High-Frequency Gravitational Waves from Phase Transitions in Nascent Neutron Stars

High-Frequency Gravitational Waves from Phase Transitions in Nascent Neutron Stars

Overview

Neutron stars provide a unique laboratory for studying quantum chromodynamics (QCD) under extreme conditions of high baryon density and relatively low temperature. According to theoretical models, the cores of sufficiently massive neutron stars could undergo a first-order phase transition from hadronic matter to a deconfined quark matter phase. This paper argues that if such a phase transition proceeds via the nucleation and expansion of quark matter bubbles, it could source high-frequency gravitational waves (GWs) at frequencies around 1 MHz.

Problem & Context

• Tentative evidence suggests that the cores of massive neutron stars consist of deconfined quark matter.
• During the formation of a neutron star in a supernova, its core may transition to this quark phase.
• The authors argue that the dynamics of this phase transition, if it proceeds via bubble nucleation and expansion, could produce high-frequency GWs detectable by upcoming experiments.
• The exact location of the QCD phase boundaries in the temperature-density plane is unknown, making neutron stars a unique laboratory for probing this regime.

Methodology

• The authors model the dynamics of the QCD phase transition inside a collapsing neutron star core:
  • Consider three possible scenarios: (1) complete transition, (2) stalled transition forming a mixed phase, or (3) microscopically mixed phase.
  • Use the Tolman–Oppenheimer–Volkoff equations to relate the mass, radius, and core pressure of the neutron star.
  • Compute the bubble nucleation rate, expansion velocity, and fraction of the core that transitions to quark matter before stalling.
• Estimate the frequency and amplitude of the resulting high-frequency GW signal using the sound shell model.

Results

• For the equations of state considered, only two cases (HS(IUF) and SRO(KDE0v1)) can simultaneously satisfy the conditions for a sizable GW signal:
  • Sufficient number of bubbles nucleating (>2)
  • Substantial fraction of the core transitioning (x_q > 0.03)
• Under these optimistic conditions, peak strain amplitudes around 10^-22 could be possible for a galactic supernova.
• The expected GW signal frequency is typically in the MHz range.

Interpretation

• The QCD phase transition in a collapsing neutron star core is a promising candidate for producing detectable high-frequency GWs, complementing other potential sources like neutron star mergers.
• Detection of such a signal could provide unique insights into the QCD phase diagram at high densities, as well as constrain the neutron star equation of state.
• Even non-detection could rule out certain equations of state that would have predicted a strong signal.

Limitations & Uncertainties

• The exact details of the phase transition dynamics, including bubble nucleation, expansion, and interactions, are highly sensitive to the still poorly understood QCD equation of state at high densities.
• The authors note that their estimates rely on several simplifying assumptions, and that the true GW signal may be weaker or stronger than their predictions.
• The expected event rate is low, with only 1-2 supernovae per century in the Milky Way, making detection challenging.

What Comes Next

• The authors propose to carefully monitor high-frequency GW data around the time of the next galactic supernova for a possible signal lasting tens of microseconds.
• Improved theoretical understanding of QCD at high densities, as well as advancements in high-frequency GW detector technology, could further enhance the prospects for detecting this novel signature.