Probing the gravitational wave background from cosmic strings with LISA

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 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.

Cylindrical systems in general relativity

“…One of such cosmological sources is the cosmic superstring network [91], which emits GWs throughout the history of the Universe, and generates a SGWB from the superposition of many uncorrelated sources, as mentioned in Introduction. Recently, it was found that LISA will be able to detect such a background with Gµ/c 2 O 10 −17 [14], while currently the most stringent bounds come from pulsar timing arrays [38], which yield the upper bound Gµ/c 2 O 10 −11 . On the other hand, depending on the string network model, LIGO-Virgo observations could constrain it to Gµ/c 2 O 10 −14 [5,6].…”

Metastable strings and dumbbells in supersymmetric hybrid inflation

“…where m M is the monopole mass and µ is the string tension. Using this decay rate and the BOS model [29] for GW emission from string loops (for a general discussion, see [3]), the SGWB generated by a metastable string network was evaluated for the parameters of the B−L model. In the LIGO-Virgo frequency band a GW signal close to the current upper limit was predicted 1 and from the PTA bounds an upper bound on the monopole-stringtension ratio was obtained, √ κ 8 [24].…”

“…We are interested in embedding U(1) B−L into a larger group such that B−L strings can break into segments connecting a monopole with an antimonopole. The simplest possibility is to extend the electroweak part of the standard model gauge group to SU(2 3 . Hence, the homotopy groups π 1 (M) and π 2 (M) are trivial and there are neither topologically stable monopoles nor strings.…”

“…While monopoles and domain walls would overclose the universe and must therefore be avoided, cosmic strings evolve towards a scaling regime where their fraction of the total energy density is constant. Cosmic strings have characteristic signatures in gravitational lensing, the cosmic microwave background and the stochastic gravitational wave background (SGWB) and are therefore a potentially very interesting messenger from the early universe (for reviews and references, see, for example, [2,3]).…”

Trans-Planckian censorship and inflation in grand unified theories

“…However, at second sight, the production of cosmic strings after DHI could also turn into virtue; namely, if they should be unstable, which would be the case if the gauge group of our model was embedded in a semisimple GUT gauge group at higher energies [73] (see also [74,75]). This is a reasonable assumption and could lead to a promising scenario where the string network first emits an observable signal in gravitational waves [76] and then decays during the radiation-dominated era, so as to circumvent the CMB bound on Gµ cs . We will come back to such a scenario in future work.…”