arXiv subjects

V. K. Onemli

Publications and source records attributed to V. K. Onemli.

13 recordsLinked to original sources

arXiv2019

Quantum Fluctuations of a Self-interacting Inflaton

We present a method to analytically compute the quantum corrected two-point correlation function of a scalar field in leading order at each loop in a homogeneous, isotropic and spatially flat spacetime where the expansion rate is time dependent and express the quantum corrected power spectrum $Δ^2(k)$ as a time derivative of the coincident correlation function evaluated at time $t_k$ of the first horizon crossing of a mode with comoving wave number $k$. To facilitate the method, we consider the simplest version of inflation driven by a massive, minimally coupled inflaton endowing a quartic self-interaction---with positive or negative self-coupling. We compute the quantum corrected two-point correlation function, power spectrum, spectral index $n(k)$ and the running of the spectral index $α(k)$ for the inflaton fluctuations at one-loop order. Numerical estimates of the $n(k)$ and $α(k)$ and the cosmological measurements are in agreement, within reasonable ranges of values for the physical parameters in the model.

gr-qc↗

arXiv2017

Quantum Effects of Mass on Scalar Field Correlations and Fluctuations during Inflation

We consider an infrared truncated massive minimally coupled scalar field with a quartic self-interaction in the locally de Sitter background of an inflating universe. We compute the two-point correlation function of the scalar and the mean squared fluctuations (variance) of the field variation analytically, at tree, one- and two-loop order. The one-loop correlator at a fixed comoving separation asymptotes to zero in the massive case but grows, at late times, like $-λ\ln^2(a)$ in the massless limit, where $a$ is the cosmic scale factor. For a fixed physical distance, on the other hand, it grows, at late times, like $-λ\ln^3(a)$ in the massless limit. This growth is severely suppressed in the massive case. In fact, the one-loop correlator asymptotes effectively to zero for masses larger than half the expansion rate. We find out also that the tree-order variance of field variation decreases when quantum corrections are included. Hence, the actual effect that any local observer perceives in the field strength as fluctuations happen does not deviate from the average effect as much as the tree-order variance implies.

gr-qc↗

arXiv2015

Quantum corrected power spectra of massless minimally coupled scalars during inflation: Effects of Yukawa coupling versus quartic self-interaction

We, in the first part, contemplate a massless minimally coupled scalar which is Yukawa-coupled to a massless Dirac fermion in a locally de Sitter background of an inflating spacetime. We compute the scalar's quantum corrected mode function, power spectrum, spectral index and the running of the spectral index at one-loop order. We find that the spectrum is slightly blue-tilted; hence, the amplitudes of fluctuations grow slightly toward the smaller scales. Then, in the second part, we apply the computation method used in the first part to a massless minimally coupled scalar with a quartic self-interaction in the same background and obtain exact analytic expressions for the associated quantities at one-loop order. In contrast to the Yukawa scalar, the spectrum in this case is slightly red-tilted; hence, the amplitudes of fluctuations grow slightly toward the larger scales.

gr-qc↗

arXiv2015

Vacuum Fluctuations of a Scalar Field during Inflation: Quantum versus Stochastic Analysis

We consider an infrared truncated massless minimally coupled scalar field with a quartic self-interaction in the locally de Sitter background of an inflating universe. We compute the two-point correlation function of the scalar at one and two-loop order applying quantum field theory. The tree-order correlator at a fixed comoving separation (that is at increasing physical distance) freezes in to a nonzero value. At a fixed physical distance, it grows linearly with comoving time. The one-loop correlator, which is the dominant quantum correction, implies a negative temporal growth in the correlation function, at this order, at a fixed comoving separation and at a fixed physical distance. We also obtain quantitative results for variance in space and time of one and two-loop correlators and infer that the contrast between the vacuum expectation value and the variance becomes less pronounced when the loop corrections are included. Finally, we repeat the analysis of the model applying a stochastic field theory and reach the same conclusions.

