An exact model for enhancing/suppressing primordial fluctuations

Domènech, Guillem; Vargas, Gerson; Vargas, Teófilo

doi:10.1088/1475-7516/2024/03/002

“…While this work was in preparation ref. [96] appeared, where it was shown that the usual δN formalism could only be applied sufficiently long after a sudden transition, such that δϕ h is dominated by the growing mode, which is consistent with our results.…”

Section: Jcap05(2024)053supporting

confidence: 92%

The separate-universe approach and sudden transitions during inflation

Jackson,

Assadullahi,

Gow

et al. 2024

J. Cosmol. Astropart. Phys.

12

0

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The separate-universe approach gives an intuitive way to understand the evolution of cosmological perturbations in the long-wavelength limit. It uses solutions of the spatially-homogeneous equations of motion to model the evolution of the inhomogeneous universe on large scales. We show that the separate-universe approach fails on a finite range of super-Hubble scales at a sudden transition from slow roll to ultra-slow roll during inflation in the very early universe. Such transitions are a feature of inflation models giving a large enhancement in the primordial power spectrum on small scales, necessary to produce primordial black holes after inflation. We show that the separate-universe approach still works in a piece-wise fashion, before and after the transition, but spatial gradients on finite scales require a discontinuity in the homogeneous solution at the transition. We discuss the implications for the δN formalism and stochastic inflation, which employ the separate-universe approximation.

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“…The detection of gravitational waves (GWs) from a binary black hole merger by the LIGO-Virgo collaboration has brought attention to the PBHs (Abbott et al 2016a(Abbott et al , 2016b(Abbott et al , 2016c(Abbott et al , 2017a(Abbott et al , 2017b(Abbott et al , 2017c(Abbott et al , 2017d. PBHs could be considered as a suitable candidate for explaining all or a portion of the dark matter in the Universe (Hawking 1971;Chapline 1975;Polnarev & Khlopov 1985;Ivanov et al 1994;Capozziello & Faraoni 2010;Capozziello & De Laurentis 2011;Nojiri & Odintsov 2011;Olmo 2011; De la Cruz-Dombriz & Sáez-Gómez 2012; Choudhury & Mazumdar 2014;Nojiri et al 2017;Hajkarim & Schaffner-Bielich 2020;Solbi & Karami 2021a;Teimoori et al 2021b;Solbi & Karami 2021b;Cai & Piao 2021;Chakraborty et al 2022;Papanikolaou et al 2022;Rezazadeh et al 2022;Papanikolaou et al 2023b;Choudhury et al 2023b;Papanikolaou et al 2023a;Cai et al 2023;Domènech et al 2024). Since PBHs originate from the gravitational collapse of primordial density perturbations, they are not subject to the Chandrasekhar mass limitation.…”

Section: Introductionmentioning

confidence: 99%

Primordial Black Holes in Scalar Field Inflation Coupled to the Gauss–Bonnet Term with Fractional Power-law Potentials

Ashrafzadeh,

Karami

2024

ApJ

6

0

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In this study, we investigate the formation of primordial black holes (PBHs) in a scalar field inflationary model coupled to the Gauss–Bonnet term with fractional power-law potentials. The coupling function enhances the curvature perturbations, then results in the generation of PBHs and detectable secondary gravitational waves (GWs). We identify three separate sets of parameters for the potential functions of the form ϕ 1/3, ϕ 2/5, and ϕ 2/3. By adjusting the model parameters, we decelerate the inflaton during the ultra-slow-roll phase and enhance curvature perturbations. Our calculations predict the formation of PBHs with masses of O ( 10 ) M ⊙ , which are compatible with LIGO-Virgo observational data. Additionally, we find PBHs with masses around O ( 10 − 6 ) M ⊙ and O ( 10 − 5 ) M ⊙ , which can explain ultra-short-timescale microlensing events in OGLE data. Furthermore, our proposed mechanism could lead to the formation of PBHs in mass scales around O ( 10 − 14 ) M ⊙ and O ( 10 − 13 ) M ⊙ , contributing to approximately 99% of the dark matter in the Universe. We also study the production of secondary GWs in our model. In all cases of the model, the density parameter of secondary GWs Ω GW 0 exhibits peaks that intersect the sensitivity curves of GW detectors, providing a means to verify our findings using data of these detectors. Our numerical results demonstrate a power-law behavior for the spectra of Ω GW 0 with respect to frequency, given by Ω GW 0 ( f ) ∼ ( f / f c ) n . Additionally, in the infrared regime where f ≪ f c , the power index takes a log-dependent form, specifically n = 3 − 2 / ln ( f c / f ) .

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“…Alternative mechanisms within the framework of single-field inflation have also been proposed to alleviate problematic fine-tuning [41][42][43][44][45][46][47][48][49][50][51][52][53][54]. Mechanisms involving steplike features in potentials have been studied [41,[43][44][45].…”

Section: Introductionmentioning

confidence: 99%

Mechanisms for Producing Primordial Black Holes from Inflationary Models beyond Fine-Tuning

Stamou

2024

Universe

2

0

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In this study, we present an analysis of the fine-tuning required in various inflationary models in order to explain the production of Primordial Black Holes (PBHs). We specifically examine the degree of fine-tuning necessary in two prominent single-field inflationary models: those with an inflection point and those with step-like features in the potential. Our findings indicate that models with step-like features generally require less fine-tuning compared to those with an inflection point, making them more viable for consistent PBH production. An interesting outcome of these models is that, in addition to improved fine-tuning, they may also predict low-frequency signals that can be detected by pulsar timing array (PTA) collaborations. Additionally, we extend our analysis to multifield inflationary models to assess whether the integration of additional fields can further alleviate the fine-tuning demands. The study also explores the role of a spectator field and its impact on the fine-tuning process. Our results indicate that although mechanisms involving a spectator field can circumvent the issue of fine-tuning parameters for PBH production, both multifield models and models with step-like features present promising alternatives. While fine-tuning involves multiple considerations, our primary objective is to evaluate various inflationary models to identify the one that most naturally explains the formation of PBHs. Hence, this study introduces a novel approach by categorizing existing PBH mechanisms, paving the way for subsequent research to prioritize models that minimize the need for extensive fine-tuning.

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An exact model for enhancing/suppressing primordial fluctuations

Cited by 14 publications

References 171 publications

The separate-universe approach and sudden transitions during inflation

The separate-universe approach and sudden transitions during inflation

Primordial Black Holes in Scalar Field Inflation Coupled to the Gauss–Bonnet Term with Fractional Power-law Potentials

Mechanisms for Producing Primordial Black Holes from Inflationary Models beyond Fine-Tuning

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