Features of heavy physics in the CMB power spectrum (1010.3693v4)

Abstract: The computation of the primordial power spectrum in multi-field inflation models requires us to correctly account for all relevant interactions between adiabatic and non-adiabatic modes around and after horizon crossing. One specific complication arises from derivative interactions induced by the curvilinear trajectory of the inflaton in a multi-dimensional field space. In this work we compute the power spectrum in general multi-field models and show that certain inflaton trajectories may lead to observationally significant imprints of `heavy' physics in the primordial power spectrum if the inflaton trajectory turns, that is, traverses a bend, sufficiently fast (without interrupting slow roll), even in cases where the normal modes have masses approaching the cutoff of our theory. We emphasise that turning is defined with respect to the geodesics of the sigma model metric, irrespective of whether this is canonical or non-trivial. The imprints generically take the form of damped superimposed oscillations on the power spectrum. In the particular case of two-field models, if one of the fields is sufficiently massive compared to the scale of inflation, we are able to compute an effective low energy theory for the adiabatic mode encapsulating certain relevant operators of the full multi-field dynamics. As expected, a particular characteristic of this effective theory is a modified speed of sound for the adiabatic mode which is a functional of the background inflaton trajectory and the turns traversed during inflation. Hence in addition, we expect non-Gaussian signatures directly related to the features imprinted in the power spectrum.

• The paper demonstrates that derivative interactions from rapid turning trajectories in multi-field inflation generate damped oscillations in the CMB power spectrum.
• The paper reveals that even heavy fields (M² ≫ H²) can influence lighter adiabatic modes, challenging standard decoupling expectations during inflation.
• The paper derives an effective low-energy theory from two-field models, predicting observable non-Gaussian signatures that offer testable insights through CMB experiments.

Overview of "Features of Heavy Physics in the CMB Power Spectrum"

The paper by Acucharo et al. examines the distinct impact of multi-field inflation models on the primordial power spectrum, focusing on the potential imprints of heavy physics phenomena. This research explores how interactions between adiabatic and non-adiabatic modes can leave significant observables in the cosmic microwave background (CMB) power spectrum, particularly when the inflaton field trajectory undergoes a rapid turning in a multi-dimensional field space.

Key Findings

The paper highlights several important results:

1. Derivative Interactions: It analyzes how the curvilinear trajectory of the inflaton in a multi-dimensional space introduces derivative interactions between adiabatic and non-adiabatic modes. This scenario leads to the formation of damped oscillations superimposed on the power spectrum, a signature of heavy physics.
2. Turning Trajectories: A primary focus is the dynamics occurring when the inflaton traverses a curve or "turn" with respect to the geodesics of the sigma model metric. Importantly, these turns can occur even in models where the field's masses approach the cutoff of the theory.
3. Effective Low-Energy Theory: In two-field inflation models, if one field is significantly massive, an effective low-energy theory can be derived that encapsulates relevant operators of the multi-field dynamics. This is characterized by a modified speed of sound for the adiabatic mode depending on the inflaton trajectory and the trajectory's turns.
4. Heavy Field Effects: The paper underscores circumstances where even heavy fields with considerable masses compared to the Hubble scale (mass ratio $M^2 \gg H^2$) can still affect the dynamics of lighter adiabatic modes, challenging the expectation of rapid decoupling during inflation.
5. Non-Gaussian Signatures: Due to these simultaneous interactions and turns, the multi-field models predict non-Gaussian signatures directly related to the features seen in the power spectrum. These effects might provide a testable signature in CMB observations that could corroborate or refute multi-field inflation models.

Implications

The research suggests that careful observations of oscillatory features and non-Gaussianities in the power spectrum can serve as a probe for multi-field inflationary dynamics and the presence of additional heavy fields. This is especially pertinent for experiments such as the Planck satellite, which aim to constrain the shape and nature of the Pprimordial power spectrum with high precision.

Theoretical and Practical Developments: Theoretically, the paper extends effective field theory tools to multi-field inflation, allowing for nuanced predictions of cosmic perturbations due to complex inflaton trajectories. Practically, these insights could inform the interpretation of CMB data, aiding in the search for traces of high-energy physics in the early universe.

Future Directions

The paper identifies several avenues for further research:

• Non-Gaussianity: Developing a more comprehensive understanding of the bispectrum and higher moments linked to non-Gaussianities as predicted by the effective theories needs further exploration.
• Beyond Two-Fields: Expanding the formalism beyond two-field models to include scenarios involving multiple heavy fields, which could have complex coupled dynamics during inflation.
• Observational Signatures: Investigating specific observational predictions that can be extracted from varying inflationary trajectories in upcoming observational datasets.

Overall, Acucharo et al.'s paper reinforces the rich landscape of inflationary trajectories and the potential of the CMB to illuminate the underpinnings of early universe physics, making a compelling case for the relevance of heavy physics in cosmological analyses.