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Universe
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Continuous Gravitational Waves
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Continuous Gravitational Waves

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Gravitation".

Deadline for manuscript submissions: closed (15 July 2022) | Viewed by 19254

Special Issue Editors

Prof. Dr. Paola Leaci

E-Mail Website
Guest Editor
Physics Department, Rome Sapienza University, Piazzale Aldo Moro 2, 00185 Rome, Italy
Interests: gravitational-wave physics; gravitational-wave data analysis; pulsar search; observational relativity and cosmology
Prof. Dr. Andrzej Królak

E-Mail Website
Guest Editor
Institute of Mathematics of Polish Academy of Sciences, Sniadeckich 8, 00-656 Warsaw, Poland
Interests: gravitation and general theory of relativity; Cosmology; Theoretical Physics; Mathematical Physics

Special Issue Information

Dear Colleagues,

Among the uppermost priorities of the gravitational-wave community, undetected feeble continuous gravitational waves (CWs) stand out, the detection of which is crucial for the comprehension of matter at extreme supranuclear densities and highly relativistic regimes. These signals are typically emitted by non-axisymmetric and rapidly rotating neutron stars (either isolated or in a binary system), but also by ultra-light boson clouds around spinning black holes (according to what has been recently and theoretically shown).

Multiple efforts are currently ongoing to detect CW signals analyzing the most sensitive advanced LIGO-Virgo observation runs via both veteran searches and developing robust and deep-learning-based algorithms.

This Special Issue aims to foster progress in the CW field. Interested colleagues are invited to submit research papers that can contribute to CW detection. This will allow us to shed light on diverse fundamental physics questions, supporting tests of general relativity (studying the polarization content of the gravitational signal) and possibly bringing to unexpected outcomes.

Prof. Dr. Paola Leaci
Prof. Dr. Andrzej Królak
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Gravitational waves
  • Data-analysis techniques
  • Neutron stars
  • Black Holes
  • Boson clouds
  • Tests of general relativity
  • Deep learning

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Published Papers (6 papers)

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Research

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8 pages, 252 KiB  
Article
A Fast Data Processing Technique for Continuous Gravitational Wave Searches
by Massimo Tinto
Universe 2021, 7(12), 486; https://doi.org/10.3390/universe7120486 - 10 Dec 2021
Viewed by 1947
Abstract
This article discusses the potential advantages of a data processing technique for continuous gravitational wave signals searches in the data measured by ground-based gravitational wave interferometers. Its main advantage over other techniques is that it does not need to search over the signal’s [...] Read more.
This article discusses the potential advantages of a data processing technique for continuous gravitational wave signals searches in the data measured by ground-based gravitational wave interferometers. Its main advantage over other techniques is that it does not need to search over the signal’s direction of propagation. Although it is a “coherent method” (i.e., it coherently processes year-long data), it is applied to a data set obtained by multiplying the original time-series with a (half-year) time-shifted copy of it. As a result, the phase modulation due to the interferometer motion around the Sun is automatically canceled in the signal of the synthesized time-series. Although the resulting signal-to-noise ratio is not as high as that of a coherent search, it equals that of current hierarchical methods. In addition, since the signal search is performed over a parameters space of smaller dimensionality, the associated false-alarm probability should be smaller than those characterizing hierarchical methods and result in an improved likelihood of detection. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
23 pages, 515 KiB  
Article
Probing Gravitational Waves from Pulsars in Brans–Dicke Theory
by Paritosh Verma
Universe 2021, 7(7), 235; https://doi.org/10.3390/universe7070235 - 9 Jul 2021
Cited by 6 | Viewed by 4855
Abstract
This paper comprises the theoretical background for the data analysis of gravitational waves (GWs) from spinning neutron stars in Brans–Dicke (BD) theory. Einstein’s general theory of relativity (GR) predicts only two tensor polarization states, but a generic metric theory of gravity can also [...] Read more.
This paper comprises the theoretical background for the data analysis of gravitational waves (GWs) from spinning neutron stars in Brans–Dicke (BD) theory. Einstein’s general theory of relativity (GR) predicts only two tensor polarization states, but a generic metric theory of gravity can also possess scalar and vector polarization states. The BD theory attempts to modify the GR by varying gravitational constant G, and it has three polarization states. The first two states are the same as in GR, and the third one is scalar polarization. We derive the response of a laser interferometric detector to the GW signal from a spinning neutron star in BD theory. We obtain a statistic based on the maximum likelihood principle to identify the signal in BD theory in the detector’s noise. This statistic generalizes the well known F-statistic used in the case of GR. We perform Monte Carlo simulations in Gaussian noise to test the detectability of the signal and the accuracy of estimation of its parameters. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
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12 pages, 836 KiB  
Communication
Continuous Gravitational-Wave Data Analysis with General Purpose Computing on Graphic Processing Units
by Iuri La Rosa, Pia Astone, Sabrina D’Antonio, Sergio Frasca, Paola Leaci, Andrew Lawrence Miller, Cristiano Palomba, Ornella Juliana Piccinni, Lorenzo Pierini and Tania Regimbau
Universe 2021, 7(7), 218; https://doi.org/10.3390/universe7070218 - 30 Jun 2021
Cited by 4 | Viewed by 1781
Abstract
We present a new approach to searching for Continuous gravitational Waves (CWs) emitted by isolated rotating neutron stars, using the high parallel computing efficiency and computational power of modern Graphic Processing Units (GPUs). Specifically, in this paper the porting of one of the [...] Read more.
We present a new approach to searching for Continuous gravitational Waves (CWs) emitted by isolated rotating neutron stars, using the high parallel computing efficiency and computational power of modern Graphic Processing Units (GPUs). Specifically, in this paper the porting of one of the algorithms used to search for CW signals, the so-called FrequencyHough transform, on the TensorFlow framework, is described. The new code has been fully tested and its performance on GPUs has been compared to those in a CPU multicore system of the same class, showing a factor of 10 speed-up. This demonstrates that GPU programming with general purpose libraries (the those of the TensorFlow framework) of a high-level programming language can provide a significant improvement of the performance of data analysis, opening new perspectives on wide-parameter searches for CWs. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
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14 pages, 734 KiB  
Article
Geometric Approach to Analytic Marginalisation of the Likelihood Ratio for Continuous Gravitational Wave Searches
by Karl Wette
Universe 2021, 7(6), 174; https://doi.org/10.3390/universe7060174 - 1 Jun 2021
Cited by 4 | Viewed by 2597
Abstract
The likelihood ratio for a continuous gravitational wave signal is viewed geometrically as a function of the orientation of two vectors; one representing the optimal signal-to-noise ratio, and the other representing the maximised likelihood ratio or F-statistic. Analytic marginalisation over the angle [...] Read more.
The likelihood ratio for a continuous gravitational wave signal is viewed geometrically as a function of the orientation of two vectors; one representing the optimal signal-to-noise ratio, and the other representing the maximised likelihood ratio or F-statistic. Analytic marginalisation over the angle between the vectors yields a marginalised likelihood ratio, which is a function of the F-statistic. Further analytic marginalisation over the optimal signal-to-noise ratio is explored using different choices of prior. Monte-Carlo simulations show that the marginalised likelihood ratios had identical detection power to the F-statistic. This approach demonstrates a route to viewing the F-statistic in a Bayesian context, while retaining the advantages of its efficient computation. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
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Review

