Cosmological Backgrounds of Gravitational Waves and eLISA/NGO: Phase Transitions, Cosmic Strings and Other Sources (1201.0983v3)

Abstract: We review several cosmological backgrounds of gravitational waves accessible to direct-detection experiments, with a special emphasis on those backgrounds due to first-order phase transitions and networks of cosmic (super-)strings. For these two particular sources, we revisit in detail the computation of the gravitational wave background and improve the results of previous works in the literature. We apply our results to identify the scientific potential of the NGO/eLISA mission of ESA regarding the detectability of cosmological backgrounds.

• The paper demonstrates that phase transitions in the early universe generate distinct gravitational wave spectra, with bubble collisions and MHD turbulence yielding characteristic frequency slopes.
• The study refines predictions by modeling bubble wall dynamics and friction effects during phase transitions, enhancing the accuracy of gravitational wave emission forecasts.
• It also evaluates cosmic string contributions, outlining how eLISA can detect multi-band signals that probe high-energy physics from the early universe.

Cosmological Backgrounds of Gravitational Waves and eLISA/NGO: Phase Transitions, Cosmic Strings, and Other Sources

The paper "Cosmological Backgrounds of Gravitational Waves and eLISA/NGO: Phase Transitions, Cosmic Strings and Other Sources" presents a comprehensive analysis of gravitational wave (GW) backgrounds that could be detected by space-based observatories such as the evolved Laser Interferometer Space Antenna (eLISA), also referred to as the New Gravitational wave Observatory (NGO). This work explores two principal cosmological sources of stochastic GW backgrounds: first-order phase transitions in the early universe and cosmic strings. The assessment provides detailed evaluations of GW generation mechanisms and potential observability by eLISA, alongside discussing implications for understanding the early universe.

Gravitational Waves from First-Order Phase Transitions

The paper focuses extensively on first-order phase transitions, which are characterized by the nucleation of bubbles of a broken symmetry phase within the false vacuum, leading to violent events that can generate stochastic GW backgrounds. Notably, the paper discusses critical parameters such as the latent heat (quantified by the parameter $\alpha$), the rate of the transition ($\beta$), and the bubble wall velocity. It highlights the inadequacy of the Jouguet detonation assumption, emphasizing the need for more nuanced models that incorporate friction effects on bubble walls exerted by the surrounding plasma. This friction introduces additional variables such as the friction coefficient $\eta$, affecting both the dynamics of bubble walls and the resulting GW spectra.

The work improves detection forecasts by integrating recent theoretical advancements into the GW emission modeling from bubble collisions and magnetohydrodynamic turbulence. The derived GW frequency spectra exhibit characteristic slopes, with bubble collisions yielding a $f^{-1}$ high-frequency tail and MHD turbulence contributing a $f^{-5/3}$ slope. Importantly, eLISA's sensitivity could probe GW signals from phase transitions occurring at energy scales between 100 GeV and 10 TeV, capturing potential cosmological events beyond the reach of current particle accelerators such as the LHC.

Gravitational Waves from Cosmic Strings

In addition to phase transitions, the paper examines GWs arising from cosmic strings—one-dimensional topological defects formed in the early universe. The analysis distinguishes between cosmic strings with small initial loop sizes $\alpha \lesssim \Gamma G \mu$ and those with larger loop sizes $\alpha \sim 0.1$. For both scenarios, it outlines the methods to calculate the stochastic GW backgrounds and considers the implications of loop production models, string tension $G\mu$, and reconnection probabilities $p$.

The GW spectra from cosmic strings span a wide frequency range, enabling potential multi-band detections across ground-based observers like LIGO and space-based interferometers like eLISA. This wide spectral reach, governed by scaling laws tied to high-energy physics beyond the standard model, allows for the dissemination of crucial information about the early universe up to energy scales nearing $10^{16}$ GeV, potentially overlapping insights gathered from cosmic microwave background analyses.

Implications and Future Perspectives

Assessing the implications for future gravitational wave observatories, the paper integrates theoretical predictions with observational strategies, spotlighting the potential discovery space of eLISA. This includes discerning limits of detectability regarding cosmological backgrounds and distinguishing cosmological GW sources from galactic binaries and instrumental noise. Consequently, eLISA could not only corroborate existing astrophysical theories but also shed light on uncharted territories of early-universe high-energy physics, influencing future experimentation and observation strategies.

Moreover, the paper paves the way for investigating unconventional scenarios, such as varied thermal histories of the universe or yet-undiscovered gravitational wave sources. By laying out the framework for integrating eLISA's observational capabilities with current GW data, this research underscores the instrument's potential to act as a pivotal tool in astrophysics and cosmology, probing epochs beyond the reach of electromagnetic signals.