Research Project: Kütle Çekiminin Nötron Yıldızlarıyla Imtihanı
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Contributors
Funders
ID
TB.00684
Authors
Ramazanoğlu, Fethi Mübin
Faculty Member
Publications
Crossing the singularity of a gravitational wave collision
(American Physical Society, 2024) Dereli, Tekin; Ünlütürk, Kıvanç İbrahim; Gurtug, Ozay; Department of Physics; Graduate School of Sciences and Engineering; Yes; College of Sciences; GRADUATE SCHOOL OF SCIENCES AND ENGINEERING
A reformulation of general relativity inspired by the Belinski-Khalatnikov-Lifshitz conjecture had been introduced by Ashtekar, Henderson, and Sloan which is based on variables closely related to the basic variables of loop quantum gravity, thereby providing a way of classically analyzing singularities that may be carried over to the quantum theory. It is reasonable to expect that these variables are regular at generic spacelike singularities. This has been shown on various examples-particularly, cosmological spacetimes. In this study we extend this analysis to the singularities of gravitational wave collision spacetimes, which are the result of the mutual focusing of the two waves. We focus on two specific examples and explicitly confirm that the said variables are regular at the singularity and can be smoothly continued beyond it.
Pervasiveness of the breakdown of self-interacting vector field theories
(American Physical Society, 2023) Ramazanoğlu, Fethi Mübin; Coates, Andrew; Department of Physics; Yes; College of Sciences
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.
Loss of hyperbolicity and tachyons in generalized Proca theories
(American Physical Society, 2023) Coates, Andrew; Ramazanoğlu, Fethi Mübin; Ünlütürk, Kıvanç İbrahim; ; Department of Physics; Graduate School of Sciences and Engineering; Yes; College of Sciences; GRADUATE SCHOOL OF SCIENCES AND ENGINEERING
Various groups recently demonstrated that the time evolution of the 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. © 2023 American Physical Society.
Subtleties in constraining gravity theories with mass-radius data
(American Physical Society, 2023) Ramazanoğlu, Fethi Mübin; Demirboğa, Ekrem; Sahin, Yakup Emre; Department of Physics; Graduate School of Sciences and Engineering; Yes; College of Sciences; GRADUATE SCHOOL OF SCIENCES AND ENGINEERING
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-Fare`se (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 massradius data can potentially constrain various theories of gravity, but the method also has limitations.
Aspects of time evolution in p-form field theories
(American Physical Society, 2024) Ünlütürk, Kıvanç İbrahim; Ramazanoğlu, Fethi Mübin; Department of Physics; Yes; College of Sciences
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 wavelike characteristic in certain parts of the configuration space, and such problematic regions can 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 form fields leads to a variety in time evolution characteristics, where hyperbolicity is lost in different parts of the configuration space as the interaction changes.
