Cosmological and High Energy Physics implications from gravitational-wave background searches in LIGO-Virgo-KAGRA's O1-O4a runs

TL;DR

The study analyzes LIGO-Virgo-KAGRA O1-O4a data to search for a cosmological gravitational-wave background across eight beyond-Standard-Model scenarios (first-order phase transitions, cosmic strings, domain walls, stiff equation of state, axion inflation, scalar-induced perturbations, primordial black holes, and parity violation). Using Bayesian cross-correlation methods and a CBC background model, the authors derive upper limits on the gravitational-wave energy density and translate them into constraints on model parameters, accounting for detector calibration and potential polarization. Across all models, no GW background is detected; the resulting limits significantly tighten previous bounds and rule out or constrain sizeable regions of parameter space for these early-Universe scenarios. The findings demonstrate that current ground-based GW data already probe high-energy physics and cosmology beyond the reach of collider experiments, guiding theoretical model-building and informing the design of future detectors and analyses. The work also emphasizes the importance of separating astrophysical and cosmological backgrounds and highlights directions for improved sensitivity in upcoming observing runs.

Abstract

We search for gravitational-wave background signals produced by various early Universe processes in the Advanced LIGO O4a dataset, combined with the data from the earlier O1, O2, and O3 (LIGO-Virgo) runs. The absence of detectable signals enables powerful constraints on fundamental physics. We derive gravitational-wave background energy density upper limits from the O1-O4a data to constrain parameters associated with various possible processes in the early Universe: first-order phase transitions, cosmic strings, domain walls, stiff equation of state, axion inflation, second-order scalar perturbations, primordial black hole binaries, and parity violation. In our analyses, the presence of an astrophysical background produced by compact (black hole and neutron star) binary coalescences throughout the Universe is also considered. We address the implications for various cosmological and high energy physics models based on the obtained parameter constraints. We conclude that LIGO-Virgo data already yield significant constraints on numerous early Universe scenarios.

Figures (20)

• Figure 1: Constraints from the LIGO-Virgo observing runs O1-O4a data on the first order phase transition parameters $\alpha_{\rm PT}$, $\beta/H_{\rm PT}$, and $T_{\rm PT}$, assuming a dominant sound wave contribution and taking into account the CBC background. The priors selected are shown in Table \ref{['table1']}. The $95\%$ and $68\%$ confidence level exclusion contours are shown in blue and red, respectively. Clearly, less conservative choices of priors could lead to stronger constraints. The green lines correspond to $\beta/H_{\rm PT} =( 8 \pi)^{1/3}$.
• Figure 2: Similarly as in Fig. \ref{['fig:SW_corner']}, the constraints from the LIGO-Virgo observing runs O1-O4a data on the first order phase transition parameters including the CBC background, but for a dominant bubble collision contribution.
• Figure 3: Exclusion regions at the 95% CL on the cosmic string paramter space $(G\mu, N_k)$.
• Figure 4: The gravitational-wave spectrum of DW networks for a benchmark for the wall tension $\sigma_{\rm DW}$ and annihilation temperature $T_\text{ann}$. The schematic in the top left box illustrates the impact of these parameters on the spectra: increasing the wall tension $\sigma_{\rm DW}$ enhances the peak amplitude, while a higher annihilation temperature $T_\text{ann}$ reduces the amplitude and shifts the spectrum to higher frequencies. In addition, the assumed shape for the overlapping astrophysical gravitational wave background is displayed in green, for a representative value of $\Omega_{\rm ref}$ (and with $\alpha=2/3$). Finally, sensitivity curves for LIGO-Virgo O3 run KAGRA:2021kbb, as well as the ones of the O4a run LIGOScientific:2025bgj and LIGO A+ detector Aplusdesign are included.
• Figure 5: Posteriors for the strength of the CBC background amplitude $\Omega_\text{ref}$, the tension $\sigma_{\rm DW}$ of the DW network and the temperature $T_\text{ann}$ at which the network annihilates using LIGO-Virgo data from O1, O2, O3 and O4a. The gray region in the bottom corresponds to a region where the DW network dominates the Universe, i.e. $T_\text{ann} \leq T_\text{dom}$.