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 Gµ 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 wellpositioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.
We study the evolution of maximally symmetric p-branes with a Sp−i ⊗ Ê i topology in flat expanding or collapsing homogeneous and isotropic universes with N + 1 dimensions (with N ≥ 3, p < N , 0 ≤ i < p). We find the corresponding equations of motion and compute new analytical solutions for the trajectories in phase space. For a constant Hubble parameter, H, and i = 0 we show that all initially static solutions with a physical radius below a certain critical value, r 0 c , are periodic while those with a larger initial radius become frozen in comoving coordinates at late times. We find a stationary solution with constant velocity and physical radius, rc, and compute the root mean square velocity of the periodic p-brane solutions and the corresponding (average) equation of state of the p-brane gas. We also investigate the p-brane dynamics for H = constant in models where the evolution of the universe is driven by a perfect fluid with constant equation of state parameter, w = Pp/ρp, and show that a critical radius, rc, can still be defined for −1 ≤ w < wc with wc = (2 − N )/N . We further show that for w ∼ wc the critical radius is given approximately by rcH ∝ (wc − w) γc with γc = −1/2 (rcH → ∞ when w → wc). Finally, we discuss the impact that the large scale dynamics of the universe can have on the macroscopic evolution of very small loops.
We develop a velocity-dependent one-scale model describing p-brane dynamics in flat homogeneous and isotropic backgrounds in a unified framework. We find the corresponding scaling laws in frictionless and friction dominated regimes considering both expanding and collapsing phases.
We consider a model where particles are described as localized concentrations of energy, with fixed rest mass and structure, which are not significantly affected by their self-induced gravitational field. We show that the volume average of the on-shell matter Lagrangian Lm describing such particles, in the proper frame, is equal to the volume average of the trace T of the energy-momentum tensor in the same frame, independently of the particle's structure and constitution. Since both Lm and T are scalars, and thus independent of the reference frame, this result is also applicable to collections of moving particles and, in particular, to those which can be described by a perfect fluid. Our results are expected to be particularly relevant in the case of modified theories of gravity with nonminimal coupling to matter where the matter Lagrangian appears explicitly in the equations of motion of the gravitational and matter fields, such as f (R, Lm) and f (R, T ) gravity. In particular, they indicate that, in this context, f (R, Lm) theories may be regarded as a subclass of f (R, T ) gravity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.