One of the universe’s strangest conundrums is the smoothness of the early cosmos following the Big Bang and how clumpy things like galaxies could have formed suddenly from it. The answer could be cosmic strings.
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Some cosmologists are embracing the possibilities. Edward Witten of Princeton University, one of the world’s foremost theoretical physicists, says, “Strings of different sizes and kinds probably exist.” Thirty years ago, Witten opposed string theory. He now believes these tiny stringlike loops of energy could be the universe’s basic form of matter and energy and that some strings could reach enormous sizes.
The current incarnation of the theory suggests cosmic strings arose after the inflationary period. The strings researchers currently propose are less massive and more stable than the ones originally thought up in the 1980s. Because of these changes, they would have less effect on the cosmos than astronomers originally thought, so they would not necessarily be ruled in or out of existence by recent observations
With the reformulation of what astronomers think strings might be, the question of whether they can be detected still hangs out there. Two research teams have reported evidence of cosmic strings in different parts of the sky, but these observations are unconfirmed. According to Alexander Vilenkin of Tufts University, however, who pioneered cosmic string theory by suggesting strings could have triggered the formation of galaxies, the discoveries have “breathed new life into this field.”
The current thinking on cosmic strings goes as follows: When inflation occurred, “cracks” in the universe’s phase transition arose, and these cracks created thin, superdense strings of matter and energy. These features might have formed like fissures in ice, along faults between transition zones. These sinewy filaments of matter might forever be frozen in a primordial state, having avoided the cosmic inflation the rest of the universe experienced.
If they exist, cosmic strings are almost unimaginably thin, yet they possess nearly unlimited length. Strings also may be incredibly dense, much denser than the matter at a neutron star’s center. With such density, cosmic strings would act as gravitational lenses if they floated in front of bright background objects, and this could be one way to find one. Yet spotting a long cosmic string could be incredibly difficult: Computer simulations suggest they would be spaced about 325 million light-years apart. The nearest long cosmic string might be 10,000 light-years away.
The possibility of detecting a cosmic string by lensing exists, but another avenue may offer better odds. Since 2015, astronomers have been looking for bursts of gravitational waves associated with cosmic strings using gravitational wave observatories such as LIGO and Virgo, though they have not yet found any evidence. “Cosmic strings might actually provide the best observational window into fundamental string theory,” says Thomas Kibble of Imperial College London. With a rebirth of study and credibility, cosmic strings will carry on as a hot topic.