On the Cosmological Implications of the String Swampland (1806.09718v2)

Abstract: We study constraints imposed by two proposed string Swampland criteria on cosmology. These criteria involve an upper bound on the range traversed by scalar fields as well as a lower bound on $|\nabla_{\phi} V|/V$ when $V >0$. We find that inflationary models are generically in tension with these two criteria. Applying these same criteria to dark energy in the present epoch, we find that specific quintessence models can satisfy these bounds and, at the same time, satisfy current observational constraints. Assuming the two Swampland criteria are valid, we argue that the universe will undergo a phase transition within a few Hubble times. These criteria sharpen the motivation for future measurements of the tensor-to-scalar ratio $r$ and the dark energy equation of state $w$, and for tests of the equivalence principle for dark matter.

• The paper analyzes two proposed Swampland criteria from string theory and examines their significant implications for both early universe inflation and dark energy models like Lambda-CDM.
• The Swampland criteria, particularly the field range and steepness conditions, introduce tension with standard single-field slow-roll inflationary models that fit observations.
• The criteria challenge the Lambda-CDM dark energy model but align better with quintessence, suggesting a near-future cosmological transition and highlighting key observational targets.

Analyzing Cosmological Implications of the String Swampland Criteria

The research paper titled "On the Cosmological Implications of the String Swampland" provides an in-depth examination of two proposed Swampland criteria within string theory and their implications for cosmology, particularly with respect to early universe inflationary models and contemporary dark energy models. This paper sheds light on the tension these criteria might introduce into current inflationary theories and the $\Lambda$CDM cosmology, potentially guiding future observational investigations.

The paper posits two Swampland criteria. The first criterion limits the range of scalars in field space to $\Delta \sim \mathcal{O}(1)$ in reduced Planck units. This constraint implies that any effective field theory is only valid within a finite field space radius before a tower of light modes significantly alters the dynamics. The second criterion necessitates a steep potential with a lower bound, $|\nabla_{\phi} V|/V > c \sim \mathcal{O}(1)$ when $V > 0$. This requirement challenges the feasibility of de Sitter (dS) vacua and imposes tight constraints on scalar potentials.

The authors explore how these criteria impact both the early and present-day universe, focusing initially on early universe inflation. Single-field slow-roll inflation models, which have been a cornerstone of modern cosmological theory, face challenges under these criteria. Specifically, the most observationally consistent inflationary models—plateau models—are shown to demand $c\lesssim 0.02$ and $\Delta \gtrsim 5$, both of which are in tension with the Swampland constraints.

In considering dark energy, the paper argues that the second criterion conflicts with the $\Lambda$CDM cosmology, but quintessence models can align with both the Swampland criteria and observational data. These models suggest constraints of $c < 0.6$ and $c < 3.5\Delta$, aligning with quintessence theories where $1 + w \gtrsim 0.15 c^2$ for the dark energy equation of state.

The authors forecast a significant cosmological transition within a few Hubble times, driven by the proposed Swampland criteria, underscoring the importance of measuring the tensor-to-scalar ratio and dark energy's equation of state. The authors also highlight the necessity to empirically test the equivalence principle in the dark sector as a further validation of these theoretical implications.

This analysis by Agrawal et al. illustrates prominent theoretical challenges introduced by the Swampland criteria, suggesting a possible reevaluation or evolution of inflationary models and the structure of dark energy, potentially spurring significant theoretical and observational advances in cosmology. Researchers are encouraged to probe these theoretical developments while refining observational techniques to either substantiate or refute these conceptual frameworks within the broader scope of string theory and quantum gravity.