Chapter 1: The Nature of Quark Matter

Quark matter, potentially found within the densest stars known as neutron stars, could represent a new state of matter, surpassing the known categories of solids, liquids, gases, and plasmas. Recent research indicates that quark matter may constitute a significant portion of the mass in certain neutron stars, challenging our understanding of these celestial bodies.

Normal matter, which forms the bulk of stars, consists of atoms with positively charged protons and neutral neutrons at their core, surrounded by negatively charged electrons. Under extreme gravitational pressure, these electron clouds disintegrate, leading to a “soup” of subatomic particles known as nuclear matter.

Listen to Dr. Tyler Gorda from the University of Virginia as he shares insights on quark matter and describes the experience of traversing a neutron star in an interview on Astronomy News with the Cosmic Companion from June 23, 2020.

As the compression intensifies, electrons and protons combine to form additional neutrons, resulting in neutron stars, which resemble giant atomic nuclei the size of cities. The density of these stars is immense; just a thimbleful of neutron star material could weigh more than Mount Everest.

During neutron star collisions, ripples in spacetime occur, as demonstrated in simulations. Video credit: Numerical Relativity Simulation: T. Dietrich (Nikhef), Wolfgang Tichy (Florida Atlantic University) and the CoRe-collaboration Scientific Visualization: T. Dietrich (Nikhef), S. Ossokine, and A. Buonanno (Max Planck Institute for Gravitational Physics) / .gif creation by The Cosmic Companion.

Although neutron stars can remain stable for extended periods, recent studies suggest that even the neutrons within these stars may disintegrate under extreme pressure. Neutrons and protons are made up of three quarks, which could potentially be freed by the immense gravitational forces, as indicated by the research.

This theoretical framework suggests the possibility of neutron stars containing quark cores. While the concept was proposed decades ago, the methods to validate it have been largely ineffective, as even advanced supercomputers struggle with the complex calculations required.

Researchers in Finland propose that by examining gravitational waves emitted during neutron star collisions and correlating this data with predictions of nuclear reactions, they have found a way to investigate the existence of quark matter.

Some neutron stars may possess quark cores, where the gravitational forces are so intense that protons and neutrons cannot withstand the pressure and break down into quarks. Jyrki Hokkanen from CSC — IT Center for Science notes that if the behavior of matter in these collapsed stars aligns with this study's findings, then quark cores could occupy more than half the volume of these stellar remnants.

“Verifying the presence of quark cores in neutron stars has been a crucial objective in neutron star physics since this possibility was first considered around 40 years ago,” remarked Aleksi Vuorinen, Associate Professor at the University of Helsinki.

There is a speculative notion that some stars could be entirely composed of nuclear matter, though such entities would exhibit peculiar characteristics — notably, the speed of sound within them would exceed half the speed of light.

Chapter 2: The Fate of Massive Stars

As massive stars evolve, they undergo cycles of collapse and expansion, fusing heavier elements during each contraction. Ultimately, the most massive stars collapse into black holes, while slightly less massive stars form neutron stars.

These dense remnants can contain between one and 1.7 times the mass of our Sun compressed into a volume comparable to a city. The first observation of a neutron star occurred in 1967, but it took three decades for astronomers to accurately measure their mass. Since then, three neutron stars have been observed that approach or exceed the conventional upper limit of two solar masses.

Further investigations have indicated that many stable, massive neutron stars behave as if they have these theorized quark cores at their centers.

Signals from neutron star collisions indicated constraints on how much these stars can deform as they near each other, despite the immense gravitational forces involved. This insight allowed scientists to conclude that neither star in a particular collision exceeded 26 km (16 miles) in diameter.

Since the first detection in 2017, the frequency of neutron star mergers observed has increased due to the focused efforts of the LIGO and Virgo observatories, aiming to comprehend the processes occurring within these enigmatic stars. The latest gravitational wave data suggests that quark cores may exist across various star systems in the universe.

Each advancement in astronomical observation — from optical and radio telescopes to ultraviolet and gamma-ray observatories — has expanded our understanding of the universe. Currently, we are on the brink of a new era in gravitational wave astronomy, which involves measuring the ripples in spacetime caused by massive objects.

“There is a strong belief that we are entering a golden age of gravitational wave astrophysics and that we will soon witness significant advancements in our comprehension of the cosmos,” speculated Vuorinen.

The analysis of this study was published in Nature Physics. James Maynard, the founder and publisher of The Cosmic Companion, resides in Tucson with his wife, Nicole, and their cat, Max.

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