Cosmological Backgrounds of Gravitational Waves (1801.04268v3)

Abstract: Gravitational waves (GWs) have a great potential to probe cosmology. We review early universe sources that can lead to cosmological backgrounds of GWs. We begin by presenting definitions of GWs in flat space-time and in a cosmological setting, and discussing the reasons why GW backgrounds from the early universe are of a stochastic nature. We recap current observational constraints on stochastic backgrounds, and discuss some of the characteristics of present and future GW detectors including advanced LIGO, advanced Virgo, the Einstein Telescope, KAGRA, LISA. We then review in detail early universe GW generation mechanisms proposed in the literature, as well as the properties of the GW backgrounds they give rise to. We classify the backgrounds in five categories: GWs from quantum vacuum fluctuations during standard slow-roll inflation, GWs from processes that operate within extensions of the standard inflationary paradigm, GWs from post-inflationary preheating and related non-perturbative phenomena, GWs from first order phase transitions (related or not to the electroweak symmetry breaking), and GWs from topological defects, in particular from cosmic strings. The phenomenology of early universe processes that can generate a stochastic background of GWs is extremely rich, and some backgrounds are within the reach of near-future GW detectors. A future detection of any of these backgrounds will provide crucial information on the underlying high energy theory describing the early universe, probing energy scales well beyond the reach of particle accelerators.

• The paper details several early-universe mechanisms that generate stochastic gravitational wave backgrounds, including inflation, preheating, and phase transitions.
• It outlines advanced detection methods using interferometers and pulsar timing arrays to set upper limits on gravitational wave energy density.
• Future observatories like LISA and the Einstein Telescope are expected to refine our understanding of high-energy physics and early cosmology.

Cosmological Backgrounds of Gravitational Waves: A Review

The paper "Cosmological Backgrounds of Gravitational Waves" provides a comprehensive examination of the potential early universe sources that lead to the formation of cosmological backgrounds of gravitational waves (GWs). This essay offers an expert-level overview of the paper's content, elucidating the theoretical frameworks, observational constraints, and detection methodologies discussed.

Gravitational Waves in Cosmology

GWs, ripples in spacetime caused by some of the most energetic processes in the universe, have unique capabilities as probes of cosmological history. They are fundamentally decoupled from matter and radiation after their production, traveling unimpeded through the universe. This makes them apt tools for investigating conditions in the early universe, reaching energy scales and epochs that are inaccessible through electromagnetic observations.

Stochastic Nature and Sources of Cosmological GW Backgrounds

The stochastic gravitational wave background (SGWB) is expected to be of stochastic nature due to the vast number of uncorrelated sources in the early universe. The paper categorizes these GW backgrounds based on their production mechanisms: quantum vacuum fluctuations during inflation, processes within extended inflationary models, post-inflationary preheating, first-order phase transitions, and cosmic string dynamics.

1. Inflationary GW Backgrounds: During the inflationary epoch, quantum fluctuations inherent to the inflationary field generate a spectrum of tensor perturbations. These perturbations, which turn into GWs, are nearly scale-invariant and serve as an "irreducible" background observable in cosmic microwave background (CMB) anisotropies.
2. Extended Inflation Scenarios: Different inflationary paradigms predict additional GW sources, such as those from temporary particle production episodes or the gravitational coupling of new fields during inflation, potentially leading to observable spectra with distinct features and enhanced high-frequency ranges.
3. Post-Inflationary Preheating: The transition from inflation to the standard big bang model often involves rapid and efficient energy transfer known as preheating. This process can produce non-perturbative phenomena like parametric resonance, leading to significant GW emissions.
4. Phase Transitions: First-order phase transitions could occur if the early universe undergoes symmetry breaking, leading to bubble collisions and associated GW generation. These events are detectable if they occur in the right energy scales and frequencies that intersect with current or planned GW observatories.
5. Cosmic Defects and Strings: GW backgrounds can also arise from topological defects such as cosmic strings. Their dynamics, driven by network evolution and loop emissions, provide distinct spectral signatures that are being actively sought with interferometers and pulsar timing arrays (PTA).

Constraints and Detection Prospects

Presently, the detection of cosmological GW backgrounds involves an interplay between theoretical predictions and observational constraints. Advanced LIGO and Virgo, among other current observatories, have established upper bounds on the GW energy density, thus tightly constraining models of early universe GW sources. PTA projects like the International Pulsar Timing Array and upcoming initiatives such as the Square Kilometer Array aim to widen the accessible frequency range into the nanohertz band, improving our ability to detect or constrain such backgrounds.

Future Implications

The ongoing development of ground-based and space-based detectors like the Einstein Telescope and LISA promises future discoveries or constraints on GW backgrounds. Such observations will impart profound insights into high-energy physics and early universe cosmology, potentially reconciling current theoretical models with empirical data and opening windows to new physics.

In conclusion, the paper successfully elucidates the diverse mechanisms behind the formation of cosmological GW backgrounds and places them within the context of current and future observational capabilities. These insights are poised to refine our understanding of the universe's origin and evolution, marking gravitational wave astronomy as an integral pillar of modern cosmology.