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13 pages, 337 KiB  
Review
Gravitational Lensing of Continuous Gravitational Waves
by Marek Biesiada and Sreekanth Harikumar
Universe 2021, 7(12), 502; https://doi.org/10.3390/universe7120502 - 17 Dec 2021
Cited by 10 | Viewed by 2891
Abstract
Continuous gravitational waves are analogous to monochromatic light and could therefore be used to detect wave effects such as interference or diffraction. This would be possible with strongly lensed gravitational waves. This article reviews and summarises the theory of gravitational lensing in the [...] Read more.
Continuous gravitational waves are analogous to monochromatic light and could therefore be used to detect wave effects such as interference or diffraction. This would be possible with strongly lensed gravitational waves. This article reviews and summarises the theory of gravitational lensing in the context of gravitational waves in two different regimes: geometric optics and wave optics, for two widely used lens models such as the point mass lens and the Singular Isothermal Sphere (SIS). Observable effects due to the wave nature of gravitational waves are discussed. As a consequence of interference, GWs produce beat patterns which might be observable with next generation detectors such as the ground based Einstein Telescope and Cosmic Explorer, or the space-borne LISA and DECIGO. This will provide us with an opportunity to estimate the properties of the lensing system and other cosmological parameters with alternative techniques. Diffractive microlensing could become a valuable method of searching for intermediate mass black holes formed in the centres of globular clusters. We also point to an interesting idea of detecting the Poisson–Arago spot proposed in the literature. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
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26 pages, 896 KiB  
Review
Search Methods for Continuous Gravitational-Wave Signals from Unknown Sources in the Advanced-Detector Era
by Rodrigo Tenorio, David Keitel and Alicia M. Sintes
Universe 2021, 7(12), 474; https://doi.org/10.3390/universe7120474 - 4 Dec 2021
Cited by 42 | Viewed by 3526
Abstract
Continuous gravitational waves are long-lasting forms of gravitational radiation produced by persistent quadrupolar variations of matter. Standard expected sources for ground-based interferometric detectors are neutron stars presenting non-axisymmetries such as crustal deformations, r-modes or free precession. More exotic sources could include decaying ultralight [...] Read more.
Continuous gravitational waves are long-lasting forms of gravitational radiation produced by persistent quadrupolar variations of matter. Standard expected sources for ground-based interferometric detectors are neutron stars presenting non-axisymmetries such as crustal deformations, r-modes or free precession. More exotic sources could include decaying ultralight boson clouds around spinning black holes. A rich suite of data-analysis methods spanning a wide bracket of thresholds between sensitivity and computational efficiency has been developed during the last decades to search for these signals. In this work, we review the current state of searches for continuous gravitational waves using ground-based interferometer data, focusing on searches for unknown sources. These searches typically consist of a main stage followed by several post-processing steps to rule out outliers produced by detector noise. So far, no continuous gravitational wave signal has been confidently detected, although tighter upper limits are placed as detectors and search methods are further developed. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
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