arXiv subjects

Fethi M. Ramazanoğlu

Publications and source records attributed to Fethi M. Ramazanoğlu.

At least 19 recordsLinked to original sources

arXiv2026

Novel Observable Signals from First-Order Gravitational Phase Transitions

Spontaneous scalarization is a well-known phenomenon in scalar-tensor theories, featuring large deviations from general relativity. However, some of its signatures are hard to detect, because it is generally studied as a continuous phase transition. We show that scalarization occurs in a discontinuous (first-order) manner in many astrophysically relevant scenarios. This leads to loud signals that provide new tests of gravity. We demonstrate this in the case of gravitational wave breathing modes from a neutron star that scalarizes due to accretion, and discuss other possibilities.

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arXiv2026

Underlying mechanisms of phase transitions in scalar-tensor theories

Spontaneous scalarization phenomenon in scalar-tensor gravity is known to be a form of phase transition, and it was recently shown that the order of this transition changes depending on the parameters of the theory. There exists a phenomenological description of this result based on Landau theory, but the underlying mechanisms which determine the coefficients of the Landau expansion were unknown. In this study we calculate these coefficients starting from first principles. To this end, we start with an energy functional that describes the nonlinear behavior of the theory, and reduce it to an energy function. This allows us to explain the previously observed, but not well-understood, features of the scalarization phase transition, and enables us to predict which phase transition order will be present for which coupling function or in which regime of the parameter space. The details of the phase transition determine certain astrophysical observables such as signals sourced by transitions from metastable states in first-order scalarization. Thus, predicting these details is an important part of understanding scalarization itself.

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arXiv2026

Potentially healthy time evolution in interacting $p$-form fields

It was recently discovered that many self-interacting vector fields can have healthy time evolution when field amplitudes are small, however, further dynamics becomes impossible if the norm of the field reaches a certain finite value. This result was also generalized to higher form fields using the fact that vector fields can also be viewed as 1-form fields. However, other recent work demonstrated that some exceptional vector field theories do not suffer from such problems if the self-interaction term is chosen in a specific way. Here, we study whether a similar exception exists for $p$-form fields in general. We show that, in 4 spacetime dimensions, 3-form fields with self interaction can indeed have healthy time evolution similarly to vectors. However, the analogous coupling terms still lead to loss of hyperbolicity for 2-form fields. We study the reasons for these differences, and comment on directions to generalize our findings. We also demonstrate that eliminating the mass term, one of the proposals to build healthy self-interacting vector field theories, moves the singularity to other parts of the equations of motion, hence, most likely does not provide well-posed time evolution.

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arXiv2026

Phase transition structure of scalarized neutron stars: the effect of rotation and linear coupling

There has been a recent revival in understanding the spontaneous scalarization phenomenon in scalar-tensor gravity as a phase transition. Using the tools of the Landau theory, we now know that first-order transitions where scalarization occurs in a discontinuous manner is more prominent than what had been considered in the literature, and this might lead to novel observation channels. However, the examples so far have been restricted to specific quadratic scalar coupling terms and spherically symmetric stars. Here we explore the phase transition structure of scalarization for more general couplings, considering linear as well as quadratic terms in the conformal scaling factor of the theory. Moreover, we also investigate the effect of rotation on the scalarization phase transition. Both of these considerations are natural choices since the coupling in a scalar-tensor theory can appear at all orders, and astrophysical neutron stars commonly have angular momentum. The introduction of linear coupling leads to a complex solution space which is harder to explore. However, we demonstrate that the Landau model of scalarization enables us to systematically find the branches of scalarized solutions that are commonly overlooked in numerical searches, providing a novel tool. On the other hand, the main effect of stellar rotation is shifting the stellar masses at which the phase transition occurs to higher values, but the qualitative picture remains similar to what happens under spherical symmetry.

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arXiv2025

Nature of phase transitions and metastability in scalar-tensor theories

Compact stars above a critical stellar mass develop large scalar fields in some scalar-tensor theories. This scenario, called spontaneous scalarization, has been an intense topic of study since it passes weak-field gravity tests naturally while providing clear observables in the strong-field regime. The underlying mechanism for the onset of scalarization is often depicted as a second-order phase transition. Here, we show that a first-order phase transition is in fact the most common mechanism. This means metastability and transitions between locally stable compact object configurations are much more likely than previously believed, opening vast new avenues for observational prospects.

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arXiv2024

Spontaneous scalarization

Scalarization is a mechanism that endows strongly self-gravitating bodies, such as neutron stars and black holes, with a scalar-field configuration. It resembles a phase transition in that the scalar configuration appears only when a certain quantity that characterizes the compact object, for example, its compactness or spin, is beyond a threshold. A critical and comprehensive review of scalarization, including the mechanism itself, theories that exhibit it, its manifestation in neutron stars, black holes and their binaries, potential extension to other fields, and a thorough discussion of future perspectives, is provided.

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arXiv2024

Aspects of time evolution in $p$-form field theories

We show that higher form fields, specifically 2- and 3-forms in four spacetime dimensions, suffer from loss of hyperbolicity when they have self interaction. The equations of motion lose their wave-like characteristic in certain parts of the configuration space, and such problematic regions can also be reached dynamically from healthy initial data. These findings are analogous to recent results for vector fields, which are equivalently 1-forms, however, the details of the pathology can be different for higher forms. We also show that the possibility of more than one self interaction term in higher forms also leads to the amelioration of hyperbolicity problems.

