The Cold Big-Bang Cosmology as a Counter-example to Several Anthropic Arguments

TL;DR

The paper challenges the efficacy of anthropic explanations in cosmology by constructing a Cold Big-Bang (CBB) framework in which the standard FRW parameters $R,\eta_\gamma,\eta_L,\eta_{\rm dm},\Lambda$, and $Q$ can differ by orders of magnitude from their hot big-bang values while still permitting sun-like stars and planetary systems. It shows that through adjustments to initial conditions, nucleogenesis, and structure formation, life-supporting environments arise across wide ranges of these parameters, including $\eta_\gamma$ of order unity and various metallicities. The analysis demonstrates that entropy-per-baryon, amplitude of fluctuations $Q$, baryon-to-dark-matter ratio $\eta_{\rm dm}$, cosmological constant $\Lambda$, and curvature scale $R$ can each be varied without qualitatively destroying the prospects for observers, and that multiple local maxima in the observer density $\xi$ can occur across a broad parameter space. Consequently, anthropic arguments lose robustness when multiple cosmological parameters are allowed to vary, unless one imposes a strongly constrained prior $P$, and the paper advocates using CBB-type models to test and refine such priors. In short, the work shows that anthropic reasoning alone cannot uniquely constrain the values of several fundamental cosmological parameters if diverse sub-universes with life are possible.

Abstract

A general Friedmann big-bang cosmology can be specified by fixing a half-dozen cosmological parameters such as the photon-to-baryon ratio Eta, the cosmological constant Lambda, the curvature scale R, and the amplitude Q of (assumed scale-invariant) primordial density fluctuations. There is currently no established theory as to why these parameters take the particular values we deduce from observations. This has led to proposed `anthropic' explanations for the observed value of each parameter, as the only value capable of generating a universe that can host intelligent life. In this paper, I explicitly show that the requirement that the universe generates sun-like stars with planets does not fix these parameters, by developing a class of cosmologies (based on the classical `cold big-bang' model) in which some or all of the cosmological parameters differ by orders of magnitude from the values they assume in the standard hot big-bang cosmology, without precluding in any obvious way the existence of intelligent life. I also give a careful discussion of the structure and context of anthropic arguments in cosmology, and point out some implications of the cold big-bang model's existence for anthropic arguments concerning specific parameters.

Figures (2)

• Figure 1: Early evolution of a sample FRW cosmology with (initial) $\eta_\gamma=1.0$, $\eta_L=2.5$ and ${\eta_{\rm dm}} \ll 1$. Baryon, photon, electron and neutrino densities are given in ${\rm g\,cm^{-3}}$. The Jeans mass is in solar units, the temperature is in units of $10^6\,$K, and $X_{\rm He}$ and $X_Z$ indicate mass fractions of baryons bound into Helium and into elements heavier than Lithium.
• Figure 2: Cooling of primordial clouds in a CBB for $\eta_\gamma = 1, {\eta_{\rm dm}} \ll 1$. The characteristic temperature $T$ and density $\rho$ as masses of $100M_{100}{\rm\,M_\odot}$ begin collapse yield an absorption $\kappa(T,\rho)$ using the fits of Bell & Lin [31]. Condensations in the region indicated by vertical lines are optically thick; others are optically thin. The shaded regions indicate condensations for which the dynamical time exceeds the cooling time.