arXiv subjects

Peter Suranyi

Publications and source records attributed to Peter Suranyi.

At least 19 recordsLinked to original sources

arXiv2023

WKB approximation to boson dark matter

Galactic dark matter halos may be composed of ultralight axions (ULAs) ($m_a \lesssim 1$ eV) with wave functions that satisfy nonlinear Schrödinger-Poisson equations (SPA). We find eigenstates of SPA in WKB approximation. The expansion parameter of the WKB approximation is $δ=1/\sqrt{S}$, where $S=2 M R G m_a^{2}$, with $M$ being the total mass, $R$ the radius of the halo, and $G$ the gravitational constant. $S\gg 1$ for almost all galaxies, even if the ULA mass is as small as $m_a=10^{-22} $ eV, making the leading order WKB approximation almost exact. As the level spacing of bound states is roughly proportional to $δ$, the number of states in the gravitational well is huge. We do not see a reason why not all or most of them contribute to the halo. Using an appropriate distribution function allows the summation of states to construct the profile of the halo as a function of the gravitational potential, which can be found solving the Poisson equation. Using various energy distribution functions, we obtain results similar to those in simulations. Future plans include investigations of collapse through time dependent generalizations, and inclusion of self-interactions, which also induce decay processes of the halo.

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arXiv2021

Global View of Axion Stars with (Nearly) Planck-Scale Decay Constants

We show that axion stars formed from axions with nearly Planck-scale decay constants $f$ are unstable to decay, and are unlikely to have phenomenological consequences. More generally, we show how results at smaller $f$ cannot be naively extrapolated to $f=\mathcal{O}(M_P)$ as, contrary to conventional wisdom, gravity and special relativity can both become relevant in the same regime. We clarify the rate of decay by reviewing and extending previous work on oscillons and axion stars, which imply a fast decay rate even for so-called dilute states at large $f$.

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arXiv2021

Density profile of multi-state fuzzy dark matter

Equations of motion for excited states of weakly self-interacting bosons forming fuzzy dark matter are solved using the WKB approximation. The contribution of self-interactions are neglected in the equations of motion. Wave functions of excited states are expressed in terms of a yet undetermined gravitational potential. At equilibrium, the contributions of states to the density distribution are summed using Bose-Einstein statistics. Combined with the Poisson equation, a differential equation is obtained for the gravitational potential, which has physically acceptable solutions only if the energy spectrum of excited states has a finite gap, corresponding to a finite virial radius. Such a gap could be created by decay processes, in first order perturbation of the self-interaction potential. The obtained density profile is found to be similar to the Burkert profile.

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arXiv2020

Galactic Condensates composed of Multiple Axion Species

Ultralight scalar dark matter has been proposed to constitute a component of dark matter, though the minimal scenarios have increasingly become constrained. In this work, we analyze scenarios where the dark matter consists of more than one ultralight boson, each with different masses. This potentially leads to formation of gravitationally-bound Bose-Einstein condensates with structures that are very different from condensates composed of a single scalar field. By generalizing from the well-understood single-flavor case, we explore a large range of input parameters, subject to stability criteria, and determine the allowed parameter space for two-flavor condensates as a function of particle physics parameters, paying particular attention to cases where such condensates could compose galactic cores. We also analyze single-flavor condensates subject to external gravity from massive inner bodies and find that such systems may mimic the size of galactic cores as well.

