Decay of ultralight axion condensates

Eby, Joshua; Ma, Michael; Surányi, P.; Wijewardhana, L. C. R.

doi:10.1007/jhep01(2018)066

Cited by 46 publications

(70 citation statements)

References 98 publications

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“…However, near the crossover to the dense branch of solutions, corrections from special relativity become large, leading to a breakdown of this criteron. In particular, ∆ = O(1) implies a large decay rate, violating the approximate N -conservation [35,36,38,49]. Further, comparing Eqs.…”

Section: Gross-pitaevskii-poisson (Gpp)mentioning

confidence: 97%

“…[ [35][36][37][38] as well as under perturbations (what we call structural stability) [30,[39][40][41]. As such, their phenomenological effects can be searched for in the dark matter halo.…”

Section: B Solutionsmentioning

confidence: 99%

“…Finally, though states above the dense minimum, Z dense 3.5, are structurally stable, it is important to remember that all condensates with central field value Z(0) 0.05, produced after the big bang would have decayed completely by the present time and cannot form even a fraction of dark matter [35,38].…”

Section: B Solutionsmentioning

confidence: 99%

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Global view of QCD axion stars

et al. 2019

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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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Section: Gross-pitaevskii-poisson (Gpp)mentioning

confidence: 97%

Section: B Solutionsmentioning

confidence: 99%

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Global view of QCD axion stars

et al. 2019

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“…These quanta give rise to a decay rate for boson stars formed from Hermitian scalars, as is the case for axions. In [36,39], we parameterized these scattering states by free spherical waves; this is altogether appropriate far from the star and is only perturbed by binding energy contributions inside the star, which are small when the star is dilute. This simplification breaks down for more strongly bound boson stars.…”

Section: )mentioning

confidence: 99%

Expansion in higher harmonics of boson stars using a generalized Ruffini-Bonazzola approach. Part 1. Bound states

Eby

Surányi

Wijewardhana

2018

J. Cosmol. Astropart. Phys.

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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 ≥ 1 and E 0 is the chemical potential of a single bound axion. The method critically depends on an expansion in a parameter ∆ ≡ 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 ≈ 463 f 2 /m, at ∆ ≈ 0.27, 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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“…Axion stars were considered first around 30 years ago, originally suggested to form from collapse of overdense miniclusters in the early universe [10,11] (see also more recent simulations [12]). Since then, many properties of axion stars have been studied extensively; these include structural stability [13][14][15][16][17][18][19][20][21][22] (including nonzero angular momentum [23][24][25]), the process of gravitational collapse [26][27][28][29][30][31][32], and their decay through emission of relativistic particles [33][34][35][36][37][38]. There has recently been a significant amount of work regarding relativistic corrections more generally to the classical field description of axion stars [39][40][41][42].…”

Section: Introductionmentioning

confidence: 99%

Approximation methods in the study of boson stars

et al. 2018

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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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Decay of ultralight axion condensates

Cited by 46 publications

References 98 publications

Global view of QCD axion stars

Global view of QCD axion stars

Expansion in higher harmonics of boson stars using a generalized Ruffini-Bonazzola approach. Part 1. Bound states

Approximation methods in the study of boson stars

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