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# 10.30 Frontier Observatories

These are exciting times for astronomy. Large new telescopes are being built, and NASA has completed the launch of a series of space telescopes that covered the electromagnetic spectrum from the far infrared to gamma rays and the agency is now entertaining proposals for ambitious new projects (limited, as always, by a budget of roughly \$18 billion that is flat or declining in real terms). The narrow visual window has been opened wide, giving access to electromagnetic information over a factor of 1015 in wavelength. Astronomers are often attracted to wilderness frontiers, far from mainstream civilization. Visionaries are already thinking of the next step forward.

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 22-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 2019. 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 2024. Largest of all, the European Southern Observatory plans an Extremely Large Telescope (ELT), with segments giving an equivalent temperature of 39 meters, and first light anticipated for 2024. These behemoths are among the most ambitious technology projects humans have ever undertaken.

Space telescopes as large as 30 to 100 meters are being considered. Such a facility would have over 2000 times the light-gathering power of the Hubble Space Telescope! The construction of such a large mirror depends on the use of thin but rigid materials that can be shaped with an accuracy much finer than the thickness of a human hair. Observatories on the Moon offer great advantages for most types of observation. Optical and infrared telescopes on the Moon could collect data in conditions of excellent darkness, with low backgrounds and no atmosphere to distort the incoming images. Radio telescopes could work free from man-made interference on the dark side of the Moon. Plans for robotic lunar observatories have been made, with possible deployment soon after a Moon base is established, within 20-30 years.

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.

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. Ice Cube 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 in a couple of year, we will have the first global neutrino observatory.

Perhaps the most exotic signal from space is a gravity wave. General relativity, Einstein's theory of gravity, predicts that rapid motions and changes in the state of matter should lead to the emission of gravity waves. Gravity waves are like ripples in the structure of space. They travel at the speed of light. The U.S. Congress funded an ambitious project to detect gravity waves; they are important because they open a new window on the universe that is completely independent of all the diverse types of electromagnetic radiation. Gravity waves are created by the death of stars, by the interactions of black holes, and by the behavior of the entire universe when it was small and dense billions of years ago.

The Laser Interferometer Gravitational Wave Observatory (LIGO) will try to detect 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 singals to confirm that it isn’ just noise, and the timing difference gives some directional sensitivity. LIGO went through a successful engineering phase from 2002 to 2010 and will start taking science data soon. The universe is alive with the gravitational signals of astronomical catastrophes, from supernovae to collisions of neutron stars. Detecting these signals is one of the most exciting frontiers in astronomy.