Answers to frequently asked questions about the pulsar timing array Hellings and Downs curve

Abstract

We answer frequently asked questions (FAQs) about the Hellings and Downs correlation curve -- the "smoking-gun" signature that pulsar timing arrays (PTAs) have detected gravitational waves (GWs). Many of these questions arise from inadvertently applying intuition about the effects of GWs on LIGO-like detectors to the case of pulsar timing, where not all of it applies. This is because Earth-based detectors, like LIGO and Virgo, have arms that are short (km scale) compared to the wavelengths of the GWs that they detect (approx 100-10,000 km). In contrast, PTAs respond to GWs whose wavelengths (tens of light-years) are much shorter than their arms (a typical PTA pulsar is hundreds to thousands of light-years from Earth). To demonstrate this, we calculate the time delay induced by a passing GW along an Earth-pulsar baseline (a "one-arm, one-way" detector) and compare it in the "short-arm" (LIGO-like) and "long-arm" (PTA) limits. This provides qualitative and quantitative answers to many questions about the Hellings and Downs curve. The resulting FAQ sheet should help in understanding the "evidence for GWs" recently announced by several PTA collaborations.

32 pages, 12 figures; revised in places to match published version

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Cited by in corpus (21)

• Dissecting the Stochastic Gravitational Wave Background with Astrometry
• Source anisotropies and pulsar timing arrays
• Beyond the Hellings-Downs curve: Non-Einsteinian gravitational waves in pulsar timing array correlations
• Pulsar Timing Array Harmonic Analysis and Source Angular Correlations
• Charting the Nanohertz Gravitational Wave Sky with Pulsar Timing Arrays
• Correlations for an anisotropic polarized stochastic gravitational wave background in pulsar timing arrays
• Optimal reconstruction of the Hellings and Downs correlation
• Measuring kinematic anisotropies with pulsar timing arrays
• The impact of large-scale galaxy clustering on the variance of the Hellings-Downs correlation: theoretical framework
• Measuring anisotropies in the PTA band with cross-correlations
• Mitigating cosmic variance in the Hellings-Downs curve: a Cosmic Microwave Background analogy
• The impact of large-scale galaxy clustering on the variance of the Hellings-Downs correlation: numerical results
• Unveiling the existence of nontensorial gravitational-wave polarizations from individual supermassive black hole binaries with pulsar timing arrays
• Detecting gravitational waves with light
• Signature of polarized ultralight vector dark matter in pulsar timing arrays
• Probing Picohertz Gravitational Waves with Pulsars
• Identifying the Quadrupolar Nature of Gravitational Wave Background through Space-based Missions
• Observable Gravitational Wave Strain at Second Order
• Spatial Correlation between Pulsars from Interfering Gravitational-Wave Sources in Massive Gravity
• Pulsar timing array analysis in a Legendre polynomial basis
• Can we hear beats with pulsar timing arrays?