NEW YORK - Astronomers announced on Monday that they had discovered what many consider the holy grail of their field: ripples in the fabric of space-time that are echoes of the massive expansion of the universe that took place just after the Big Bang.Predicted by Albert Einstein nearly a century ago, the discovery of the ripples, called gravitational waves, would be a crowning achievement in one of the greatest triumphs of the human intellect: an understanding of how the universe began and evolved into the cornucopia of galaxies and stars, nebulae and vast stretches of nearly empty space that constitute the known universe."This detection is cosmology's missing link," Marc Kamionkowski, a physicist of Johns Hopkins University and one of the researchers on the collaboration that made the finding, told reporters on Monday at a press conference at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.Gravitational waves are feeble, primordial undulations that propagate across the cosmos at the speed of light. Astronomers have sought them for decades because they are the missing evidence for two theories.One is Einstein's general theory of relativity, first published in 1915, which launched the modern era of research into the origins and evolution of the cosmos. The general theory explains gravity as the deformation of space by massive bodies. Einstein posited that space is like a flimsy blanket, with embedded stars and planets causing it to curve rather than remain flat.Those curvatures of space are not stationary, Einstein said. Instead, they propagate like water in a lake or seismic waves in Earth's crust and so are "gravitational waves" that "alternately squeeze space in one direction and stretch it in the other direction," Jamie Bock, a physicist at the California Institute of Technology in Pasadena and one of the lead scientists on the collaboration, told Reuters.The other, much more recent theory that predicted gravitational waves is called cosmic inflation. Developed in the 1980s, it starts with the well-accepted idea that the universe began in a Big Bang, an explosion of space-time, 13.8 billion years ago.An instant later, according to the theory, the infant cosmos expanded exponentially, inflating in size by 100 trillion trillion times. That made the cosmos remarkably uniform across vast expanses of space and also super-sized tiny fluctuations in gravity, producing gravitational waves. Although the theory of cosmic inflation received a great deal of experimental support, the failure to find the gravitational waves it predicted caused many cosmologists to hold off endorsing it.That may change after the announcement on Monday."These results are not only a smoking gun for inflation, they also tell us when inflation took place and how powerful the process was," Harvard University physicist Avi Loeb said in a statement. The strength of the gravitational waves' signal is tied to how powerfully the universe expanded during the brief era of inflation.The measurements announced by the astronomers on Monday are nearly twice as large as cosmologists predicted for gravitational waves, suggesting a great deal more could be learned about how inflation worked.SOUTH POLE TELESCOPEThe gravitational waves were detected by a radio telescope called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization). The instrument, which scans the sky from the South Pole, examines what is called the cosmic microwave background, the extremely weak radiation that pervades the universe. Its discovery in 1964 by astronomers at Bell Labs in New Jersey was hailed as the best evidence to date that the universe began in an immensely hot explosion.The microwave background radiation, which has been bathing the universe since 380,000 years after the Big Bang, is a mere 3 degrees above absolute zero, having cooled to near non-existence from the immeasurably hot plasma that was the universe in the first fractions of a second of its existence. The background radiation is not precisely uniform. And like light, the relic radiation is polarized as the result of interacting with electrons and atoms in space.Computer models predicted a particular curl pattern in the background radiation that would match what would be expected with the universe's inflation after the Big Bang."It's mind-boggling to go looking for something like this and actually find it," Clem Pryke, a physicist at the University of Minnesota and another lead scientist on the collaboration, told reporters. "Theorists are forever sending the experimentalists on wild goose-chase missions. When we first saw hints of a signal we totally didn't believe it."It will be up to other teams of scientists, working with an array of Earth-based, balloon-launched and space telescopes, to verify the findings."This is the smoking gun for inflation," Kamionkowski said. But even if the results hold up, "we've learned only that inflation has sent us a telegram, encoded on gravitational waves and transcribed on the cosmic microwave background sky."It will be essential, he added, "to follow through with more detailed and precise measurements to infer fully what this telegram is telling us."QUANTUM HINTSThe detection of gravitational waves may help physicists realize a dream of Einstein's that he died before achieving: unifying all the forces of nature.Three of the four forces have been unified, which means that physicists have shown that they are facets of the same basic force. But the fourth, Einstein's beloved gravity, remains the odd man out: it seems to be a property of space rather than a consequence of subatomic, or quantum, particles as the other forces are.The three quantum-based forces are electromagnetism, the weak nuclear force (responsible for radioactivity) and the strong nuclear force (which glues together the protons and neutrons in atomic nuclei).Because cosmic inflation was powered by quantum effects, with the universe springing from a volume smaller than a subatomic particle, primordial gravitational waves were also created by quantum processes, cosmologists believe. If so, then by scrutinizing the gravitational waves that pervade today's cosmos, scientists might finally show that all four forces of nature arise from a single uber-force, achieving Einstein's dream.