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Fast-Roll Inflation

High Energy Physics - Theory · Submitted 21 Oct 2001 · 34 references

Pith's verdict: A short, honest summary paper: the growth-rate formula and the two side observations are the durable content, and the e-fold numbers are illustrative rather than predictive — which the author mostly says himself.

T0 · AI review

How this review was made: An AI referee (selfhosted/deepseek-v4.1-flash) read the paper on November 13, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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arXiv IDhep-th/0110195 (version 2)
Pith IDZF3NUF4I, Pith's permanent record of this paper and its review
CitationAndrei Linde. "Fast-Roll Inflation." arXiv:hep-th/0110195 (2001). https://pith.science/paper/ZF3NUF4I
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Referee report
The paper

The paper claims

Linde argues that inflation does not need slow roll: a scalar field falling fast from a hilltop, with a tachyonic mass of order the Hubble scale, can still stretch the universe by many e-folds, provided its mass is small in Planck units. He

The reading

The paper asks whether inflation really requires the slow-roll condition $|m^2| = |V''|/V$ to be much smaller than $H^2$. Linde's answer is no: in the simplest symmetry-breaking potentials a field sitting near a maximum with a tachyonic mass of order the Hubble scale grows as $V(0)$, $|m^2| = |V''|/V$, and the total expansion $e^{Ht_*}$ depends on the ratio $V(0)$ raised to $1/F$, where $F = V(0)$. Because that exponent is large when $m$ is small in Planck units, a stage that badly violates slow roll can still deliver 60 or more e-folds, enough to account for the present accelerated expansion and enough to prepare initial conditions for a later slow-roll stage. The paper then makes two general points: a universe that inflated long enough to become flat can never come to be dominated by a negative cosmological constant, and potentials with a stable minimum at $V(0)$ are as dangerous cosmologically as potentials unbounded from below, because the field is driven into a kinetic-energy-dominated collapse whose late-time behaviour hardly depends on the shape of the potential.

What carries the argument

The load-bearing object is the growth exponent $F(m^2/H^2) = V(0)$, which interpolates between the slow-roll result $F = V(0)$ and the fast-roll result $F = V(0)$, together with the expansion formula $e^{Ht_*} = (V(0))^{1/F}$ and its numerical inverses $F^{-1}(1) = 3.3$, $F^{-1}(6) = 0.73$, $F^{-1}(12) = 0.44$. This single function converts a mass ratio into an amount of inflation and is what lets the paper compare spontaneous-symmetry-breaking models, $N=8$ supergravity, and quintessence on one footing. The second piece of machinery is the flatness argument: after long inflation the curvature term $k/a^2$ is negligible, so $H^2 = V(0)$ has no solution with $V(0)$, and anti-de Sitter cannot be reached. The third is the collapse solution $a(0)$ with $V(0)$, which shows why the singularity's structure is fixed by kinetic energy rather than by the potential.

Extended reading notes

Core claim

The central claim is that fast-roll inflation is a genuine regime, not a failed limit. For $V(0)$ near a maximum, the homogeneous mode obeys $V(0)$ and grows as $V(0)$ with $F = V(0)$; the total expansion is $e^{Ht_*} = (V(0))^{1/F}$. Slow roll is the small-$m$ limit $F = V(0)$, fast roll is the large-$m$ limit $F = V(0)$, and the interesting middle case $m^2 = O(H^2)$ is where the paper's estimates live. Taking the initial amplitude to be the level of the exponentially growing quantum fluctuations, $V(0)$, Linde finds expansions of order $10^{200}$ for $m = H = 10^{-60}M_p$, $10^{56}$ for the electroweak scale, and still $10^{44}$ in the gauged $N=8$ supergravity case where $|V''| = 2V$, i.e. $m^2 = 6H^2$. He is explicit that fast-roll inflation at the very start produces adiabatic perturbations of order one with a steep red spectrum, so it is safe only if followed by slow roll or if it occurs at the present epoch; a short fast-roll stage at an intermediate scale risks overproducing black holes.

Load-bearing premise

The size of every quoted expansion factor rests on taking the starting displacement of the field to be about one tenth of its mass, $V(0)$, a value borrowed from simulations of the growth of quantum fluctuations rather than derived within the model; a starting value nearer the hilltop edge would shrink the predicted inflation to almost nothing.

