Chapter 10: Detecting Radiation from Space

rtist's concept of the completed Giant Magellan Telescope which will be situated in the Atacama Desert some 115 km (71 mi) north-northeast of La Serena, Chile.

Ground-based telescopes went through a renaissance in the 1990s and 2000s, after forty years where the Palomar 5-meter telescope (and a Russian 6-meter that was compromised by a poor location) were the largest in the world. A dozen optical telescopes of 8 to 10-meter aperture were constructed and are now in operation. The cost of a telescope scales faster than the square of the aperture, so doubling the size increases the cost by a factor of 5-6. The frontier for optical astronomy rests with a handful of projects to build 20 to 30-meter telescopes, each costing a billion dollars or more. The Giant Magellan Telescope (GMT) will have seven 8.4-meter mirrors arranged like flower petals around a central segment, giving it an equivalent light-gathering power to a 24.5-meter telescope. The mirrors are being cast and polished at the University of Arizona and the telescope is expected to have first light in Chile in 2028. The Thirty Meter Telescope (TMT) will have 492 individual mirror segments combining for an equivalent aperture of 30 meters. It is planned for Mauna Kea in Hawaii, with first light expected in 2029. Largest of all, the European Southern Observatory is building an Extremely Large Telescope (ELT), with segments giving an equivalent aperture of 39 meters, and first light anticipated for 2027. These behemoths are among the most ambitious technology projects humans have ever undertaken.

Main elements of the Pierre Auger Observatory: Surface Detector (1600 dots), Fluorescence Detector (rays of telescope views), and other extensions.

Using new technological advances, astronomers are hoping to detect not only the full spectrum of electromagnetic waves but also nature's messages from space that reach the Earth in other forms. For example, cosmic rays are high-energy charged particles produced in distant parts of the galaxy. Traveling at very close to the speed of light, they have energies up to a million times larger than can be produced in the largest particle accelerators. The highest energy cosmic rays are protons traveling close to the speed of light with the kinetic energy of a well-thrown baseball! As they reach the upper atmosphere, they hit atoms of our air and create showers of secondary particles. Optical radiation from these cascades can be collected by optical "concentrators" on high, dry mountaintops, thus allowing astronomers to monitor cosmic ray activity. The current premier facility is the Pierre Auger Observatory, located in Argentina near the Andes. The detector spans 3000 square kilometers, larger than Rhode Island, and 500 scientists from over 100 institutions work there or share the data collected. This is the first observatory large enough to collect exceptionally high energy, but very rare, cosmic rays.

Photograph of the inside of the MiniBooNE neutrino detector.

Astronomers have also begun trying to detect a tiny, ghostly particle called the neutrino. These tiny particles are produced in the cores of stars, in active galaxies, and as a relic from the early universe. Vast tanks of fluid are used as detectors, hoping to catch the light flash from the very occasional neutrino interaction with atomic particles in the fluid. The detectors have been placed deep under the sea, under mountains, and in mine shafts to shield them from signals due to other particle events. Neutrino astronomy began with an observation of neutrinos from the Sun, and it received an enormous boost with the detection of a burst of neutrinos from a distant supernova in the Large Magellanic Cloud in 1987. The field is in its infancy, and the current best method to create a large volume detector is to suspend it in ice. IceCube is located at the South Pole and it uses chains of instruments dropped down holes melted in the ice to create a detector volume of a cubic kilometer. When it is operating with two similar facilities, we will have the first global neutrino observatory. In 2018, the IceCube Observatory detected an extremely high-energy neutrino from a galaxy 3.7 billion light-years away, the first time that a neutrino had been detected from a distant object in space.

The LIGO Hanford Control Room. 

The Laser Interferometer Gravitational-Wave Observatory (LIGO) detects gravity waves by measuring the vibration or "ringing" of a large mass as the gravity wave passes through. It's the most precise physics experiment ever built, looking for a signal at a level of 1 part in 10¹⁵! The challenge is to rule out the inevitable vibrations associated with the restless Earth and to amplify the tiny astronomical signal so that it can be detected. LIGO has identical facilities in two geologically quiet areas: one in Washington State and the other in Louisiana. Having two detectors allows the coincidence in signals to confirm that it isn't just noise, and the timing difference gives some directional sensitivity. LIGO went through a successful engineering phase from 2002 to 2010 and achieved its first detection in 2016, a stunning success given the engineering challenge of the experiment. The universe is alive with the gravitational signals of astronomical catastrophes, from supernovae to collisions of neutron stars and black holes. Detecting these signals is one of the most exciting frontiers in astronomy.