NANOGrav结果对非标准宇宙学中原初引力波的意义
Implications of the NANOGrav result on primordial gravitational waves in nonstandard cosmologies
摘要：
最近，NANOGrav合作报告了共谱随机过程的证据，这可能被解释为有史以来第一次检测到随机引力波（GW）背景。我们讨论了在非标准宇宙学历史中产生一阶和二阶引力波的可能性。我们表明，NANOGrav观测可以解释的一阶GWs在非标准的热历史与早期的物质占主导地位的时代，而所需的参数空间来解释NANOGrav观测在标准宇宙学或在非标准时代的kination支配被排除的BBN和CMB观测。对于由大的原初标量涨落产生的二阶巨阱，我们研究了标准辐射支配和两种特殊的非标准情形，并给出了几种形式的原初功率谱$P_ζ(k)$，以获得丰富的原初黑洞（PBH）产生.我们发现，NANOGrav观测可以解释与所有这些$P_ζ(k)$的标准辐射支配。此外，尘埃时代导致丰富的PBH形成的较低幅度的$P_ζ(k)$比辐射为主的情况下，并符合NANOGrav观察只有少数的所有$P_ζ(k)$形式在这里考虑，其中的峰值波数大于波数范围探测的NANOGrav。在这个非标准的时代，对于一个宽的功率谱，PBH产生在一个广泛的质量范围内的行星质量制度。如果要满足NANOGrav结果，则非标准的Kination支配时期不能为任何$P_ζ(k)$产生足够的PBH。
英文摘要：
Recently, the NANOGrav collaboration has reported evidence for a common-spectrum stochastic process, which might be interpreted as the first ever detection of stochastic gravitational wave (GW) background. We discuss the possibility of the signal arising from the first and second-order GWs in nonstandard cosmological history. We show that the NANOGrav observation can be explained by the first order GWs in the nonstandard thermal history with an early matter-dominated era, whereas the parameter space required to explain the NANOGrav observation in the standard cosmology or in the nonstandard epoch of kination domination is ruled out by the BBN and CMB observations. For the second-order GWs arising from the large primordial scalar fluctuations, we study the standard radiation domination and two specific nonstandard cases with a few forms of the primordial power spectrum $P_ζ(k)$ to achieve abundant primordial black hole (PBH) production. We find that the NANOGrav observation can be explained with standard radiation domination for all of these $P_ζ(k)$. Furthermore, a dustlike epoch leads to abundant PBH formation for a lower amplitude of $P_ζ(k)$ than the radiation dominated case and complies with the NANOGrav observation only for a few of the all $P_ζ(k)$ forms considered here, where the peak wavenumber is larger than the wavenumber range probed by the NANOGrav. In this nonstandard epoch, for a broad power spectrum, PBHs are produced in a wide mass range in the planetary mass regime. A nonstandard epoch of kination domination cannot produce enough PBH for any of the $P_ζ(k)$ if the NANOGrav result is to be satisfied.
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