Measuring the spectrum of primordial gravitational waves with CMB, PTA and Laser Interferometers (2007.04241v2)

Abstract: We investigate the possibility of measuring the primordial gravitational wave (GW) signal across 21 decades in frequencies, using the cosmic microwave background (CMB), pulsar timing arrays (PTA), and laser and atomic interferometers. For the CMB and PTA experiments we consider the LiteBIRD mission and the Square Kilometer Array (SKA), respectively. For the interferometers we consider space mission proposals including the Laser Interferometer Space Antenna (LISA), the Big Bang Observer (BBO), the Deci-hertz Interferometer Gravitational wave Observatory (DECIGO), the $\mu$Ares experiment, the Decihertz Observatory (DO), and the Atomic Experiment for Dark Matter and Gravity Exploration in Space (AEDGE), as well as the ground-based Einstein Telescope (ET) proposal. We implement the mathematics needed to compute sensitivities for both CMB and interferometers, and derive the response functions for the latter from the first principles. We also evaluate the effect of the astrophysical foreground contamination in each experiment. We present binned sensitivity curves and error bars on the energy density parameter, $\Omega_{GW}h^2$, as a function of frequency for two representative classes of models for the stochastic background of primordial GW: the quantum vacuum fluctuation in the metric from single-field slow-roll inflation, and the source-induced tensor perturbation from the spectator axion-SU(2) inflation models. We find excellent prospects for joint measurements of the GW spectrum by CMB and space-borne interferometers mission proposals.

Measuring the Spectrum of Primordial Gravitational Waves with CMB, PTA, and Laser Interferometers

This paper presents a detailed investigation into the potential for measuring the primordial gravitational wave (GW) signal across a vast range of frequencies, making use of data from the cosmic microwave background (CMB), pulsar timing arrays (PTA), and a variety of laser and atomic interferometers. The authors consider several cutting-edge missions and proposals, including but not limited to the LiteBIRD mission for CMB, the Square Kilometer Array (SKA) for PTA, and several interferometer projects like LISA, BBO, DECIGO, and AEDGE.

The paper is anchored in comparing two theoretical models for the stochastic background of primordial GWs: the signal expected from single-field slow-roll inflation and the tensor perturbation from the spectator axion-SU(2) inflation models. These models are useful in understanding the early universe's physics, particularly the inflationary epoch, which could not otherwise be probed with terrestrial particle colliders.

The authors compute the sensitivities of various experiments across 21 decades in frequency using rigorous mathematical frameworks. They derive response functions for interferometers from first principles and examine the impact of astrophysical foreground contamination on these measurements.

Significantly, the paper provides binned sensitivity curves and error bars for the energy density parameter, $\Omega_{GW}h^2$, as a function of frequency. This enables a thorough analysis of the prospects for joint GW spectrum measurements by CMB and proposed space-borne interferometer missions.

Key outcomes include:

• Sensitivity Prospects: The paper highlights excellent future prospects for detecting the GW spectrum across different frequency bands. Notably, projects like LiteBIRD and SKA are poised to effectively constrain the GW spectrum in low-frequency regimes, while DECIGO and BBO are expected to perform similarly across a different part of the frequency spectrum.
• Model Implications: The ability to characterize the entire GW spectrum is critical for distinguishing between standard inflation models and alternative models that predict different spectral shapes, such as axion-SU(2) models. These sourced GWs provide distinct observational signatures, including strong non-Gaussianity and potential chirality, which differ from vacuum fluctuation predictions.
• Foreground Challenges: A substantial portion of the paper is dedicated to assessing the impact of astrophysical foregrounds, which can significantly affect space-based interferometer readings. Strategies for mitigating these effects are discussed, including leveraging external astrophysical data to improve foreground modeling and subtraction.
• Theoretical and Practical Impacts: By covering a wide range of frequencies, the combination of these various instruments could offer unprecedented insights into the inflationary period of the universe. Practically, this could refine our understanding of early-universe physics and potentially distinguish between models predicting different initial conditions or fundamental forces in the early universe.

In conclusion, this paper provides a strong theoretical and computational foundation for future experimental efforts to probe primordial gravitational waves. Such measurements are crucial for advancing our understanding of the universe's infancy and testing the predictive power of inflationary theories. Future developments in AI could further enhance data analysis and interpretation in this field, offering even greater accuracy and insight into the cosmos' earliest moments.