Browsing by Author "Lozanov, Kaloian D."

Now showing 1 - 5 of 5
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    Cosmological dynamics of Higgs potential fine tuning
    (American Physical Society, 2019) Amin, Mustafa A.; Fan, JiJi; Lozanov, Kaloian D.; Reece, Matthew
    The 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.
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    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).
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    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.
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    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.
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    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.