- The paper demonstrates that gravitational-wave bursts from cosmic string kinks contribute nearly as significantly to the stochastic background as those from cusps.
- It employs analytical approximations and numerical integration to derive radiation expressions based on key parameters like string tension and loop size.
- Observational constraints from LIGO and projections for LISA help narrow cosmic string model parameters, enhancing our understanding of early-universe physics.
Gravitational-Wave Stochastic Background from Kinks and Cusps on Cosmic Strings
This paper provides a robust analytical and numerical paper of the contribution of kinks and cusps on cosmic string loops to the stochastic background of gravitational waves (SBGW). Topological defects such as cosmic strings are relics of symmetry-breaking processes in the early universe and are predicted by a variety of grand unified theories. Such defects, particularly cosmic strings, have long been hypothesized to be sources of gravitational waves, among other astrophysical phenomena.
Overview and Contributions
The paper first contextualizes cosmic strings within theoretical frameworks and outlines their evolution into loops as the universe expands. It distinguishes between field-theoretical cosmic strings and their cosmic superstring counterparts. The primary focus is the stochastic gravitational wave background generated by kinks and cusps on these cosmic string loops.
Cosmic string loops exhibit features known as cusps and kinks. Cusps result from configurations where the string momentarily reaches a speed close to that of light, focusing energy and potentially causing detectable bursts of gravitational waves due to their large Lorentz factors. Kinks, on the other hand, are localized "sharp" features formed from cosmic string intercommutations. The paper extends previous gravitational wave background computations, which mainly considered cusps, by incorporating the contribution from kinks.
Methodology
Using the weak-field approximation, the paper derives expressions for the gravitational radiation from cosmic string loops. The authors calculate the energy-momentum tensor of both kinks and cusps on these loops, further translating these metrics into expressions for the gravitational radiation observable by current and future detectors like LIGO and LISA.
Their formulation addresses the impact of cosmic string parameters such as the string tension (Gµ), reconnection probability (p), and loop size determined by gravitational back-reaction (α), on the gravitational wave spectrum. This involves evaluating a double integral over redshift and loop length to derive the stochastic background spectrum.
Results
The paper presents compelling numerical results demonstrating that kinks contribute to the SBGW at almost the same level as cusps. This finding contradicts some earlier assumptions that cusps would dominate the gravitational wave signal. Observational data from gravitational wave detectors constrain various cosmic string model parameters. For instance, the paper finds that gravitational wave backgrounds arising from cosmic string configurations with certain tensions (Gµ) and reconnection probabilities (p) are accessible to current LIGO observations and future LISA sensitivity.
Implications and Speculations
These findings present significant implications for cosmic string models and their detectability within gravitational wave data. The sensitivity of current detectors to various cosmic string parameter spaces emphasizes the importance of multi-messenger astronomy in probing the structure of the early universe. As LISA and more advanced gravitational wave observatories come online, it is expected that detections or even more stringent constraints on cosmic string scenarios can be achieved.
The contribution of kinks suggests a more complex structure to the stochastic background than previously thought. Their inclusion into models enhances our understanding of potential gravitational wave backgrounds, aiding the exploration of physics beyond the Standard Model, such as theories involving extra dimensions and cosmic superstrings.
Conclusion
This paper methodically identifies the contributions of distinct cosmic string features to the gravitational wave spectrum and constraints various cosmic string scenarios through comparison with observational limits. As the field evolves, the methods and results presented in this work will serve as key references for interpreting data from ongoing and future gravitational wave experiments, which continue to explore both cosmic phenomena and the fundamental nature of gravity.