Effective Field Theory in Cosmological Inflation

Effective Field Theory (EFT) has emerged as a unifying framework for describing the physics of cosmological inflation by systematically organising possible interactions of the inflaton—the hypothetical scalar field driving the accelerated expansion of the early Universe—according to their relevance at observable energy scales. By treating the broken time‐translation symmetry during inflation as a source of a Goldstone boson, EFT encapsulates both single‐field and multi‐field scenarios within a common language. In this approach, operators are ordered by the number of derivatives or powers of the fluctuation field, with higher‐order terms suppressed by a characteristic cutoff scale. This hierarchy permits robust predictions for correlation functions of primordial density perturbations, including the power spectrum and higher‐order spectra, while remaining agnostic about the ultraviolet completion of the theory. EFT techniques have refined our understanding of non‐Gaussian signatures, revealing how distinct shapes of the bispectrum or trispectrum probe heavy fields, non‐canonical kinetic terms and possible violations of Lorentz or boost invariance. The framework also links inflationary observables to effective sound speeds and higher‐derivative dispersion relations, thereby broadening the scope of phenomenological studies and guiding the interpretation of forthcoming cosmic microwave background and large-scale structure measurements.

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Technical terms

Effective Field Theory: A framework organising interactions by energy scale to describe low‐energy phenomena without specifying high‐energy details.

Inflaton: The scalar degree of freedom whose potential energy drives the rapid expansion of the early Universe.

Non‐Gaussianity: Deviations from a purely Gaussian distribution in the statistics of primordial density fluctuations, characterised by higher‐order correlation functions.

Bispectrum: The three‐point correlation function in momentum space, encoding the leading non‐Gaussian signature of inflation.

Slow‐roll: A regime in which the inflaton field evolves gradually down its potential, ensuring a near‐exponential expansion and small departures from scale invariance.

Goldstone boson: A massless mode arising from the spontaneous breaking of a continuous symmetry, here associated with time‐translation symmetry during inflation.

References

1. Cosmological Flow of Primordial Correlators. Physical Review Letters (2024).
2. Bootstrapping multi-field inflation: non-Gaussianities from light scalars revisited. Journal of Cosmology and Astroparticle Physics (2023).
3. Non-Gaussianity as a particle detector. Journal of High Energy Physics (2016).
4. The effective field theory of multifield inflation. Journal of High Energy Physics (2012).