Editorial extensions

If this is right

  • A present-day accelerated expansion driven by a field near a hilltop with $|V''| = O(H^2)$ is consistent with the data, since $V(0)$ stays nearly constant during the fast roll, so the equation of state is indistinguishable from a cosmological constant over the observed epoch.
  • A short fast-roll stage at nearly Planckian density can homogenise a region of Planck size and thereby supply the initial conditions that low-scale slow-roll and hybrid inflation otherwise lack.
  • Because $V(0)$ can be enormous for small $m$, models with potentials unbounded from below, or with minima at $V(0)$, remain viable descriptions of the current acceleration even though they eventually collapse.
  • Fast-roll inflation generates isocurvature perturbations with a flat spectrum from light fields, which can be converted into adiabatic perturbations if those fields later decay into radiation.
  • Inflationary cosmology predicts that we cannot end up in an anti-de Sitter universe, which is a constraint on any proposed resolution of the cosmological constant problem.
  • A fast-roll stage at the electroweak scale or below can add the $10^4$ to $10^5$ e-folds needed to dilute moduli and gravitinos, and can extend thermal inflation beyond the point where the field leaves the top of its potential.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The paper's expansion estimates are exponentially sensitive to the assumed initial displacement: since $e^{Ht_*} = (V(0))^{1/F}$, any mechanism that places the field closer to the hilltop than the fluctuation level $V(0)$ lengthens inflation dramatically, and any mechanism that places it further shortens it. This makes the initial-condition story, not the growth law, the place where a numerical si
  • The observation that a flat universe cannot reach anti-de Sitter can be turned around: a detection of negative spatial curvature at the percent level would reopen the anti-de Sitter route, so the claim is in principle a cosmological test of the inflationary prediction $\Omega = 1$.
  • The statement that minima with $V(0)$ are as dangerous as unbounded potentials suggests a selection criterion for quintessence model building: what matters is not whether the potential is bounded below, but how long the field lingers near a positive plateau before the kinetic energy takes over.
  • The paper treats the fast-roll and slow-roll stages as sequential; a natural extension is to ask whether the transition itself leaves a feature in the scalar spectrum at the scale that left the horizon when $F$ changed, which would be a distinctive signature of a two-stage early history.
Original abstract

We show that in the simplest theories of spontaneous symmetry breaking one can have a stage of a fast-roll inflation. In this regime the standard slow-roll condition |m^2| << H^2 is violated. Nevertheless, this stage can be rather long if |m| is sufficiently small. Fast-roll inflation can be useful for generating proper initial conditions for the subsequent stage of slow-roll inflation in the very early universe. It may also be responsible for the present stage of accelerated expansion of the universe. We also make two observations of a more general nature. First of all, the universe after a long stage of inflation (either slow-roll or fast-roll) cannot reach anti-de Sitter regime even if the cosmological constant is negative. Secondly, the theories with the potentials with a "stable" minimum at V(\phi)<0 in the cosmological background exhibit the same instability as the theories with potentials unbounded from below. This instability leads to the development of singularity with the properties practically independent of V(\phi). However, the development of the instability in some cases may be so slow that the theories with the potentials unbounded from below can describe the present stage of cosmic acceleration even if this acceleration occurs due to the fast-roll inflation.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,427 input tokens, $0.0003 reported.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.47 · human participants 0.01 · controlled comparison 0.03 · randomized assignment 0.01 · states quantitative result 0.10 · compares with prior results 0.16 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

