Probing ultra-light axion dark matter from 21 cm tomography using Convolutional Neural Networks

We present forecasts on the detectability of Ultra-light axion-like particles (ULAP) from future 21 cm 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 × 10-20 eV although this can decrease to M X < 5.25 × 10-21 eV if we relax our assumptions on the astrophysical modeling by treating those astrophysical parameters as nuisance parameters.

Missing scalars at the cosmological collider

Light scalar fields typically develop spatially varying backgrounds during inflation. Very often they do not directly affect the density perturbations, but interact with other fields that do leave nontrivial signals in primordial perturbations. In this sense they become “missing scalars” at the cosmological collider. We study potentially observable signals of these missing scalars, focusing on a special example where a missing scalar distorts the usual oscillatory features in the squeezed bispectrum. The distortion is also a useful signal distinguishing the de Sitter background induced thermal mass from a constant intrinsic mass.

Effect of density fluctuations on gravitational wave production in first-order phase transitions

We study the effect of density perturbations on the process of first-order phase transitions and gravitational wave production in the early Universe. We are mainly interested in how the distribution of nucleated bubbles is affected by fluctuations in the local temperature. We find that large-scale density fluctuations (H * < k * < β) result in a larger effective bubble size at the time of collision, enhancing the produced amplitude of gravitational waves. The amplitude of the density fluctuations necessary for this enhancement is 𝒫ζ (k *) ≳ (β/H *)-2, and therefore the gravitational wave signal from first-order phase transitions with relatively large β/H * can be significantly enhanced by this mechanism even for fluctuations with moderate amplitudes.

Coagulation Instability in Protoplanetary Disks: A Novel Mechanism Connecting Collisional Growth and Hydrodynamical Clumping of Dust Particles

We present a new instability driven by a combination of coagulation and radial drift of dust particles. We refer to this instability as “coagulation instability” and regard it as a promising mechanism to concentrate dust particles and assist planetesimal formation in the very early stages of disk evolution. Because of dust-density dependence of collisional coagulation efficiency, dust particles efficiently (inefficiently) grow in a region of positive (negative) dust density perturbations, leading to a small radial variation of dust sizes and as a result radial velocity perturbations. The resultant velocity perturbations lead to dust concentration and amplify dust density perturbations. This positive feedback makes a disk unstable. The growth timescale of coagulation instability is a few tens of orbital periods even when dust-to-gas mass ratio is on the order of 10−3. In a protoplanetary disk, radial drift and coagulation of dust particles tend to result in dust depletion. The present instability locally concentrates dust particles even in such a dust-depleted region. The resulting concentration provides preferable sites for dust–gas instabilities to develop, which leads to further concentration. Dust diffusion and aerodynamical feedback tend to stabilize short-wavelength modes, but do not completely suppress the growth of coagulation instability. Therefore, coagulation instability is expected to play an important role in setting up the next stage for other instabilities, such as streaming instability or secular gravitational instability, to further develop toward planetesimal formation.

Modulational Instability of Fast Sausage Mode as One of the Possible Mechanisms for Quasiperiodic Pulsations during Solar Flares

Quasiperiodic pulsations (QPPs) are frequently observed in the entire range of the electromagnetic spectrum during solar flares, and there can be many possible mechanisms leading to this phenomenon. In the present work, we demonstrate the possibility of the generation of QPPs by a nonlinear fast sausage mode in a coronal loop. The coronal loop itself is represented by an infinitely long homogenous magnetic flux tube, which in many cases is a good approximation, and the nonlinearity of the fast sausage mode is modeled by the nonlinear Schrödinger equation (NSE) with a cubic nonlinearity. We have shown that the frequency-renormalized plane wave solution, which happens to be an exact solution of the NSE, transforms into a series of quasiperiodic oscillations (QPOs) due to the so-called modulational instability or the Benjamin–Feir instability. Our numerical solutions show that such QPOs evolve at almost every point above a certain height along the magnetic flux tube, which represents the coronal loop. As the fast sausage mode perturbs the plasma density strongly, the density perturbations caused by the QPOs of the nonlinear fast sausage mode correspondingly modulate the radiation throughout the electromagnetic spectrum, resulting in the emergence of the corresponding QPPs. This mechanism should therefore be able to describe some of the observed QPPs.

The growth of structure

The growth of structure by gravitational collapse from initially small perturbations is described. The Jeans instability is calculated. The structure equations are obtained and solved in various cases (radiation-dominated, matter-dominated and others) via a linearized treatment. Hence the main features of the growth of density perturbations are obtained. The observed spectrum in the present is used to infer the primordial spectrum. The scale-invariant (Harrison-Zol’dovich) spectrum is described. The process of baryon acoustic oscillations is outlined and the sound horizon is defined. The chapter concludes with brief notes on galaxy formatiom.

Polychromatic Hilbert diagnostics of phase and temperature disturbances, induced by candle flame in air

The work is motivated by the scientific and practical significance of the problem of non-disturbing diagnostics of phase and temperature fields induced in a gas medium by a flame of a torch (candle). The spatial conditions in which the fields are studied satisfy the model of axial symmetry of the torch associated with the vertical orientation of the candle. A method adequate to the problem to be solved has been developed, based on polychromatic Hilbert visualization of phase optical density fields, measurement of the temperature profile in selected sections of the medium under study, registration and selection of RAW images recorded by the photomatrix in RGB channels. The visualized Hilbert structures contain information on the phase optical density perturbations induced by the temperature field. The reliability of the results is confirmed by comparing the experimentally obtained hilbertograms and those reconstructed from phase structures using the Abel transform.