Science with the space-based interferometer LISA. IV: Probing inflation with gravitational waves (1610.06481v2)

Abstract: We investigate the potential for the LISA space-based interferometer to detect the stochastic gravitational wave background produced from different mechanisms during inflation. Focusing on well-motivated scenarios, we study the resulting contributions from particle production during inflation, inflationary spectator fields with varying speed of sound, effective field theories of inflation with specific patterns of symmetry breaking and models leading to the formation of primordial black holes. The projected sensitivities of LISA are used in a model-independent way for various detector designs and configurations. We demonstrate that LISA is able to probe these well-motivated inflationary scenarios beyond the irreducible vacuum tensor modes expected from any inflationary background.

• The paper demonstrates that LISA can probe inflation by detecting a stochastic gravitational wave background, particularly from particle production and non-Gaussian signals.
• It compares four distinct inflationary scenarios, including spectator fields and broken-symmetry EFT models, which predict blue-tilted tensor spectra observable by LISA.
• The study also links primordial black hole formation during inflation with measurable gravitational waves, offering insights into dark matter and early-universe physics.

Probing Inflation with the LISA Space-Based Interferometer

The paper addresses the potential of the LISA (Laser Interferometer Space Antenna) space-based interferometer in detecting the stochastic gravitational wave background produced by various mechanisms during the inflationary period of the universe. Specifically, the paper examines several well-founded inflationary scenarios which can generate gravitational wave (GW) spectra with sufficiently high amplitudes and non-standard spectra that LISA might detect.

The authors contribute significantly by considering four distinct scenarios of gravitational wave production during inflation:

1. Particle Production During Inflation: In this scenario, the inflaton is coupled to gauge fields, which leads to particle production and subsequent gravitational wave generation. The research highlights that these waves are highly non-Gaussian and exhibit parity violation. They propose that LISA could potentially observe these gravitational waves if the parameter $\xi$, determining the coupling strength, takes values that lead to significantly strong particle production without conflicting with constraints from larger and smaller scales.
2. Spectator Fields During Inflation: This scenario considers additional scalar fields, termed spectator fields, which can source both scalar and tensor perturbations with a speed of sound parameter $c_s$. A varying $c_s$ results in a blue-tilted tensor spectrum, which increases the possibility for LISA detection. The authors note that while current CMB measurements constrain these fields, LISA could provide complementary limits, especially in the event of additional non-Gaussianities being discovered in the CMB.
3. Effective Field Theories with Broken Symmetries: These theories propose different symmetry breaking during inflation, potentially giving mass to the inflaton and influencing the tensor modes. As a consequence, a blue tilt in the tensor spectrum might occur, where gravitational waves become more detectable at shorter scales, as LISA provides. The authors advocate for the nuances of these EFT models generating signatures distinct from scalar perturbations, offering new avenues for testing inflationary cosmology.
4. Primordial Black Hole Formation: Certain inflationary models lead to large peaks in the matter power spectrum, which can later collapse into primordial black holes (PBHs). These PBHs can merge, and the stochastic GW background resulting from these mergers could be significant for LISA. This scenario is particularly compelling as PBHs could account for dark matter while providing an observable GW signal at high redshifts.

The authors conduct their investigation with an emphasis on understanding the model-independent constraints and combine them with existing observations from other GW detectors, like LIGO. They provide a detailed exploration of the LISA detector configurations, assessing their sensitivity to a host of inflation-related gravitational wave signals. Moreover, they anticipate LISA's potential to probe inflationary mechanics, opening up a wholly new regime of cosmological GW astronomy.

Importantly, this paper acknowledges that these findings could enable discriminating between different sources of GWs. Indeed, the ability to detect a stochastic GW background could inform the inflationary history of our universe, potentially discriminating between inflationary scenarios that otherwise produce similar predictions in observables like the CMB.

Overall, this comprehensive paper by the collaboration forms an essential stepping stone in utilizing next-generation GW observatories like LISA to conquer the challenges of inflation and to explore the early universe with unprecedented depth. Future work might focus on refining the theoretical predictions of GW signatures and calibrating the parameter spaces of various inflationary models to explore the full potential of LISA in uncovering the universe's primal phases.