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

Deadline for manuscript submissions: closed (31 December 2020) | Viewed by 17556

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Special Issue Editors

Prof. Dr. Philippe Jetzer

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Guest Editor

Department of Physics, University of Zürich, Winterthuerstrasse 190, CH-8057 Zürich, Switzerland
Interests: general relativity; gravitational waves; gravitational microlensing; dark matter

Prof. Dr. Domenico Giulini

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Guest Editor

Gottfried Wilhelm Leibniz Universität, Hannover, Germany
Interests: special and general relativity; exact solutions to Einstein’s equations; quantum mechanics and gravity; quantum equivalence principle

Special Issue Information

Dear Colleagues,

The purpose of this Special Issue is to collect contributions on current and proposed tests of general relativity. In particular, we aim to provide the reader with an up-to-date overview of the status of such tests and also on current ideas for future improvements. Recent milestones include the improved limits on the Eötvös parameter provided by the MICROSCOPE satellite, the local position invariance tested by the two GALILEO satellites 5 and 6, and, last but not least, the discovery of gravitational waves. The latter opened up a variety of possibilities for new tests of general relativity, in particular in the strong-gravity regime. Important progress has also been achieved with observations of several binary pulsars.

Next year ACES (Atomic Clock Ensemble in Space), which is composed of an ensemble of atomic clocks, will be brought on board the International Space Station (ISS). ACES will compare the onboard clocks to clocks based on the ground using microwave and optical links. One of its primary goals is to refine the test of local position invariance through the accurate measurement of the gravitational redshift, which is a central prediction of Einstein’s theory of general relativity and a fundamental constituent of the so-called Einstein Equivalence Principle (EEP), which in turn provides the central experimental underpinning of all metric theories of gravitation.

Given the current exciting and accelerating developments in many branches of gravitational physics, we think that it is time to publish such a Special Issue that comprises many of the relevant topics concerning the tests themselves, as well as theoretical considerations of the implications that possible deviations from the predictions of general relativity might have. We wish to invite both original and review papers to this Special Issue along the lines mentioned above; however, suggestions are welcome.

Prof. Dr. Philippe Jetzer
Prof. Dr. Domenico Giulini
Guest Editors

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Keywords

Gravitational physics
General relativity
Experimental and observational relativity
Einstein’s Equivalence Principle
Gravitational waves

Published Papers (7 papers)

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Editorial

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2 pages, 166 KiB

Open AccessEditorial

Current and Future Tests of General Relativity

by Domenico Giulini and Philippe Jetzer

Universe 2022, 8(3), 143; https://doi.org/10.3390/universe8030143 - 23 Feb 2022

Cited by 1 | Viewed by 1402

Abstract

General Relativity (GR) holds a special place amongst all fundamental theories of physics: on one hand, it is the theory of all gravitational phenomena; on the other hand, it is also a theory of spacetime [...] Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

Research

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12 pages, 489 KiB

Open AccessArticle

On the 2PN Periastron Precession of the Double Pulsar PSR J0737–3039A/B

by Lorenzo Iorio

Universe 2021, 7(11), 443; https://doi.org/10.3390/universe7110443 - 17 Nov 2021

Cited by 2 | Viewed by 1393

Abstract

One of the post-Keplerian (PK) parameters determined in timing analyses of several binary pulsars is the fractional periastron advance per orbit

k^{PK}

. Along with other PK parameters, it is used in testing general relativity once it is translated into the periastron [...] Read more.

