John C. Menzies
An interesting paper in Science by Daniele Bertacca and colleagues discusses a new model for the early universe that challenges the traditional inflationary paradigm, which relies on a hypothetical “inflaton” field. Instead, this model proposes that gravitational waves, arising from quantum fluctuations within an expanding universe, are responsible for creating the initial seeds of cosmic structures.
The conventional understanding of inflation involves a period of exceptionally rapid expansion, powered by the inflaton, which also laid down the seeds of the first structures by expanding quantum fluctuations in spacetime. However, the nature of the inflaton, what powered it, and why it turned off are unknown, and there is no conclusive evidence that inflation actually happened. This has led researchers to consider alternative models, particularly those that do not invoke new, mysterious ingredients.
The new model suggests that, in an expanding universe, quantum foam releases gravitational waves, which spread outwards, collide, and amplify themselves. The imprints these amplified gravitational waves make in space are similar across various length scales, which aligns with observations from the cosmic microwave background (CMB). These imprints are the seeds from which stars and galaxies eventually form.
The model further proposes that scalar perturbations, which are density fluctuations that seed structure, arise from the second-order effects of tensor perturbations, which are gravitational waves. These scalar perturbations can become significantly enhanced, allowing them to dominate over the linear tensor modes. The mechanism for this enhancement relates to the instability of de Sitter space. De Sitter space is a hypothetical, maximally symmetric space-time with positive curvature. It’s a solution to Einstein’s equations that describes how spacetime curves in the absence of matter or energy. The inherent instability of de Siter provides a natural means for the end of inflation as it transitions to a radiation-dominated phase over time.
In de Sitter space-time, gravitational waves naturally arise from quantum vacuum oscillations, and scalar fluctuations are generated via second-order tensor effects. Unlike the standard inflationary model, this new scenario does not depend on a specific scalar field or a model-dependent construction. It provides a way to generate nearly scale-invariant scalar adiabatic perturbations in pure de Sitter space without an inflaton, with fluctuations generated outside the horizon from tensor perturbations.
The team also calculated the power spectrum of the scalar fluctuations, finding it to be consistent with near scale-invariance. In this new framework, inflation is driven by de Sitter space where tensor metric fluctuations (i.e., gravitational waves) naturally arise from quantum vacuum oscillations and scalar fluctuations are generated via second order tensor effects. This model also predicts a unique signature in the non-Gaussian features of the large-scale structure, which can be tested observationally.
In summary, while the “Inflation without an Inflaton” model provides a compelling alternative to traditional inflationary models, it is still in its infancy. It would likely however resolve a nagging issue with inflation and that is the inhomogeneous distribution of matter in the universe and may explain how galaxies formed so quickly after the initial appearance of matter. It is also possible that our measure of the age of the universe is flawed.
Reference
Bertacca, D., Jimenez, R., Matarrese, S., & Ricciardone, A. (2024). Inflation without an Inflaton. arXiv. https://doi.org/10.48550/arxiv.2412.14265