did not vary with the time of day or the season, which ruled out the Sun and any single celestial object. Suspecting a fault in their equipment, the pair checked every connection, cooled their receiver to reduce internal interference, and even removed a pair of pigeons that had been nesting inside the antenna, scrubbing away the droppings the birds had left behind. Yet the mysterious hum persisted. The explanation, it turned out, lay not in the antenna but in the universe itself. A short distance away, a group of physicists at Princeton University had been predicting that if the universe had begun in an extremely hot, dense state and had been expanding and cooling ever since, a faint glow of radiation should still pervade all of space. This relic radiation, stretched and weakened over billions of years, would now be detectable as a faint signal of microwaves coming uniformly from every direction. When the two groups made contact, they realised that Penzias and Wilson had stumbled upon exactly this predicted glow. The noise they had tried so hard to eliminate was the cosmic microwave background, the oldest light in existence and a direct echo of the universe's fiery origins. The discovery had far-reaching consequences for our understanding of cosmology. For decades, scientists had debated two competing theories about the history of the universe. One held that the cosmos had begun at a definite moment in a colossal expansion, while a rival proposal argued that the universe had always existed in essentially the same state, with new matter being created continuously to fill the gaps left by expansion. The existence of a uniform background radiation was a natural prediction of the first model but had no comfortable place in the second. The detection of the cosmic microwave background therefore provided powerful evidence in favour of an expanding universe with a definite beginning, and within a few years the rival theory had largely been abandoned. In 1978, Penzias and Wilson were awarded the Nobel Prize in Physics for their accidental but momentous contribution. In the decades since, increasingly sophisticated instruments, including satellites placed above the distorting effects of the atmosphere, have mapped the background radiation in remarkable detail. These maps reveal that the glow is not perfectly smooth: it contains extremely subtle variations in temperature, differing by only a few parts in a hundred thousand. Faint though they are, these tiny ripples are immensely important, because they represent the slight unevenness in the early universe from which galaxies and clusters of galaxies eventually grew. What began as an unwanted hiss in a telephone engineer's receiver has thus become one of the richest sources of information about the structure and history of everything we can observe.