One of the post-Keplerian (PK) parameters determined in timing analyses of several binary pulsars is the fractional periastron advance per orbit

k^{PK}

. Along with other PK parameters, it is used in testing general relativity once it is translated into the periastron precession

{\dot{ω}}^{PK}

. It was recently remarked that the periastron

ω

of PSR J0737–3039A/B may be used to measure/constrain the moment of inertia of A through the extraction of the general relativistic Lense–Thirring precession

{\dot{ω}}^{LT, A} ≃ - 0 . 00060^{\circ} {yr}^{- 1}

from the experimentally determined periastron rate

{\dot{ω}}_{obs}

provided that the other post-Newtonian (PN) contributions to

{\dot{ω}}_{\exp}

can be accurately modeled. Among them, the 2PN seems to be of the same order of magnitude of

{\dot{ω}}^{LT, A}

. An analytical expression of the total 2PN periastron precession

{\dot{ω}}^{2 PN}

in terms of the osculating Keplerian orbital elements, valid not only for binary pulsars, is provided, thereby elucidating the subtleties implied in correctly calculating it from

k^{1 PN} + k^{2 PN}

and correcting some past errors by the present author. The formula for

{\dot{ω}}^{2 PN}

is demonstrated to be equivalent to that obtainable from

k^{1 PN} + k^{2 PN}

by Damour and Schäfer expressed in the Damour–Deruelle (DD) parameterization.

{\dot{ω}}^{2 PN}

actually depends on the initial orbital phase, hidden in the DD picture, so that

- 0 . 00080^{\circ} {yr}^{- 1} \leq {\dot{ω}}^{2 PN} \leq - 0 . 00045^{\circ} {yr}^{- 1}

. A recently released prediction of

{\dot{ω}}^{2 PN}

for PSR J0737–3039A/B is discussed. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

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12 pages, 355 KiB

Open AccessArticle

On the 2PN Pericentre Precession in the General Theory of Relativity and the Recently Discovered Fast-Orbiting S-Stars in Sgr A*

by Lorenzo Iorio

Universe 2021, 7(2), 37; https://doi.org/10.3390/universe7020037 - 4 Feb 2021

Cited by 10 | Viewed by 1966

Abstract

Recently, the secular pericentre precession was analytically computed to the second post-Newtonian (2PN) order by the present author with the Gauss equations in terms of the osculating Keplerian orbital elements in order to obtain closer contact with the observations in astronomical and astrophysical scenarios of potential interest. A discrepancy in previous results from other authors was found. Moreover, some of such findings by the same authors were deemed as mutually inconsistent. In this paper, it is demonstrated that, in fact, some calculation errors plagued the most recent calculations by the present author. They are explicitly disclosed and corrected. As a result, all of the examined approaches mutually agree, yielding the same analytical expression for the total 2PN pericentre precession once the appropriate conversions from the adopted parameterisations are made. It is also shown that, in the future, it may become measurable, at least in principle, for some of the recently discovered short-period S-stars in Sgr A*, such as S62 and S4714. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

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13 pages, 415 KiB

Open AccessEditor’s ChoiceArticle

Benefit of New High-Precision LLR Data for the Determination of Relativistic Parameters

by Liliane Biskupek, Jürgen Müller and Jean-Marie Torre

Universe 2021, 7(2), 34; https://doi.org/10.3390/universe7020034 - 3 Feb 2021

Cited by 15 | Viewed by 3000

Abstract

Since 1969, Lunar Laser Ranging (LLR) data have been collected by various observatories and analysed by different analysis groups. In the recent years, observations with bigger telescopes (APOLLO) and at infra-red wavelength (OCA) are carried out, resulting in a better distribution of precise LLR data over the lunar orbit and the observed retro-reflectors on the Moon. This is a great advantage for various investigations in the LLR analysis. The aim of this study is to evaluate the benefit of the new LLR data for the determination of relativistic parameters. Here, we show current results for relativistic parameters like a possible temporal variation of the gravitational constant

\dot{G} / G_{0} = (- 5.0 \pm 9.6) \times 10^{- 15} {yr}^{- 1}

, the equivalence principle with

Δ {(m_{g} / m_{i})}_{EM} = (- 2.1 \pm 2.4) \times 10^{- 14}

, and the PPN parameters

β - 1 = (6.2 \pm 7.2) \times 10^{- 5}

and

γ - 1 = (1.7 \pm 1.6) \times 10^{- 4}

. The results show a significant improvement in the accuracy of the various parameters, mainly due to better coverage of the lunar orbit, better distribution of measurements over the lunar retro-reflectors, and last but not least, higher accuracy of the data. Within the estimated accuracies, no violation of Einstein’s theory is found and the results set improved limits for the different effects. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

