Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFormation of Relativistic Axion Stars
97
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Axions and axion-like particles are compelling candidates for the missing dark matter of the universe. As they undergo gravitational collapse, they can form compact objects such as axion stars or even black holes. In this paper, we study the formation and distribution of such objects. First, we simulate the formation of compact axion stars using numerical relativity with aspherical initial conditions that could represent the final stages of axion dark matter structure formation. We show that the final states of such collapse closely follow the known relationship of initial mass and axion decay constant $f_a$. Second, we demonstrate with a toy model how this information can be used to scan a model density field to predict the number densities and masses of such compact objects. In addition to being detectable by the LIGO/VIRGO gravitational wave interferometer network for axion mass of $10^{-9} < m_a < 10^{-11}$ eV, we show using peak statistics that for $f_a < 0.2M_{pl}$, there exists a mass gap between the masses of axion stars and black holes formed from collapse.
rate research
Read More
The classical equations of motion for an axion with potential $V(phi)=m_a^2f_a^2 [1-cos (phi/f_a)]$ possess quasi-stable, localized, oscillating solutions, which we refer to as axion stars. We study, for the first time, collapse of axion stars numerically using the full non-linear Einstein equations of general relativity and the full non-perturbative cosine potential. We map regions on an axion star stability diagram, parameterized by the initial ADM mass, $M_{rm ADM}$, and axion decay constant, $f_a$. We identify three regions of the parameter space: i) long-lived oscillating axion star solutions, with a base frequency, $m_a$, modulated by self-interactions, ii) collapse to a BH and iii) complete dispersal due to gravitational cooling and interactions. We locate the boundaries of these three regions and an approximate triple point $(M_{rm TP},f_{rm TP})sim (2.4 M_{pl}^2/m_a,0.3 M_{pl})$. For $f_a$ below the triple point BH formation proceeds during winding (in the complex $U(1)$ picture) of the axion field near the dispersal phase. This could prevent astrophysical BH formation from axion stars with $f_all M_{pl}$. For larger $f_agtrsim f_{rm TP}$, BH formation occurs through the stable branch and we estimate the mass ratio of the BH to the stable state at the phase boundary to be $mathcal{O}(1)$ within numerical uncertainty. We discuss the observational relevance of our findings for axion stars as BH seeds, which are supermassive in the case of ultralight axions. For the QCD axion, the typical BH mass formed from axion star collapse is $M_{rm BH}sim 3.4 (f_a/0.6 M_{pl})^{1.2} M_odot$.
We investigate the physics of black hole formation from the head-on collisions of boosted equal mass Oscillatons (OS) in full numerical relativity, for both the cases where the OS have equal phases or are maximally off-phase (anti-phase). While unboosted OS collisions will form a BH as long as their initial compactness $mathcal{C}equiv GM/R$ is above a numerically determined critical value $mathcal{C}>0.035$, we find that imparting a small initial boost counter-intuitively emph{prevents} the formation of black holes even if $mathcal{C}> 0.035$. If the boost is further increased, at very high boosts $gamma>1/12mathcal{C}$, BH formation occurs as predicted by the hoop conjecture. These two limits combine to form a stability band where collisions result in either the OS passing through (equal phase) or bouncing back (anti-phase), with a critical point occurring around ${cal C}approx 0.07$. We argue that the existence of this stability band can be explained by the competition between the free fall and the interaction timescales of the collision.
Light axions ($m_a lesssim 10^{-10}$ eV) can form dense clouds around rapidly rotating astrophysical black holes via a mechanism known as rotational superradiance. The coupling between axions and photons induces a parametric resonance, arising from the stimulated decay of the axion cloud, which can rapidly convert regions of large axion number densities into an enormous flux of low-energy photons. In this work we consider the phenomenological implications of a superradiant axion cloud undergoing resonant decay. We show that the low energy photons produced from such events will be absorbed over cosmologically short distances, potentially inducing massive shockwaves that heat and ionize the IGM over Mpc scales. These shockwaves may leave observable imprints in the form of anisotropic spectral distortions or inhomogeneous features in the optical depth.
We study scalar-tensor-tensor cross correlation $langle zeta hh rangle$ generated by the dynamics of interacting axion and SU(2) gauge fields during inflation. We quantize the quadratic action and solve the linear equations by taking into account mixing terms in a non-perturbative manner. Combining that with the in-in formalism, we compute contributions from cubic interactions to the bispectrum $B_{zeta hh}$. We find that the bispectrum is peaked at the folded configuration, which is a unique feature encoded by the scalar mixing and localized production of tensor modes. With our parameter choice, the amplitude of the bispectrum is $k^6 B_{zeta hh} sim 10^{-16}$. The unique shape dependence, together with the parity-violating nature, is thus a distinguishing feature to search for in the CMB observables.
Blue axion isocurvature perturbations are both theoretically well-motivated and interesting from a detectability perspective. These power spectra generically have a break from the blue region to a flat region. Previous investigations of the power spectra were analytic, which left a gap in the predicted spectrum in the break region due to the non-applicability of the used analytic techniques. We therefore compute the isocurvature spectrum numerically for an explicit supersymmetric axion model. We find a bump that enhances the isocurvature signal for this class of scenarios. A fitting function of three parameters is constructed that fits the spectrum well for the particular axion model we study. This fitting function should be useful for blue isocurvature signal hunting in data and making experimental sensitivity forecasts.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
