The second data release from the European Pulsar Timing Array: IV. Implications for massive black holes, dark matter and the early Universe (2306.16227v2)

Abstract: The European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array (InPTA) collaborations have measured a low-frequency common signal in the combination of their second and first data releases respectively, with the correlation properties of a gravitational wave background (GWB). Such signal may have its origin in a number of physical processes including a cosmic population of inspiralling supermassive black hole binaries (SMBHBs); inflation, phase transitions, cosmic strings and tensor mode generation by non-linear evolution of scalar perturbations in the early Universe; oscillations of the Galactic potential in the presence of ultra-light dark matter (ULDM). At the current stage of emerging evidence, it is impossible to discriminate among the different origins. Therefore, in this paper, we consider each process separately, and investigate the implications of the signal under the hypothesis that it is generated by that specific process. We find that the signal is consistent with a cosmic population of inspiralling SMBHBs, and its relatively high amplitude can be used to place constraints on binary merger timescales and the SMBH-host galaxy scaling relations. If this origin is confirmed, this is the first direct evidence that SMBHBs merge in nature, adding an important observational piece to the puzzle of structure formation and galaxy evolution. As for early Universe processes, the measurement would place tight constraints on the cosmic string tension and on the level of turbulence developed by first-order phase transitions. Other processes would require non-standard scenarios, such as a blue-tilted inflationary spectrum or an excess in the primordial spectrum of scalar perturbations at large wavenumbers. Finally, a ULDM origin of the detected signal is disfavoured, which leads to direct constraints on the abundance of ULDM in our Galaxy.

• The paper reveals that a common low-frequency gravitational wave background likely originates from a widespread population of inspiraling supermassive black hole binaries, constraining merger timescales and galaxy scaling relations.
• The paper demonstrates that integrating EPTA and InPTA data refines galaxy formation models and necessitates revisions in the SMBH-bulge mass relationship in semi-analytic frameworks.
• The paper also examines early Universe theories, such as inflationary models and cosmic strings, while challenging the applicability of ultra-light dark matter scenarios based on pulsar timing observations.

Insights from the EPTA's Second Data Release: Implications on Astrophysical Phenomena and Cosmology

The paper utilizes the European Pulsar Timing Array's (EPTA) second data release, combined with the Indian Pulsar Timing Array (InPTA) data, to investigate a common low-frequency signal that exhibits the characteristics of a gravitational wave background (GWB). This signal, detected across multiple pulsar timing array (PTA) collaborations, could potentially originate from a variety of cosmological and astrophysical sources, each offering transformative insights into the Universe's evolution.

The primary candidate for best explaining the GWB in the nanohertz frequency range is the stochastic background produced by an ensemble of inspiraling supermassive black hole binaries (SMBHBs). The amplitude and spectral characteristics of the detected signal suggest constraints on the merger timescales and scaling relations of host galaxies and their central black holes. The findings indicate that the detected GWB is consistent with a widespread cosmic distribution of SMBHBs, which adds remarkable weight to the hypothesis that such binaries eventually merge.

In terms of theoretical constructs, this detection has implications for galaxy formation models. The constraint on the normalization of the SMBH-bulge mass relation and merger timelines might necessitate revisions to current semi-analytic models (SAMs) like those based on the L-galaxies framework and Barausse's models. The data suggests rapid SMBHB merging post-galactic coalescence, indicating that sub-parsec SMBHBs are prevalent in galaxy centers.

Moreover, alternative scenarios rooted in early Universe physics provide competitive explanations. The detected signal aligns with some parameter spaces of inflationary models where the consistency relation is broken, potentially creating a blue-tilted spectrum. This spectral shape implies the involvement of primordial phenomena like cosmic strings or (M)HD turbulence at the QCD energy scale. Each hypothesis provides a window into the conditions of the early Universe, often demanding non-standard scenarios to match current observations. For instance, cosmic string networks could, under varied theoretical assumptions about intercommutation and loop dynamics, produce a compatible GWB signal.

The search for ultra-light dark matter (ULDM) provides another intriguing layer. Although constraints placed by the absence of signal at the expected local dark matter density challenge an ULDM explanation, this paper emphasizes the versatility of pulsar timing arrays in probing the astrophysical and cosmological candidates.

In summary, the EPTA's second data release provides compelling data that align with several candidate sources for the low-frequency GWB. While a population of SMBHBs remains the stronghold explanation, the exploration of early Universe implications showcases the complex interplay between general relativity principles and cosmological evolution. Future work within this domain will likely focus on improved detection significance and resolving overlap in multiple GWB sources, which is critical for unveiling the nature of the universe's dark components and gravitational interactions at cosmological distances.