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arxiv: 2605.18286 · v1 · pith:LZPCERQNnew · submitted 2026-05-18 · 🌌 astro-ph.CO · gr-qc· hep-th

Primary gravitational waves at high frequencies II: Emergence of the exponential cut-off in the power spectrum

Alipriyo Hoory , Jerome Martin , Arnab Paul , L. Sriramkumar This is my paper

Pith reviewed 2026-05-20 00:24 UTC · model grok-4.3

classification 🌌 astro-ph.CO gr-qchep-th
keywords primary gravitational wavespower spectruminflation transitionexponential suppressionBorn approximationeffective potentialhigh frequenciesregularization
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The pith

Infinitely smooth transitions from inflation cause exponential suppression in the power spectrum of primary gravitational waves at high frequencies.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates how the regularized power spectrum of primary gravitational waves behaves on small scales as the transition from inflation becomes smoother and smoother. It finds that infinitely differentiable effective potentials lead to an exponential suppression at high wave numbers, rather than oscillations or power-law decay. A sympathetic reader would care because this cutoff shape might allow future observations to determine the energy scale and timing of the end of inflation.

Core claim

Using the Born approximation, the paper shows that for transitions from inflation to radiation and matter domination described by infinitely differentiable effective potentials, the regularized power spectrum of primary gravitational waves exhibits an exponential suppression on small scales.

What carries the argument

The Born approximation applied to the tensor mode equation with increasingly smoothed effective potentials governing gravitational wave evolution.

If this is right

  • Linear smoothing of the transition produces oscillations in the regularized PS whose amplitude falls as one over k on small scales.
  • Increasing the smoothness makes the suppression at high wave numbers progressively sharper.
  • Infinitely differentiable potentials produce a clean exponential cutoff in the regularized PS.
  • Observing the exponential drop could fix the energy scale and the time at which inflation ended.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The differentiability of the post-inflationary transition may leave an observable signature in any high-frequency cosmological signal.
  • Future detectors operating at very high frequencies could use the cutoff shape to distinguish between different models of how inflation ends.
  • Similar exponential tails might appear in other perturbation spectra if the transition is modeled with the same level of smoothness.

Load-bearing premise

That an infinitely differentiable effective potential can represent a physically realizable transition from inflation to radiation domination.

What would settle it

A direct measurement of the high-frequency gravitational wave spectrum that either confirms or rules out the presence of an exponential cutoff at scales set by the end of inflation.

read the original abstract

[Abridged] In slow roll inflation, the power spectrum (PS) of primary gravitational waves (PGWs) generated from the quantum vacuum rises as $k^2$ over wave numbers $k$ which never leave the Hubble radius. In fact, over such small scales, the PS exhibits a similar behavior at any time after inflation. In a recent work, we had argued that the PS of PGWs has to be regularized to truncate the unphysical quadratic rise at large wave numbers. Assuming instantaneous transitions from inflation to the epochs of radiation and matter domination, we had shown that the regularized PS oscillates with a constant amplitude about a vanishing mean over small scales during these epochs. We had also smoothed the transition (actually, the `effective potential' governing the equation of motion of GWs) from inflation to radiation domination using a linear function and evaluated the regularized PS of PGWs post inflation. In such a case, we had shown that, over small scales, while the regularized PS continues to oscillate about zero, its amplitude decreases as $k^{-1}$. In this work, using the Born approximation, we examine the behavior of the regularized PS of PGWs over small scales when they are evolved through smoother and smoother transitions from inflation to the epochs of radiation and matter domination. We illustrate that, at small scales or high frequencies, the suppression in the regularized PS of PGWs occurs more and more sharply as the transition is smoothed further and further. With the help of examples, we also show that, in the case of transitions described by infinitely differentiable `effective potentials', the regularized PS of PGWs exhibits an exponential suppression on small scales. We argue that the observation of the exponential drop in the PS of PGWs can help us determine the energy scale and the time of the end of inflation. We clarify related issues and discuss the wider implications.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript extends prior work on regularizing the unphysical quadratic rise in the power spectrum of primary gravitational waves (PGWs) at high wave numbers. It uses the Born approximation to evolve the tensor mode functions through increasingly smooth transitions from inflation to radiation and matter domination, showing that infinitely differentiable effective potentials produce an exponential suppression in the regularized power spectrum at small scales/high frequencies, with the sharpness increasing as the transition is smoothed further. Explicit examples illustrate the behavior, and the authors argue this cutoff could observationally constrain the energy scale and end time of inflation.

