Gravitational Waves From High-temperature Vacuum Decay Reveal Nanohertz And Millihertz Regimes

The early universe may have undergone a dramatic phase transition, creating ripples in spacetime known as gravitational waves, and a new analysis explores the potential signals from this event. Ahmad Mohamadnejad from Lorestan University and colleagues investigate high-temperature vacuum decay, a process theorised to have occurred in the very early universe, and calculate the resulting gravitational wave spectrum. Their work reveals that, depending on the specific conditions, this decay generates gravitational waves at two distinct frequencies, opening up possibilities for detection with current and future observatories. The team’s calculations predict signals at nanohertz frequencies, potentially within reach of Pulsar Timing Arrays, and at millihertz frequencies, ideal targets for the planned Laser Interferometer Space Antenna, offering a promising avenue for probing the physics of the early universe and validating models of cosmic evolution.

Cosmological Phase Transitions and Gravitational Waves

This research details the generation of gravitational waves from cosmological phase transitions within models featuring scale-invariant dark matter. Scientists investigate how these transitions, occurring in the early universe, could produce detectable ripples in spacetime, potentially revealing information about dark matter and the conditions of the early universe. The spectrum of these waves carries crucial information about the transition’s temperature, strength, and the speed at which bubbles of new vacuum formed and expanded. The team connects particle physics models, which predict the existence of scale-invariant dark matter, to observable cosmological phenomena, namely gravitational waves.

Detecting these waves would provide evidence for physics beyond the Standard Model and potentially reveal the nature of dark matter. Scientists employed the effective potential approach to study the phase transition, calculating the potential energy of the system to determine the stability of the vacuum and the nature of the transition. They then calculated the rate of bubble nucleation, or formation, during the phase transition, and the velocity of the bubble walls, considering both subsonic and supersonic scenarios. The team calculated the gravitational wave spectrum using various methods, including envelope approximations and hydrodynamic simulations, to model bubble collisions accurately.

They considered contributions from bubble collisions, turbulence, and acoustic waves, identifying scenarios producing the strongest gravitational wave signals. The research demonstrates that, under certain conditions, gravitational waves generated from these phase transitions could be detectable by current and future observatories like LISA and NANOGrav. The strength and frequency of the signal depend on the parameters of the dark matter model and the details of the phase transition, allowing for distinctions between varying dark matter masses and bubble wall velocities. Turbulence significantly contributes to the gravitational wave spectrum, especially at lower frequencies.

This research provides a potential pathway for indirectly detecting dark matter through its influence on cosmological phase transitions and the resulting gravitational waves. Detecting these waves would strongly indicate new physics beyond the Standard Model and complement direct dark matter detection experiments. The results highlight the importance of future gravitational wave observatories for probing the early universe and searching for evidence of new physics.

Sphaleron Dominated Vacuum Decay at High Temperatures

This work presents a comprehensive analysis of vacuum decay at high temperatures and the resulting stochastic gravitational wave background, utilizing scale-invariant models to explore the dynamics of the early universe. Scientists constructed an effective potential, incorporating tree-level calculations, the Coleman-Weinberg correction, finite-temperature effects, and Daisy resummation techniques, to accurately model the conditions of the early universe. Investigations into the first-order phase transition revealed critical and nucleation temperatures, alongside the supercooling parameter. The research demonstrates that vacuum decay is dominated by sphaleron processes rather than quantum tunneling, altering understanding of phase transition mechanisms.

Detailed calculations produced the full gravitational wave spectrum arising from bubble collisions, sound waves, and turbulence, allowing scientists to map model parameters to observable signatures. Measurements confirm that the critical temperature for vacuum decay varies significantly with model parameters, influencing the strength and duration of the resulting gravitational wave signal. The strength parameter directly correlates with the amplitude of the gravitational wave spectrum, while the inverse duration parameter impacts the frequency range of the emitted gravitational waves. These findings establish a clear link between theoretical model parameters and observable gravitational wave signatures.

Sphaleron Decay and Stochastic Gravitational Waves

This work presents a comprehensive analysis of vacuum decay and the resulting stochastic gravitational wave background within scale-invariant models of particle physics. Researchers constructed a high-temperature effective potential, incorporating quantum corrections and finite-temperature effects, to accurately model a first-order phase transition in the early universe. Calculations reveal that this decay is dominated by sphaleron processes rather than quantum tunneling, influencing the transition’s characteristics and the subsequent gravitational wave production. The team computed the gravitational wave spectrum generated by bubble collisions, turbulence, and sound waves during the phase transition, identifying two distinct regimes within the model’s parameter space. Future research directions include refining these calculations and exploring the implications of these findings for electroweak baryogenesis, potentially explaining the observed matter-antimatter asymmetry in the universe.