Level 5 Reading 10A

Reading 10A

COSMIC DAWN High in the Andes Mountains, an array of telescopes larger than many cities reveals the secrets of the universe. An Eye on the Heavens Telescopes have come a long way since the first ones were invented early in the 17th century. Traditional telescopes allow astronomers to view objects in space thanks to the visible light those objects emit or reflect. However, for modern telescopes, any electromagnetic radiation will suffice for the purpose of viewing objects in space. Extremely hot objects, such as stars, emit not only visible light but also high-energy gamma radiation. Specialized telescopes—such as NASA's space-based Chandra X-ray Observatory— are built to detect such radiation. Cold objects—like comets and asteroids—emit low-energy radiation, which is invisible to the naked eye. Much of the universe is even colder than this. The clouds of dust and gas of which stars are made are only slightly warmer than absolute zero— the temperature at which atoms stop moving. To capture images of cold objects, astronomers use radio telescopes. A radio telescope is a device that typically uses a large dish antenna to collect the low-energy radiation emitted by objects in space. In the 1960s, astronomers started using them to view asteroids, planets, comets, and other objects. However, it was challenging to get a clear image of those objects using ground-based antennas because low-energy radiation is absorbed and distorted by water vapor in the Earth's atmosphere. The signal that a dish antenna on the ground finally receives is therefore weak. The signal can be strengthened by positioning the antenna on a site with very dry air, and it can be made even stronger by arranging several antennas in an array, combining their signals so that they function together as a single, more powerful

telescope. By the 1980s, several small arrays were operating in Japan, Europe, and the United States. Technological advances soon made much larger arrays possible, provided that a high, flat site could be found where the antennas could be set up. And if the antennas were portable, the distance between them could be adjusted to change the sensitivity of the telescope. Placed far apart, they could zoom in to focus on a small target, such as a planet. Grouping the antennas closer together would have the effect of zooming out, which would be useful for capturing images of an object as large as a galaxy. In the 1990s, astronomers began searching the world for the ideal place to set up a large array of antennas. The Perfect Location One May morning, two small trucks passed through the quiet town of San Pedro in Chile's Atacama Desert and headed up a mountainside. It was 1994, and the men inside the trucks were trying to find the highest, driest, flattest place on Earth. They had already spent a week and a half checking other locations in this desert. Now, guided by a map and a Chilean astronomer named Hernan Quintana, they were searching for the Chajnantor plateau. At 5,000 meters, it is almost as high as the two base camps used by climbers on Mount Everest (Qomolangma). The Atacama Desert is one of the driest places on Earth, with less than 1.2 centimeters of rain a year on average. The desert's remoteness and thin, dry air make the area ideal for observing the night sky. Astronomers from several countries were very interested in setting up observatories there. For the most part, these were observatories to view the fraction of the universe visible using the portion of the light spectrum that the human eye can see. Quintana and his companions were looking for a location to set up an array of dish antennas for a large radio telescope. Quintana felt that the Atacama Desert was likely the best possible place— but it wasn't easy to get to. "The trip was slow and painful, because the tires kept getting stuck in the sand," remembers Riccardo Giovanelli of Cornell University, one of the researchers who accompanied Quintana. When they finally arrived, the group was not disappointed. "The sky was beautiful—it was the deepest blue one can expect to see," remembered Giovanelli. "There was no doubt that somewhere nearby was the place." One of the astronomers brought along an instrument to measure water vapor. It was lower than any other place they had been. News of the discovery inspired a cooperative effort among astronomers in the United States, Europe, and Japan. Scientists in these countries began to realize that by working together, they could build a larger and more powerful array than any one of them could alone. In 1999, an agreement was signed to cooperate on a telescope project. Assembling the Array It would take more than 10 years to transform one of the world's loneliest spots into a busy modern observatory. Land mines planted decades before by the Chilean military had to be located and removed, and an oil company needed to be convinced to choose a different route for a planned pipeline that would have crossed the site. Prototype antennas were redesigned after testing in New Mexico, and new ones needed to be manufactured. The infrastructure for the installation had to be built from scratch, including many kilometers of service roads. The first of the dish antennas—12 meters in diameter and weighing more than a hundred tons— arrived from the United States at the Chilean port of Antofagasta in April 2007. Escorted by police cars, a truck carried the massive dish up the mountain, its progress occasionally interrupted by herds of llamas crossing the road. Over the next five years the dishes continued to arrive. Two specially made 28-wheel transporters—nicknamed Otto and Lore— stood ready to move the antennas to new locations on the plateau as needed. Setting the dishes up to work together required astonishing precision: They would need to turn together on command and point at the same target in the sky within a second and a half of one another. A massive supercomputer had to be installed on-site to coordinate their movements and interpret the signals received. Early Discoveries The installation was officially opened in March 2013 and named the Atacama Large Millimeterf/submillimeter Array, or ALMA for short. Even before it was completely set up, it had already produced results. The year before, with only 16 antennas in operation, researchers led by Caltech's Joaquin Vieira had used ALMA to view 26 distant galaxies. From the images they gathered, they were able to deduce that many stars in those galaxies were a billion years younger than astronomers had previously thought. Since the official opening of ALMA, there has been a steady stream of other discoveries of great interest to astronomers. In July 2013, the telescope's high-resolution images provided clues that may help answer a question that has long puzzled astronomers: Why are massive galaxies so rare in the universe? ALMA is also helping researchers understand how planets are born, by providing the first-ever images of the planet-forming process. One of the ALMA's most notable achievements is the role it played in creating the first image of a black hole. ALMA collaborated with six other observatories to form an array that functions as one Earth-sized telescope. This network collected and processed more than a petabyte of data while staring at a black hole in the distant Messier 87 galaxy. The image—a circular void surrounded by a ring of orange light—made headline news in April 2019. These achievements are just the beginning. In the future, ALMA will show us even finer details of galaxies and star systems. On a dry plateau a few kilometers from where shepherds once slept, our eyes will open upon an unseen universe.

ALMA'S TEN-MILE-WIDE ZOOM Elevation: 5,000 meters Rearranging antennas on the wide plateau is like adjusting a camera's zoom. When the antennas are far apart, the telescope focuses on tight sections of the sky and fine details. Grouping the antennas closer together is like using a wide-angle lens and takes in wider areas of the sky. ALMA consists of two telescope arrays working together. Two massive transporters relocate antennas in the main array with submillimeter precision. The Morita Array, a separate group of 16 antennas built by Japan, targets large-scale structures in the universe.

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