Significance. If the central mathematical result holds, the work provides a concrete link between the analytic properties of the effective potential and an exponential high-frequency cutoff in the PGW spectrum, offering a potential probe of the end of inflation. Credit is due for the forward evolution under chosen smoothing functions, the explicit demonstration that the exponential form emerges from the potential's analytic properties rather than being inserted by hand, and the scoping of the claim to C^infty cases without asserting that all physical transitions must satisfy this smoothness.

major comments (1)
  1. [smoothing the transition and Born approximation] Section on smoothing the transition and use of Born approximation: the exponential suppression is illustrated via the Born approximation and specific smoothing examples, but the manuscript provides no error estimates, convergence checks, or direct comparison against exact numerical solutions for the mode functions. This is load-bearing for the central claim about the sharpness of the cutoff at high k.
minor comments (1)
  1. [abstract] The abstract would benefit from briefly stating the explicit functional forms of the infinitely differentiable effective potentials used in the examples.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive assessment of our work and the constructive comment. We address the major point below and will revise the manuscript accordingly to strengthen the presentation of the Born approximation results.

read point-by-point responses

  1. Referee: [smoothing the transition and Born approximation] Section on smoothing the transition and use of Born approximation: the exponential suppression is illustrated via the Born approximation and specific smoothing examples, but the manuscript provides no error estimates, convergence checks, or direct comparison against exact numerical solutions for the mode functions. This is load-bearing for the central claim about the sharpness of the cutoff at high k.

    Authors: We agree that the absence of explicit error estimates and numerical benchmarks is a limitation in the current draft. The Born approximation is employed because it isolates the leading high-k contribution arising directly from the analytic properties of the C^infty effective potential; the exponential decay follows from the rapid fall-off of the Fourier transform of such functions. Higher-order Born terms involve additional convolutions that are parametrically more suppressed at large k. Nevertheless, to make this rigorous, we will add a new appendix containing (i) a bound on the remainder of the Born series for the chosen smoothing functions and (ii) a direct comparison, for one representative C^infty transition, between the Born-approximated tensor mode functions and those obtained by numerical integration of the exact mode equation. This will quantify the accuracy of the approximation in the regime k ≫ transition scale where the exponential cutoff appears. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected in derivation chain

full rationale

The paper derives the exponential suppression in the regularized PGW power spectrum for infinitely differentiable effective potentials by applying the Born approximation to forward-evolved mode equations under progressively smoother transitions. This behavior is illustrated via explicit examples rather than being presupposed or fitted. The result follows from the analytic properties of the chosen smoothing functions and the mathematical structure of the tensor mode equation. No load-bearing self-citations reduce the central claim to prior inputs, and the paper explicitly scopes its conclusions to the imposed C^infty condition without claiming universality for all physical transitions. The derivation remains self-contained against the stated assumptions.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central result rests on the validity of the Born approximation for the mode evolution and on the ability to prescribe arbitrarily smooth effective potentials without violating the background cosmology. No new particles or forces are introduced.

free parameters (1)
  • transition smoothness parameter
    The width or differentiability order of the smoothing function for the effective potential is chosen to illustrate progressively sharper cutoffs.
axioms (2)
  • domain assumption Born approximation is sufficient to capture the leading behavior of the tensor modes across the transition
    Invoked to evaluate the regularized power spectrum after the transition.
  • domain assumption The effective potential can be made infinitely differentiable while preserving the slow-roll background and the definition of the regularization
    Required for the exponential suppression to appear.

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Reference graph

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