We add an $\textit{S}$ extra dimension (size $\textit{L}$) to the well known $\textit{D}$ = 4 static, spherically symmetric $\textit{Q}$-balls and boson stars. We show that the resulting $\textit{uniform horizonless boson strings}$ possess a static zero-mode for a critical value of $\textit{L}$. This is at the threshold of a Gregory-Laflamme instability of these objects, occurring for larger values of $\textit{L}$. The non-linear continuation of the zero-mode yields $\textit{D}$ = 5 $\textit{non-uniform boson strings}$. In addition, there are also intrinsic $\textit{D}$ = 5 solutions describing $\textit{localized boson stars on the S}$ , supported by the contribution of the scalar field Kaluza-Klein modes. Basic properties of all three types of aforementioned solutions are discussed, together with their phase space.

Portugal

C. Herdeiro

E. Radu

Gregory-Laflamme-type instability of boson strings and related phases in $\textit{D}$ = 5 Kaluza-Klein theory

The physical properties of a (3+1)-D gravastar in the context of massive gravity are discussed in this work. In present investigation, the field equations have been solved for a static, uncharged sphere in order to achieve the gravastar model as proposed by Mazur and Mottola [Mazur and Mottola in Report No. LA-UR-01-5067, (2001); Mazur and Mottola, Proc. Natl. Acad. Sci. USA 101 (2004) 9545]. We address length of thin shell, energy, and entropy for the thin shell containing an ultra-relativistic stiff fluid. Israel matching criteria are used to ensure that the inner and outside geometries join smoothly. The present study within dRGT gravity differs markedly and fundamentally from true gravastars described in [ <inter-ref xmlns:ns0='http://www.w3.org/1999/xlink' ns0:role='http://www.elsevier.com/xml/linking-roles/preprint' ns0:href='arxiv:2302.09690' id='inf0010'>arXiv:2302.09690</inter-ref>; Mazur and Mottola, Class. Quant. Grav. 32 (2015) 215024], which requires the phase transition occured at <math altimg='si1.svg'><mi>R</mi><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>2</mn><mi mathvariant='script'>M</mi></math>. It turns out that the existence of the graviton mass entirely alters the behavior of the gravastar. Particularly, when <math altimg='si2.svg'><mi>m</mi><mo stretchy='false'>&#8594;</mo><mn>0</mn></math>, our findings precisely matched the outcomes of general relativity.

India

Piyali Bhar

Existence and stability of (3+1)-dimensional gravastar model in de Rham–Gabadadze–Tolley like massive gravity

<p>We study inflation in a recently proposed gravitational effective field theory describing the trace anomaly. The theory requires an additional scalar which is massless in the early universe. This scalar—referenced as an anomalyon—couples to the familiar matter and radiation through the gauge field trace anomaly. We derive a class of cosmological solutions that deviate from the standard inflationary ones only slightly, in spite of the fact that the anomalyon has a sizable time dependent background. On the other hand, the scalar cosmological perturbations in this theory are different from the conventional inflationary perturbations. The inflaton and anomalyon perturbations mix, and one of the diagonal combinations gives the standard nearly scale-invariant adiabatic spectrum, while the other combination has a blue power spectrum at short distance scales. We argue that this blue spectrum can lead to the formation of primordial black holes (PBHs) at distance scales much shorter than the ones tested in cosmic microwave background observations. The resulting PBHs can be heavy enough to survive to the present day Universe. For natural values of the parameters involved the PBHs would constitute only a tiny fraction of the dark matter, but with fine-tunings perhaps all of dark matter could be accounted by them. We also show that the theory predicts primordial gravitational waves which are almost identical to the standard inflationary ones.</p>