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arXiv2023

Treatments and placebos for the pathologies of effective field theories

We demonstrate some shortcomings of "fixing the equations," an increasingly popular remedy for time evolution problems of effective field theories (EFTs). We compare the EFTs and their "fixed" versions to the UV theories from which they can be derived in two cases: K-essence and nonlinear Proca theory. We find that when an EFT breaks down due to loss of hyperbolicity, fixing does not approximate the UV theory well if the UV solution does not quickly settle down to vacuum. We argue that this can be related to the EFT approximation itself becoming invalid, which cannot be rectified by fixing.

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arXiv2023

Loss of hyperbolicity and tachyons in generalized Proca theories

Various groups recently demonstrated that the time evolution of simplest self-interacting vector fields, those with self-interaction potentials, can break down after a finite duration in what is called loss of hyperbolicity. We establish that this is not an isolated issue, and other generalizations of the Proca theory suffer from the same problem. Specifically, we show that vector field theories with derivative self-interactions have a similar pathology. For this, we derive the effective metric that governs the dynamics, and show that it can change signature during time evolution. We also show that, generalized Proca theories may suffer from tachyonic instabilities as well, which lead to another form of unphysical behavior.

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arXiv2023

Subtleties in constraining gravity theories with mass-radius data

Simultaneous measurements of neutron star masses and radii can be used to constrain deviations from general relativity (GR) as was recently demonstrated for the spontaneous scalarization model of Damour and Esposito-Farèse (DEF). Here, we investigate the general applicability of the same procedure beyond this single example. We first show that a simple variation of the DEF model renders the same mass-radius measurements ineffective for obtaining constraints. On the other hand, a recently popular and distinct model of scalarization that arises in scalar-Gauss-Bonnet theory can be constrained similarly to the original DEF model, albeit due to a slightly different underlying mechanism. These establish that using the mass-radius data can potentially constrain various theories of gravity, but the method also has limitations.

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arXiv2023

Pervasiveness of the breakdown of self-interacting vector field theories

Various groups recently argued that self-interacting vector field theories lack a well-defined time evolution when the field grows to large amplitudes, which has drastic consequences for models in gravity and high energy theory. Such field amplitudes can be a result of an external driving mechanism, or occur intrinsically, due to large values of the field and its derivatives in the initial data. This brings a natural question: is small amplitude initial data guaranteed to evolve indefinitely in these theories in the absence of an outside driving term? We answer this question in the negative, demonstrating that arbitrarily low amplitude initial data can still lead to the breakdown of the theory. Namely, ingoing spherically symmetric wave packets in more than one spatial dimensions grow as an inverse power of the radius, and their amplitudes can generically reach high enough values where time evolution ceases to exist. This simple example further establishes the pervasiveness of the pathology of self-interacting vector field theories.

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arXiv2023

Coordinate singularities of self-interacting vector field theories

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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arXiv2022

The intrinsic pathology of self-interacting vector fields

We show that self-interacting vector field theories exhibit unphysical behaviour 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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arXiv2022

Constraining scalar-tensor theories using neutron star mass and radius measurements

We use neutron star mass and radius measurements to constrain the spontaneous scalarization phenomenon in scalar-tensor theories using Bayesian analysis. Neutron star structures in this scenario can be significantly different from the case of general relativity, which can be used to constrain the theory parameters. We utilize this idea to obtain lower bounds on the coupling parameter $β$ for the case of massless scalars. These constraints are currently weaker than the ones coming from binary observations, and they have relatively low precision due to the approximations in our method. Nevertheless, our results clearly demonstrate the power of the mass-radius data in testing gravity, and can be further improved with future observations. The picture is different for massive scalars, for which the same data is considerably less effective in constraining the theory parameters in an unexpected manner. We identify the main reason for this to be a large high-likelihood region in the parameter space where deviations from general relativity are relatively small. We hope this initial study to be an invitation to use neutron star structure measurements more commonly to test alternative theories in general.

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arXiv2022

The ghost of vector fields in compact stars

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.

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arXiv2021

Instability of vectorized stars

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 non-perturbative 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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arXiv2019

Generalized disformal coupling leads to spontaneous tensorization

We show that gravity theories involving disformally transformed metrics in their matter coupling lead to spontaneous growth of various fields in a similar fashion to the spontaneous scalarization scenario in scalar-tensor theories. Scalar-dependent disformal transformations have been investigated in this context, and our focus is understanding the transformations that depend on more general fields. We show that vector-dependent disformal couplings can be obtained in various different ways, each leading to spontaneous vectorization as indicated by the instabilities in linearized equations of motion. However, we also show that spontaneous growth is not evident beyond vectors. For example, we could not identify a spontaneous growth mechanism for a spinor field through disformal transformations, even though there is a known example for conformal transformations. This invites further work on the fundamental differences between the two types of metric transformations. We argue that our results are relevant for observations in strong gravity such as gravitational wave detections due to their promise of large deviations from general relativity in this regime.

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arXiv2019

Spontaneous tensorization from curvature coupling and beyond

We generalize the recently introduced extended scalar-tensor-Gauss-Bonnet (ESTGB) theories and their close relatives to include spontaneous growth of nonscalar fields such as vectors. This is analogous to the program that developed spontaneous tensorization from the original spontaneous scalarization theory of Damour and Esposito-Farèse (DEF). The new larger family of theories conserves the appeal of the DEF theory in terms of conforming to weak-field tests and also providing large signals in strong gravity. Moreover, they provide a much richer phenomenology including spontaneous tensorization of black holes as in ESTGB. These theories, together with other possible future extensions that we discuss, testify to the ubiquity of spontaneous growth in gravity. We also note that theories with derivative coupling require special attention since they can lead to potentially problematic higher derivatives in the equations of motion.

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