Axion stars, gravitationally bound states of low-energy axion particles, have a maximum mass allowed by gravitational stability. Weakly bound states obtaining this maximum mass have sufficiently large radii such that they are dilute, and as a result, they are well described by a leading-order expansion of the axion potential. Heavier states are susceptible to gravitational collapse. Inclusion of higher-order interactions, present in the full potential, can give qualitatively different results in the analysis of collapsing heavy states, as compared to the leading-order expansion. In this work, we find that collapsing axion stars are stabilized by repulsive interactions present in the full potential, providing evidence that such objects do not form black holes. In the last moments of collapse, the binding energy of the axion star grows rapidly, and we provide evidence that a large amount of its energy is lost through rapid emission of relativistic axions.

We investigate the decay of condensates of scalars in a field theory defined by

l29 I Mossbauer spectra of #-Ge x Sei_ x alloys display a local maximum in the site-intensity ratio I B /IA (X) at the critical composition x =x c == 0.23 ±0.02. This observation is consistent with the realization of mechanical critical behavior in a covalent network glass recently predicted by Phillips and Thorpe. We identify x c with the onset of percolation of a specific molecular fragment based on the layered form of c-GeSe2.PACS numbers: 61.40. + b, 76.80. + y Phillips 1 and independently Thorpe 2 have predicted that when the average coordination number (m) of a three-dimensional covalent network in mechanical equilibrium equals an optimal value of 2.4, a condition for mechanical critical behavior exists. In undercoordinated {(m) < 2.4) networks, the number JV C of interatomic forces (bond-bending and bond-stretching) per atom visualized to act as constraints is less than the network dimensionality N d ( = 3), the number of degrees of freedom per atom. Under a shearing force such undercoordinated networks easily deform and thus possess a finite number of zero-frequency modes f=**N d -N c . In overcoordinated networks {{m) > 2.4), N c exceeds N d , and in general such networks contain macroscopic rigid domains. When (m) =2.4, N c = N d , and /^O, the system is at the mean-field vector percolation threshold. This model represents the mathematical realization of the intuitive idea that a glass is a frozen liquid of very high viscosity, because a liquid is conventionally regarded as having no resistance to shear at zero frequency, while the high viscosity of the glass ( ~-10 15 times that of a normal liquid) reflects the proximity of the material to the percolation of mechanical rigidity.Binary Ge x Se!-x glasses offer an attractive test system for these ideas because the coordination numbers m of Ge and Se are respectively 4 and 2 over the composition range 0 < x < \. The ease of preparing bulk glasses by water quenching and the flexibility of tuning (m)=2(x"fl)of the glass network by merely changing the alloy composition x is convenient in practice. Theoretically, the threshold composition x* c corresponding to complete satisfaction of mechanical critical behavior is given by 2.4 = 2 (x/ +1), i.e., x c ' = 0.20. We have studied these binary glasses in the range 0 < x < j by use of 129 I Mossbauer emission spectroscopy. The application of this method to probe the morphological structure of network glasses is discussed elsewhere. 3,4 In the present work we show that the Mossbauer site-intensity ratios studied as a function of x provide dramatic evidence of threshold behavior at x = x c = 0.23(2). This behavior is found to correlate well with molar volumes 5 and Raman-mode frequencies, 6 both studied as a function of x by previous workers. The present microscopic experiments provide new insights into the nature of the spontaneously rigid domains prevailing in the overconstrained (x > x c ) networks. These domains are identified with assemblies of specific molecular clusters described later. It appears that the ...

Following Ruffini and Bonazzola, we use a quantized boson field to describe condensates of axions forming compact objects. Without substantial modifications, the method can only be applied to axions with decay constant, f a , satisfying δ = (f a / M P ) 2 1, where M P is the Planck mass. Similarly, the applicability of the Ruffini-Bonazzola method to axion stars also requires that the relative binding energy of axions satisfies1, where E a and m a are the energy and mass of the axion. The simultaneous expansion of the equations of motion in δ and ∆ leads to a simplified set of equations, depending only on the parameter, λ = √ δ / ∆ in leading order of the expansions. Keeping leading order in ∆ is equivalent to the infrared limit, in which only relevant and marginal terms contribute to the equations of motion. The number of axions in the star is uniquely determined by λ. Numerical solutions are found in a wide range of λ. At small λ the mass and radius of the axion star rise linearly with λ. While at larger λ the radius of the star continues to rise, the mass of the star, M , attains a maximum at λ max 0.58. All stars are unstable for λ > λ max . We discuss the relationship of our results to current observational constraints on dark matter and the phenomenology of Fast Radio Bursts.

We study black hole solutions in R 4 × S 1 space, using an expansion to fourth order in the ratio of the radius of the horizon, µ, and the circumference of the compact dimension, L. A study of geometric and thermodynamic properties indicates that the black hole fills the space in the compact dimension at ǫ(µ/L) 2 ≃ 0.1. At the same value of ǫ the entropies of the uniform black string and of the black hole are approximately equal. PACS numbers: 04.50.+h, 04.70.Bw, 04.70.Dy I. INTRODUCTIONIf the topology of space-time is R 4 × S 1 then the only known exact solution representing a black object is the uniform black string with horizon topology S 2 × S 1 [1]. Though this solution exists for all values of the mass it is unstable below a critical value, M GL , as shown by Gregory and Laflamme [2].Horowitz and Maeda [3] argued that a uniform black string cannot change its topology into a black hole in finite affine time, making the possibility of such a transition questionable. They suggested the possibility of a transition to a nonuniform black string. Gubser [4] showed the existence of nonuniform black string solutions. The non-uniform black string solution in 6 dimensions was investigated numerically [5] for a range of the mass values above M GL .Nonuniform black string configurations do not exist for masses below the Gregory-Laflamme point, in the region where the uniform black string solution is unstable [5]. A natural candidate for a black object in this mass range is a black hole. Unfortunately, no exact black hole solutions are known in a 5 (or more) dimensional space with a compactified dimension. Still, on the basis of physical intuition, a black hole solution should exist for very small values of the mass. When the radius of the horizon is much smaller than the size of the compactified dimension, i.e. when M → 0, the black hole should be unaware of the compactification. Then a Myers-Perry [6] solution should become asymtotically exact. Indeed, a numerical solution, extending the Myers-Perry solution to larger values of the mass was found by Harmark and Obers [7,8] and Kudoh and Wiseman [9].Using general arguments Kol [10] suggested that the black hole branch and the nonuniform black string branch meet at a point when the black hole fills the compact dimension. Further arguments for such a transition were presented in [11,12].Recently, we have studied a related problem, namely the existence of black holes in 14] theories. We used [15,16] an approximation scheme based on the expansion of solutions in the ratio of the radius of the horizon of the black hole to the ADS curvature. In this paper we employ a similar strategy by expanding the metric and other relevant quantities in the ratio of the two natural lengths associated with a black hole configuration, (i) the five dimensional Schwarzschild radius, µ, associated with the mass µ = 8G 5 M/(3π) and (ii) the compactification length, L, defined as the proper circumference of the compact dimension in the region far away from the mass. The dimensionless ratio of the...