Stephen R. Taylor

VIPER · Vanderbilt University

Research

Gravitational waves across the nanohertz–millihertz band — pulsar timing arrays, supermassive black-hole binaries, LISA, and the methods that connect them.

I direct the Vanderbilt Initiative for Probes of Extreme Relativity (VIPER), an intensely collaborative research group investigating gravitational waves across the nanohertz–millihertz frequency band. VIPER develops astrostatistics methods, Bayesian parameter estimation tools, AI-driven data analysis pipelines, and large-scale simulations to probe the universe’s most extreme environments: supermassive black holes, relativistic binaries, neutron stars, and early-Universe processes.

Nanohertz gravitational waves

Illustration of a pulsar timing array: Earth at the center of a distorted spacetime grid, with lines to pulsars around it
A pulsar timing array: gravitational waves passing between Earth and the pulsars perturb the arrival times of their pulses.

The main thrust of my research is searching for nanohertz gravitational waves from distant sources using a web of Milky Way pulsars (a Pulsar Timing Array), and unveiling the properties of the supermassive black holes, cosmic strings, or early Universe processes that produce these GWs.

Pulsars are rapidly rotating, highly magnetized neutron stars that emit beams of radiation along their magnetic field axis. Millisecond pulsars (MSPs) spin hundreds of times a second with extraordinarily stable pulse arrival times, making them natural clocks in the sky.

June 29, 2023 — I announced NANOGrav’s evidence for a cosmic background of nanohertz-frequency gravitational waves at NSF Headquarters, alongside Kip Thorne and Jocelyn Bell Burnell.

The Hellings–Downs curveExpected correlation between the timing residuals of two pulsars as a function of their angular separation, for an isotropic gravitational-wave background in general relativity: one half at zero separation, a minimum of about −0.15 near 82 degrees, and one quarter at 180 degrees.0°30°60°90°120°150°180°00.250.5Angular separation between pulsars, ζCorrelation, Γ(ζ)
The Hellings–Downs curve: the correlation an isotropic gravitational-wave background induces between the timing residuals of two pulsars, as a function of their angular separation on the sky. Its shape distinguishes a background from clock errors (a monopole) and solar-system ephemeris errors (a dipole).

I have developed many of the signal models, statistical techniques, and coding infrastructure underlying the global pulsar timing array effort — including Bayesian spectral characterization, open-source analysis pipelines, forward models for population forecasting, and host-galaxy identification frameworks for supermassive black hole binaries.

Selected papers

  1. The NANOGrav 15 yr data set: evidence for a gravitational-wave background

    NANOGrav Collaboration: G. Agazie, A. Anumarlapudi, A. M. Archibald, et al. (incl. S. R. Taylor)

    The Astrophysical Journal Letters 951, L8 2023Co-lead authorDOIarXiv:2306.16213

  2. The dawn of gravitational wave astronomy at light-year wavelengths: insights from pulsar timing arrays

    S. R. Taylor

    Astrophysics and Space Science 370, 124 2025ReviewDOIarXiv:2511.08966

  3. The astrophysics of nanohertz gravitational waves

    S. Burke-Spolaor, S. R. Taylor, M. Charisi, T. Dolch, J. S. Hazboun, et al.

    Astronomy and Astrophysics Review 27, 5 2019Co-lead author · ReviewDOIarXiv:1811.08826

Supermassive black-hole binaries

The gravitational-wave background is expected to be dominated by the combined signal of a cosmic population of supermassive black-hole binaries. My group forecasts when pulsar timing arrays will resolve the first individual binaries, develops targeted searches for candidates identified electromagnetically, and builds frameworks for identifying the host galaxies of binaries found in gravitational waves — connecting gravitational-wave detections to the galaxies and quasars around them.

Selected papers

  1. Identifying the host galaxies of supermassive black hole binaries found by pulsar timing arrays

    P. Petrov, S. R. Taylor, M. Charisi, C.-P. Ma

    The Astrophysical Journal 976, 129 2024arXiv:2406.04409

  2. Efficient large-scale, targeted gravitational-wave probes of supermassive black-hole binaries

    M. Charisi, S. R. Taylor, C. A. Witt, J. Runnoe

    Physical Review Letters 132, 061401 2024DOI

  3. Expectations for the first supermassive black-hole binary resolved by PTAs I: Model efficacy

    L. Schult, P. Petrov, S. R. Taylor, N. Pol, N. Laal, M. Charisi, C.-P. Ma

    Submitted to Physical Review D 2025arXiv:2510.01317

LISA and the millihertz band

As one of six US scientists appointed to the ESA–NASA LISA Science Team, I am developing population-inference methods for LISA sources within the global fit, targeting stellar-mass binaries, massive black-hole mergers, and extreme mass-ratio inspirals across the millihertz band.

LISA — three spacecraft exchanging laser beams across 2.5-million-kilometer arms — was adopted by ESA in 2024 and is planned for launch in the 2030s.

Selected papers

  1. Flexible spectral separation of multiple isotropic and anisotropic stochastic gravitational wave backgrounds in LISA

    A. W. Criswell, S. Banagiri, J. Lawrence, L. Schult, S. Rieck, S. R. Taylor, V. Mandic

    Submitted to The Astrophysical Journal 2025arXiv:2508.20308

  2. Inferring the properties of a population of compact binaries in presence of selection effects

    S. Vitale, D. Gerosa, W. M. Farr, S. R. Taylor

    Handbook of Gravitational Wave Astronomy (Springer) 2021arXiv:2007.05579

Methods and software

The common thread is statistical: turning noisy, heterogeneous data into reliable astrophysical inference. My group’s work includes Bayesian spectral characterization of the gravitational-wave background, Gibbs-sampling schemes for full PTA analyses, generalized optimal statistics for mapping the background across the sky and spectrum, and normalizing flows and neural emulators for fast inference. Our code is open source — see github.com/stevertaylor.

Selected papers

  1. Solving the PTA data analysis problem with a global Gibbs scheme

    N. Laal, S. R. Taylor, R. van Haasteren, W. G. Lamb, X. Siemens

    Physical Review D 111, 063067 2025DOIarXiv:2410.11944

  2. Fast parameter inference on pulsar timing arrays with normalizing flows

    D. Shih, M. Freytsis, S. R. Taylor, J. A. Dror, N. Smyth

    Physical Review Letters 133, 011402 2024DOIarXiv:2310.12209

  3. Deep neural emulation of the supermassive black hole binary population

    N. Laal, S. R. Taylor, L. Z. Kelley, J. Simon, K. Gültekin, et al.

    The Astrophysical Journal 982, 55 2025DOIarXiv:2411.10519

Funding

Total as PI / Co-PI / Institutional PI: $6 million. Selected active grants:

  • NASA LISA Preparatory Science (2026–2028): Population inference within the LISA global fit
  • NSF Windows on the Universe (2023–2026): Mapping host galaxies of low-frequency GW sources
  • NASA FINESST (2023–2026): Optimizing multi-messenger searches for supermassive black-hole binaries
  • NSF CAREER (2021–2026): Unveiling the nanohertz GW discovery landscape

The full list is in the CV.

Collaborations

  • NANOGrav — North American Nanohertz Observatory for Gravitational Waves
  • IPTA — International Pulsar Timing Array
  • LISA — Laser Interferometer Space Antenna
  • LIGO — Laser Interferometer Gravitational-wave Observatory
  • OzGrav — ARC Centre of Excellence for Gravitational Wave Discovery
  • MeerKAT PTA — MeerKAT Pulsar Timing Array
  • GWIC — Gravitational Wave International Committee