United States

Gregory Gabadadze

David N. Spergel

Giorgi Tukhashvili

Inflation with the trace anomaly action and primordial black holes

Thin enough black strings are unstable to growing ripples along their length, eventually pinching and forming a naked singularity on the horizon. We investigate how string theory can resolve this singularity. First, we study the string-scale version of the static non-uniform black strings that branch off at the instability threshold: “string-ball strings”, which are linearly extended, self-gravitating configurations of string balls obtained in the Horowitz-Polchinski (HP) approach to near-Hagedorn string states. We construct non-uniform HP strings in spatial dimensions $\textit{d}$ ≤ 6 and show that, as the inhomogeneity increases, they approach localized HP balls. We also examine the thermodynamic properties of the different phases in the canonical and microcanonical ensembles. We find that, for a sufficiently small mass, the uniform HP string will be stable and not evolve into a non-uniform or localized configuration. Building on these results and independent evidence from the evolution of the black string instability with $\textit{α}$ corrections, we propose that, at least in $\textit{d}$ = 4, 5, string theory slows and eventually halts the pinching evolution at a classically stable stringy neck. In $\textit{d}$ ≥ 6 this transition is likely to occur into a puffed-up string ball. The system then enters a slower phase in which the neck gradually evaporates into radiation. We discuss this scenario as a framework for understanding how string theory resolves the formation of naked singularities.

Spain

Roberto Emparan

France

Mikel Sanchez-Garitaonandia

Netherlands

Marija Tomašević

String theory in a pinch: resolving the Gregory-Laflamme singularity

In the conventional method of studying wormhole (WH) geometry, traversability requires the presence of exotic matter, which also provides negative gravity effects to keep the wormhole throat open. In de Rham–Gabadadze–Tolley (dRGT) massive gravity theory, we produce two types of WH solutions in our present paper. We obtain the field equations for exact WH solutions by selecting a static and spherically symmetric metric for the background geometry. We derive the WH geometry completely for the two different choices of redshift functions. The obtained WH solutions violate all the energy conditions, including the null energy condition (NEC). Various plots are used to illustrate the behavior of the wormhole for a suitable range of  $$m^2c_1$$  <math> <mrow> <msup> <mi>m</mi> <mn>2</mn> </msup> <msub> <mi>c</mi> <mn>1</mn> </msub> </mrow> </math>  , where m is the graviton mass. It is observed that the photon deflection angle becomes negative for all values of  $$m^2c_1$$  <math> <mrow> <msup> <mi>m</mi> <mn>2</mn> </msup> <msub> <mi>c</mi> <mn>1</mn> </msub> </mrow> </math>   as a result of the repulsive action of gravity. It is also shown that the repulsive impact of massive gravitons pushes the spacetime geometry so strongly that the asymptotic flatness is affected. The volume integral quantifier (VIQ) is computed to determine the amounts of matter that violate the null energy condition. The complexity factor of the proposed model is also discussed.

Properties of the wormhole model in de Rham–Gabadadze–Tolley like massive gravity with specific matter density

The characteristics of wormhole models in the context of the de Rham–Gabadadze–Tolley-like massive gravity theory are examined in this article. Dark matter density profiles of Thomas Fermi and Einasto spike are used to find the wormhole shape functions. By exploiting these formed shape functions, we create a wormhole geometry that connects asymptotically flat regions of spacetime while fulfilling all necessary requirements. Through a comprehensive analytical and graphical investigation, we explore the characteristics of exotic matter in these wormhole structures and examine their material composition within the context of energy conditions. The volume integral quantifier is used to quantify the exotic matter. We also discuss the phenomena of the complexity factor for all wormhole models and conclude that it approaches zero for increasing values of the radial coordinate, indicating the homogeneity of the energy density and the isotropic behavior of the pressure. Moreover, the repulsive nature of these wormhole solutions, a critical characteristic for their possible traversability is revealed by our analysis of the anisotropy parameter.

Jitendra Kumar

Oman

S. Maurya

Sweeti Kiroriwal

Dark matter influences on wormhole stability in de Rham–Gabadadze–Tolley like massive gravity

