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S. P. de Alwis

Publications and source records attributed to S. P. de Alwis.

At least 19 recordsLinked to original sources
arXiv2026

Notes on LSZ, i epsilon Prescriptions and Perturbation Theory, in QFT and Cosmology

We review the original argument of Lehmann et al (LSZ) that relates the flat space S-matrix to the correlation function of field operators and clarify some confusing issues. Next we discuss the origin of the $iε$ prescriptions following Weinberg, but without assuming that the vacuum is free at asymptotic times. Then we discuss the corresponding argument in inflationary cosmology, emphasizing that unitarity violating contour deformations are unnecessary.

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arXiv2025

Notes on Quantum Effective Actions

We first note that, at least in perturbation theory, there is a well-defined (subject to regularization) Lorentzian definition of the quantum effective action in both flat and curved space including (perturbative) gravity. The advantage of the latter is that we do not need to deal with the conformal factor problems of Euclidean quantum gravity. We then make some remarks on the Euclidean version (in flat space) and convexity and resolve a puzzle that highlights the importance of keeping the initial and final states in the functional integral. Next we discuss the gauge invariant effective action of Vilkovisky and DeWitt and show its gauge fixing independence. We conclude with the expression for the Wilsonian effective action in this framework.

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arXiv2024

Revisiting Vacuum decay in Field Theory

We revisit the formalism for tunneling in quantum field theory developed by Coleman and collaborators. In particular using the generalization of WKB methods for tunneling in quantum mechanics we avoid the problems with negative eigenvalues and convexity issues associated with Coleman's approach. While the exponential factor is the same, we find differences in the pre-factor. Then we point out that to actually discuss the time evolution of the state, we need a wave packet formulation which we proceed to discuss. Next we address the problem of justifying the application of semi-classical tunneling calculations to the decay of the standard model vacuum, where the classical potential signifies absolute stability, though the effective potential appears to imply the possibility of meta-stability (with more than one local minimum). This is in contrast to the usual situation in applications of the formalism for tunneling, where the \textit{classical} potential has more than one local minimum.

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arXiv2023

Comments on Entropy Calculations in Gravitational Systems

We discuss the logic of, and some puzzles in, the various approaches to thermodynamics of gravitational systems. In particular the blackhole, deSitter (dS), black hole in dS (SdS) and in Anti-deSitter SAdS backgrounds are considered. After reviewing the original calculations of Hawking and Gibbons we discuss an alternative Hamiltonian method. This justifies the lowest order Euclidean calculation but is free of the problems associated with the latter when going to higher orders. To conclude we address the sign issue in dS thermodynamics.

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arXiv2021

Radiative Generation of dS from AdS

The large volume scenario (LVS) of type IIB string compactifications has a robust supersymmetry breaking minimum with a negative cosmological constant (CC). We argue that radiative corrections below the Kaluza-Klein (KK) scale can result in a positive CC, though the string theory generated CC is negative, if some mild conditions on the spectrum of low energy fluctuations are satisfied. This would make the so-called deSitter swampland conjecture (even if true at a high scale) physically irrelevant.

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arXiv2021

Wilsonian Effective Field Theory and String Theory

We argue that deriving an effective field theory from string theory requires a Wilsonian perspective with a physical cutoff. Employing proper time regularization we demonstrate the decoupling of states and contrast this with what happens in dimensional regularization. In particular we point out that even if the cosmological constant (CC) calculated from some classical action at some ultra-violet scale is negative, this does not necessarily imply that the CC calculated at cosmological scales is also negative, and discuss the possible criteria for achieving a positive CC starting with a CC at the string/KK scale which is negative. Obviously this has implications for swampland claims.

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arXiv2020

Cosmological Trans-Planckian Conjectures are not Effective

It is remarkable that the primordial fluctuations as revealed by the CMB coincide with what quantum fluctuations would look like if they were stretched across the sky by accelerated cosmic expansion. It has been observed that this same stretching also brings very small -- even trans-Planckian -- length scales up to observable sizes if extrapolated far enough into the past. This potentially jeopardizes later descriptions of late-time cosmology by introducing uncontrolled trans-Planckian theoretical errors into all calculations. Recent speculations, such as the Trans-Planckian Censorship Conjecture (TCC), have been developed to avoid this problem. We revisit old arguments why the consistency of (and control over) the Effective Field Theory (EFT) governing late-time cosmology is not necessarily threatened by the descent of modes due to universal expansion, even if EFT methods may break down at much earlier times. Failure of EFT methods only poses a problem if late-time predictions rely on non-adiabatic behaviour at these early times (such as is often true for bouncing cosmologies, for example). We illustrate our arguments using simple non-gravitational examples such as slowly rolling scalar fields and the spacing between Landau levels for charged particles in slowly varying magnetic fields, for which similar issues arise and are easier to understand. We comment on issues associated with UV completions. Our arguments need not invalidate speculative ideas like the TCC but suggest they are not required by the present evidence.

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arXiv2019

The Wave Function of the Universe and CMB Fluctuations

The Hartle-Hawking and Tunneling (Vilenkin) wave functions are treated in the Hamiltonian formalism. We find that the leading (i.e. quadratic) terms in the fluctuations around a maximally symmetric background, are indeed Gaussian (rather than inverse Gaussian), for both types of wave function, when properly interpreted. However the suppression of non-Gaussianities and hence the recovery of the Bunch-Davies state is not transparent.

