Inflationary Theory versus Ekpyrotic/Cyclic Scenario

TL;DR

The paper critically evaluates inflationary cosmology and its proposed alternatives, arguing that the original ekpyrotic scenario fails to solve the core problems of homogeneity, flatness, and entropy. It then analyzes cyclic variants, showing they effectively rely on inflation and may not constitute true alternatives. The author argues that introducing a post-singularity inflationary stage, or removing problematic negative- potential minima, collapses the cyclic idea into chaotic inflation, suggesting inflation remains the simplest and most predictive framework. Throughout, the work emphasizes the unresolved issues surrounding singularities and perturbations in brane-based models and frames inflation as the most viable path forward for concordance cosmology.

Abstract

I will discuss the development of inflationary theory and its present status, as well as some recent attempts to suggest an alternative to inflation. In particular, I will argue that the ekpyrotic scenario in its original form does not solve any of the major cosmological problems. Meanwhile, the cyclic scenario is not an alternative to inflation but rather a complicated version of inflationary theory. This scenario does not solve the flatness and entropy problems, and it suffers from the singularity problem. We describe many other problems that need to be resolved in order to realize a cyclic regime in this scenario, produce density perturbations of a desirable magnitude, and preserve them after the singularity. We propose several modifications of this scenario and conclude that the best way to improve it is to add a usual stage of inflation after the singularity and use that inflationary stage to generate perturbations in the standard way. This modification significantly simplifies the cyclic scenario, eliminates all of its numerous problems, and makes it equivalent to the usual chaotic inflation scenario.

Figures (8)

• Figure 1: The harmonic oscillator scalar field potential $V(\phi) = V_0 + m^2\phi^2/2$ in a simplest version of chaotic inflation.
• Figure 2: Evolution of the scalar field and the scale factor in the model $V(\phi) = {m^2\over 2} \phi^2 + V_0$ with $V_0 > 0$. In the beginning we have a stage of inflation with the field $\phi$ linearly decreasing at $\phi > 1$. Then the field enters a stage of oscillations with a gradually decreasing amplitude of the field. When the energy of the oscillations becomes smaller than $V_0$, the universe enters a second stage of inflation, which corresponds to the present stage of acceleration of the universe.
• Figure 3: Evolution of scalar fields $\phi$ and $\Phi$ during the process of self-reproduction of the universe. The height of the distribution shows the value of the field $\phi$ which drives inflation. The surface is painted black in those parts of the universe where the scalar field $\Phi$ is in the first minimum of its effective potential, and white where it is in the second minimum. Laws of low-energy physics are different in the regions of different color. The peaks of the "mountains" correspond to places where quantum fluctuations bring the scalar fields back to the Planck density. Each of such places in a certain sense can be considered as a beginning of a new Big Bang.
• Figure 4: Cyclic scenario potential used in Ref. cyclic. The potential at $\phi>1$ approaches a very small constant value $V_0 \sim 10^{-120}$. At $\phi \ll -40$ the potential vanishes.
• Figure 5: Symmetric scalar field potential in the bicycling scenario. At large values of $|\phi|$ one has $V(\phi) \approx V_0 \sim 10^{-120}$ and there is a minimum at $\phi = \phi_0$.