We show that in the simplest theories of spontaneous symmetry breaking one can have a stage of a fast-roll inflation. In this regime the standard slow-roll condition |m^2| << H^2 is violated. Nevertheless, this stage can be rather long if |m| is sufficiently small. Fast-roll inflation can be useful for generating proper initial conditions for the subsequent stage of slow-roll inflation in the very early universe. It may also be responsible for the present stage of accelerated expansion of the universe. We also make two observations of a more general nature. First of all, the universe after a long stage of inflation (either slow-roll or fast-roll) cannot reach anti-de Sitter regime even if the cosmological constant is negative. Secondly, the theories with the potentials with a "stable" minimum at V(\phi)<0 in the cosmological background exhibit the same instability as the theories with potentials unbounded from below. This instability leads to the development of singularity with the properties practically independent of V(\phi). However, the development of the instability in some cases may be so slow that the theories with the potentials unbounded from below can describe the present stage of cosmic acceleration even if this acceleration occurs due to the fast-roll inflation.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We show that in the simplest theories of spontaneous symmetry breaking one can have a stage of a fast-roll inflation. contribution 0.55 0.92 0.64 asserted 0.95
s1 In this regime the standard slow-roll condition |m^2| << H^2 is violated. result 0.88 0.46 0.08 asserted 1.00
s2 Nevertheless, this stage can be rather long if |m| is sufficiently small. result 0.99 0.73 0.10 hedged possibility 0.64
s3 Fast-roll inflation can be useful for generating proper initial conditions for the subsequent stage of slow-roll inflation in the very early universe. interpretation 0.96 0.42 0.22 hedged possibility 0.87
s4 It may also be responsible for the present stage of accelerated expansion of the universe. interpretation 0.95 0.30 0.12 hedged possibility 1.00
s5 We also make two observations of a more general nature. contribution 0.74 0.83 0.03 unclear 0.63
s6 First of all, the universe after a long stage of inflation (either slow-roll or fast-roll) cannot reach anti-de Sitter regime even if the cosmological constant is negative. result 0.99 0.73 0.13 asserted 1.00
s7 Secondly, the theories with the potentials with a "stable" minimum at V(\phi)<0 in the cosmological background exhibit the same instability as the theories with potentials unbounded from below. result 0.99 0.70 0.13 asserted 1.00
s8 This instability leads to the development of singularity with the properties practically independent of V(\phi). result 0.97 0.73 0.14 asserted 1.00
s9 However, the development of the instability in some cases may be so slow that the theories with the potentials unbounded from below can describe the present stage of cosmic acceleration even if this acceleration occurs due to the fast-roll inflation. result 0.54 0.60 0.13 hedged possibility 1.00

What kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 0.91.

What was examined, and what was not

sources read
  • arXiv listing abstract v2
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Evolution of the scalar field and the scale factor in the model with V (φ) = V0 − m2φ 2 2 . The second model has the standard potential used for the description of spontaneous symmetry breaking, Eq. (2.29), but we added to it a negative cosmological constant Λ < 0 so that V (φ) = λ 4 (φ 2 − v 2 ) 2 + Λ becomes negative in the minimum of V (φ) at φ = v. As we see in
Figure 2. Evolution of the scalar field and the scale factor in the model with V (φ) = λ 4 (φ 2−v 2 ) 2+Λ, with Λ < 0. not depend on the choice of the effective potential, unless the potential is exponentially steep. Indeed, let us assume for a second that in our investigation of the singularity one can neglect V (φ), and then check whether such an approximation is self-consistent. In this approximation the energy density is determined not by V (φ) but by φ˙2/2, and the corresponding equation of state is p = ρ. This leads to the collapse of the universe with a(τ ) = τ 1/3 , where τ is th

Figures reproduced from arXiv:hep-th/0110195 by the authors.

References
How the literature uses it

40 arXiv papers cite this in 58 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 45 sentences without a confident label

Compares with it 3 papers
Cites it as background 7 papers
  • We emphasize here that it is far from trivial to build a model in which the universe is initially expanding, then contracts, and finally expands again, although each step was relatively well studied (see Refs. [ref] for the turnaround to contraction by a negative potential and Refs. [ref] for the bounce with positive curvature) There are many attempts to realize bouncing or cyclic cosmology by violating the NEC [ref] , but we do not consider such a possibility throughout the paper.

    Bouncing Universe from Nothing · Introduction · 0.92

  • See, for example [ref] , for different cases of `fast roll' inflation, depending on which (and how many) of the slow roll parameters in ([ref]) are actually large during the inflationary period.

    On Non-slow Roll Inflationary Regimes · Other non-slow roll regimes · 0.97

  • They encompass fast roll [ref] and constant rate of roll [ref] models.

    A Gravity Dual of Ultra-slow Roll Inflation · Ultra-slow roll inflation · 0.91

Discussion

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