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

TL;DR

The paper addresses the detectability of cosmological gravitational-wave backgrounds in the eLISA/NGO band, focusing on first-order phase transitions and cosmic strings as the most promising sources. It advances the field by relaxing restrictive assumptions (e.g., Jouguet detonations) and by integrating improved models of bubble collisions with MHD turbulence, as well as by refining the calculation of the stochastic background from both small and large cosmic-string loops while accounting for the early-universe cosmology. The authors provide updated predictions for the GW spectra and deliver detection forecasts showing that eLISA could probe TeV-scale phase transitions and a broad region of cosmic-string parameter space, with complementary constraints from ground-based detectors and pulsar timing arrays. Overall, the work strengthens the case for eLISA as a powerful probe of high-energy physics and early-universe dynamics, guiding both theoretical modeling and observational strategies.

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.

Figures (13)

• Figure 1: Sensitivity curve of eLISA computed using the expected instrumental noise and the confusion noise generated by unresolved galactic binaries yellowbook.
• Figure 2: GW spectra for the EWPT in the SM augmented by dimension six operators arXiv:0709.2091. Left panel, $\eta=0.2$, Right panel, $\eta=1$. The red line shows the eLISA noise curve. The GW spectra correspond, from bottom to top, to increasing values of $\alpha$ as given in Table 1 of Huber and Konstandin 2008 arXiv:0709.2091: $\alpha=0.128\,,~\alpha=0.201\,,~\alpha=0.311\,,~\alpha=0.586\,,~\alpha=1.197\,,~\alpha=2.268$. The values of $\beta/H_*$ and $T_*$ decrease accordingly (see discussion in the main text).
• Figure 3: GW spectra for the holographic PT hep-ph/0607158arXiv:1007.1468. The black line shows the eLISA noise curve. The solid lines are the GW spectra for $T_*= 100$ GeV, and from top to bottom $\beta/H_*=6$ and $\beta/H_*=15$; the dashed lines are for $T_*= 10^4$ GeV, and from top to bottom again $\beta/H_*=6$ and $\beta/H_*=15$.
• Figure 4: Contour plots in the $(\alpha\,,\beta/H_*)$ parameter space for different values of $T_*$ and two values of the friction $\eta$. eLISA is most sensitive to PTs occuring at temperatures of the order of 10 TeV, for which it can detect a wider range of values of $\alpha$ and $\beta/H_*$. This means that if the PT takes place at high temperature, eLISA can detect its GW signal even if the PT is not very strong and occurs quite fast. The shadowed regions correspond to a signal to noise ratio of at least one.
• Figure 5: Contour plots in the $(T_*\,,\beta/H_*)$ parameter space for very strong PTs, $\alpha\gtrsim 10$, and reasonable values of the friction $\eta\lesssim 1$. Again, we see that eLISA is most sensitive to PT at temperatures around $T_*\simeq 10$ TeV, where it can detect PTs occurring slowly (small $\beta/H_*$) with a signal to noise ratio higher than 10.