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arXiv 2407.09405astro-ph.COgr-qchep-th
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宇宙学引力波背景的暴胀初始条件

Inflationary Initial Conditions for the Cosmological Gravitational Wave Background

Lorenzo Valbusa Dall'Armi, Alina Mierna, Sabino Matarrese, Angelo Ricciardone

备注：
Accepted for publication on JCAP. arXiv admin note: text overlap with arXiv:2307.11043
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AI总结：

本研究针对宇宙学引力波背景初始条件计算难题，提出通过微扰引力场能量-动量张量的自洽方法，发现单场暴胀等场景下存在大的非绝热初始条件，可提升角功率谱与对早期退耦粒子的探测敏感度。

摘要：

宇宙学起源随机引力波背景（CGWB）各向异性的初始条件在很大程度上取决于引力波的产生机制。由于CGWB预计是非热的，其初始条件的计算可能比宇宙微波背景（CMB）更具挑战性——CMB在早期宇宙中与其他粒子的相互作用使其具有黑体谱。本文中，我们证明暴胀期间度规量子涨落产生的宇宙学背景的初始条件偏离绝热性。这些原初引力波实际上由两个独立自由度（引力子的两个偏振态）的量子涨落产生。此外，CGWB在爱因斯坦方程中的作用可忽略不计，因为其能量密度相对于普通物质处于次主导地位。因此，计算暴胀引力子初始条件的唯一可行方法是对由度规小尺度张量涨落定义的引力场能量-动量张量进行微扰。这一全新且自洽的方法表明，即便在单场暴胀期间，也存在大的非绝热初始条件。与标准绝热情形相比，这一贡献提升了CGWB的总角功率谱，同时也提高了各向异性对早期宇宙中相对论性退耦粒子存在的敏感度。本研究还证明，我们的发现具有相当的普适性，既适用于单场暴胀，也适用于其他由度规量子涨落产生CGWB的情形，例如curvaton（曲率子）模型。

英文摘要：

The initial conditions on the anisotropies of the stochastic gravitational-wave background of cosmological origin (CGWB) largely depend on the mechanism that generates the gravitational waves. Since the CGWB is expected to be non-thermal, the computation of the initial conditions could be more challenging w.r.t. the Cosmic Microwave Background (CMB), whose interactions with other particles in the early Universe lead to a blackbody spectrum. In this paper, we show that the initial conditions for the cosmological background generated by quantum fluctuations of the metric during inflation deviate from adiabaticity. These primordial gravitational waves are indeed generated by quantum fluctuations of two independent degrees of freedom (the two polarization states of the gravitons). Furthermore, the CGWB plays a negligible role in the Einstein's equations, because its energy density is subdominant w.r.t. ordinary matter. Therefore, the only possible way to compute the initial conditions for inflationary gravitons is to perturb the energy-momentum tensor of the gravitational field defined in terms of the small-scale tensor perturbation of the metric. This new and self-consistent approach shows that a large, non-adiabatic initial condition is present even during the single-field inflation. Such a contribution enhances the total angular power spectrum of the CGWB compared to the standard adiabatic case, increasing also the sensitivity of the anisotropies to the presence of relativistic and decoupled particles in the early Universe. In this work we have also proved that our findings are quite general and apply to both single-field inflation and other scenarios in which the CGWB is generated by the quantum fluctuations of the metric, like the curvaton.

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