We investigate Color Flavor Locked (CFL) Strange Stars using the most basic de Rham–Gabadadze–Tolley massive gravity model, which explains the massive graviton in the absence of a ghost propagating mode. In the current study, the compact star model has been achieved by solving the field equations for a static, uncharged sphere (Li et al., in Phys Dark Universe 42: 101308, 2023). Here, we provide some analytical and numerical findings and examine the effects of the massive gravity and anisotropy characteristics on the structure of this stellar object. The modified Tolman–Oppenheimer–Volkoff equation is obtained by taking into account the hydrostatic equilibrium. In order to verify stability of the stellar object, we have taken into account the redshift function and adibatic index. Furthermore, it is observed that the model parameters have a substantial impact on the mass, radius, and compactness of the star object. Finally, our findings provide a possible structure of hypothetical super massive CFL strange stars, which may be placed in the lower “mass-gap” region  $$2~M_{\odot }-5~M_{\odot }$$  <math> <mrow> <mn>2</mn> <mspace width="3.33333pt"></mspace> <msub> <mi>M</mi> <mo>⊙</mo> </msub> <mo>-</mo> <mn>5</mn> <mspace width="3.33333pt"></mspace> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </mrow> </math>   and remains stable without collapsing into a black hole corresponding to our considered model parameters.

Krishna Das

Akashdip Karmakar

Ujjal Debnath

Color flavor locked strange stars in de Rham–Gabadadze–Tolley like massive gravity

The wormhole solution in dRGT massive gravity is examined in this paper in the background of non-commutative geometry. In order to derive the wormhole model, along with the zero tidal force, we assume that the matter distribution is given by the Gaussian and Lorentzian distributions. The shape function in both models involves the massive gravity parameters <math><msup><mrow><mi>m</mi></mrow><mrow><mn>2</mn></mrow></msup><msub><mrow><mi>c</mi></mrow><mrow><mn>1</mn></mrow></msub></math> and <math><msup><mrow><mi>m</mi></mrow><mrow><mn>2</mn></mrow></msup><msub><mrow><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msub></math>. But the spacetime loses its asymptotic flatness due to the action of the massive gravity parameter. It is noticed that the asymptotic flatness is affected by the repulsive effect induced in the massive gravitons that push the spacetime geometry very strongly. We observed that each model violates the null energy criteria, indicating the presence of exotic matter which is necessary to sustain the wormholes. The exotic matter is measured using the volume integral quantifier. Moreover, it is discovered that the model is stable under the hydrostatic equilibrium condition by utilizing the TOV equation. Finally, our research encompassed an exploration of the repulsive influence exerted by gravity. Our findings demonstrated that the presence of repulsive gravity results in a negative deflection angle for photons following null geodesics. Remarkably, we consistently observed negative values for the deflection angle across all values of <math><msub><mrow><mi>r</mi></mrow><mrow><mn>0</mn></mrow></msub></math> in the two scenarios examined. This consistent negativity unequivocally signifies the manifestation of the repulsive gravity effect.

China

Allah Ditta

South Africa

Abdelghani Errehymy

Noncommutative wormhole in de Rham-Gabadadze-Tolley like massive gravity

The question of building a local diff-invariant effective gravitational action for the trace anomaly is reconsidered. General Relativity (GR) combined with the existing action for the trace anomaly is an inconsistent low energy effective field theory. This issue is addressed by extending GR into a certain scalar-tensor theory, which preserves the GR trace anomaly equation, up to higher order corrections. The extension introduces a new mass scale – assumed to be below the Planck scale – that governs four high dimensional terms in a local diff-invariant trace anomaly action. Such terms can be kept, while an infinite number of Planck-suppressed invariants are neglected. The resulting theory maintains two derivative equations of motion. In a certain approximation it reduces to the conformal Gallileon, which could have physical consequences.

A new gravitational action for the trace anomaly

A recently proposed effective action for the trace anomaly describes a tensor-scalar theory that is weakly coupled up to a certain high energy scale, where it becomes strongly interacting. Its ultraviolet completion is obtained by coupling to gravity a quantum field theory in which conformal invariance is spontaneously broken. In this paper, we show that if the field theory that gives rise to the trace anomaly is a large <math><msub><mi>N</mi><mi>c</mi></msub></math> conformal field theory, then the trace anomaly action has a completion above the strong scale in a holographic Randall-Sundrum two-brane theory, with the radion as a low energy remnant of the spontaneously broken conformal symmetry. Furthermore, we note that the subleading <math><msub><mi>N</mi><mi>c</mi></msub></math> terms can be derived by adding localized fields to the UV brane, so that the theory remains weakly coupled. The subleading terms are also obtained by introducing the Weyl squared terms in the 5D bulk. These, however, exhibit strongly coupled behavior at the respective sub-Planckian energy scales.