Academic Reading — Test 75

Reading passage

When astronomers wish to understand how the universe began, they cannot travel back in time to witness its earliest moments. Instead, they study the faint signals that have travelled across space for billions of years. The most important of these signals is the cosmic microwave background, often shortened to the CMB. This is a faint glow of radiation that fills the entire sky in every direction. Although it is invisible to the human eye, it can be detected by specialised instruments that are sensitive to microwaves. The discovery and study of this radiation has provided some of the strongest evidence for the theory known as the Big Bang. According to the Big Bang model, the universe began roughly 13.8 billion years ago in an extremely hot and dense state. In its first moments, it was so hot that ordinary atoms could not exist. Instead, matter consisted of a dense soup of charged particles, and light was constantly scattered by these particles, so it could not travel freely. As the universe expanded, it gradually cooled. After about 380,000 years, the temperature had fallen far enough for electrons and protons to combine into neutral atoms. At this point, light was suddenly able to move through space without being blocked. The radiation released during this event has been travelling ever since, and it is this ancient light that we now observe as the cosmic microwave background. The radiation was first detected by accident in 1965 by two American scientists, Arno Penzias and Robert Wilson. They were testing a large antenna and noticed a persistent hum of background noise that they could not eliminate. At first they suspected that the noise came from a fault in their equipment, or even from the droppings of pigeons that had nested inside the antenna. However, after careful checking, they realised that the signal came from every part of the sky and could not be removed. Quite separately, a group of physicists nearby had predicted that such radiation should exist as a leftover from the early universe. The two teams soon became aware of each other's work, and the noise was recognised as the predicted glow. Penzias and Wilson were later awarded a Nobel Prize for their finding. One of the most striking features of the cosmic microwave background is its remarkable uniformity. Wherever astronomers look, the temperature of the radiation is almost exactly the same, at about 2.7 degrees above absolute zero. This smoothness is itself powerful support for the Big Bang, because it suggests that the early universe was extremely even in its distribution of matter and energy. Yet the radiation is not perfectly uniform. Extremely small variations in temperature, known as anisotropies, can be measured across the sky. These tiny differences, amounting to only a few parts in a hundred thousand, are believed to mark the regions where matter was slightly more concentrated. Over billions of years, gravity pulled material into these denser regions, eventually forming the galaxies and clusters of galaxies that we see today. To study these faint variations in detail, scientists have launched a series of satellites above the atmosphere, where measurements are not disturbed by the air or by heat from the ground. The first of these, named COBE, confirmed in the early 1990s that the radiation matched the predictions of the Big Bang with great precision. Later missions, including the WMAP satellite and the European Planck spacecraft, produced increasingly detailed maps of the temperature variations. By analysing the size and pattern of these variations, researchers have been able to estimate the age of the universe, its overall composition, and the rate at which it is expanding. The Planck mission, in particular, supplied figures that are still widely used by cosmologists today. The cosmic microwave background therefore acts as a kind of photograph of the universe in its infancy, capturing it as it was only a few hundred thousand years after it began. No rival theory of the universe's origin has been able to explain the existence and precise properties of this radiation as successfully as the Big Bang model. For this reason, the study of the CMB remains a central part of modern cosmology. As instruments continue to improve, scientists hope to learn even more about the conditions that prevailed in the first fraction of a second of cosmic history, a period that still holds many unanswered questions.