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arXiv2019

Global view of QCD axion stars

Taking a comprehensive view, including a full range of boundary conditions, we reexamine QCD axion star solutions based on the relativistic Klein-Gordon equation (using the Ruffini-Bonazzola approach) and its non-relativistic limit, the Gross-Pitaevskii equation. A single free parameter, conveniently chosen as the central value of the wavefunction of the axion star, or alternatively the chemical potential with range $-m<μ< 0$ (where $m$ is the axion mass), uniquely determines a spherically-symmetric ground state solution, the axion condensate. We clarify how the interplay of various terms of the Klein-Gordon equation determines the properties of solutions in three separate regions: the structurally stable (corresponding to a local energy minimum) dilute and dense regions, and the intermediate, structurally unstable transition region. From the Klein-Gordon equation, one can derive alternative equations of motion including the Gross-Pitaevskii and Sine-Gordon equations, which have been used previously to describe axion stars in the dense region. In this work, we clarify precisely how and why such methods break down as the binding energy increases, emphasizing the necessity of using the full relativistic Klein-Gordon approach. Finally, we point out that, even after including perturbative axion number violating corrections, solutions to the equations of motion, which assume approximate conservation of axion number, break down completely in the regime with strong binding energy, where the magnitude of the chemical potential approaches the axion mass.

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arXiv2018

Stability of Condensed Fuzzy Dark Matter Halos

Stability properties of gravitationally bound condensates composed of ultralight axionic Fuzzy Dark Matter (FDM) are studied. Previous work has shown that astrophysical collisions could make self-gravitating condensates structurally unstable, making them prone to collapse and decay; in the context of FDM, we reexamine the relevant timescales using the time-dependent variational method. We show that FDM condensates can be made unstable through gravitational interactions with central black holes, for black hole masses in a phenomenologically relevant range. Instability could also be stimulated by galaxy collisions. The subsequent decay takes place over a period lasting as long as many thousands of years. We also discuss the possible relevance of FDM condensates to understanding the composition of Ultracompact Dwarf (UCD) Galaxies. Future observation of extremely massive black holes in the central regions of UCDs can constrain this interpretation.

astro-ph.CO↗

arXiv2018

On Approximation Methods in the Study of Boson Stars

We analyze the accuracy of the variational method in computing physical quantities relevant for gravitationally bound Bose-Einstein condensates. Using a variety of variational ansätze found in existing literature, we determine physical quantities and compare them to exact numerical solutions. We conclude that a "linear+exponential" wavefunction proportional to $(1 + ξ)\exp(-ξ)$ (where $ξ$ is a dimensionless radial variable) is the best fit for attractive self-interactions along the stable branch of solutions, while for small particle number $N$ it is also the best fit for repulsive self-interactions. For attractive self-interactions along the unstable branch, a single exponential is the best fit for small $N$, while a sech wavefunction fits better for large $N$. The Gaussian wavefunction ansatz, which is used often in the literature, is exceedingly poor across most of the parameter space, with the exception of repulsive interactions for large $N$. We investigate a "double exponential" ansatz with a free constant parameter, which is computationally efficient and can be optimized to fit the exact solutions in different limits. We show that the double exponential can be tuned to fit the sech ansatz, which is computationally slow. We also show how to generalize the addition of free parameters in order to create more computationally efficient ansätze using the double exponential. Determining the best ansatz, according to several comparison parameters, will be important for analytic descriptions of dynamical systems. Finally, we examine the underlying relativistic theory, and critically analyze the Thomas-Fermi approximation often used in the literature.

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arXiv2018

Expansion in Higher Harmonics of Boson Stars using a Generalized Ruffini-Bonazzola Approach, Part 1: Bound States

The method pioneered by Ruffini and Bonazzola (RB) to describe boson stars involves an expansion of the boson field which is linear in creation and annihilation operators. In the nonrelativistic limit, the equation of motion of RB is equivalent to the nonlinear Schrödinger equation. Further, the RB expansion constitutes an exact solution to a non-interacting field theory, and has been used as a reasonable ansatz for an interacting one. In this work, we show how one can go beyond the RB ansatz towards an exact solution of the interacting operator Klein-Gordon equation, which can be solved iteratively to ever higher precision. Our Generalized Ruffini-Bonazzola approach takes into account contributions from nontrivial harmonic dependence of the wavefunction, using a sum of terms with energy $k\,E_0$, where $k\geq1$ and $E_0$ is the chemical potential of a single bound axion. The method critically depends on an expansion in a parameter $Δ\equiv \sqrt{1 - E_0{}^2/m^2} < 1$, where $m$ is the mass of the boson. In the case of the axion potential, we calculate corrections which are relevant for axion stars in the transition or dense branches of solutions. We find with high precision the local minimum of the mass, $M_{min}\approx 463\,f^2/m$, at $Δ\approx0.27$, where $f$ is the axion decay constant. This point marks the crossover from the transition branch to the dense branch of solutions, and a corresponding crossover from structural instability to stability.

