New limits on cosmic strings from gravitational wave observation (1709.02434v2)

Abstract: We combine new analysis of the stochastic gravitational wave background to be expected from cosmic strings with the latest pulsar timing array (PTA) limits to give an upper bound on the energy scale of the possible cosmic string network, $G\mu < 1.5\times 10^{-11}$ at the 95% confidence level. We also show bounds from LIGO and to be expected from LISA and BBO. Current estimates for the gravitational wave background from supermassive black hole binaries are at the level where a PTA detection is expected. But if PTAs do observe a background soon, it will be difficult in the short term to distinguish black holes from cosmic strings as the source, because the spectral indices from the two sources happen to be quite similar. If PTAs do not observe a background, then the limits on $G\mu$ will improve somewhat, but a string network with $G\mu$ substantially below $10^{-11}$ will produce gravitational waves primarily at frequencies too high for PTA observation, so significant further progress will depend on intermediate-frequency observatories such as LISA, DECIGO and BBO.

• The paper establishes new upper limits on cosmic string tension by modeling the stochastic gravitational wave background across cosmic history.
• It employs state-of-the-art techniques that integrate cosmic emissions and redshifting, constraining the tension parameter to below 1.5×10⁻¹¹.
• The research highlights that future observatories like LISA and BBO will enhance detection capabilities and clarify overlapping spectral contributions.

Overview of Cosmic String Limits from Gravitational Wave Observations

The research paper focuses on establishing new upper limits on the potential energy scale of cosmic string networks by leveraging the latest data from pulsar timing arrays (PTAs) along with an analysis of the stochastic gravitational wave background expected from cosmic strings. The paper also incorporates gravitational wave observations from LIGO and anticipates future data from observatories like LISA and BBO.

The concept of cosmic strings, proposed initially as topological defects emerging in unified field theories or as fundamental strings within string theory, manifests through gravitational interactions observable as bursts or a continuous stochastic background of gravitational waves. Specifically, this research centers on the stochastic background of radiation emitted by oscillating loops of cosmic strings throughout cosmic history.

Methodology and Key Findings

The examination utilizes recent advancements in modeling the gravitational wave background spectrum, significantly enhancing previous estimates by considering additional physical effects not accounted for in older analyses. The team employs state-of-the-art techniques to calculate the stochastic gravitational wave background by integrating emissions across cosmological epochs, accommodating pertinent factors like redshifting.

Key findings from the analysis include:

• The updated observational constraints restrict the cosmic string network energy scale $G\mu$ (where $\mu$ denotes tension and $G$ is Newton's constant) to values substantially lower than past limits derived from CMB data.
• Current estimates place the possible $G\mu$ values below $1.5 \times 10^{-11}$, primarily led by constraints from PTAs, such as the Parkes PTA and NANOGrav.
• Observations suggest that if PTAs detect a gravitational wave background, distinguishing between contributions from cosmic strings and supermassive black hole (SMBH) binaries will be challenging due to overlapping spectral characteristics.
• LIGO’s contribution, while significant in exploring portions of the gravitational wave spectrum, is less competitive in constraining cosmic strings due to frequency limitations compared to PTAs.
• The paper anticipates that forthcoming observation stations like LISA and BBO will better probe cosmic string signals at distinct spectral locations, exceeding current capabilities.

Implications and Future Prospects

The upper bounds revealed in this paper bear substantial implications for theoretical physics and cosmology:

• The constraints drastically limit the cosmic string tension, affecting predictions about structure formation and eliminating certain unified field theories that favor higher string tension.
• Continued non-detection of cosmic string signals in gravitational wave data narrows the parameter space for cosmic strings, influencing the plausibility of various cosmological models involving these entities.
• Should a gravitational wave background soon be detected, rigorous analysis will be imperative to conclusively attribute the source to either cosmic strings or SMBHs, necessitating heterogeneous datasets and advanced sensitivity in gravitational wave detection.
• Future high-frequency observatories are poised to enhance the detection potential of cosmic strings, affording greater clarity on the role of these exotic objects in the universe’s history.

In conclusion, the paper marks a pivotal point in the quest to define the properties of cosmic strings, leveraging gravitational wave observations as a vital tool in cosmology and particle physics, with the anticipation of future instrumental advancements providing even greater insight.