Inflation, Quantum Cosmology and the Anthropic Principle

TL;DR

The paper argues that inflationary cosmology naturally generates a multiverse in which many vacua and laws of low-energy physics realize across exponentially large domains. By combining chaotic and eternal inflation with quantum fluctuations, it proposes mechanisms (including baby universes and inter-universal dynamics) that diversify fundamental constants and Λ across regions, providing a framework for weak and strong anthropic principles. It discusses the challenges of calculating probabilities across the multiverse and suggests pragmatic approaches alongside models that can address the cosmological constant problem and the nature of dark energy. The work highlights the conceptual shift from a single, unique universe to a landscape where anthropic selection can explain observed parameters without requiring extreme fine-tuning, while noting unresolved measure and consistency issues. Overall, inflation plus multiverse concepts offer a coherent stance on why our universe has properties compatible with life and how the small observed Λ may arise from selection effects in a broader cosmic ensemble.

Abstract

Anthropic principle can help us to understand many properties of our world. However, for a long time this principle seemed too metaphysical and many scientists were ashamed to use it in their research. I describe here a justification of the weak anthropic principle in the context of inflationary cosmology and suggest a possible way to justify the strong anthropic principle using the concept of the multiverse.

Figures (2)

• Figure 1: Motion of the scalar field in the theory with $V(\phi) = {m^2\over 2} \phi^2$. Several different regimes are possible, depending on the value of the field $\phi$. If the potential energy density of the field is greater than the Planck density $\rho \sim M_p^4 \sim 10^{94}$ g/cm$^3$, quantum fluctuations of space-time are so strong that one cannot describe it in usual terms. Such a state is called space-time foam. At a somewhat smaller energy density (region A: $m M_p^3 < V(\phi) < M_p^4$) quantum fluctuations of space-time are small, but quantum fluctuations of the scalar field $\phi$ may be large. Jumps of the scalar field due to quantum fluctuations lead to a process of eternal self-reproduction of inflationary universe which we are going to discuss later. At even smaller values of $V(\phi)$ (region B: $m^2 M_p^2 < V(\phi) < m M_p^3$ ) fluctuations of the field $\phi$ are small; it slowly moves down as a ball in a viscous liquid. Inflation occurs both in the region A and region B. Finally, near the minimum of $V(\phi)$ (region C) the scalar field rapidly oscillates, creates pairs of elementary particles, and the universe becomes hot.
• Figure 2: A typical distribution of scalar fields $\phi$ and $\chi$ 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 $\chi$ 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.