An unknown object, perhaps a cosmic string, was detected at the Milky Way’s galactic center in 2016 that could have profound implications for understanding gravity, space-time and the universe itself. Cosmic strings, galaxy-sized filaments of raw energy, may be threaded through spacetime, according to some theories. At the Big Bang, our universe exploded into being, expanded at a fantastic speed and cooled, perhaps cracking the fabric of the universe with hairline fractures.
These cosmic strings, seen mathematically as invisible threads of pure energy, thinner than an atom but light-years long. The huge amount of energy they contain would also makes them extremely heavy –a few centimeters might weigh as much as Mount Everest.
However, the strings might have been formed with too low an energy to give off any signals “detectable in the near future”. or the possibility that ancient cosmic strings radiated away their energy and faded to nothingness too quickly after the Big Bang to have left a lasting impression, reports Cathal O’Connell in Cosmos.
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“The fact that strings come up all the time makes me confident that they exist,” says Thibault Damour, a theoretical physicist at the Institute of Advanced Scientific Studies near Paris, who focuses mainly on the physics of black holes and string theory cosmology.
However, writes O’Connell, as time capsules of the early universe, these vanishingly thin, intergalactic filaments should retain fantastic energies – more than a billion times greater than those released by smashing particles at the Large Hadron Collider, says Ken Olum, a theoretical physicist at Tufts University. “You can’t build an accelerator to test physics at that scale.”
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Olum’s research is the study of cosmic strings, in particular observable effects, such as gravitational waves, that might enable us to detect the cosmic string network, if it exists. Cosmic strings are one of the potential sources of a stochastic background of gravitational waves that may be detected by pulsar timing arrays or observatories such as LIGO. With collaborators he has developed a large-scale simulation of cosmic strings.
For some physicists, like Columbia University’s Peter Woit, a theory that can’t be tested is not even wrong,” viewing cosmic strings in the same category as “string theory” at the other extreme of the size scale. String theory invokes vibrating strings tinier than any subatomic particle as the building blocks of the universe.
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For Matthew Bailes, an astrophysicist at Swinburne University of Technology in Melbourne, cosmic strings are a “mathematical curiosity” or worse, “an exotic fantasy”. Cosmic strings sit on the shelf alongside other beautiful ideas that could complete our understanding of the universe, but lack empirical support. “This is the beauty and the danger of physics,” Damour says. “Sometimes things exist that we can never see.”
The possibility that the discovery of a snake-like object lurking at the chaotic center of the Milky Way near its supermassive black hole could be a cosmic string, would provide the first evidence for a highly speculative theory with profound implications for understanding gravity, space-time and the universe itself.
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Cosmic strings are theoretical one-dimensional loops thought to have formed when regions of spacetime with different properties contacted each other during the very early stages of the universe. They are thought to be as thin as a proton, but with an extremely high density. As such they have very high gravitational forces and can act as cylindrical gravitational lenses, producing the double images of galaxies. It may be possible to discover cosmic strings by looking for pairs of identical galaxies.
British field theorist Tom Kibble, who died in June 2016, created the idea of cosmic strings in 1976. He was musing, writes O Connell, “about the first split second after the Big Bang when the universe underwent a rapid expansion, then cooled rapidly. This, he suggested, caused a phase change in the quantum fields, like water freezing to ice. Kibble conjectured that the quantum phase changes in the early universe would have caused the fields to align in different orientations, again causing cracks – cosmic strings.
“Kibble’s predicted the existence of a fundamental particle that imparts mass to all others, now known as the Higgs boson –a discovery of that in 2012 won the Nobel prize.
Kibble spent much of his later career contemplating how, after the Big Bang, the universe cooled and went through successive phase transitions (when a medium changes form, such as liquid freezing into solid). Most significant, reports The New York Times, he predicted that topological defects would emerge in the universe at each phase transition, similar to the cracks that form when water freezes.
“Cosmic strings, however, were particularly problematic to put to the test. They would only appear at the edges of vast regions about as big as the observable universe. That is why, in Kibble’s original 1976 scheme, he wrote that ‘looking for cosmic strings directly would be pointless.’
