New findings by NANOGrav, the North American Nanohertz Observatory for Gravitational Waves, could become the “discovery of the century” if the data has been generated by a network of giant filaments –cosmic strings–left over from the birth of the universe.
Invisible Threads of Pure Energy
One of the most controversial theories of the 21st century, commonly referred to as a “theory of everything,” string theory suggests that the universe is a reflections of pointlike elementary particles, extremely tiny theoretical one-dimensional loops of vibrating strings seen mathematically as invisible threads of pure energy, thinner than an atom but light-years long at the Planck length of 10 ^-33 cm, a billion billion times smaller than a proton that carry mass and electric currents. Their extremely high density, thought to have formed when regions of spacetime with different properties contacted each other in the first split second after the Big Bang when the universe underwent a rapid expansion, then cooled rapidly, may have caused a phase change in the quantum fields, like water freezing to ice.
Quantum Phase Changes in the Early Universe
These quantum phase changes in the early universe –the idea that the universe, symmetrical at its start, lost symmetry and acquired defects as it expanded and cooled– said theorist Tom Kibble, who created the idea of cosmic strings, one-dimensional defects that stretch like strings across the universe in 1976, “would have caused the fields to align in different orientations, again causing cracks – cosmic strings that would only appear at the edges of vast regions about as big as the observable universe.”
The enormous amount of energy they contain would also makes them extremely heavy –a few centimeters might weigh as much as Mount Everest. Cosmic strings, however, were particularly problematic said Kibble, who wrote that “looking for cosmic strings directly would be pointless.”
Fantastic Energies –Billion Times Greater than LHC
A contrary conjecture suggests that the strings might have been formed with too low an energy to give off any detectable signals, or that ancient cosmic strings radiated away their energy and faded to nothingness too quickly after the Big Bang. “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. “You can’t build an accelerator to test physics at that scale.” says Ken Olum, a theoretical physicist at Tufts University who studies 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, observes Olum, are a potential source of a stochastic background of gravitational waves that may be detected by pulsar timing arrays or observatories such as LIGO.
In 2016, an enigmatic snake-like object, perhaps a cosmic string –galaxy-sized filaments of raw energy, that may be threaded through spacetime–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 was detected by astrophysicist Farhad Yusef-Zadeh of Northwestern University who reported the unusual filament using the NSF’s Karl G. Jansky Very Large Array (VLA). The filament could have profound implications for understanding gravity, space-time and the universe itself.
“But we still have more work to do to find out what the true nature of this filament is,” says Mark Morris at UCLA, who led the study and uses radio, infrared, and X-ray observatories to study the Galactic Center. Theorists had predicted that cosmic strings, if they exist, would migrate to the centers of galaxies, where it might be captured once a portion of the string crosses the event horizon of the black hole.
A Game Changer? Pulsar Data from NANOGrav
Now new pulsar data from NANOGrav, suggests the presence of cosmic strings. Pulsars exotic, dead stars, beam out pulses of radio waves from their poles, so that from Earth we see regular flashes each time the beams sweep by our line of sight, like the flashes of a lighthouse, known as a Pulsar Timing Array, or PTA. Subtle aberrations in the clockwork blinking of these dead, relic stars could become “the result of the century” if the distortions are produced by a network of giant filaments left over from the birth of the universe.
When their PTAs are distorted, physicists know something is up. NANOGrav uses the Milky Way to detect gravitational waves with the help pulsars — In particular, the researchers look for distortions caused by gravitational waves — ripples in space-time that, when they pass through the pulsars, change the blips’ arrival time on Earth. These gravitational waves new conjectures suggest could come from the vibrations of cosmic strings –“melodies” that can be written on strings corresponding to the laws of chemistry– collisions of supermassive black holes, or other violent cosmic processes.
NANOGrav’s latest analysis, reports Quanta, posted online on Sept. 9, “pulls together more than 12 years of observations of dozens of pulsars from radio telescopes dotted across North America. The paper is still being peer-reviewed, but the researchers found that something was distorting the blips emitted by all of the pulsars in the same way, and with frequencies that are expected of gravitational waves. It remains possible that this pattern is instead coming from some unknown, common source of noise in the pulsars, or in the clocks that measure the blips’ arrival on Earth.”
“We cannot say for sure if it is noise or it’s a gravitational wave signal,” said Alberto Sesana, former chair of the International Pulsar Timing Array, a consortium of projects including NANOGrav.
NANOGrav’s paper provoked a speedy rejoinder among physicists who work on cosmic strings, with a paper coauthored with Vedran Brdar, “Has NANOGrav found first evidence for cosmic strings?” arguing that the data could be interpreted as coming from cosmic strings, if the strings were created when the universe had a certain ultra-high temperature. This temperature “will always ring a bell” among physicists, paper co-author, Kai Schmitz said, because it’s the point at the Big Bang which the strong, weak and electromagnetic forces are believed to have been unified.
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