Using gravitational waves to see the first second of the Universe (2401.04388v3)

Abstract: Gravitational waves are a unique probe of the early Universe, as the Universe is transparent to gravitational radiation right back to the end of inflation. In this article, we summarise detection prospects and the wide scope of primordial events that could lead to a detectable stochastic gravitational wave background. Any such background would shed light on what lies beyond the Standard Model, sometimes at remarkably high scales. We overview the range of strategies for detecting a stochastic gravitational wave background before delving deep into three major primordial events that can source such a background. Finally, we summarize the landscape of other sources of primordial backgrounds.

• The paper demonstrates that gravitational wave detection can probe the Universe’s first second by revealing signals from high-energy primordial events.
• It details innovative detection strategies, including pulsar timing arrays, astrometry, and interferometry, to capture the stochastic background.
• The study’s implications extend to testing physics beyond the Standard Model and exploring high-energy phase transitions in the early cosmos.

Using Gravitational Waves to See the First Second of the Universe

The exploration of gravitational waves (GWs) as a tool to probe the early Universe offers a compelling insight into the primordial events that could lead to a detectable stochastic gravitational wave background. The paper authored by Rishav Roshan and Graham White, titled "Using gravitational waves to see the first second of the Universe," provides an in-depth examination of the detection prospects of these primordial sources. It underscores that the stochastic background from gravitational waves can illuminate aspects of physics beyond the Standard Model and explore new regimes of energy and scale, occasionally reaching remarkably high scales.

The Significance of Gravitational Waves

Gravitational waves, as predicted by general relativity, have been pivotal to modern cosmology and have been confirmed by observations such as the aLIGO detections. While electromagnetic observations are limited to data after recombination, gravitational waves allow us to explore back to the end of inflation because the Universe is transparent to GWs right from the beginning.

Detection Strategies and Prospects

The paper extensively reviews the technology and methodologies involved in the detection of stochastic gravitational wave backgrounds. It outlines several strategies:

1. Pulsar Timing Arrays (PTAs): These facilities measure deviations in the periodic signals from pulsars. While they are an effective tool for detecting GWs in the nanoHz to µHz range, current PTAs are yet to observe a stochastic background from the early Universe.
2. Astrometry: The precision of measuring the positions of stars across the sky over extended periods could potentially reveal the influence of GWs via astrometric microlensing and proper motion variations. Projects like Gaia and future missions such as Theia could significantly contribute to this approach.
3. Interferometry: Ground-based interferometers like LIGO, VIRGO, and KAGRA, as well as future space-based detectors like LISA and TianQin, are crucial for detecting GWs from higher frequency sources typically produced by very high-energy phase transitions.

Primordial Sources of Gravitational Waves

The paper discusses various primordial events that may generate a detectable GW background:

1. First-order Phase Transitions: Phase transitions in the early universe, when symmetries are broken, can produce GWs through bubble nucleation, collisions, and subsequent turbulence in the plasma. The strength and frequency of the signal can provide insights into conditions like the energy scale of the transition and the speed of sound in the medium.
2. Topological Defects: Cosmic strings or other defects resulting from symmetry breaking can source gravitational waves over a wide range of scales.
3. Scalar-induced Gravitational Waves: These are generated from large perturbations in the scalar fields during ultra-slow roll phases of inflation or from abrupt changes in the equation of state, which can induce secondary gravitational radiation.

Implications and Future Directions

Understanding these signals requires precise predictions of their spectra, which involve complex computational challenges. The potential discovery of primordial stochastic gravitational waves could revolutionize our understanding of high-energy physics and cosmology. It is implied that future research might focus on enhancing detector sensitivity, refining theoretical models, and potentially reconciling signals observed across different GW observatory modalities.

In conclusion, the paper highlights the transformative potential of gravitational wave cosmology and the pursuit of probing the Universe's infancy. As experimental and theoretical techniques advance, GWs may unveil new physics and mysteries of the cosmos, providing a unique glimpse into the first second of creation.