The astrophysical gravitational wave stochastic background (1101.2762v3)

Abstract: A gravitational wave stochastic background of astrophysical origin may have resulted from the superposition of a large number of unresolved sources since the beginning of stellar activity. Its detection would put very strong constrains on the physical properties of compact objects, the initial mass function or the star formation history. On the other hand, it could be a 'noise' that would mask the stochastic background of cosmological origin. We review the main astrophysical processes able to produce a stochastic background and discuss how it may differ from the primordial contribution by its statistical properties. Current detection methods are also presented.

• The paper presents a detailed analysis of the astrophysical gravitational wave background by identifying key sources such as binary neutron star mergers, core collapse supernovae, and black hole formations.
• It applies cross-correlation techniques to distinguish between continuous Gaussian noise and discrete popcorn noise from isolated events, enhancing detection capabilities.
• The study constrains models of star formation history and the evolution of compact objects, urging the development of precise simulations to separate astrophysical and cosmological signals.

Overview of "The Astrophysical Gravitational Wave Stochastic Background"

The paper "The Astrophysical Gravitational Wave Stochastic Background" by T. Regimbau extensively examines the potential sources and characteristics of the astrophysical background of gravitational waves (GWs), distinct from the cosmological background. The astrophysical GW stochastic background results from the superposition of numerous unresolved sources spanning from the inception of stellar formation. It's crucial in understanding compact objects, the initial mass function, and star formation history, while also potentially obscuring cosmological signals.

Key Characteristics and Sources

The paper categorizes potential sources of the astrophysical stochastic background into several key areas, each contributing uniquely to the spectrum and its statistical properties:

1. Binary Neutron Stars: The coalescence of binary neutron stars is considered a significant contributor due to their high GW emission during inspiral, providing energy density peaks around hundreds of hertz. For realistic estimations, the local coalescence rates are derived from galactic merger rates and projected onto a cosmological scale.
2. Rotating Neutron Stars: Though most contributions from rotating stars, such as pulsars, are negligible under typical conditions, particular neutron stars, like magnetars with strong magnetic distortions, might contribute significantly when their spin down is dominated by GW emission.
3. Core Collapse Supernovae: These explosions can emit GWs during the formation of neutron stars or black holes. Different models of supernova mechanisms, such as bar-mode instabilities and r-mode post-collapse instabilities, are discussed, giving rise to diverse spectra, often situating their peaks around kilohertz frequencies.
4. Black Hole Formation and Quasinormal Modes: The gravitational collapse into black holes is another potent emitter of GWs. The characteristic spectrum is determined by the black hole mass and spin parameters, which affect the frequency and amplitude of the ringdown phase signals.
5. Captures by Supermassive Black Holes: The inspiral and eventual capture of compact objects by supermassive black holes (SMBHs) contribute a background observable by space-based detectors like LISA. These events are significant due to their occurrence rates and cumulative GW emission over long durations.

Detection Methods and Challenges

Detection of the astrophysical GW stochastic background necessitates sophisticated methodologies due to its potential overlap and interference with the cosmological background. The paper discusses the optimal use of cross-correlation techniques to differentiate between Gaussian noise-like signals typically associated with continuous backgrounds and the popcorn noise arising from less frequent, isolated events.

The challenge lies in distinguishing between these competing signals and extracting useful information about the Universe's astrophysical processes. The spectral characteristics are integral in achieving this, as they allow researchers to apply filters tailored to specific background profiles over the frequency bands detected by instruments like LIGO, Virgo, and future observatories like the Einstein Telescope.

Theoretical Implications and Future Directions

The research highlights the implications of detecting astrophysical GWs, such as providing constraints on the physics of compact objects, assessing star formation across cosmic time, and examining the initial mass function. As detection capabilities advance, astrophysical backgrounds could offer unprecedented information about high-energy processes and cosmological parameters throughout the Universe's history.

The paper concludes by urging the development of more precise models to simulate the astrophysical background accurately, which will aid in distinguishing it from cosmological backgrounds. These efforts are vital to getting a clearer observational window into both the signals originating from massive stellar objects and the theoretical frameworks that shape these phenomena.

The paper effectively illustrates the intricate nature of the astrophysical GW stochastic background while illuminating the prospects and complexities ahead in uncovering the vast astrophysical history embedded within.