Browsing by Author "Amin, Mustafa A."
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Item A Lower Bound on Dark Matter Mass(American Physical Society, 2024) Amin, Mustafa A.; Mirbabayi, MehrdadWe argue that there is a lower bound of order 10−19 eV on dark matter mass if it is produced after inflation via a process with finite correlation length. We rely on nondetection of free-streaming suppression and white-noise enhancement of density perturbations as the observational inputs.Item Beyond Schrödinger-Poisson: nonrelativistic effective field theory for scalar dark matter(Springer Nature, 2021) Salehian, Borna; Zhang, Hong-Yi; Amin, Mustafa A.; Kaiser, David I.; Namjoo, Mohammad HosseinMassive scalar fields provide excellent dark matter candidates, whose dynamics are often explored analytically and numerically using nonrelativistic Schrödinger-Poisson (SP) equations in a cosmological context. In this paper, starting from the nonlinear and fully relativistic Klein-Gordon-Einstein (KGE) equations in an expanding universe, we provide a systematic framework for deriving the SP equations, as well as relativistic corrections to them, by integrating out ‘fast modes’ and including nonlinear metric and matter contributions. We provide explicit equations for the leading-order relativistic corrections, which provide insight into deviations from the SP equations as the system approaches the relativistic regime. Upon including the leading-order corrections, our equations are applicable beyond the domain of validity of the SP system, and are simpler to use than the full KGE case in some contexts. As a concrete application, we calculate the mass-radius relationship of solitons in scalar dark matter and accurately capture the deviations of this relationship from the SP system towards the KGE one.Item Cosmological dynamics of Higgs potential fine tuning(American Physical Society, 2019) Amin, Mustafa A.; Fan, JiJi; Lozanov, Kaloian D.; Reece, MatthewThe Higgs potential appears to be fine-tuned, hence very sensitive to values of other scalar fields that couple to the Higgs. We show that this feature can lead to a new epoch in the early Universe featuring violent dynamics coupling the Higgs to a scalar modulus. The oscillating modulus drives tachyonic Higgs particle production. We find a simple parametric understanding of when this process can lead to rapid modulus fragmentation, resulting in gravitational wave production. A nontrivial equation of state arising from the nonlinear dynamics also affects the time elapsed from inflation to the CMB, influencing fits of inflationary models. Supersymmetric theories automatically contain useful ingredients for this picture.Item Dipole radiation and beyond from axion stars in electromagnetic fields(Springer Nature, 2021) Amin, Mustafa A.; Long, Andrew J.; Mou, Zong-Gang; Saffin, Paul M.We investigate the production of photons from coherently oscillating, spatially localized clumps of axionic fields (oscillons and axion stars) in the presence of external electromagnetic fields. We delineate different qualitative behaviour of the photon luminosity in terms of an effective dimensionless coupling parameter constructed out of the axion-photon coupling, and field amplitude, oscillation frequency and radius of the axion star. For small values of this dimensionless coupling, we provide a general analytic formula for the dipole radiation field and the photon luminosity per solid angle, including a strong dependence on the radius of the configuration. For moderate to large coupling, we report on a non-monotonic behavior of the luminosity with the coupling strength in the presence of external magnetic fields. After an initial rise in luminosity with the coupling strength, we see a suppression (by an order of magnitude or more compared to the dipole radiation approximation) at moderately large coupling. At sufficiently large coupling, we find a transition to a regime of exponential growth of the luminosity due to parametric resonance. We carry out 3+1 dimensional lattice simulations of axion electrodynamics, at small and large coupling, including non-perturbative effects of parametric resonance as well as backreaction effects when necessary. We also discuss medium (plasma) effects that lead to resonant axion to photon conversion, relevance of the coherence of the soliton, and implications of our results in astrophysical and cosmological settings.Item Engineering and revealing Dirac strings in spinor condensates(American Physical Society, 2024) Xu, Gui-Sheng; Jain, Mudit; Zhou, Xiang-Fa; Guo, Guang-Can; Amin, Mustafa A.; Pu, Han; Zhou, Zheng-WeiArtificial monopoles have been engineered in various systems, yet there has been no systematic study of the singular vector