Inflation with strongly non-geodesic motion: theoretical motivations and observational imprints

Abstract

A new class of inflationary attractors characterized by a strongly non-geodesic motion has been discovered in the past few years. I describe how they naturally arise in negatively curved field space, allowing to inflate on potentials that are steep in Planck units, albeit without alleviating the ever-present naturalness issue of inflation. In these scenarios, fluctuations often experience a transient tachyonic instability, which can be described by a single-field effective field theory with an imaginary speed of sound. Independently of the precise ultraviolet origin of the latter, this leaves a peculiar imprint in the form of a high-level of primordial non-Gaussianities of flattened type for all higher-order correlation functions. On small scales, a transient phase of strongly nongeodesic motion provides a mechanism to generate primordial black holes and can leave specific signatures in the form of oscillations in the frequency profile of the stochastic gravitational wave background.

Hybrid cosmological attractors

“…Nevertheless, it is important to know that such models do exist, and can be easily constructed in the familiar framework of hybrid inflation. Other mechanisms which may lead to a premature end of inflation were reviewed for example in [40].…”

The inflaton that could: primordial black holes and second order gravitational waves from tachyonic instability induced in Higgs-R2inflation

“…Also, in this work we only considered Gaussian fluctuations in the cosmological perturbations. It is well known that multi-field inflation, especially those incorporating a tachyonic instability may exhibit high levels on non-Gaussianity, which will alter the predictions of SGWB, and PBH abundances [159][160][161][162][163]. We leave the computation of non-Gaussianity and its impact on small scale observables for a future work.…”

Cosmic acceleration and turns in the Swampland

“…( 4. 19) and (5.17), we can get an upper bound for the absolute value of the angular velocity θ(s) ≤ 2k n θ 0 cos θ(s) Φ .…”

“…Strong non-geodesic motion, characterized by rapid turns in field space with Ω ≫ 1 (in Hubble units), can lead to intriguing and rich phenomenology. Examples have been provided in the context of inflation (see, e.g., [7][8][9][10][11][12][13][14][15][16][17][18][19][20]) and also for quintessence models (see, e.g., [21][22][23][24][25][26][27][28]). These can be modifications of the inflationary power spectrum [10,12], production of primordial black holes [29][30][31], possibility to inflate on a steep potential [15,23,25] (namely with large potential gradient) and also enhanced growth of large-scale structure [23].…”