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Title: The NANOGrav 15 yr Data Set: Search for Signals from New Physics
Abstract The 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) shows positive evidence for the presence of a low-frequency gravitational-wave (GW) background. In this paper, we investigate potential cosmological interpretations of this signal, specifically cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls. We find that, with the exception of stable cosmic strings of field theory origin, all these models can reproduce the observed signal. When compared to the standard interpretation in terms of inspiraling supermassive black hole binaries (SMBHBs), many cosmological models seem to provide a better fit resulting in Bayes factors in the range from 10 to 100. However, these results strongly depend on modeling assumptions about the cosmic SMBHB population and, at this stage, should not be regarded as evidence for new physics. Furthermore, we identify excluded parameter regions where the predicted GW signal from cosmological sources significantly exceeds the NANOGrav signal. These parameter constraints are independent of the origin of the NANOGrav signal and illustrate how pulsar timing data provide a new way to constrain the parameter space of these models. Finally, we search for deterministic signals produced by models of ultralight dark matter (ULDM) and dark matter substructures in the Milky Way. We find no evidence for either of these signals and thus report updated constraints on these models. In the case of ULDM, these constraints outperform torsion balance and atomic clock constraints for ULDM coupled to electrons, muons, or gluons.  more » « less
Award ID(s):
2020275 2111738 2207267 1847938 2106552 1909933 2146016 2114721 2020265 2202388
Author(s) / Creator(s):
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Date Published:
Journal Name:
The Astrophysical Journal Letters
Page Range / eLocation ID:
Medium: X
Sponsoring Org:
National Science Foundation
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  1. MCMC chains for the GWB analyses performed in the paper "The NANOGrav 15 yr Data Set: Search for Signals from New Physics". 

    The data is provided in pickle format. Each file contains a NumPy array with the MCMC chain (with burn-in already removed), and a dictionary with the model parameters' names as keys and their priors as values. You can load them as

    with open ('path/to/file.pkl', 'rb') as pick: temp = pickle.load(pick) params = temp[0] chain = temp[1]

    The naming convention for the files is the following:

    • igw: inflationary Gravitational Waves (GWs)
    • sigw: scalar-induced GWs
      • sigw_box: assumes a box-like feature in the primordial power spectrum.
      • sigw_delta: assumes a delta-like feature in the primordial power spectrum.
      • sigw_gauss: assumes a Gaussian peak feature in the primordial power spectrum.
    • pt: cosmological phase transitions
      • pt_bubble: assumes that the dominant contribution to the GW productions comes from bubble collisions.
      • pt_sound: assumes that the dominant contribution to the GW productions comes from sound waves.
    • stable: stable cosmic strings
      • stable-c: stable strings emitting GWs only in the form of GW bursts from cusps on closed loops.
      • stable-k: stable strings emitting GWs only in the form of GW bursts from kinks on closed loops.
      • stable-m: stable strings emitting monochromatic GW at the fundamental frequency.
      • stable-n: stable strings described by numerical simulations including GWs from cusps and kinks.
    • meta: metastable cosmic strings
      • meta-l: metastable strings with GW emission from loops only.
      • meta-ls metastable strings with GW emission from loops and segments.
    • super: cosmic superstrings.
    • dw: domain walls
      • dw-sm: domain walls decaying into Standard Model particles.
      • dw-dr: domain walls decaying into dark radiation.

    For each model, we provide four files. One for the run where the new-physics signal is assumed to be the only GWB source. One for the run where the new-physics signal is superimposed to the signal from Supermassive Black Hole Binaries (SMBHB), for these files "_bhb" will be appended to the model name. Then, for both these scenarios, in the "compare" folder we provide the files for the hypermodel runs that were used to derive the Bayes' factors.

    In addition to chains for the stochastic models, we also provide data for the two deterministic models considered in the paper (ULDM and DM substructures). For the ULDM model, the naming convention of the files is the following (all the ULDM signals are superimposed to the SMBHB signal, see the discussion in the paper for more details)

    • uldm_e: ULDM Earth signal.
    • uldm_p: ULDM pulsar signal
      • uldm_p_cor: correlated limit
      • uldm_p_unc: uncorrelated limit
    • uldm_c: ULDM combined Earth + pulsar signal direct coupling 
      • uldm_c_cor: correlated limit
      • uldm_c_unc: uncorrelated limit
    • uldm_vecB: vector ULDM coupled to the baryon number
      • uldm_vecB_cor: correlated limit
      • uldm_vecB_unc: uncorrelated limit 
    • uldm_vecBL: vector ULDM coupled to B-L
      • uldm_vecBL_cor: correlated limit
      • uldm_vecBL_unc: uncorrelated limit
    • uldm_c_grav: ULDM combined Earth + pulsar signal for gravitational-only coupling
      • uldm_c_grav_cor: correlated limit
        • uldm_c_cor_grav_low: low mass region  
        • uldm_c_cor_grav_mon: monopole region
        • uldm_c_cor_grav_low: high mass region
      • uldm_c_unc: uncorrelated limit
        • uldm_c_unc_grav_low: low mass region  
        • uldm_c_unc_grav_mon: monopole region
        • uldm_c_unc_grav_low: high mass region

    For the substructure (static) model, we provide the chain for the marginalized distribution (as for the ULDM signal, the substructure signal is always superimposed to the SMBHB signal)

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