gr-qc↗

arXiv2013

The Quantum Corrected Mode Function and Power Spectrum for a Scalar Field during Inflation

We compute the one- and two-loop corrected mode function of a massless minimally coupled scalar endowed with a quartic self-interaction in the locally de Sitter background of an inflating universe for a state which is released in Bunch-Davies vacuum at time $t=0$. We then employ it to correct the scalar's tree-order scale invariant power spectrum $Δ^2_φ$. The corrections are secular, and have scale dependent part that can be expanded in even powers of $k/(Ha)$, where $k$ is the comoving wave number, $H$ is the expansion rate and $a$ is the cosmic scale factor. At one-loop, the scale invariant shift in the power spectrum grows as $(Ht)^2$ in leading order. The $k$-dependent shifts, however, are constants for each mode, in the late time limit. At two-loop order, on the other hand, the scale invariant shift grows as $(Ht)^4$ whereas the $k$-dependent shifts grow as $(Ht)^2$, in leading order. We finally calculate the scalar's spectral index $n_φ$ and the running of the spectral index $α_φ$. They imply that the spectrum is slightly red-tilted; hence, the amplitudes of fluctuations grow slightly towards the larger scales.

astro-ph.CO↗

arXiv2010

The Zeta-Zeta Correlator Is Time Dependent

We comment on the recent arguments by Senatore and Zaldarriaga that loop corrections to the zeta-zeta correlator cannot grow with time after first horizon crossing. We first emphasize the need to search for such secular dependence in corrections whose in-out matrix elements are infrared singular on an infinite spatial manifold. Then we give examples of such time dependence from pure quantum gravity and from scalar potential models. Finally, we point out that this time dependence arises from inflationary particle production and is therefore unlikely to endanger the preservation of super-horizon correlations as a record of inflation.

astro-ph.CO↗

arXiv2009

A Completely Regular Quantum Stress Tensor with $w < -1$

For many quantum field theory computations in cosmology it is not possible to use the flat space trick of obtaining full, interacting states by evolving free states over infinite times. State wave functionals must be specified at finite times and, although the free states suffice to obtain the lowest order effects, higher order corrections necessarily involve changes of the initial state. Failing to correctly change the initial state can result in effective field equations which diverge on the initial value surface, or which contain tedious sums of terms that redshift like inverse powers of the scale factor. In this paper we verify a conjecture from 2004 that the lowest order initial state correction can indeed absorb the initial value divergences and all the redshifting terms of the two loop expectation value of the stress tensor of a massless, minimally coupled scalar with a quartic self interaction on nondynamical de Sitter background.

gr-qc↗

arXiv2007

Quantum Stability of a w < - 1 Phase of Cosmic Acceleration

We consider a massless, minimally coupled scalar with a quartic self-interaction which is released in Bunch-Davies vacuum in locally de Sitter background of an inflating universe. It was shown, in this system, that quantum effects can induce a temporary phase of super-acceleration causing a violation of the Weak Energy Condition on cosmological scales. In this paper we investigate the system's stability by studying the behavior of linearized perturbations in the quantum-corrected effective field equation at one and two-loop order. We show that the time dependence we infer from the quantum-corrected mode function is in perfect agreement with the system developing a positive mass squared. The maximum induced mass remains perturbatively small and it does not go tachyonic. Thus, the system is stable.

gr-qc↗

arXiv2006

Probing Cold Dark Matter Cusps by Gravitational Lensing

I elaborate on my prediction that an indirect detection of cold dark matter (CDM) may be possible by observing the gravitational lensing effects of the CDM cusp caustics at cosmological distances. Cusps in the distribution of CDM are plentiful once density perturbations enter the nonlinear regime of structure formation. Caustic ring model of galactic halo formation provides a well defined density profile and geometry near the cusps of the caustic rings. I calculate the gravitational lensing effects of the cusps in this model. As a pointlike background source passes behind a cusp of a cosmological foreground halo, the magnification in its image may be detected by present instruments. Depending on the strength of detected effect and the time scale of brightness change, it may even be possible to discriminate between the CDM candidates: axions and weakly interacting massive particles.

