The inflationary expansion of the early universe in the moments immediately following the Big Bang is believed to have occurred at faster-than-lightspeed. This may indicate higher-than-expected strength of the gravitational waves that occurred during the early Big Bang. The value of a key parameter r has been estimated to be 0.2, which is significantly higher than the 0.11 which had been predicted or expected. This approach is especially well-suited to detecting low-frequency waves produced by the merging of supermassive black holes-among the most energetic events in the cosmos, and yet so far invisible to us.The B-mode polarization of the Cosmic Microwave Background has been measured by the BICEP2 experiment from Antarctica, revealing important details about the early moments of the Big Bang, and giving strong validation of the inflationary universe theories: They are monitoring groups of pulsars, stellar remnants that emit extremely repetitive pulses of radiation, and searching for subtle time shifts caused by passing gravitational waves ( right). Four teams are currently attempting an even grander effort to scan for gravitational waves across interstellar distances. The European Space Agency’s Laser Interferometer Space Antenna, or LISA, will use three laser spacecraft ( middle) flying in formation to create an interferometer in which each arm is 2.5 million kilometers long. Other efforts will extend the search for gravitational wave searches beyond Earth. Further ahead, a European consortium is studying the underground Einstein Telescope ( concept drawing, left), which would vastly surpass LIGO by using nested laser arrays in a triangular configuration. A planned third LIGO facility in India will boost the sensitivity and precision of the combined facility starting in the mid-2020s. Japan’s Kamioka Gravitational Wave Detector (KAGRA), scheduled to begin operation this year, is a LIGO-like interferometer built underground for greater isolation. Next-generation detectors will broaden the reach of gravitational-wave astronomy. And analysis of the stretching of gravitational waves is providing a novel way to measure the expansion rate of the universe. One LIGO event was produced by colliding neutron stars, exposing new details of how this explosive process seeds space with heavy elements. Five of the events detected so far came from merging black holes-all of them more massive than the known stellar black holes in our galaxy, for unclear reasons. The discovery of gravitational waves is already yielding major astronomical insights. A forlorn Zenneck lamented, “All attempts to connect gravitation with other phenomena in a satisfying way are to be regarded as unsuccessful or as yet not adequately established.” However, none of the schemes mentioned in the encyclopedia article-including Gerber’s-resembled a modern relativistic theory of gravity. One modified theory of gravity, devised by the German physicist Paul Gerber (1854–1909), astoundingly gave the correct answer for Mercury’s movements. Some proposals were designed to explain the notorious riddle of Mercury’s perihelion shift: The longitude of the planet’s closest approach to the Sun kept advancing by the small but mysterious angle of 43 seconds of arc per century, and no known Newtonian forces could account for it. Heaviside’s work does not appear to have made much of an impression on Zenneck, who relegates it to a footnote, but Zenneck does describe the work of several other contemporaries who likewise assumed that gravitational effects propagated at the speed of light.
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