Lectures on the Cosmological Constant Problem (1502.05296v1)

Abstract: These lectures on the cosmological constant problem were prepared for the X Mexican School on Gravitation and Mathematical Physics. The problem itself is explained in detail, emphasising the importance of radiative instability and the need to repeatedly fine tune as we change our effective description. Weinberg's no go theorem is worked through in detail. I review a number of proposals including Linde's universe multiplication, Coleman's wormholes, the fat graviton, and SLED, to name a few. Large distance modifications of gravity are also discussed, with causality considerations pointing towards a global modification as being the most sensible option. The global nature of the cosmological constant problem is also emphasized, and as a result, the sequestering scenario is reviewed in some detail, demonstrating the cancellation of the Standard Model vacuum energy through a global modification of General Relativity.

• The paper details how radiative instability forces extreme fine-tuning of the cosmological constant due to its sensitivity to ultraviolet physics.
• The paper examines symmetry-based approaches and modified gravity frameworks as potential mechanisms to mitigate the vacuum energy discrepancy.
• The paper calls for clearer conceptual frameworks and empirical exploration to bridge quantum field theory with gravitational phenomena.

Overview of "Lectures on the Cosmological Constant Problem" by Antonio Padilla

The paper "Lectures on the Cosmological Constant Problem" by Antonio Padilla provides an extensive overview of one of the most persistent challenges in theoretical physics: the cosmological constant problem. This issue arises from the vast discrepancy between the theoretical expectations of the vacuum energy density, derived from quantum field theories, and the observed value of the cosmological constant, which appears to be exceedingly small. In this meticulously composed lecture series, Padilla navigates through the historical development of the problem, the theoretical implications, and the myriad proposed solutions, while prioritizing radiative instability as a central concern that underpins the cosmological constant's sensitivity to the ultraviolet (UV) physics.

Central Thesis and Discussion

Padilla begins by tracing historical perspectives, citing Pauli’s early calculations in the 1920s, which humorously underscored the gravity of zero-point energies by suggesting the Universe would not extend to the Moon. He then encapsulates Zel'dovich's contributions from the late 1960s, establishing a foundation for understanding the cosmological constant as "the mother of all physics problems."

The core of the paper tackles the cosmological constant problem with particular emphasis on radiative instability. This issue arises because higher-loop quantum corrections persistently require extreme fine-tuning to maintain the cosmological constant's small observed value, indicating its linear dependence on unknown UV physics parameters. Padilla articulates how effective field theories, often utilized to bridge large-scale physics with microphysics, falter in providing a stable solution due to this sensitivity.

Proposed Solutions and Concepts

Padilla explores several proposed resolutions to the cosmological constant problem, ranging from symmetry arguments to modifications of gravity. Notably:

• Symmetry-Based Approaches: Padilla discusses symmetry proposals such as supersymmetry and scale invariance, which theoretically could stabilize the cosmological constant through underlying symmetries. Supersymmetric models argue for cancellations between bosonic and fermionic vacuum contributions, although observational constraints on supersymmetry breaking scales render this less plausible.
• Modified Gravity Theories: He examines large-distance modifications to gravity, such as the cosmological model of sequestering, which aims to nullify the Standard Model vacuum energy. This approach modifies gravity globally without locally altering General Relativity, suggesting a framework wherein cosmological constant cancellations might occur at infra-red scales, evading the ghostly pitfalls faced by many massive gravity models.
• Coleman’s Wormholes and Exotic Mechanisms: Padilla revisits niche proposals like Coleman’s wormholes, which suggest a probabilistic reshaping of vacuum states, although later critiques revealed serious pathologies in maintaining causality and perturbative unitarity.

Implications and Conclusions

Throughout the lectures, Padilla emphasizes the need for clearer conceptualization and empirical exploration to redefine our understanding of cosmological measurements. He positions the problem as an interplay between quantum field theory philosophies and gravitational paradigms, driving a deeper inquiry into both foundational and speculative physics.

In conclusion, the paper advocates for a nuanced appreciation of robustness in General Relativity while pragmatically exploring avenues to stabilize the cosmological constant against radiative corrections. Although these lectures do not present a definitive solution, they serve as a compendium of critical knowledge and observations directing future cosmological exploration and theoretical advancement. Padilla’s insights offer experienced researchers a cohesive narrative to further probe the cosmological constant's elusive nature within an ever-expansive landscape of possible theories and models.