快速膨胀宇宙中NANOGrav结果与UV冻结的意义
Implications of NANOGrav results and UV freeze-in in a fast-expanding Universe
AI总结：
本研究提出含$Z_4$对称性的暗区模型，通过BBN前快速膨胀宇宙中的强一阶相变解释NANOGrav低频引力波信号，同时结合暗物质遗迹丰度约束快速膨胀参数需满足$n\lesssim 4$。
摘要：
NANOGrav合作组最新报告的脉冲星计时数据表明，频率$f\sim 10^{-8}\\\\ \rm Hz$处存在随机引力波（GW）背景。我们证明，由标准模型（SM）规范单态费米子$\chi$和单态标量$\phi$构成的暗区（二者均满足$Z_4$对称性），可通过强一阶相变（SFOPT）产生此类低频引力波，这一过程修正了标准宇宙学历史：我们假设大爆炸核合成（BBN）前时期存在由新宇宙学组分$\varphi$驱动的超常规快速膨胀，其能量密度随标度因子的红移关系为$\rho_\varphi\propto a^{-\left(4+n\right)}$。根据快速膨胀参数、再加热温度以及理论有效标度的选择，该模型还可通过冻结机制解释正确的暗物质（DM）遗迹丰度。我们的研究表明，能成功解释NANOGrav结果的一阶相变，结合PLANCK观测到的暗物质丰度，会对快速膨胀参数施加约束，要求$n\lesssim 4$才能同时解释二者。
英文摘要：
Recent pulsar timing data reported by the NANOGrav collaboration indicates the existence of a stochastic gravitational wave (GW) background at a frequency $f\sim 10^{-8}~\rm Hz$. We show that a dark sector consisting of a Standard Model (SM) gauge singlet fermion $χ$ and a singlet scalar $ϕ$, both charged under a $Z_4$ symmetry, is capable of generating such a low frequency GW via strong first order phase transition (SFOPT) through the modification of the standard cosmological history, where we assume faster-than-usual expansion at pre-BBN times driven by a new cosmological species $φ$ whose energy density red-shifts with the scale factor as $ρ_φ\propto a^{-\left(4+n\right)}$. Depending on the choice of the fast expansion parameters, reheat temperature and effective scale of the theory, it is also possible to address correct dark matter (DM) relic abundance via freeze-in. We show that a successful first order phase transition explaining NANOGrav results together with PLANCK observed DM abundance put bound on the fast expansion parameters requiring $n\lesssim 4$ to explain both.
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