[论文解读] Unraveling the early universe's equation of state and primordial black hole production with PTA, BBN, and CMB observations

QingHua Zhu, Zhichao Zhao+2 more|arXiv (Cornell University)|Jul 25, 2023

Cosmology and Gravitation Theories被引用 12

一句话总结

论文对早期宇宙状态方程W进行贝叶斯推断，利用与PTA数据相关的标量感应引力波，同时结合BBN和CMB约束，发现w=1/3兼容，w=0在>2σ下被排除，f*和A被严格约束。

ABSTRACT

Pulsar timing array (PTA) data releases showed strong evidence for a stochastic gravitational-wave background in the nanohertz band. When the signal is interpreted by a scenario of scalar-induced gravitational waves (SIGWs), we encounter overproduction of primordial black holes (PBHs). We wonder if varying the equation of state (EoS) of the early Universe can resolve this issue and thereby lead to a consistent interpretation of the PTA data. Analyzing a data combination of PTA, big-bang nucleosynthesis, and cosmic microwave background, we find that an epoch with EoS $w\sim\mathcal{O}(10^{-2})$ between the end of inflation and the onset of radiation domination can significantly suppress the production of PBHs, leading to alleviation of the PBH-overproduction issue. With the inferred interval $w=0.44_{-0.40}^{+0.52}$ at 95\% confidence level, our scenario can interpret the PTA data just as well as the conventional scenario of SIGWs produced during the radiation domination.

研究动机与目标

• Investigate the equation of state parameter w of the early universe between inflation and radiation domination using SIGW.
• Constrain the SIGW energy-density spectrum parameters (A, f*, w) with multi-dataset observations.
• Assess how BBN and CMB bounds on relativistic degrees of freedom refine SIGW-based inferences.
• Examine the impact of assuming a monochromatic primordial curvature perturbation spectrum on EoS inferences.

提出的方法

• Model SIGWs produced during an early-time epoch with a variable EoS w in [0,1] transitioning to radiation domination (w=1/3).
• Derive the energy-density fraction spectrum Omega_GW(k, eta) and its present value h^2 Omega_GW,0(k) incorporating the transition at eta_R.
• Use a monochromatic power spectrum P_zeta = A k_* delta(k - k_*) and compute resultant SIGW spectra.
• Perform Bayesian parameter inference on (log10 A, log10(f*/Hz), w) with priors and two dataset combinations (NG15 PTA alone; NG15 PTA plus BBN/CMB bounds).

实验结果

研究问题

• RQ1 What values of the EoS parameter w during the early universe are compatible with SIGW interpretations of PTA data when combined with BBN and CMB constraints?
• RQ2 What ranges of spectral amplitude A and peak frequency f* for a monochromatic curvature perturbation spectrum are allowed by the joint datasets?
• RQ3 How does the transition from an early-time w to w=1/3 affect the SIGW spectrum and its observability in PTA, BBN, and CMB data?
• RQ4 Do BBN and CMB bounds tighten the SIGW-inferred parameter space compared to PTA data alone?

主要发现

• Spectral amplitude A is constrained to roughly 10^-3 to 10^-1 across scenarios.
• Spectral peak frequency f* is constrained to roughly 10^-7 to 10^-6 Hz across scenarios.
• Early matter domination with w=0 is excluded at more than 2σ confidence with the joint data.
• Radiation domination with w=1/3 remains compatible with the joint datasets.
• A kination-dominated early epoch (w=1) is not forbidden by the current data.
• The results depend on the assumed k_* η_R values (10, 100, 1000) and the inclusion of BBN/CMB priors.