Stochastic Gravitational Wave Backgrounds (1811.08797v1)

Abstract: A stochastic background of gravitational waves can be created by the superposition of a large number of independent sources. The physical processes occurring at the earliest moments of the universe certainly created a stochastic background that exists, at some level, today. This is analogous to the cosmic microwave background, which is an electromagnetic record of the early universe. The recent observations of gravitational waves by the Advanced LIGO and Advanced Virgo detectors imply that there is also a stochastic background that has been created by binary black hole and binary neutron star mergers over the history of the universe. Whether the stochastic background is observed directly, or upper limits placed on it in specific frequency bands, important astrophysical and cosmological statements about it can be made. This review will summarize the current state of research of the stochastic background, from the sources of these gravitational waves, to the current methods used to observe them.

• The paper details a comprehensive review of stochastic gravitational wave backgrounds, emphasizing novel detection techniques and the classification of cosmological and astrophysical sources.
• It explains how correlation methods between advanced detectors are employed to identify subtle signals within noisy data, advancing the field of multi-messenger astronomy.
• The review outlines future prospects with improved observatory sensitivity, highlighting the potential to probe early universe physics and validate predictions of general relativity.

Overview of "Stochastic Gravitational Wave Backgrounds" by Nelson Christensen

The paper "Stochastic Gravitational Wave Backgrounds" by Nelson Christensen provides a comprehensive review of the current state of research on stochastic gravitational wave backgrounds (SGWBs). The author examines the potential sources of SGWBs, the methods utilized to detect them, and the implications these waves have for astrophysics and cosmology.

Introduction to Gravitational Waves

Gravitational waves (GWs) are ripples in spacetime, first predicted by Einstein in 1916 as a consequence of his general theory of relativity. These waves are generated by the acceleration of massive objects, akin to how moving charges generate electromagnetic waves. The first direct detection by Advanced LIGO in 2015 marked a significant milestone, providing empirical evidence for these theoretical predictions. Christensen emphasizes that detecting gravitational waves has opened a new era in multi-messenger astronomy, with Advanced LIGO and Advanced Virgo making notable contributions through various detections, such as binary black hole mergers.

Stochastic Background of Gravitational Waves

Stochastic gravitational wave backgrounds are produced by the superposition of numerous unresolved and incoherent gravitational wave sources. These could originate from early cosmic events, much like the cosmic microwave background, or from a plethora of astrophysical processes throughout the universe's history. Notably, recent observations suggest SGWBs from binary black hole and neutron star mergers. While such backgrounds are immensely challenging to detect directly due to their noise-like nature in individual detectors, correlation methods between multiple detectors are employed to identify them.

Sources and Detection

Christensen categorizes the sources of SGWBs into two major classes: cosmological and astrophysical. Cosmological sources include primordial fluctuations from the inflationary epoch, cosmic strings, and first-order phase transitions in the early universe. Astrophysical sources encompass supernovae, magnetars, and compact binary coalescences, such as those observed by LIGO and Virgo.

Detection of SGWBs relies primarily on correlating the output of two or more gravitational wave detectors to enhance the signal-to-noise ratio. The paper reviews several methods employed in these detections, noting that Advanced LIGO and Virgo leverage such techniques effectively to place upper limits on SGWBs. Notable is the application of spherical harmonic decomposition and radiometer analyses to search for anisotropies and directional SGWBs.

Implications and Future Prospects

The detection of a SGWB would offer profound insights into both the astrophysical processes across cosmic history and the physics of the early universe. The potential to glean information extending back to moments shortly after the Big Bang is of exceptional scientific interest. Christensen speculates on the future of gravitational wave astronomy, positing that the improved sensitivities of upcoming detectors, like LISA and DECIGO, and enhanced analysis techniques could further our understanding of SGWBs.

The ongoing advancements in detector technology, analysis methods, and extended observational runs suggest a promising horizon where SGWBs may soon move from the field of theoretical possibility to empirical reality. This progression would not only validate aspects of general relativity on a cosmological scale but would also potentially illuminate new physics beyond the current theoretical frameworks.

In summary, Christensen's review encapsulates the pivotal roles both contemporary observations and theoretical advancements play in the quest to understand stochastic gravitational wave backgrounds. It underscores the interdisciplinary nature of this research, bridging gaps in understanding across the fields of astrophysics, cosmology, and gravitational physics.