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1. Estimating the impact of foregrounds on the future detection of Rayleigh scattering

   https://doi.org/10.1088/1475-7516/2022/09/048  

   Zhu, Yijie; Beringue, Benjamin; Choi, Steve K.; Battaglia, Nicholas; Meerburg, P. Daniel; Meyers, Joel (September 2022, Journal of Cosmology and Astroparticle Physics)

   Abstract Rayleigh scattering of the cosmic microwave background (CMB) by neutral hydrogen shortly after recombination leaves frequency-dependent imprints on intensity and polarization fluctuations. High signal-to-noise observations of CMB Rayleigh scattering would provide additional insight into the physics of recombination, including greater constraining power for parameters like the primordial helium fraction, the light relic density, and the sum of neutrino masses. However, such a measurement of CMB Rayleigh scattering is challenging due to the presence of astrophysical foregrounds, which are more intense at the high frequencies, where the effects of Rayleigh scattering are most prominent. Here we forecast the detectability of CMB Rayleigh scattering including foreground removal using blind internal linear combination methods for a set of near-future surveys. We show that atmospheric effects for ground-based observatories and astrophysical foregrounds pose a significant hindrance to detecting CMB Rayleigh scattering with experiments planned for this decade, though a high-significance measurement should be possible with a future CMB satellite. 

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2. Unraveling cosmological anisotropies within stochastic gravitational wave backgrounds

   https://doi.org/10.1088/1475-7516/2023/10/064  

   Cui, Yanou; Kumar, Soubhik; Sundrum, Raman; Tsai, Yuhsin (October 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract Identifying the anisotropies in a cosmologically sourced stochastic gravitational wave background (SGWB) would be of significance in shedding light on the nature of primordial inhomogeneities.For example, if SGWB carries isocurvature fluctuations, it would provide evidence for a multi-field inflationary origin of these inhomogeneities.However, this is challenging in practice due to finite detector sensitivity and also the presence of the astrophysical foregrounds that can compete with the cosmological signal.In this work, we explore the prospects for measuring cosmological SGWB anisotropies in the presence of an astrophysical counterpart and detector noise.To illustrate the main idea, we perform a Fisher analysis using a well-motivated cosmological SGWB template corresponding to a first order phase transition,and an astrophysical SGWB template corresponding to extra-galactic binary mergers, and compute the uncertainty with which various parameters characterizing the isotropic and anisotropic components can be extracted.We also discuss some subtleties and caveats involving shot noise in the astrophysical foreground.Overall, we show that upcoming experiments, e.g., LISA, Taiji, Einstein Telescope, Cosmic Explorer, and BBO, can all be effective in discovering plausible anisotropic cosmological SGWBs. 

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3. Disentangling Multiple Stochastic Gravitational Wave Background Sources in PTA Data Sets

   https://doi.org/10.3847/1538-4357/ac86cc  

   Kaiser, Andrew R.; Pol, Nihan S.; McLaughlin, Maura A.; Chen, Siyuan; Hazboun, Jeffrey S.; Kelley, Luke Zoltan; Simon, Joseph; Taylor, Stephen R.; Vigeland, Sarah J.; Witt, Caitlin A. (October 2022, The Astrophysical Journal)

   Abstract With strong evidence of a common-spectrum stochastic process in the most recent data sets from the NANOGrav Collaboration, the European Pulsar Timing Array (PTA), Parkes PTA, and the International PTA, it is crucial to assess the effects of the several astrophysical and cosmological sources that could contribute to the stochastic gravitational wave background (GWB). Using the same data set creation and injection techniques as in Pol et al., we assess the separability of multiple GWBs by creating single and multiple GWB source data sets. We search for these injected sources using Bayesian PTA analysis techniques to assess recovery and separability of multiple astrophysical and cosmological backgrounds. For a GWB due to supermassive black hole binaries and an underlying weaker background due to primordial gravitational waves with a GW energy-density ratio of Ω PGW /Ω SMBHB = 0.5, the Bayes’ factor for a second process exceeds unity at 17 yr, and increases with additional data. At 20 yr of data, we are able to constrain the spectral index and amplitude of the weaker GWB at this density ratio to a fractional uncertainty of 64% and 110%, respectively, using current PTA methods and techniques. Using these methods and findings, we outline a basic protocol to search for multiple backgrounds in future PTA data sets. 

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4. Uncertainty and Bias of Cosmology and Astrophysical Population Model from Statistical Dark Sirens

   https://doi.org/10.3847/1538-4357/ac9da0  

   Yu, Hang; Seymour, Brian; Wang, Yijun; Chen, Yanbei (December 2022, The Astrophysical Journal)

   Abstract Gravitational-wave (GW) radiation from a coalescing compact binary is a standard siren, as the luminosity distance of each event can be directly measured from the amplitude of the signal. One possibility to constrain cosmology using the GW siren is to perform statistical inference on a population of binary black hole (BBH) events. In essence, this statistical method can be viewed as follows. We can modify the shape of the distribution of observed BBH events by changing the cosmological parameters until it eventually matches the distribution constructed from an astrophysical population model, thereby allowing us to determine the cosmological parameters. In this work, we derive the Cramér–Rao bound for both cosmological parameters and those governing the astrophysical population model from this statistical dark siren method by examining the Fisher information contained in the event distribution. Our study provides analytical insights and enables fast yet accurate estimations of the statistical accuracy of dark siren cosmology. Furthermore, we consider the bias in cosmology due to unmodeled substructures in the merger rate and mass distribution. We find that a 1% deviation in the astrophysical model can lead to a more than 1% error in the Hubble constant. This could limit the accuracy of dark siren cosmology when there are more than 10⁴BBH events detected. 

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5. Methods for estimating neutron star parameters using multiple mechanisms for gravitational wave emission associated with pulsar glitches

   https://doi.org/10.1093/mnras/staf2176  

   Ball, Matthew; Frey, Raymond (December 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Several mechanisms for gravitational wave (GW) emission are believed to be associated with pulsar glitches. This emission may be split between long-duration continuous waves and short-duration bursts. In the Advanced LIGO era, searches for GWs associated with pulsar glitches have only considered continuous wave emission. The increasing sensitivity of the detectors and the prospects for future detectors suggest that astrophysically motivated analyses involving multiple mechanisms may be possible. Here, we present a framework for combining two simple models for GW emission – long-duration continuous waves and short-duration bursts – to derive more constraining astrophysical implications than a single model would allow. The best limits arise from using models that predict a specific amount of GW emission; however, there are relatively few models that make such predictions. We apply these methods to the 2016 December Vela pulsar glitch and make predictions for how well future observing runs and detectors would improve results. As part of this analysis, we performed a targeted search for GW bursts associated with this glitch and find no signal. 

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