Pulsar timing array (PTA) collaborations in North America, Australia, and Europe, have been exploiting the exquisite timing precision of millisecond pulsars over decades of observations to search for correlated timing deviations induced by gravitational waves (GWs). PTAs are sensitive to the frequency band ranging just below 1 nanohertz to a few tens of microhertz. The discovery space of this band is potentially rich with populations of inspiraling supermassive black hole binaries, decaying cosmic string networks, relic post-inflation GWs, and even non-GW imprints of axionic dark matter. This article aims to provide an understanding of the exciting open science questions in cosmology, galaxy evolution, and fundamental physics that will be addressed by the detection and study of GWs through PTAs. The focus of the article is on providing an understanding of the mechanisms by which PTAs can address specific questions in these fields, and to outline some of the subtleties and difficulties in each case. The material included is weighted most heavily toward the questions which we expect will be answered in the near-term with PTAs; however, we have made efforts to include most currently anticipated applications of nanohertz GWs.

北美、澳大利亚和欧洲的脉冲星计时阵列（PTA）合作，在数十年的观测中一直在利用毫秒脉冲星的精确计时精度来寻找由引力波（GW）引起的相关计时偏差。 PTA 对略低于 1 纳赫兹到几十微赫兹的频段敏感。该带的发现空间可能充满了激励的超大质量黑洞双星、衰变的宇宙弦网络、暴胀后引力波遗迹，甚至轴子暗物质的非引力波印记。本文旨在提供对宇宙学、星系演化和基础物理学中令人兴奋的开放科学问题的理解，这些问题将通过 PTA 检测和研究引力波来解决。本文的重点是提供对 PTA 解决这些领域具体问题的机制的理解，并概述每种情况下的一些微妙之处和困难。所包含的材料主要针对我们预计将在短期内通过 PTA 得到解答的问题；然而，我们已经努力将目前最预期的纳赫兹引力波应用纳入其中。