In 2017, an international group of astronomers and physicists excitedly reported the first simultaneous detection of light and gravitational waves from the same source–a merger of two neutron stars. In the world of astrophysics, Aug. 17, 2017, was a red-letter day. “This is a game-changer for astrophysics,” said UC Santa Barbara faculty member Andy Howell, who leads the supernova group at the Las Cumbres Observatory (LCO). “A hundred years after Einstein theorized gravitational waves, we’ve seen them and traced them back to their source to find an explosion with new physics of the kind we’ve only dreamed about.”
In an email to The Daily Galaxy, Howell wrote: “There are a few big areas where we have gaps in our knowledge of astrophysics because we can’t reproduce the right conditions on Earth. One is the synthesis of the heaviest, neutron-rich elements. We think the merger of these neutron stars produces the heaviest elements on the periodic table.
“There are still many questions though,” Howell explained. “How much do other processes like supernovae contribute? How does the relative size of the two neutron stars affect the production of these elements? For these newly synthesized elements, we also only have a rough idea of how they block and transmit light. In a supernova we can just read off the elements present by their characteristic absorption lines we see in the spectra. But in a kilonova we don’t know this stuff from first principles because the physics is too complicated.”
Unknown –the dividing line between neutron stars and black holes
“And finally,” Howell concludes in his email, “we don’t really know what neutron star matter behaves like at the extremes. We don’t know the dividing line between neutron stars and black holes. Is something that weighs 2.7 solar masses a neutron star or a black hole? By studying these mergers we can get some information about the ratio of pressure to density in a neutron star, which we call the equation of state. That’s one of the only ways to probe matter at these extremes.”
Like cramming New York City into a sugar cube
The visible mass in the Universe emerged at the Big Bang when hadrons — the building blocks of atomic nuclei — formed from a hot fireball made of quarks and gluons, as described in an article in Nature. When temperatures raise to trillions of degrees, particles deep inside the atoms start to shift into new, non-atomic states. Protons and neutrons inside the fireball of a neutron-star merger reach a density analogous to the effect of cramming New York City into a sugar cube.
Physicists are mapping these exotic phases, which probably occurred during the Big Bang, and are believed to arise in neutron star collisions and powerful cosmic ray impacts in an experiment in Germany called the High Acceptance DiElectron Spectrometer (HADES). The experiment, reported in Nature Physics, was the first to measure the temperature of quark matter — a phase of matter at extremely high temperature and/or high density, composed of the elementary particles quarks and gluons — under conditions akin to the inside of a neutron star collision, where most particles are matter (as opposed to antimatter).
Kilonovas –shaking the fabric of space-time
When neutron stars — the super-dense cores of dead stars where a teaspoon of neutron star goo would weigh about 10 million tons — or a neutron star and a black hole merge, they shake the fabric of space-time and trigger explosions called kilonovas.
HADES -Glimpse of the “quark matter” phases
To mimic these conditions, the HADES team slammed gold atoms moving at nearly the speed of light into a gold target, creating a mass of hundreds of protons and neutrons so dense that the theory couldn’t conclusively predict what would happen. The resulting explosion was over in a flash, and electron-positron pairs piled up in the detector surrounding the crash site.
HADES has provided a more direct glimpse of the “quark matter” phases thought to fill the cores of merging neutron stars. “It’s a point in a region where nobody else has touched as far as I know,” said Gene Van Buren, a physicist at the Relativistic Heavy Ion Collider (RHIC) in New York, which probes a higher-energy variety of quark matter called quark-gluon plasma. “That’s pretty exciting.”
“The theory of the strong force, called quantum chromodynamics (QCD),” writes Charlie Wood in Quanta, “is so complicated that no one has been able to predict exactly how matter will behave at high temperatures and densities. Theorists have developed a number of approximation schemes that are valid in certain situations, but large uncertainties make it hard to extend them. Experiments like HADES aim to manually fill the gaps left by the theory.”
With the HADES collaboration almost as soon as the quark matter forms, it starts making short-lived composite particles called rho mesons, each composed of a quark and an antiquark. The rho mesons immediately transform into fleeting “virtual” photons, each of which splits into an electron and its antimatter twin, the positron. These particles carry information about the matter’s early moments all the way out to the HADES detector.
“There are no other observables that could really bring such rich information,” said Tetyana Galatyuk, one of the 200 members of the HADES collaboration.
Neutron Stars Could Reveal Dark Side of Our Universe
“If we did discover exotic neutron decays, then we would in the same stroke also learn something amazing about the dark side of our universe—the survival of massive neutron stars would then immediately tell us that there isn’t just one dark matter particle, but a whole set of dark particles with their own dark forces.” said Jessie Shelton, physicist at the University of Illinois who has won awards from MIT and the LHC Theory Initiative working on a broad range of topics in particle physics beyond the Standard Model, with particular interests in dark matter, top quarks, and the Higgs boson.
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