Galaxy Formation Efficiency and the Multiverse Explanation of the Cosmological Constant with EAGLE Simulations (1801.08781v2)

Abstract: Models of the very early universe, including inflationary models, are argued to produce varying universe domains with different values of fundamental constants and cosmic parameters. Using the cosmological hydrodynamical simulation code from the eagle collaboration, we investigate the effect of the cosmological constant on the formation of galaxies and stars. We simulate universes with values of the cosmological constant ranging from Lambda = 0 to Lambda_0 = 300, where Lambda_0 is the value of the cosmological constant in our Universe. Because the global star formation rate in our Universe peaks at t = 3.5 Gyr, before the onset of accelerating expansion, increases in Lambda of even an order of magnitude have only a small effect on the star formation history and efficiency of the universe. We use our simulations to predict the observed value of the cosmological constant, given a measure of the multiverse. Whether the cosmological constant is successfully predicted depends crucially on the measure. The impact of the cosmological constant on the formation of structure in the universe does not seem to be a sharp enough function of Lambda to explain its observed value alone.

• The paper uses EAGLE simulations to explore galaxy formation efficiency under varying cosmological constants and evaluates multiverse explanations for its observed value.
• It finds that moderate increases in the cosmological constant do not significantly alter early star formation, but large increases lead to baryonic mass loss due to feedback.
• The analysis of multiverse measures like causal patch and causal diamond provides stronger constraints on predicting the cosmological constant than the mass-weighted approach.

Analysis of "Galaxy Formation Efficiency and the Multiverse Explanation of the Cosmological Constant with EAGLE Simulations"

The paper "Galaxy Formation Efficiency and the Multiverse Explanation of the Cosmological Constant with EAGLE Simulations" by L. Barnes et al. explores the impact of varying the cosmological constant ($\Lambda$) on galaxy formation and examines its implications for multiverse theories. Using the EAGLE simulation framework, which is a cosmological hydrodynamical code, the authors simulate universes with different values of $\Lambda$, aiming to understand its effects on star formation and structure formation efficiency.

Key Findings and Methodology

1. Simulation Framework:
   • The authors utilize EAGLE, a robust computational cosmology tool modeled on hydrodynamics to simulate galaxy formation across different universe models. The simulations range $\Lambda$ from zero to 300 times the observed value in our universe ($\Lambda_0$) to explore the efficiency of galaxy formation under various conditions.
2. Star Formation and Galaxy Structure:
   • One insightful result is the timing of the star formation rate peaking at $t = 3.5$ Gyr in our universe, which occurs prior to significant $\Lambda$-driven accelerating expansion. This observation highlights that moderate increases in $\Lambda$ do not substantially alter star formation due to this early formation peak.
3. Cosmological Constant Impact:
   • The paper discusses that $\Lambda$ significantly influences the baryonic processes more than the dark matter halo growth due to the ejection dynamics enforced by galactic and black hole feedback. For $\Lambda$ values significantly larger than $\Lambda_0$, this leads to a net loss in baryonic mass due to its ejection to the intergalactic medium.
4. Multiverse Model Analysis:
   • The analysis investigates the anthropic reasons for the small observed $\Lambda$ in our universe by evaluating multiverse measures: mass-weighted, causal patch, and causal diamond. The authors conclude that these measures, notably the latter two, have pronounced constraining effects on predicting $\Lambda$.
5. Distribution of $\Lambda$:
   • The simulated data illustrate discrepancies in predictions depending on the observer model and multiverse measure employed. Notably, the causal patch and causal diamond measures indicate a higher likelihood of observing a $\Lambda$ close to the actual value compared to the mass-weighted approach, which suggests larger values as more typical.

Implications and Future Research

• Understanding Universe Fine-Tuning:

The findings add to the understanding of the universe’s fine-tuning for life and the role of multiverse theories in explaining the cosmological constant. The complexity introduced by varying $\Lambda$ offers a larger parameter space to evaluate galaxy formation and observer probabilities, testing anthropic principles.

• Future Exploration:

Future research will likely hone in on further exploration of multiverse measures and alternative models beyond $\Lambda$ variations, possibly incorporating data on other fundamental constants or initial conditions.

• Simulation Advancements:

Enhancing the simulation detail, especially concerning late-time galaxy evolution and long-term baryonic processes, may help refine these predictions and provide greater insight into multiverse predictions and the practicality of existing measures.

The paper is a significant contribution to the intersection of cosmology and theoretical physics, particularly concerning the grounds on which multiverse hypotheses are formulated and tested. These findings underscore the necessity for a well-grounded measure within such models to make any conclusive arguments about the observed cosmological constants. The research paints a complex picture of how slight changes in fundamental constants can profoundly influence cosmic history, underlining the theoretical challenge in understanding our own universe’s parameters within a potentially vast multiverse.