potentials associated with the monopole field. We show that the Dirac string, the line singularity of the vector potential, can be engineered, manipulated, and made manifest in a spinor atomic condensate. We elucidate the connection among spin, orbital degrees of freedom, and the artificial gauge, and show that there exists a mapping between the vortex filament and the Dirac string. We also devise a proposal where preparing initial spin states with relevant symmetries can result in different vortex patterns, revealing an underlying correspondence between the internal spin states and the spherical vortex structures. Such a mapping also leads to a new way of constructing spherical Landau levels, and monopole harmonics. Our observation provides insights into the behavior of quantum matter possessing internal symmetries in curved spaces.Item Equation of State and Duration to Radiation Domination after Inflation(American Physical Society, 2017) Lozanov, Kaloian D.; Amin, Mustafa A.We calculate the equation of state after inflation and provide an upper bound on the duration before radiation domination by taking the nonlinear dynamics of the fragmented inflaton field into account. A broad class of single-field inflationary models with observationally consistent flattening of the potential at a scale M away from the origin, V ( ϕ ) ∝ | ϕ | 2 n near the origin, and where the couplings to other fields are ignored, is included in our analysis. We find that the equation of state parameter w → 0 for n = 1 and w → 1 / 3 (after sufficient time) for n ≳ 1 . We calculate how the number of e -folds to radiation domination depends on both n and M when M ∼ m Pl , whereas when M ≪ m Pl , we find that the duration to radiation domination is negligible. Our results are explained in terms of a linear instability analysis in an expanding universe and scaling arguments, and are supported by 3 + 1 -dimensional lattice simulations. We show that our upper bound on the postinflationary duration before radiation domination reduces the uncertainty in inflationary observables even when couplings to additional light fields are included (at least under the assumption of perturbative decay).Item Gravitational perturbations from oscillons and transients after inflation(American Physical Society, 2019) Lozanov, Kaloian D.; Amin, Mustafa A.We study the scalar and tensor perturbations generated by the fragmentation of the inflaton condensate into oscillons or transients after inflation, using nonlinear classical lattice simulations. Without including the backreaction of metric perturbations, we find that the magnitude of scalar metric perturbations never exceeds a few ×10−3, whereas the maximal strength of the gravitational wave signal today is O(10−9) for standard postinflationary expansion histories. We provide parameter scalings for the α-attractor models of inflation, which can be easily applied to other models. We also discuss the likelihood of primordial black hole formation, as well as conditions under which the gravitational wave signal can be at observationally interesting frequencies and amplitudes. Finally, we provide an upper bound on the frequency of the peak of the gravitational wave signal, which applies to all preheating scenarios.Item Gravitational waves from asymmetric oscillon dynamics?(American Physical Society, 2018) Amin, Mustafa A.; Braden, Jonathan; Copeland, Edmund J.; Giblin, John T. Jr.; Solorio, Christian; Weiner, Zachary J.; Zhou, Shuang-YongIt has been recently suggested that oscillons produced in the early universe from certain asymmetricpotentials continue to emit gravitational waves for a number of e-folds of expansion after their formation, leading to potentially detectable gravitational wave signals. We revisit this claim by conducting a convergence study using graphics processing unit (GPU)-accelerated lattice simulations and show that numerical errors accumulated with time are significant in low-resolution scenarios, or in scenarios where the run-time causes the resolution to drop below the relevant scales in the problem. Our study determines that the dominant, growing high frequency peak of the gravitational wave signals in the fiducial “hill-top model” by Antusch et al., [Phys. Rev. Lett. 118, 011303 (2017).] is a numerical artifact. This finding prompts the need for a more careful analysis of the numerical validity of other similar results related to gravitational waves from oscillon dynamics.Item Polarized solitons in higher-spin wave dark matter(American Physical Society, 2022) Jain, Mudit; Amin, Mustafa A.We first show that the effective nonrelativistic theory of gravitationally interacting, massive integer-spin fields (spin-0, 1, and 2 in particular) is described by a 2s+1 