View Full Version : Boson stars are eating my brain

Roger E. Moore
2018-Aug-07, 01:53 PM

Okay, I decided I wanted to understand what a boson star was, so I looked up the above paper and halfway through the first paragraph my brain was squashed like a Mario Kart down a black hole.

What is a boson star? Pretend I am very stupid but still want to know. You can speak slowly. And thank you for trying. :)

2018-Aug-07, 04:19 PM
A star made of bosons. Helpful, I know.

The problem is that most bosons wouldn't form a star, so boson stars are only really a thing if you postulate the existence of axions. If axions exist and if they have certain properties (to do with how they interact with themselves - they need to be able to interact repulsively in order to maintain the structure) then at certain cosmological epochs simulations show (https://arxiv.org/pdf/hep-ph/9303313.pdf)that small over-densities of them could condense further. If they do this you get a dense ball of bosons. This ball doesn't emit light or interact with matter other than via gravity so it just kind of sits there. That paper has some unfamiliar uses of terms in it - normally the galactic halo is not thought of as a boson star as it is too diffuse and formed too late. Plus I am really not sold on how they get to the claim that the halo is a BEC.

Roger E. Moore
2018-Aug-07, 09:00 PM
Ah, Alcubierre, he invented that interstellar drive. Let's see what he says...


ℓ-Boson stars

Miguel Alcubierre, Juan Barranco, Argelia Bernal, Juan Carlos Degollado, Alberto Diez-Tejedor, Miguel Megevand, Dario Nunez, Olivier Sarbach
(Submitted on 29 May 2018)

We present new, fully nonlinear numerical solutions to the static, spherically symmetric Einstein-Klein-Gordon system for a collection of an arbitrary odd number N of complex scalar fields with an internal U(N) symmetry and no self-interactions. These solutions, which we dub ℓ-boson stars, are parametrized by an angular momentum number ℓ=(N−1)/2, an excitation number n, and a continuous parameter representing the amplitude of the fields. They are regular at every point and possess a finite total mass. For ℓ=0 the standard spherically symmetric boson stars are recovered. We determine their generalizations for ℓ>0 , and show that they give rise to a large class of new static configurations which might have a much larger compactness ratio than ℓ=0 stars.



Head-on collisions of Proca stars

Sanchis-Gual, Nicolas; Herdeiro, Carlos; Font, Josť A.; Radu, Eugen

Proca stars, $\textit{aka}$ vector boson stars, are self-gravitating Bose-Einstein condensates obtained as numerical stationary solutions of the Einstein-(complex)-Proca system. These solitonic objects can achieve a compactness comparable to that of black holes, thus yielding an example of a black hole mimicker, which, moreover, can be both stable and form dynamically from generic initial data by the mechanism of gravitational cooling. In this paper we further explore the dynamical properties of these solitonic objects by performing head-on collisions of equal mass Proca stars, using fully non-linear numerical evolutions. We show that the end point and the gravitational waveform from these collisions depends on the compactness of the Proca star. Proca stars with sufficiently small compactness collide emitting gravitational radiation and leaving a stable Proca star remnant. But more compact Proca stars collide to form a transient ${\it hypermassive}$ Proca star, which ends up decaying into a black hole, albeit temporarily surrounded by Proca quasi-bound states. The unstable intermediate stage can leave an imprint in the waveform, making it distinct from that of a head-on collision of black holes. The final quasi-normal ringing matches that of Schwarzschild black hole, even though small deviations may occur, as a signature of sufficiently non-linear and long-lived Proca quasi-bound states.



Brief History of Ultra-light Scalar Dark Matter Models

Jae-Weon Lee
(Submitted on 17 Apr 2017 (v1), last revised 11 Jan 2018 (this version, v2))

This is a review on the brief history of the scalar field dark matter model also known as fuzzy dark matter, BEC dark matter, wave dark matter, or ultra-light axion.
In this model ultra-light scalar dark matter particles with mass m=O(10−22)eV condense in a single Bose-Einstein condensate state and behave collectively like a classical wave. Galactic dark matter halos can be described as a self-gravitating coherent scalar field configuration called boson stars.
At the scale larger than galaxies the dark matter acts like cold dark matter, while below the scale quantum pressure from the uncertainty principle suppresses the smaller structure formation so that it can resolve the small scale crisis of the conventional cold dark matter model.



Phase transitions between dilute and dense axion stars

Pierre-Henri Chavanis
(Submitted on 10 Oct 2017)

We study the nature of phase transitions between dilute and dense axion stars interpreted as self-gravitating Bose-Einstein condensates. We develop a Newtonian model based on the Gross-Pitaevskii-Poisson equations for a complex scalar field with a self-interaction potential V(|ψ|2) involving an attractive |ψ|4 term and a repulsive |ψ|6 term. Using a Gaussian ansatz for the wave function, we analytically obtain the mass-radius relation of dilute and dense axion stars for arbitrary values of the self-interaction parameter λ≤0. We show the existence of a critical point |λ|c∼(m/MP)2 above which a first order phase transition takes place. We qualitatively estimate general relativistic corrections on the mass-radius relation of axion stars. For weak self-interactions |λ|<|λ|c, a system of self-gravitating axions forms a stable dilute axion star below a general relativistic maximum mass M dilute max, GR∼M2P/m and collapses into a black hole above that mass. For strong self-interactions |λ|>|λ|c, a system of self-gravitating axions forms a stable dilute axion star below a Newtonian maximum mass M dilute max,N=5.073MP/|λ|−−√, collapses into a dense axion star above that mass, and collapses into a black hole above a general relativistic maximum mass M dense max,GR∼|λ|−−√M3P/m2. Dense axion stars explode below a Newtonian minimum mass M dense in, N∼m/|λ|−−√ and form dilute axion stars of large size or disperse away. We determine the phase diagram of self-gravitating axions and show the existence of a triple point... forget it.

the system is down for maintenance

Roger E. Moore
2018-Aug-08, 03:11 PM
Will try to read these two from December 2017 and see if anything clicks.


Expansion in Higher Harmonics of Boson Stars using a Generalized Runi-Bonazzola Approach, Part 1: Bound States
Joshua Eby, Peter Suranyi, and L.C.R. Wijewardhana

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 Schr"odinger 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 kE 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 Δ≡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 ≈463f 2 /m , at Δ≈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.


On Profiles of Boson Stars with Self-Interactions

Felix Kling, Arvind Rajaraman

Under the influence of gravity, light scalar fields can form bound compact objects called boson stars. We use the technique of matching asymptotic expansions to obtain the profile for boson stars where the constituent particles have self-interactions. We obtain parametric representations of these profiles as a function of the self-interactions, including the case of very strong self-interactions. We show that our methods agree with solutions obtained by purely numerical methods. Significant distortions are found as compared to the noninteracting case.

Roger E. Moore
2018-Aug-14, 11:33 PM
I surrender. I cannot grok this.