Gravitational wave cosmology

TL;DR

This review maps the landscape of gravitational-wave cosmology, detailing cosmological GW sources such as inflationary GWs, phase-transition remnants, domain walls, and PBHs, and how they encode early-Universe physics. It synthesizes how GW propagation, iSW effects, and polarization content probe the large-scale structure and fundamental symmetries of gravity, including tests of Lorentz and parity violation and non-GR polarization modes. It also highlights the role of GW standard Sirens in measuring the Hubble constant and expansion history, with an emphasis on forthcoming space-based detectors (LISA, Taiji, TianQin) and next-generation ground-based observatories (ET, CE). Collectively, the work demonstrates the potential of multimessenger GW observations to constrain high-energy physics, refine cosmological parameters, and test the foundations of gravity across a wide range of frequencies and cosmic epochs.

Abstract

Gravitational waves (GWs) originating from cosmological sources offer direct insights into the physics of the primordial Universe, the fundamental nature of gravity, and the cosmic expansion of the Universe. In this review paper, we present a comprehensive overview of our recent advances in GW cosmology, supported by the national key research and development program of China, focusing on cosmological GW sources and their implications for fundamental physics and cosmology. We first discuss the generation mechanisms and characteristics of stochastic gravitational wave backgrounds generated by physical processes occurred in the early Universe, including those from inflation, phase transitions, and topological defects, and summarize current and possible future constraints from pulsar timing array and space-based detectors. Next, we explore the formation and observational prospects of primordial black holes as GW sources and their potential connection to dark matter. We then analyze how GWs are affected by large-scale structure, cosmological perturbations, and possible modifications of gravity on GW propagation, and how these effects can be used to test fundamental symmetry of gravity. Finally, we discuss the application of GW standard sirens in measuring the Hubble constant, the expansion history, and dark energy parameters, including their combination with electromagnetic observations. These topics together show how GW observations, especially with upcoming space-based detectors, such as LISA, Taiji, and Tianqin, can provide new information about the physics of the early Universe, cosmological evolution, and the nature of gravity.

Figures (19)

• Figure 1: The present energy spectra of total induced GWs. The blue solid line represents GW produced from inflation and the red solid line represents the one from the radiation-dominated era . The orange and purple dashed lines represent the sensitivity curves of LISA Audley:2017drz and Taiji Ruan:2018tsw, respectively. Copied from Ref. Peng:2021zon with permission.
• Figure 2: Present energy spectra for induced GWs and primordial GWs for Set 1, respectively. The red solid line represents induced GWs in the radiation-dominated era, and the blue represents PGWs generated during inflation. Other dashed lines are the expected sensitivity curve of the future GW projects summarized in Ref. Moore:2014lga. The shaded regions represent the existing constraints on GWs Kohri:2018awvLentati:2015qwp. Copied from Ref. Peng:2022ttg with permission.
• Figure 3: The plot of $\delta\chi^2$ with the peak mass of PBHs ($m_{\mathrm{pbh}}^*/M_\odot \approx2.3\times10^{18}\left({2\pi H_{0}}/{k_*}\right)^{2}$) generated by the log-normal power spectrum with $\sigma_*=0.5$. The dashed line corresponds to $\delta\chi^2=28.74$. Copied from Ref. Yuan:2019wwo with permission.
• Figure 4: The non-Gaussian components of the SIGWs energy spectrum generated by a log-normal power spectrum with $\sigma_*=0.2$. The dashed line corresponds to $\Omega_{\rm GW}(k) \propto k^3 \ln^2 \left( \frac{4k_*^2}{3k^2} \right)$. Copied from Ref. Yuan:2023ofl with permission.
• Figure 5: The non-minimal coupling functions for model G, model R and model O. Here we set $\Delta=0.1$, $\Lambda=0.01$, $\xi=0.001$ and the values of $A_\text{G}$, $A_\text{R}$ and $A_\text{O}$ are chosen to let PBHs make up all of the DM.