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Register a new userProbing Oscillons of Ultra-Light Axion-like Particle by 21cm Forest
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Ultra-Light Axion-like Particle (ULAP) is motivated as one of the solutions to the small scale problems in astrophysics. When such a scalar particle oscillates with an $mathcal{O}(1)$ amplitude in a potential shallower than quadratic, it can form a localized dense object, oscillon. Because of its longevity due to the approximate conservation of the adiabatic invariant, it can survive up to the recent universe as redshift $z sim mathcal{O}(10)$. The scale affected by these oscillons is determined by the ULAP mass $m$ and detectable by observations of 21cm line. In this paper, we examine the possibility to detect ULAP by 21cm line and find that the oscillon can enhance the signals of 21cm line observations when $m lesssim 10^{-19} {rm eV}$ and the fraction of ULAP to dark matter is much larger than $10^{-2}$ depending on the form of the potential.
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Ultra-light axion-like particle (ULAP) is one of attractive candidates for cold dark matter. Because the de Broglie wavelength of ULAP with mass $sim 10^{-22} {rm eV}$ is $mathcal{O}({rm kpc})$, the suppression of the small scale structure by the uncertainty principle can solve the core-cusp problem. Frequently, ULAP is assumed to be uniformly distributed in the present universe. In typical ULAP potentials, however, strong self-resonance at the beginning of oscillation invokes the large fluctuations, which may cause the formation of the dense localized object, oscillon. % Such a dense object lives for a long time, it may affect the cosmological evolution. In this paper, we confirm the oscillon formation in a ULAP potential by numerical simulation and analytically derive its lifetime.
In string theory, the simultaneous existence of many Axion-Like Particles (ALPs) are suggested over a vast mass range, and a variety of potentials have been developed in the context of inflation. In such potentials shallower than quadratic, the prominent instability can produce localized dense objects, oscillons. Because of the approximate conservation of their adiabatic invariant, oscillons generally survive quite long, maybe up to the current age of the universe in the case of ultra-light ALPs with $m sim 10^{-22} {rm eV}$. Such oscillons can have significant effects on the evolution of the recent universe. In this paper, we investigate the oscillons of the pure-natural type potential by classical lattice simulation to explore the key quantities necessary for phenomenological application: the number density of oscillons, the oscillon mass distribution, the energy ratio of oscillons to the ALP field, and the power spectrum. Then, we evolve these values in consideration of the analytic decay rate.
We present forecasts on the detectability of Ultra-light axion-like particles (ULAP) from future 21cm radio observations around the epoch of reionization (EoR). We show that the axion as the dominant dark matter component has a significant impact on the reionization history due to the suppression of small scale density perturbations in the early universe. This behavior depends strongly on the mass of the axion particle. Using numerical simulations of the brightness temperature field of neutral hydrogen over a large redshift range, we construct a suite of training data. This data is used to train a convolutional neural network that can build a connection between the spatial structures of the brightness temperature field and the input axion mass directly. We construct mock observations of the future Square Kilometer Array survey, SKA1-Low, and find that even in the presence of realistic noise and resolution constraints, the network is still able to predict the input axion mass. We find that the axion mass can be recovered over a wide mass range with a precision of approximately 20%, and as the whole DM contribution, the axion can be detected using SKA1-Low at 68% if the axion mass is $M_X<1.86 times10^{-20}$eV although this can decrease to $M_X<5.25 times10^{-21}$eV if we relax our assumptions on the astrophysical modeling by treating those astrophysical parameters as nuisance parameters.
Cosmological observations are used to test for imprints of an ultra-light axion-like field (ULA), with a range of potentials $V(phi)propto[1-cos(phi/f)]^n$ set by the axion-field value $phi$ and decay constant $f$. Scalar field dynamics dictate that the field is initially frozen and then begins to oscillate around its minimum when the Hubble parameter drops below some critical value. For $n!=!1$, once dynamical, the axion energy density dilutes as matter; for $n!=!2$ it dilutes as radiation and for $n!=!3$ it dilutes faster than radiation. Both the homogeneous evolution of the ULA and the dynamics of its linear perturbations are included, using an effective fluid approximation generalized from the usual $n=1$ case. ULA models are parameterized by the redshift $z_c$ when the field becomes dynamical, the fractional energy density $f_{z_c} equiv Omega_a(z_c)/Omega_{rm tot}(z_c)$ in the axion field at $z_c$, and the effective sound speed $c_s^2$. Using Planck, BAO and JLA data, constraints on $f_{z_c}$ are obtained. ULAs are degenerate with dark energy for all three potentials if $1+z_c lesssim 10$. When $3times10^4 gtrsim 1+z_c gtrsim 10 $, $f_{z_c}$ is constrained to be $ lesssim 0.004 $ for $n=1$ and $f_{z_c} lesssim 0.02 $ for the other two potentials. The constraints then relax with increasing $z_c$. These results strongly constrain ULAs as a resolution to cosmological tensions, such as discrepant measurements of the Hubble constant, or the EDGES measurement of the global 21 cm signal.
We investigate the bursts of electromagnetic and scalar radiation resulting from the collision, and merger of oscillons made from axion-like particles using 3+1 dimensional lattice simulations of the coupled axion-gauge field system. The radiation into photons is suppressed before the merger. However, it becomes the dominant source of energy loss after the merger if a resonance condition is satisfied. Conversely, the radiation in scalar waves is dominant during initial merger phase but suppressed after the merger. The backreaction of scalar and electromagnetic radiation is included in our simulations. We evolve the system long enough to see that the resonant photon production extracts a significant fraction of the initial axion energy, and again falls out of the resonance condition. We provide a parametric understanding of the time, and energy scales involved in the process and discuss observational prospects of detecting the electromagnetic signal.
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