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19 pages, 511 KiB

Open AccessArticle

Revisiting the 2PN Pericenter Precession in View of Possible Future Measurements

by Lorenzo Iorio

Universe 2020, 6(4), 53; https://doi.org/10.3390/universe6040053 - 13 Apr 2020

Cited by 11 | Viewed by 2555

Abstract

At the second post-Newtonian (2PN) order, the secular pericenter precession

{\dot{ω}}^{2 PN}

At the second post-Newtonian (2PN) order, the secular pericenter precession

{\dot{ω}}^{2 PN}

of either a full two-body system made of well-detached non-rotating monopole masses of comparable size and a restricted two-body system composed of a point particle orbiting a fixed central mass have been analytically computed so far with a variety of approaches. We offer our contribution by analytically computing

{\dot{ω}}^{2 PN}

in a perturbative way with the method of variation of elliptical elements by explicitly calculating both the direct contribution due to the 2PN acceleration

A^{2 PN}

, and also an indirect part arising from the self-interaction of the 1PN acceleration

A^{1 PN}

in the orbital average accounting for the instantaneous shifts induced by

A^{1 PN}

itself. Explicit formulas are straightforwardly obtained for both the point particle and full two-body cases without recurring to simplifying assumptions on the eccentricity e. Two different numerical integrations of the equations of motion confirm our analytical results for both the direct and indirect precessions. The values of the resulting effects for Mercury and some binary pulsars are confronted with the present-day level of experimental accuracies in measuring/constraining their pericenter precessions. The supermassive binary black hole in the BL Lac object OJ 287 is considered as well. A comparison with some of the results appeared in the literature is made. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

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Review

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21 pages, 2002 KiB

Open AccessReview

Testing General Relativity with Gravitational Waves: An Overview

by N. V. Krishnendu and Frank Ohme

Universe 2021, 7(12), 497; https://doi.org/10.3390/universe7120497 - 16 Dec 2021

Cited by 21 | Viewed by 2803

Abstract

The detections of gravitational-wave (GW) signals from compact binary coalescence by ground-based detectors have opened up the era of GW astronomy. These observations provide opportunities to test Einstein’s general theory of relativity at the strong-field regime. Here we give a brief overview of the various GW-based tests of General Relativity (GR) performed by the LIGO-Virgo collaboration on the detected GW events to date. After providing details for the tests performed in four categories, we discuss the prospects for each test in the context of future GW detectors. The four categories of tests include the consistency tests, parametrized tests for GW generation and propagation, tests for the merger remnant properties, and GW polarization tests. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

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38 pages, 2458 KiB

Open AccessFeature PaperReview

Gravity Tests with Radio Pulsars

by Norbert Wex and Michael Kramer

Universe 2020, 6(9), 156; https://doi.org/10.3390/universe6090156 - 22 Sep 2020

Cited by 37 | Viewed by 3249

Abstract

The discovery of the first binary pulsar in 1974 has opened up a completely new field of experimental gravity. In numerous important ways, pulsars have taken precision gravity tests quantitatively and qualitatively beyond the weak-field slow-motion regime of the Solar System. Apart from the first verification of the existence of gravitational waves, binary pulsars for the first time gave us the possibility to study the dynamics of strongly self-gravitating bodies with high precision. To date there are several radio pulsars known which can be utilized for precision tests of gravity. Depending on their orbital properties and the nature of their companion, these pulsars probe various different predictions of general relativity and its alternatives in the mildly relativistic strong-field regime. In many aspects, pulsar tests are complementary to other present and upcoming gravity experiments, like gravitational-wave observatories or the Event Horizon Telescope. This review gives an introduction to gravity tests with radio pulsars and its theoretical foundations, highlights some of the most important results, and gives a brief outlook into the future of this important field of experimental gravity. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

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