Last February, scientists made the
groundbreaking discovery of gravitational waves produced by two colliding black
holes. Now researchers are expecting to detect similar gravitational wave
signals in the near future from collisions involving neutron stars—for example,
the merging of two neutron stars to form a black hole, or the merging of a
neutron star and a black hole.
In a new study published in Physical
Review Letters, Aleksi Kurkela at CERN and the University of Stavanger in
Norway and Aleksi Vuorinen at the University of Helsinki in Finland have
developed an improved method of analyzing the ultradense matter called
"quark matter" that is thought to exist in the cores of neutron
stars. Their method makes theoretical predictions regarding the properties of
neutron star matter that researchers working with the future data will
hopefully be able to test.
So far, the best quantitative
description of quark matter works only at a temperature of absolute zero.
Although this zero-temperature approximation is adequate for describing dormant
neutron stars, neutron star collisions would have such drastically higher
temperatures that thermal corrections are essential.
In the new study, Kurkela and Vuorinen
have accounted for high-temperature effects and incorporated them into the
equation of state that describes quark matter, generalizing the equation to
relatively small but non-zero temperatures. This modified framework provides a
much more accurate description of quark matter that is valid in the hot
conditions present in neutron star mergers.
Quark matter
As their name implies, neutron stars are
made mostly of neutrons, and like all known matter, neutrons are made of
quarks. Usually quarks are tightly bound together in groups of three, but the
enormous density and pressure in the core of a neutron star is thought to break
the structure of the neutrons, so that the quarks separate and form quark
matter. Whereas atoms are the basic constituents of the atomic matter that
we're familiar with, the basic constituents of quark matter are quarks (along
with gluons that hold the quarks together).
Currently, quark matter is not very
well understood, mainly because it does not exist naturally on Earth.
Researchers can produce quark-gluon plasma at high-energy particle
colliders, such as the Large Hadron Collider (LHC), but it only exists for a
fraction of a second before decaying because of the difficulty in maintaining
the extreme conditions it requires.
Gravitational waves from neutron stars
An alternative to producing quark
matter is to search for it in space. Using techniques similar to those that
were recently used to detect gravitational waves from black hole collisions,
researchers are currently searching for gravitational
waves from neutron star collisions. Detecting the signal of such a
collision would provide scientists with a wealth of new information on quark
matter.
"The hope is that the
gravitational wave signal from a merger of two neutron stars or a neutron star
and a black hole would provide detailed information about the structure of
neutron stars," Kurkela told Phys.org. "This in turn would
enable researchers to infer the equation of state of the matter the stars are
composed of, i.e., the thermodynamic properties of nuclear and quark
matter."
If experimentally detecting quark
matter is difficult, theoretically describing it is equally as challenging.
This is because the description involves applying the strong force (which is
mediated by the gluons) to the extremely high-energy matter of neutron stars.
"Our goal as particle/nuclear
theorists is to predict the equation of state from first principles, i.e.,
starting from the basic properties of the theory of strong interactions, quantum
chromodynamics (QCD)," Vuorinen said. "This is a long and very
demanding challenge, but if we are successful, then one day when neutron star
observations are accurate enough, our results can be used to interpret the
observational data from neutron star mergers, and ultimately tell whether
neutron stars have quark matter cores."
The results here also apply to the
quark-gluon plasma produced in particle accelerators, which the scientists
explain is somewhat different than the quark matter predicted to exist in neutron
stars.
"The quark-gluon plasma that is
produced in heavy ion collisions can be thought of as a hot but not very dense
soup of quarks and gluons, while quark matter is a very dense and cold,
essentially solid state, of matter," Kurkela said. "Our work in fact
bridges the gap between these two systems, as our result is applicable at all
temperatures, unlike any of the previous results."
In the future, the researchers plan to
further refine their method to improve its predictions.
"Together with our collaborators
both from Europe and the US, we are actively working towards improving the
current state-of-the-art results for the zero-temperature equation of state of
quark matter," Vuorinen said. "We hope to have the next orders of the
so-called weak coupling expansion of the equation of state available still
during this year, which will allow a refined prediction of the properties of
cold quark matter."
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