Free Astronomy Magazine March-April 2016

COSMOLOGY Hemisphere. According to general relativity, a pair of black holes orbit- ing around each other lose energy through the emission of gravitation- al waves, causing them to gradually approach each other over billions of years, and then much more quickly in the fi- nal minutes. During the final fraction of a sec- ond, the two black holes collide into each other at nearly one-half the speed of light and form a single more mas- sive black hole, converting a portion of the combined black holes’ mass to energy, accor- ding to Einstein’s formula E=mc 2 . This ener- gy is emitted as a final strong burst of grav- itational waves. It is these gravitational waves that LIGO has observed. The existence of gravitational waves was first demonstrated in the 1970s and 80s by Joseph Taylor, Jr., and colleagues. Taylor and Russell Hulse discovered in 1974 a binary sys- tem composed of a pulsar in orbit around a neutron star. Taylor and Joel M. Weisberg in 1982 found that the orbit of the pulsar was slowly shrinking over time because of the release of energy in the form of gravitation- al waves. For discovering the pulsar and show- ing that it would make possible this parti- cular gravitational wave measurement, Hul- se and Taylor were awarded the Nobel Prize in Physics in 1993. The new LIGO discovery is the first observa- tion of gravitational waves themselves, made by measuring the tiny disturbances the waves make to space and time as they pass through the Earth. “Our observation of gravitational waves ac- complishes an ambitious goal set out over 5 decades ago to directly detect this elusive phenomenon and better understand the uni- verse, and, fittingly, fulfills Einstein’s legacy on the 100 th anniversary of his general theory of relativity,” says Caltech’s David H. Reitze, ex- ecutive director of the LIGO Laboratory. The discovery was made possible by the en- hanced capabilities of Advanced LIGO, a T he Laser Inter- ferometer Gravitational- Wave Observa- tory (LIGO) consists of two widely separated installations with- in the United States, one in Hanford, Washing- ton (left), and the other in Living- ston, Louisiana (below), operated in unison as a sin- gle observatory. [LIGO Laboratory] major upgrade that increases the sensitivi- ty of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe pro- bed—and the discovery of gravitational waves during its first observation run. The US National Science Foundation leads in financial support for Advanced LIGO. LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and re- search institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing mem- bers of the collaboration. “This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality,” says Gabriela Gonza ́ lez, LSC