Primordial gravitational waves: Model effects on time evolution and spectrum (2502.16643v1)

Abstract: With the detection of gravitational waves (GW) in recent years, many papers that use simulated GW datasets to constrain dark energy models have been published. However, these works generally consider GW generated by massive astrophysical objects, such as mergers of black holes or neutron stars. In this paper, we analyse the evolution and spectrum of GW generated in the inflationary epoch, assuming a standard slow-roll single-field inflationary scenario. Three models for the background are considered: a field theory model of interacting dark energy - the Interacting Holographic Tachyonic Model, the Holographic Dark Energy Model, and the {\Lambda}CDM. The results show significant dependence on the cosmological model, especially for the spectrum, and in particular show that an interaction between dark energy and dark matter can leave a significant imprint. Therefore, future primordial gravitational waves (PGW) datasets could be very useful for constraining dark energy models, including to probe an interaction in the dark sector of the universe.

Analysis of Primordial Gravitational Waves and Their Dependence on Cosmological Models

The paper titled "Primordial Gravitational Waves: Model Effects on Time Evolution and Spectrum" by Sandro M. R. Micheletti presents an in-depth examination of the time evolution and spectral characteristics of primordial gravitational waves (PGWs) within the framework of various cosmological models. This research focuses particularly on the implications of different dark energy models during the inflationary epoch and highlights the potential of PGWs to serve as probes of high-energy physics and the universe's expansion history.

Key Insights and Methodology

Micheletti's work extends the broader discourse on gravitational waves beyond the conventional focus on those emitted by massive astrophysical entities like black holes and neutron stars. Instead, this research investigates PGWs posited to have originated from quantum fluctuations during the inflationary era. This presents a distinctive opportunity to examine quantum aspects of gravitation as well as the interactions within the dark sector of the universe.

The paper explores three background cosmological scenarios:

1. Interacting Holographic Tachyonic Model (IHTM): This model combines the dynamics of a tachyonic field with dark matter interactions, parameterized with a coupling constant indicative of dark energy to dark matter decay processes.
2. Holographic Dark Energy Model (HDE): A model that operates under the constraint of holographic principles dictating the energy density of the universe and involves interactions without the tachyonic field's specifics.
3. ΛCDM Model: Serving as a baseline, this model represents a standard cosmological model with a cosmological constant and cold dark matter but no interactions.

The PGWs are examined under these models using a slow-roll single-field inflationary hypothesis. The gravitational wave spectrum hinges significantly on the selected cosmological model, primarily affecting the energy density spectrum in the frequency range of $10^{-19}\, \text{Hz} < f < 10^{-17}\, \text{Hz}$, where dark energy interactions may make substantial differences.

Results and Implications

One of the critical findings accentuated by this paper is the pronounced sensitivity of the PGW spectrum to the underlying dark energy model and its parameters. The research delineates how variations in the coupling constant within the IHTM and the parameters of the HDE affect the amplitude and phase of the PGW spectrum. Differences in spectra can reach variance in amplitude up to 50%, and the relative spectral energy densities ($\Omega_g$) can diverge by an order of magnitude, contingent on specific frequency values.

Furthermore, Micheletti highlights that even relatively early PGW modes, entering the horizon during or before the matter era, exhibit alterations in their spectra contingent on the dark energy model due to impacts on the comoving horizon's evolution. These findings underscore that PGWs can effectively constrain different dark energy models, potentially revealing interactions in the universe's dark sector.

Future Directions

With the development of next-generation experiments like SKA, LISA, and BBO, which promise improved sensitivity to a wide range of PGW frequencies, there is potential to further exploit PGWs as tools to explore the dark energy effects predicted by this paper. The intersection of theoretical expectations with experimental sensitivity forecasts the possibility of refining cosmological models, thus enhancing our understanding of the inflationary paradigm and providing new insights into quantum gravity. Future data from these technological advancements may notably contribute to disentangling the dense parameters of cosmological models and addressing any degeneracies with other astrophysical datasets.

Conclusion

Micheletti's research exemplifies an essential progression in cosmology, emphasizing the role of PGWs in evaluating dark energy models within an inflationary framework. The paper offers substantial evidence that PGWs possess untapped potential as indicators of both cosmological interactions and high-energy universe dynamics, serving as pivotal tools for theoretical verification and providing a robust framework for interpreting results from future observational data.