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arXiv2018

Decay of Ultralight Axion Condensates

Axion particles can form macroscopic condensates, whose size can be galactic in scale for models with very small axion masses $m\sim10^{-22}$ eV, and which are sometimes referred to under the name of Fuzzy Dark Matter. Many analyses of these condensates are done in the non-interacting limit, due to the weakness of the self-interaction coupling of axions. We investigate here how certain results change upon inclusion of these interactions, finding a decreased maximum mass and a modified mass-radius relationship. Further, these condensates are, in general, unstable to decay through number-changing interactions. We analyze the stability of galaxy-sized condensates of axion-like particles, and sketch the parameter space of stable configurations as a function of a binding energy parameter. We find a strong lower bound on the size of Fuzzy Dark Matter condensates which are stable to decay, with lifetimes longer than the age of the universe.

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arXiv2018

On Non-slow Roll Inflationary Regimes

We summarize our work on constant roll inflationary models. It was understood recently that constant roll inflation, in a regime beyond the slow roll approximation, can give models that are in agreement with the observational constraints. We describe a new class of constant roll inflationary models and investigate the behavior of scalar perturbations in them. We also comment on other non-slow roll regimes of inflation.

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arXiv2017

Systematics of Constant Roll Inflation

We study constant roll inflation systematically. This is a regime, in which the slow roll approximation can be violated. It has long been thought that this approximation is necessary for agreement with observations. However, recently it was understood that there can be inflationary models with a constant, and not necessarily small, rate of roll that are both stable and compatible with the observational constraint $n_s \approx 1$. We investigate systematically the condition for such a constant-roll regime. In the process, we find a whole new class of inflationary models, in addition to the known solutions. We show that the new models are stable under scalar perturbations. Finally, we find a part of their parameter space, in which they produce a nearly scale-invariant scalar power spectrum, as needed for observational viability.

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arXiv2017

Collisions of Dark Matter Axion Stars with Astrophysical Sources

If QCD axions form a large fraction of the total mass of dark matter, then axion stars could be very abundant in galaxies. As a result, collisions with each other, and with other astrophysical bodies, can occur. We calculate the rate and analyze the consequences of three classes of collisions, those occurring between a dilute axion star and: another dilute axion star, an ordinary star, or a neutron star. In all cases we attempt to quantify the most important astrophysical uncertainties; we also pay particular attention to scenarios in which collisions lead to collapse of otherwise stable axion stars, and possible subsequent decay through number changing interactions. Collisions between two axion stars can occur with a high total rate, but the low relative velocity required for collapse to occur leads to a very low total rate of collapses. On the other hand, collisions between an axion star and an ordinary star have a large rate, $Γ_\odot \sim 3000$ collisions/year/galaxy, and for sufficiently heavy axion stars, it is plausible that most or all such collisions lead to collapse. We identify in this case a parameter space which has a stable region and a region in which collision triggers collapse, which depend on the axion number ($N$) in the axion star, and a ratio of mass to radius cubed characterizing the ordinary star ($M_s/R_s^3$). Finally, we revisit the calculation of collision rates between axion stars and neutron stars, improving on previous estimates by taking cylindrical symmetry of the neutron star distribution into account. Collapse and subsequent decay through collision processes, if occurring with a significant rate, can affect dark matter phenomenology and the axion star mass distribution.

astro-ph.CO↗

arXiv2017

Collapse of Axion Stars

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.

astro-ph.CO↗

arXiv2017

QCD Axion Star Collapse with the Chiral Potential

In a previous work, we analyzed collapsing axion stars using the low-energy instanton potential, showing that the total energy is always bounded and that collapsing axion stars do not form black holes. In this paper, we provide a proof that the conclusions are unchanged when using instead the more general chiral potential for QCD axions.

