Researcher:
Coates, Andrew

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Andrew

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Coates

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Coates, Andrew

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2020 - 20236

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Researcher Of Publications: search.filters.isAuthorOfPublication.22ffbf13-08bb-4ca2-9497-fa4782085dd0

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Now showing 1 - 6 of 6
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    Coordinate singularities of self-interacting vector field theories
    (American Physical Society (APS), 2023) Department of Physics; Department of Physics; Coates, Andrew; Ramazanoğlu, Fethi Mübin; Researcher; Faculty Member; Department of Physics; College of Sciences; College of Sciences; N/A; 254225
    Self-interacting vectors are seeing a burst of interest where various groups demonstrated that the field evolution ends in finite time. Two nonequivalent criteria have been offered to identify this breakdown: (i) the vector constraint equation cannot be satisfied beyond a point where the breakdown occurs, (ii) the dynamics is governed by an effective metric that becomes singular at the breakdown. We show that (i) identifies a coordinate singularity, and can be removed by a change of coordinates. Hence, it does not signify a physical problem, and cannot determine the validity of a theory.
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    PublicationOpen Access
    Instability of vectorized stars
    (American Physical Society (APS), 2022) Department of Physics; Coates, Andrew; Ramazanoğlu, Fethi Mübin; Researcher; Faculty Member; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; N/A; 254225; N/A
    In recent papers it has been shown that a large class of vectorization mechanisms in gravity, which involve the vector fields becoming apparently tachyonic in some regime, are actually dominated by ghosts and nonperturbative behavior. Despite this, vectorized compact object solutions have previously been found, which raises the question of how, and if, the newly discovered ghosts are quenched in these cases. Here we develop the tools to study the perturbations of vectorized compact objects, and demonstrate that they suffer from ghosts and gradient instabilities as well. Thus, these vectorized objects do not represent the stable end point of a quenched instability unlike their scalarized counterparts in the spontaneous scalarization literature.
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    PublicationOpen Access
    Intrinsic pathology of self-interacting vector fields
    (American Physical Society (APS), 2022) Department of Physics; Ramazanoğlu, Fethi Mübin; Coates, Andrew; Faculty Member; Department of Physics; College of Sciences; 254225; N/A
    We show that self-interacting vector field theories exhibit unphysical behavior even when they are not coupled to any external field. This means any theory featuring such vectors is in danger of being unphysical, an alarming prospect for many proposals in cosmology, gravity, high energy physics, and beyond. The problem arises when vector fields with healthy configurations naturally reach a point where time evolution is mathematically ill defined. We develop tools to easily identify this issue, and provide a simple and unifying framework to investigate it.
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    PublicationOpen Access
    On black hole area quantization and echoes
    (Institute of Physics (IOP) Publishing, 2022) Volkel, Sebastian H.; Kokkotas, Kostas D.; Department of Physics; Coates, Andrew; Researcher; Department of Physics; College of Sciences
    In this work we argue that black hole (BH) area quantization of Bekenstein and Mukhanov should not give rise to measurable effects in terms of so-called gravitational wave echoes during BH mergers. We outline that the quantum spectrum of a BH should be washed out during and after BH mergers, and hence one should not expect echoes in this scenario. The extreme broadening of the spectrum is due to the large particle emission rate during ringdown. Our results question key assumptions being made in recent literature on this topic.
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    PublicationOpen Access
    Gravitational Higgs mechanism and resulting observational effects
    (American Physical Society (APS), 2020) Krall, Verena; Kokkotas, Kostas D.; Department of Physics; Coates, Andrew; Researcher; Department of Physics; College of Sciences
    Recently, a toy model was introduced to demonstrate that screening mechanisms in alternative theories of gravitation can hide additional effects. In this model a scalar field is charged under a U(1) symmetry. In sufficiently compact objects the scalar field spontaneously grows, i.e., the object scalarizes, spontaneously breaking the U(1) symmetry. Exactly as in the U(1) Higgs mechanism this leads to the emergence of a mass for the gauge field. The aim of this paper is to provide an example of the physical consequences if we consider this toy model as a prototype of Weak Equivalence Principle (WEP) violations. We model neutron stars with a dipolar magnetic field to compare the magnetic field behavior of stars in Einstein-Maxwell theory on the one hand and in scalar-tensor theory with the, so-called, gravitational Higgs mechanism on the other hand.
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    Ghost of vector fields in compact stars
    (American Physical Society (APS), 2022) Silva, Hector O.; Sotiriou, Thomas P.; Department of Physics; Coates, Andrew; Ramazanoğlu, Fethi Mübin; Researcher; Faculty Member; Department of Physics; College of Sciences; N/A; 254225
    Spontaneous scalarization is a mechanism that allows a scalar field to go undetected in weak gravity environments and yet develop a nontrivial configuration in strongly gravitating systems. At the perturbative level it manifests as a tachyonic instability around spacetimes that solve Einstein's equations. The endpoint of this instability is a nontrivial scalar field configuration that can significantly modify a compact object's structure and can produce observational signatures of the scalar field's presence. Does such a mechanism exists for vector fields? Here we revisit the model that constitutes the most straightforward generalization of the original scalarization model to a vector field and perform a perturbative analysis. We show that a ghost appears as soon as the square of the naive effective mass squared becomes negative anywhere. This result poses a serious obstacle in generalizing spontaneous scalarization to vector fields.