astro-ph↗

arXiv2005

Gravitational Lensing and Structural Stability of Dark Matter Caustic Rings

Gravitational lensing by the dual cusp catastrophes of the cold dark matter (CDM) caustic rings at cosmological distances may provide the tantalizing opportunity to detect CDM indirectly, and discriminate between axions and weakly interacting massive particles (WIMPs). Caustics are places where the CDM particles are naturally focussed. Our focus is upon the caustic rings which are closed tubes whose cross-section is an elliptic umbilic catastrophe with three dual cusps. A caustic ring has a specific density profile, a specific geometry and, therefore, precisely calculable gravitational lensing signatures. The magnification monotonically increases as the line of sight approaches to the cusps where it diverges in the limit of zero velocity dispersion. In this limit, we find 37% magnification at a sample point near the outer cusps of the CDM caustic rings at cosmological distances. In the presence of finite velocity dispersion, the lower and upper bounds of the effective velocity dispersions of the axion and WIMP flows in galactic halos may be used to constrain the lensing effects at the cusps. For a cosmological axion caustic ring, we find that the magnification may range between 3% and 2800% at the outer cusp, and between 2% and 46% at the non-planer cusps. For a cosmological WIMP caustic ring, on the other hand, we constrain the magnification between 3% and 28% at the outer cusp, and between 2% and 5% at the non-planer cusps. The images of extended sources may also show distortions that can be unambiguously attributed to lensing by dark matter caustics. Finally, we derive the Catastrophe Function of the triaxial caustic rings. We obtain the flow equations as the equilibrium points of this Catastrophe Function. The analysis of the Stability (Hessian) Matrix show that the caustic rings are structurally stable.

astro-ph↗

arXiv2004

Two Loop Scalar Self-Mass during Inflation

We work in the locally de Sitter background of an inflating universe and consider a massless, minimally coupled scalar with a quartic self-interaction. We use dimensional regularization to compute the fully renormalized scalar self-mass-squared at one and two loop order for a state which is released in Bunch-Davies vacuum at t=0. Although the field strength and coupling constant renormalizations are identical to those of lfat space, the geometry induces a non-zero mass renormalization. The finite part also shows a sort of growing mass that competes with the classical force in eventually turning off this system's super-acceleration.

gr-qc↗

arXiv2004

Quantum effects can render w<-1 on cosmological scales

We report on a revision of our previous computation of the renormalized expectation value of the stress-energy tensor of a massless, minimally coupled scalar with a quartic self-interaction on a locally de Sitter background. This model is important because it demonstrates that quantum effects can lead to violations of the weak energy condition on cosmological scales - on average, not just in fluctuations - although the effect in this particular model is far too small to be observed. The revision consists of modifying the propagator so that dimensional regularization can be used when the dimension of the renormalized theory is not four. Although the finite part of the stress-energy tensor does not change (in D=4) from our previous result, the counterterms do. We also speculate that a certain, finite and separately conserved part of the stress tensor can be subsumed into a natural correction of the initial state from free Bunch-Davies vacuum.

gr-qc↗

arXiv2002

Super-Acceleration from Massless, Minimally Coupled $ϕ^4$

We derive a simple form for the propagator of a massless, minimally coupled scalar in a locally de Sitter geometry of arbitrary spacetime dimension. We then employ it to compute the fully renormalized stress tensor at one and two loop orders for a massless, minimally coupled phi^4 theory which is released in Bunch-Davies vacuum at t=0 in co-moving coordinates. In this system the uncertainty principle elevates the scalar above the minimum of its potential, resulting in a phase of super-acceleration. With the non-derivative self-interaction the scalar's breaking of de Sitter invariance becomes observable. It is also worth noting that the weak energy condition is violated on cosmological scales. An interesting subsidiary result is that canceling overlapping divergences in the stress tensor requires a conformal counterterm which has no effect on purely scalar diagrams.

gr-qc↗