Probing the gravitational wave background from cosmic strings with LISA (1909.00819v1)

Abstract: Cosmic string networks offer one of the best prospects for detection of cosmological gravitational waves (GWs). The combined incoherent GW emission of a large number of string loops leads to a stochastic GW background (SGWB), which encodes the properties of the string network. In this paper we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background, considering leading models of the string networks. We find that LISA will be able to probe cosmic strings with tensions $G\mu \gtrsim \mathcal{O}(10^{-17})$, improving by about $6$ orders of magnitude current pulsar timing arrays (PTA) constraints, and potentially $3$ orders of magnitude with respect to expected constraints from next generation PTA observatories. We include in our analysis possible modifications of the SGWB spectrum due to different hypotheses regarding cosmic history and the underlying physics of the string network. These include possible modifications in the SGWB spectrum due to changes in the number of relativistic degrees of freedom in the early Universe, the presence of a non-standard equation of state before the onset of radiation domination, or changes to the network dynamics due to a string inter-commutation probability less than unity. In the event of a detection, LISA's frequency band is well-positioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.

• The paper demonstrates LISA's ability to probe stochastic gravitational wave backgrounds from cosmic strings, tightening Gμ constraints by nearly six orders of magnitude.
• It employs three cosmic string evolution models to analyze the gravitational wave energy density spectrum and predict observable signatures.
• The study indicates that LISA could bridge theory and observation, advancing early Universe physics by testing symmetry-breaking predictions.

Analysis of Gravitational Wave Backgrounds from Cosmic Strings with LISA

The paper entitled "Probing the gravitational wave background from cosmic strings with LISA" explores the potential of the Laser Interferometer Space Antenna (LISA) to detect the gravitational wave background produced by cosmic strings. The authors focus on determining the constraints that LISA could place on the properties of cosmic string networks, particularly the string tension parameter $G\mu$. Such constraints could significantly improve upon those established by current and upcoming pulsar timing array (PTA) observations.

Cosmic strings, hypothesized as stable topological defects, are anticipated to arise from symmetry-breaking phase transitions in the early Universe or as extended objects from string theory. These strings are sources of gravitational waves (GWs), predominantly from oscillating loops that emit waves as they shrink via gravitational radiation. This background is stochastic, arising from the incoherent superposition of GWs emitted over the Universe's history.

Significant focus is placed on the parameter $G\mu$, where $G$ is Newton's constant and $\mu$ is the string tension, roughly related to the energy density per unit length of the string. The paper highlights LISA's potential to improve current $G\mu$ constraints by six orders of magnitude over PTAs, thus extending the sensitivity to $G\mu \gtrsim \mathcal{O}(10^{-17})$.

The analysis of the gravitational wave energy density spectrum $\mathrm{\Omega}_{\rm gw}(f)$ centers on three models for the loop number density, corresponding to different scenarios of cosmic string evolution. These include:

• Model I: An analytical approach where all loops are initially assumed to form at a fixed fraction of the characteristic long string length, obtained from global energy conservation.
• Model II: A simulation-informed model by Blanco-Pillado, Olum, and Shlaer (BOS), calibrated from numerical simulations predicting a significant fraction of energy in large loops.
• Model III: The Lorenz, Ringeval, and Sakellariadou (LRS) model, which includes a wider distribution of loop sizes and incorporates theoretical predictions of massive backreaction effects.

The spectral analysis considers the scale-dependent responses due to early Universe phenomena such as changes in the effective number of relativistic degrees of freedom and potential early phases such as matter domination or kination. These effects imprint features upon the SGWB, potentially accessible within LISA’s operational frequency range of $\approx 0.1$ mHz to 1 Hz.

By considering cases of reduced intercommuting probabilities, where cosmic strings interact less frequently, as with cosmic superstrings in certain string theory scenarios, the paper extends the possible bounds on $G\mu$ beyond those for simple Nambu-Goto strings. This has implications for the formation and evolution of these defect networks.

In summary, the investigation promises that LISA’s detection capabilities will provide a significant advance in constraining the properties of cosmic strings. The thorough exploration of the GW spectrum ensures that LISA will be crucial in testing the cosmological implications of particle physics theories that predict such topological defects. This work sets the stage for LISA not only to detect the GW signals from cosmic strings but also to provide insights into the physics of the early Universe, further bridging the gap between theory and observation in modern cosmology and astrophysics. Future advancements in detector sensitivity and data analysis techniques will be required to fully exploit LISA’s observational capabilities and to test the theoretical models presented in this research.