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arXiv2019

Higher Derivative Corrections to Lower Order RG Flow Equations

We show that the RG flow equation for the cosmological constant (CC) receives contributions (in addition to those coming from the CC the Einstein-Hilbert term and $R^{2}$ and $R_{μν}^{2}$ terms) only from terms with just two powers of curvature, but having also powers of the covariant derivative, in the Wilsonian effective action. In pure gravity our argument implies that just considering $f(R)$ theories will miss this effect which arises from terms such as $"R"\square^{n}"R",\,n=0,1,2,\ldots$. We expect similar contributions for the flow equation of the Einstein-Hilbert term as well. Finally we argue that the perturbative ghosts coming from curvature squared terms in the action are in fact spurious since they are at the cutoff scale and can be removed by (cutoff dependent) field redefinitions.

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arXiv2018

Exact RG Flow Equations and Quantum Gravity

We discuss the different forms of the functional RG equation and their relation to each other. In particular we suggest a generalized background field version that is close in spirit to the Polchinski equation as an alternative to the Wetterich equation to study Weinberg's asymptotic safety program for defining quantum gravity, and argue that the former is better suited for this purpose. Using the heat kernel expansion and proper time regularization we find evidence in support of this program in agreement with previous work.

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arXiv2016

Constraints on Dbar Uplifts

We discuss constraints on KKLT/KKLMMT and LVS scenarios that use anti-branes to get an uplift to a deSitter vacuum, coming from requiring the validity of an effective field theory description of the physics. We find these are not always satisfied or are hard to satisfy.

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arXiv2016

Cosmological fluctuations: Comparing Quantum and Classical Statistical and Stringy Effects

The theory of cosmological fluctuations assumes that the pre-inflationary state of the universe was the quantum vacuum of a scalar field(s) coupled to gravity. The observed cosmic microwave background fluctuations are then interpreted as quantum fluctuations. Here we consider alternate interpretations of the classic calculations of scalar and tensor power spectra by replacing the Bunch-Davies quantum vacuum with a classical statistical distribution, which may have been the consequence of a pre-inflationary process of decoherence as in the quantum cosmology literature. Mathematically they are essentially identical calculations. However if one takes the latter interpretation then one might replace the Planck length by for instance the fundamental length scale of string theory. In particular this changes the relation between the scale of inflation and the scalar power spectrum but leaves the parameter(s) characterizing the bi-spectrum unchanged at leading order. Differences will occur however at higher order in the loop expansion. We also discuss the relation to theories with low sound speed and/or a period of dissipation during inflation (warm inflation).

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arXiv2015

One PI and Wilsonian Actions in SUSY theories

The soft breaking terms in supersymmetric theories are calculated at some high scale characterizing the hidden supersymmetry breaking sector, and then evolved down to the TeV scale. These parameters are usually presented as the ones that should be compared to experiment. The physical parameters however are those occurring in the quantum effective (1PI) action - in particular the physical mass is the location of the pole in the full quantum propagator. Here we discuss the relation between the two and the possible existence of additive contributions to the gaugino mass. We argue that infra red effects which violate non-renormalization theorems are absent (for the 1PI action) if the calculation is done at a generic point in field space so that an effective IR cutoff is present. It follows that if a gaugino mass term is absent in the Wilsonian action it is absent in the 1PI action.

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arXiv2013

Gauge Threshold Corrections and Field Redefinitions

We review the argument for field redefinitions arising from threshold corrections to heterotic string gauge couplings, and the relation between the linear and the chiral multiplet. In the type IIB case we argue that the necessity for moduli mixing at one loop order has not been clearly established, since this is based on extending the background field expansion way beyond its regime of validity. We also resolve some issues related to the form of non-perturbative terms resulting from gaugino condensation. This enables us to estimate the effective cutoff in the field theory by evaluating the non-perturbative superpotential by two different methods, and find that it is around the Kaluza-Klein scale, as one might have expected on general grounds of self-consistency.

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arXiv2013

AMSB and the Logic of Spontaneous SUSY Breaking

A cardinal principle of any theory of spontaneous (${\cal N}=1$) supersymmetry breaking, is that the order parameter is a linear combination of the F and D terms. Also as long as the supersymmetry of the action is preserved at the quantum level, this principle should be valid after the appropriate corrections are incorporated into the Kaehler potential, the superpotential, and the gauge coupling functions that define the theory. The claim that in AMSB there is an extra term for the gaugino mass that is proportional to the gravitino mass, is then equivalent to the statement that quantum effects cause an explicit breaking of (local) SUSY. Expanding on previous work, we argue that this arises from confusing the scalar compensator with the density compensator. We comment on various recent papers on AMSB.

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arXiv2012

A Local Evaluation of Global Issues in SUSY breaking

It is well known that there are different global (i.e. $M_{P}\rightarrow\infty$) limits of N=1 supergravity. We distinguish between these limits and their relevance to low energy phenomenology. We discuss a) fermion mass matrices and recently proved theorems in global SUSY b) stability issues and SUSY breaking d) R-symmetry and a recently derived bound on the superpotential and e) FI terms in global and local SUSY.

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arXiv2012

Constraints on LVS Compactifications of IIB String Theory

We argue that once all theoretical and phenomenological constraints are imposed on the different versions of the Large Volume Scenario (LVS) compactifications of type IIB string theory, one particular version is favored. This is essentially a sequestered one in which the soft terms are generated by Weyl anomaly and RG running effects. We also show that arguments questioning sequestering in LVS models are not relevant in this case.

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