Planets in Other Universes: Habitability constraints on density fluctuations and galactic structure (1505.06158v2)

Abstract: Motivated by the possibility that different versions of the laws of physics could be realized within other universes, this paper delineates the galactic parameters that allow for habitable planets and revisits constraints on the amplitude $Q$ of the primordial density fluctuations. Previous work indicates that large values of $Q$ lead to galaxies so dense that planetary orbits cannot survive long enough for life to develop. Small values of $Q$ lead to delayed star formation, loosely bound galaxies, and compromised heavy element retention. This work generalizes previous treatments: [A] We consider models for the internal structure of galaxies and find the fraction of galactic real estate that allows stable, long-lived planetary orbits. [B] We perform a large ensemble of numerical simulations to estimate cross sections for the disruption of planetary orbits due to interactions with passing stars. [C] We consider disruption due to the background radiation fields produced by the galaxies. [D] One consequence of intense galactic background radiation fields is that some portion of the galaxy, denoted as the Galactic Habitable Zone, will provide the right flux levels to support habitable planets for essentially any planetary orbit. As $Q$ increases, the fraction of stars in a galaxy that allow for habitable planets decreases due to both orbital disruption and the intense background radiation. However, the outer parts of the galaxy always allow for habitable planets, so that the value of $Q$ does not have a well-defined upper limit. Moreover, some Galactic Habitable Zones are large enough to support more potentially habitable planets than the galaxies found in our universe. These results suggest that the possibilities for habitability in other universes are somewhat more favorable and far more diverse than previously imagined.

• The paper quantifies how changes in primordial density fluctuations influence galactic formation and preserve a significant fraction of stable planetary orbits.
• The analysis employs internal galactic modeling, numerical simulations of stellar scattering, and assessments of galactic radiative impacts to evaluate habitability.
• The study highlights that even in denser galaxies, extensive galactic habitable zones may exist, challenging conventional views on optimal conditions for life.

Habitability in Alternative Cosmological Models: Analyzing the Influence of Primordial Density Fluctuations

This paper by Adams, Coppess, and Bloch addresses the viability of habitable planets in universes characterized by varying amplitudes of primordial density fluctuations, a parameter denoted as $Q$. While our universe exhibits a relatively small value of $Q \sim 10^{-5}$, informing moderate, stable galactic structures, the paper investigates the implications of expanded $Q$ values on a hypothetical cosmological scale. It explores the structural dynamics within such varied universes, tenor of galactic formation, and the resulting domains conducive to habitability.

Framework and Methodology

The analysis is rooted in cosmological models that predicate galactic evolution on the amplitude of primordial density fluctuations. The authors summarize the traditional view that an increase in $Q$ exacerbates structural density, leading to accelerated star formation and denser galactic systems. Under this framework, the authors examine several mechanisms that could impede habitability, including the gravitational disruption of planetary orbits by proximal stellar bodies and the overwhelming radiative flux from dense galactic environs.

Three methodological components underpin the paper:

1. Internal Galactic Structure Models: By modeling galaxies across a spectrum of densities, the paper quantifies the long-term stability of planetary orbits in varied galactic environments.
2. Numerical Simulations for Stellar Scattering: Ensemble calculations evaluate perturbative interactions capable of disrupting planetary orbits and assess the feasibility of sustained habitability in systems within a galactic construct.
3. Assessment of Galactic Radiative Influence: The paper examines constraints where intrinsic galactic radiance supersedes that of host stars, postulating conditions that could render regions of the galaxy inhospitable due to excessive thermal influence.

Findings

The paper asserts that extensions to the traditional model depict more nuanced possibilities for habitability across universal constructs. Despite augmented values of $Q$, which catalyze dense galactic formations, viable segments for life-supporting conditions remain extant. The thesis is supported by several key results:

• Viable Orbital Fractions: Even within intensified $Q$ environments, a non-negligible fraction of planetary systems may avoid disruptive gravitational encounters. For fluctuation amplitudes up to $Q=1$, survival fractions across galaxies of differing masses indicate considerable resilience against orbital disruption, albeit decreasing with increased $Q$.
• Potential of Galactic Habitable Zones (GHZ): A novel exploration posits that dense galaxies might harbor extensive GHZs where background galactic radiation could sustain biospheres irrespective of planetary orbital specifics. This implies an abundance of potentially habitable zones beyond those traditionally envisaged within analogous constructs to our universe.
• Comparative Habitability: The notion that increased $Q$ zones might support more life-bearing planets due to GHZ prevalence challenges traditional perceptions of our universe's optimal conditions for habitability.

Implications and Prospective Directions

From a theoretical perspective, the paper suggests a multifaceted landscape for understanding habitability, with broader cosmological implications for the multiverse hypothesis. Practically, the research encourages reconsideration of the parameters contributing to habitability, suggesting that attributes like $Q$ could yield viable conditions across fundamentally different galactic architectures.

The paper's extrapolation of the GHZ aligns with ongoing astrophysical inquiry, emphasizing the potential for observations targeting such cosmic structures to validate or refute the proposed models. Ultimately, future developments should contemplate additional fundamental variables such as varying gravitational constants or cosmic energy densities in congruence with $Q$, fostering a holistic evaluation of alternative universes pivotal to unraveling cosmic habitability.

This paper enriches the discourse on cosmological diversity and habitable potential, advocating a recalibration of our participatory assumptions in cosmic habitability paradigms.