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Fast forward to 2016: Farhad Yusef-Zadeh of Northwestern University reported the discovery of an unusual filament near the center of the Milky Way Galaxy using the NSF’s Karl G. Jansky Very Large Array (VLA). The filament is about 2.3 light years long and curves around to point at the supermassive black hole, called Sagittarius A* (Sgr A*), located in the Galactic center.
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Another team of astronomers employed a pioneering technique to produce the highest-quality image yet obtained of this curved object. “With our improved image, we can now follow this filament much closer to the Galaxy’s central black hole, and it is now close enough to indicate to us that it must originate there,” said Mark Morris of the University of California, Los Angeles, who led the study and uses radio, infrared, and X-ray observatories to study the Galactic Center “However, we still have more work to do to find out what the true nature of this filament is.”
The team’s three main explanations for the filament
The first is that it is caused by high-speed particles kicked away from the supermassive black hole. A spinning black hole coupled with gas spiraling inwards can produce a rotating, vertical tower of magnetic field that approaches or even threads the event horizon, the point of no return for infalling matter. Within this tower, particles would be sped up and produce radio emission as they spiral around magnetic field lines and stream away from the black hole.
The second, more fantastic, possibility is that the filament is a cosmic string, theoretical, as-yet undetected objects that are long, extremely thin objects that carry mass and electric currents. Previously, theorists had predicted that cosmic strings, if they exist, would migrate to the centers of galaxies. If the string moves close enough to the central black hole it might be captured once a portion of the string crosses the event horizon.
The final option is that the position and the direction of the filament aligning with the black hole are merely coincidental superpositions, and there is no real association between the two. This would imply it is like dozens of other known filaments found farther away from the center of the Galaxy. However, such a coincidence is quite unlikely to happen by chance.
Each of the scenarios being investigated would provide intriguing insight if proven true. For example, if the filament is caused by particles being ejected by Sgr A*, this would reveal important information about the magnetic field in this special environment, showing that it is smooth and orderly rather than chaotic.
The second option, the cosmic string, would provide the first evidence for a highly speculative theory with profound implications for understanding gravity, space-time and the Universe itself.
Evidence for the idea that particles are being magnetically kicked away from the black hole would come from observing that particles further away from Sgr A* are less energetic than those close in. A test for the cosmic string idea will capitalize on the prediction by theorists that the string should move at a high fraction of the speed of light. Follow-up observations with the VLA should be able to detect the corresponding shift in position of the filament.
Even if the filament is not physically tied to Sgr A*, the bend in the shape of this filament is still unusual. The bend coincides with, and could be caused by, a shock wave, akin to a sonic boom, where the blast wave from an exploded star is colliding with the powerful winds blowing away from massive stars surrounding the central black hole.
“We will keep hunting until we have a solid explanation for this object,” said co-author Miller Goss, from the National Radio Astronomy Observatory in Socorro, New Mexico. “And we are aiming to next produce even better, more revealing images.”
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With the dawn of the era of gravitational wave astronomy we may finally be able to test the existence of cosmic strings. Gravitational wave detectors such as LIGO and Virgo might be able to hear the high-pitched thrums and snaps created as cosmic strings whip through space. In September 2015 the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves reverberating from colliding black holes, adding a new dimension to astronomers’ ability to scan the universe. “After LIGO’s discovery,” Damour says, “I immediately thought, ‘Aha! Now it would be good if cosmic strings were detected.’”
“What happens is like a whip,” explains Damour, who worked out the idea with Vilenkin in 2000. A cosmic string loop wiggles and bounces, some parts would be whipped up to the speed of light – and emit a burst of gravitational waves. The two physicists, reports Cathal O’Connell calculated such a burst might be detectable by LIGO but the difficulty in detecting the crack is that it would only be emitted in a particular direction, like the beam of a flashlight. LIGO would have to be right in the path of the beam.
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The next level up in the search for cosmic strings, and perhaps our only hope of a definitive answer, will come with the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector due to launch in 2034, which will listen to the frequency band between the high-pitched chirps caught by LIGO and the sub-bass murmurs to which pulsar timing arrays are attuned.
Read Cathal O;Donnell’s “Cracks in the Universe” here
The Daily Galaxy, Sam Cabot, via Harvard CfA and Cosmos