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arXiv2016

The Lifetime of Axion Stars

We investigate the decay of condensates of scalars in a field theory defined by $V({\cal A})=m{}^2\,f{}^2\,[1-\cos({\cal A}\,/\,f)]$, where $m$ and $f$ are the mass and decay constant of the scalar field. An example of such a theory is that of the axion, in which case the condensates are called axion stars. The axion field, $\cal A$, is self adjoint. As a result the axion number is not an absolutely conserved quantity. Therefore, axion stars are not stable and have finite lifetimes. Bound axions, localized on the volume of the star, have a coordinate uncertainty $Δx \sim R \sim 1/(m_a Δ)$, where $R$ is the radius of the star and $Δ= \sqrt{1-E_0{}^2\,/\,m_a^2}$. Here $m_a$ and $E_0$ are the mass, and the ground state energy of the bound axion. Then the momentum distribution of axions has a width of $Δp\sim m_a\,Δ$. At strong binding, $Δ={\cal O}(1)$, bound axions can easily transfer a sufficient amount of momentum to create and emit a free axion, leading to fast decay of the star with a transition rate $Γ\sim m_a$. However, when $Δ\ll 1$, the momentum distribution is more restricted, and as shown in this paper, the transition rate for creating a free axion decreases as $\exp (-p \, Δx) \sim \exp (-Δ^{-1})$. Then sufficiently large, weakly bound axion stars, produced after the big bang, survive until the present time. We plot the region of their stability, limited by decay through axion loss and by gravitational instability, as a function of the mass of the axion and the mass of the star.

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arXiv2015

De Sitter Space in Gauge/Gravity Duality

We investigate gauge/gravity duality for gauge theories in de Sitter space. More precisely, we study a five-dimensional consistent truncation of type IIB supergravity, which encompasses a wide variety of gravity duals of strongly coupled gauge theories, including the Maldacena-Nunez solution and its walking deformations. We find several solutions of the 5d theory with dS_4 spacetime and nontrivial profiles for (some of) the scalars along the fifth (radial) direction. In the process, we prove that one of the equations of motion becomes dependent on the others, for nontrivial warp factor. This dependence reduces the number of field equations and, thus, turns out to be crucial for the existence of solutions with (A)dS_4 spacetime. Finally, we comment on the implications of our dS_4 solutions for building gravity duals of Glueball Inflation.

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arXiv2015

Toward a Gravity Dual of Glueball Inflation

We summarize and extend our work on nonsupersymmetric solutions of a 5d consistent truncation of type IIB supergravity, that is relevant for gauge/gravity duality. The fields in this 5d theory are the five-dimensional metric and a set of scalars. We find solutions of the 5d equations of motion, which represent dS_4 foliations over the fifth (radial) dimension. In each solution at least one scalar has a nontrivial radial profile. These scalars are interpreted as glueballs in the dual gauge theory, living in 4d de Sitter space. We explain why this lays a foundation for building gravity duals of glueball inflation models.

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arXiv2015

Axion Stars in the Infrared Limit

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 $δ=(f_a\,/\,M_P)^2\ll 1$, 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 satisfies $Δ=\sqrt{1-(E_a\,/\,m_a)^2}\ll1$, 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 $Δ$ leads to a simplified set of equations, depending only on the parameter, $λ=\sqrtδ\,/\,Δ$ in leading order of the expansions. Keeping leading order in $Δ$ 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 $λ_{\rm max}\simeq 0.58$. All stars are unstable for $λ>λ_{\rm max}$ . We discuss the relationship of our results to current observational constraints on dark matter and the phenomenology of Fast Radio Bursts.

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