component Schrödinger-Poisson action, where s is the spin of the field. We then construct s+1 distinct, gravitationally supported solitons in this nonrelativistic theory from identically polarized plane waves. Such solitons are extremally polarized, with macroscopically large spin, but no orbital angular momentum. These s+1 solitons form a basis set, out of which partially polarized solitons can be constructed. All such solitons are ground states, have a spherically symmetric energy density but not field configurations. We discuss how solitons in higher-spin fields can be distinguished from scalar solitons, and potential gravitational and nongravitational probes of them.Item Prethermalization production of dark matter(American Physical Society, 2018) Garcia, Marcos A.G.; Amin, Mustafa A.At the end of inflation, the inflaton field decays into an initially nonthermal population of relativistic particles which eventually thermalize. We consider the production of dark matter via freeze-in from this relativistic plasma, focusing on the prethermal phase. We find that for a production cross section σ(E)∼En with n>2, the present dark matter abundance is produced during the prethermal phase of its progenitors. For n≤2, entropy production during reheating makes the nonthermal contribution to the present dark matter abundance subdominant compared to that produced thermally. As specific examples, we verify that the nonthermal contribution is irrelevant for gravitino production in low scale supersymmetric models (n=0) and is dominant for gravitino production in high scale supersymmetry models (n=6).Item Self-resonance after inflation: Oscillons, transients, and radiation domination(American Physical Society, 2018) Lozanov, Kaloian D.; Amin, Mustafa A.Homogeneous oscillations of the inflaton after inflation can be unstable to small spatial perturbations even without coupling to other fields. We show that for inflaton potentials ∝|ϕ|2n near |ϕ|=0 and flatter beyond some |ϕ|=M, the inflaton condensate oscillations can lead to self-resonance, followed by its complete fragmentation. We find that for nonquadratic minima (n>1), shortly after backreaction, the equation of state parameter, w→1/3. If M≪mPl, radiation domination is established within less than an e-fold of expansion after the end of inflation. In this case self-resonance is efficient and the condensate fragments into transient, localised spherical objects which are unstable and decay, leaving behind them a virialized field with mean kinetic and gradient energies much greater than the potential energy. This end-state yields w=1/3. When M∼mPl we observe slow and steady, self-resonance that can last many e-folds before backreaction eventually shuts it off, followed by fragmentation and w→1/3. We provide analytical estimates for the duration to w→1/3 after inflation, which can be used as an upper bound (under certain assumptions) on the duration of the transition between the inflationary and the radiation dominated states of expansion. This upper bound can reduce uncertainties in CMB observables such as the spectral tilt ns, and the tensor-to-scalar ratio r. For quadratic minima (n=1), w→0 regardless of the value of M. This is because when M≪mPl, long-lived oscillons form within an e-fold after inflation, and collectively behave as pressureless dust thereafter. For M∼mPl, the self-resonance is inefficient and the condensate remains intact (ignoring long-term gravitational clustering) and keeps oscillating about the quadratic minimum, again implying w=0.Item The charged inflaton and its gauge fields: preheating and initial conditions for reheating(IOP Publishing, 2016) Lozanov, Kaloian D.; Amin, Mustafa A.We calculate particle production during inflation and in the early stages of reheating after inflation in models with a charged scalar field coupled to Abelian and non-Abelian gauge fields. A detailed analysis of the power spectra of primordial electric fields, magnetic fields and charge fluctuations at the end of inflation and preheating is provided. We carefully account for the Gauss constraints during inflation and preheating, and clarify the role of the longitudinal components of the electric field. We calculate the timescale for the back-reaction of the produced gauge fields on the inflaton condensate, marking the onset of non-linear evolution of the fields. We provide a prescription for initial conditions for lattice simulations necessary to capture the subsequent nonlinear dynamics. On the observational side, we find that the primordial magnetic fields generated are too small to explain the origin of magnetic fields on galactic scales and the charge fluctuations are well within observational bounds for the models considered in this paper.