Recent study explores how axion fields affect quark matter and hybrid stars. Axions stiffen quark matter, lowering the density where quark matter forms in hybrid stars. While this slightly reduces the star's maximum mass and radius, it significantly increases the size of the quark-matter core, allowing massive neutron stars (around 2 solar masses) to host large quark cores.
Neutron stars are fascinating objects formed when massive stars explode in supernovae. These incredibly dense stars pack more mass than the Sun into a tiny space, about 10 kilometers in radius. Recent observations using advanced instruments like the Neutron Star Interior Composition Explorer (NICER) and the LIGO/Virgo gravitational-wave detectors have provided detailed insights into neutron stars' masses, radii, and internal structure.
One of the biggest mysteries is what lies inside neutron stars. Scientists believe their cores might host an exotic state of matter called quark matter, where particles called quarks exist freely instead of being bound together in protons and neutrons. This transition from regular matter (hadronic matter) to quark matter could occur at extremely high densities, significantly affecting the star's structure and behavior.
The Role of Axions in Neutron Stars
To understand the strange physics inside neutron stars, researchers are also studying theoretical particles like axions. Axions were originally proposed to solve a problem in particle physics known as CP violation, but they are now considered potential candidates for dark matter, the mysterious substance that makes up most of the universe’s mass.
In neutron stars, axions may interact with dense matter and photons (light particles), influencing the star's properties. These interactions might also explain unusual phenomena, like certain stars cooling faster than expected. Axions become even more interesting in binary neutron star mergers, where two neutron stars collide, creating hot and dense environments ideal for studying such exotic particles.
How Axions Affect Quark Matter
Recent research by scientists, including Liu and colleagues, explored how axions impact neutron stars using a specialized theoretical model. They found that axions influence the transition from hadronic to quark matter in surprising ways.
The study showed that axions can lower the density at which quark matter starts to form, making it appear earlier in a neutron star's life. Axions also increase the pressure in quark matter, making it stiffer and more resistant to collapse. However, in regions where hadronic and quark matter mix, axions slightly soften the star's structure, reducing its maximum possible mass and radius by a small amount.
Bigger Quark Cores in Massive Neutron Stars
One of the most exciting findings is that axions can significantly increase the size and mass of the quark-matter core inside massive neutron stars. Even in neutron stars weighing about two times the mass of the Sun (2 solar masses), axions help create large quark cores.
This discovery is crucial because it aligns with recent observations of massive neutron stars. It suggests that these stars may host quark matter in their cores, confirming long-standing theories about dense matter physics.
What This Means for Astrophysics
These findings open new possibilities for studying neutron stars and testing theories about exotic particles like axions. For example, gravitational waves from neutron star mergers could carry signatures of axion-induced changes in quark matter. Observing these waves could help scientists confirm the presence of quark cores and axions in neutron stars.
Additionally, studying axions in neutron stars might explain puzzling phenomena like unusual cooling rates in certain stars. This makes neutron stars not just astronomical objects but also natural laboratories for testing fundamental physics.
Looking Ahead
The study of axions and quark matter in neutron stars is still in its early stages, but the future looks promising. Upcoming missions and advanced detectors will provide even more precise data, allowing researchers to test these ideas further.
While challenges remain—such as directly detecting axions or validating theoretical models—neutron stars offer a unique opportunity to explore the universe's most extreme conditions. These efforts may help unravel the mysteries of dense matter, exotic particles, and the nature of the cosmos itself.
By combining observations and theory, scientists are making strides in understanding neutron stars and their hidden quark cores, bringing us closer to unlocking the secrets of the universe.
Reference: He Liu, Yu-Heng Liu, Yong-Hang Yang, Min Ju, Xu-Hao Wu, Hong-Ming Liu, Peng-Cheng Chu, "Axion effects on quark matter and quark-matter cores in massive hybrid stars", Arxiv, 2024. https://arxiv.org/abs/2412.17624
Technical terms
1. Neutron Stars (NSs):
Neutron stars are the remnants of massive stars that have exploded as supernovae. They are extremely dense—just a teaspoon of their material would weigh billions of tons. These stars are primarily made of neutrons and have very strong gravity.
2. Quark Matter:
Quarks are tiny particles that make up protons and neutrons. In normal conditions, quarks are tightly bound together. However, under extreme pressure and density, like inside neutron stars, quarks might break free and form a state of matter called quark matter.
3. Equation of State (EOS):
The EOS describes how matter behaves under different pressures and densities. For neutron stars, the EOS tells us how dense matter reacts to the immense pressure in the star’s core, helping predict the star’s mass and size.
4. First-Order Phase Transition:
This is a sudden change from one type of matter to another. In neutron stars, it refers to the transition from hadronic matter (normal matter made of protons and neutrons) to quark matter at extremely high densities.
5. Hybrid Stars:
These are neutron stars that contain both hadronic matter and quark matter in their cores. They are called "hybrid" because they mix two different states of matter.
6. Axions:
Axions are theoretical particles introduced to solve a physics problem related to the violation of certain symmetries (charge-parity or CP violation). They are also considered candidates for dark matter, a mysterious substance that makes up most of the universe but doesn’t emit light or energy.
7. NICER (Neutron Star Interior Composition Explorer):
NICER is a NASA mission that measures the size and mass of neutron stars by observing their X-rays. This helps scientists understand the dense matter inside these stars.
8. Gravitational Waves:
These are ripples in space-time caused by massive objects like colliding neutron stars. Observing these waves helps scientists study extreme conditions in space, including neutron stars’ internal structure.
9. Mixed Phase:
In neutron stars, the mixed phase refers to a region where hadronic matter and quark matter coexist during the phase transition.
10. Nambu–Jona-Lasinio Model:
This is a mathematical framework used in physics to study the behavior of particles like quarks under certain conditions, including how axions affect quark matter.