A bstract It is well-known that first-order phase transitions in the early universe can be a powerful source of observable stochastic gravitational wave backgrounds. Any such gravitational wave background must exhibit large-scale anisotropies at least as large as those seen in the CMB 10 − 5 , providing a valuable new window onto the (inflationary) origins of primordial fluctuations. While significantly larger fractional anisotropies are possible (for example, in multi-field inflation) and would be easier to interpret, it has been argued that these can only be consistent with CMB bounds if the gravitational wave signal is correspondingly smaller. In this paper, we show that this argument, which relies on assuming radiation dominance of the very early universe, can be evaded if there is an era of early matter dominance of a certain robust type. This allows large gravitational wave anisotropies to be consistent with observable signals at proposed future gravitational wave detectors. Constraints from the CMB on large scales, as well as primordial black hole and mini-cluster formation on small scales, and secondary scalar-induced gravitational waves are all taken into account.
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Disentangling the primordial nature of stochastic gravitational wave backgrounds with CMB spectral distortions
The recent detection of a stochastic gravitational wave background (SGWB) at nanohertz frequencies by pulsar timing arrays (PTAs) has sparked a flurry of interest. Beyond the standard interpretation that the progenitor is a network of supermassive black hole binaries, many exotic models have also been proposed, some of which can potentially offer a better fit to the data. We explore how the various connections between gravitational waves (GWs) and cosmic microwave background (CMB) spectral distortions (SDs) can be leveraged to help determine whether an SGWB was generated primordially or astrophysically. To this end, we present updated k-space window functions that can be used for distortion parameter estimation on enhancements to the primordial scalar power spectrum. These same enhancements can also source GWs directly at second order in perturbation theory, so-called scalar-induced GWs (SIGWs), and indirectly through the formation of primordial black holes (PBHs). We perform a mapping of scalar power spectrum constraints into limits on the GW parameter space of SIGWs for δ-function features. We highlight that broader features in the scalar spectrum can explain the PTA results while simultaneously producing an SD within reach of future experiments. We additionally update PBH constraints from μ- and y-type SDs. Refined treatments of the distortion window functions widen existing SD constraints, and we find that a future CMB spectrometer could play a pivotal role in unravelling the origin of GWs imprinted at or below CMB anisotropy scales.
more » « less- NSF-PAR ID:
- 10487228
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 528
- Issue:
- 1
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 883-897
- Size(s):
- ["p. 883-897"]
- Sponsoring Org:
- National Science Foundation
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Abstract CMB-S4—the next-generation ground-based cosmic microwave background (CMB) experiment—is set to significantly advance the sensitivity of CMB measurements and enhance our understanding of the origin and evolution of the universe. Among the science cases pursued with CMB-S4, the quest for detecting primordial gravitational waves is a central driver of the experimental design. This work details the development of a forecasting framework that includes a power-spectrum-based semianalytic projection tool, targeted explicitly toward optimizing constraints on the tensor-to-scalar ratio, r , in the presence of Galactic foregrounds and gravitational lensing of the CMB. This framework is unique in its direct use of information from the achieved performance of current Stage 2–3 CMB experiments to robustly forecast the science reach of upcoming CMB-polarization endeavors. The methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments, given a desired scientific goal. To form a closed-loop process, we couple this semianalytic tool with map-based validation studies, which allow for the injection of additional complexity and verification of our forecasts with several independent analysis methods. We document multiple rounds of forecasts for CMB-S4 using this process and the resulting establishment of the current reference design of the primordial gravitational-wave component of the Stage-4 experiment, optimized to achieve our science goals of detecting primordial gravitational waves for r > 0.003 at greater than 5 σ , or in the absence of a detection, of reaching an upper limit of r < 0.001 at 95% CL.more » « less
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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
- uldm_c_grav_cor: correlated limit
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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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
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