The Gravitational-Wave Physics

TL;DR

This review surveys gravitational-wave production across three broad channels: primordial GWs from inflation and preheating, GWs from first-order phase transitions in the early Universe, and GWs from compact binary coalescences. It explains the theoretical frameworks for GW generation (inflationary tensor modes, bubble dynamics, and post-Newtonian/numerical relativity/perturbation approaches for binaries) and discusses how GWs can serve as standard sirens for cosmology. It also covers observational constraints from CMB B-modes and prospects for future detectors across a wide frequency range, highlighting the potential to probe fundamental physics, cosmology, and astrophysics. The review emphasizes the synergy between GW data and electromagnetic or other cosmological probes to illuminate early Universe physics and the evolution of the cosmos.

Abstract

The direct detection of gravitational wave by Laser Interferometer Gravitational-Wave Observatory indicates the coming of the era of gravitational-wave astronomy and gravitational-wave cosmology. It is expected that more and more gravitational-wave events will be detected by currently existing and planned gravitational-wave detectors. The gravitational waves open a new window to explore the Universe and various mysteries will be disclosed through the gravitational-wave detection, combined with other cosmological probes. The gravitational-wave physics is not only related to gravitation theory, but also is closely tied to fundamental physics, cosmology and astrophysics. In this review article, three kinds of sources of gravitational waves and relevant physics will be discussed, namely gravitational waves produced during the inflation and preheating phases of the Universe, the gravitational waves produced during the first-order phase transition as the Universe cools down and the gravitational waves from the three phases: inspiral, merger and ringdown of a compact binary system, respectively. We will also discuss the gravitational waves as a standard siren to explore the evolution of the Universe.

Figures (7)

• Figure 1: The schematic illustration of the inflaton potential. In the slow-roll inflationary scenario, an accelerated expansion of the Universe occurs when the inflaton rolls slowly along its potential. After inflation ends, the inflaton oscillates around the minimum of its potential, whose energy is converted into radiations.
• Figure 2: Marginalized joint 68% and 95% confidence level regions for $n_s$ and $r_{0.002}$ from the Planck 2015 data, compared to the theoretical predictions of some inflationary models. This figure is quoted from Ade:2015lrj.
• Figure 3: The schematic illustration of shooting algorithm for the bounce equation, which is equivalent to a particle moving in the inverse potential with Hubble friction. The exit point of field profile $\phi_\mathrm{exit}$ presents the field value at the center of nucleated bubble, which can be found by the shooting algorithm with errors and trials of under/over shooting until an exact shooting is achieved. Here, the bubble profile is rescaled by the temperature $M_\phi$ with respect to the bubble size rescaled by the mass scale $M_R$ of effective potential.
• Figure 4: The schematic illustration of envelope approximation that the GWs are mainly generated from the uncollided envelopes of colliding bubble walls and any GWs from the overlap region can be neglected.
• Figure 5: The spectrum of GWs from PTs that originated from dimension-six operator with respect to the sensitivity curves of various configurations of eLISA project Binetruy:2012ze.