TASI Lectures on Early Universe Cosmology: Inflation, Baryogenesis and Dark Matter

Abstract: These lectures, presented at TASI 2018, provide a concise introduction to inflation, baryogenesis, and aspects of dark matter not covered by the other lectures. The emphasis for inflation is an intuitive understanding and techniques for constraining inflationary models. For baryogenesis we focus on two examples, leptogenesis and electroweak baryogenesis, with attention to singlet-assisted two-step phase transitions. Concerning dark matter, we review different classes of models distinguished by their mechanisms for obtaining the observed relic density, including thermal freeze-out, asymmetric dark matter, freeze-in, SIMP dark matter, the misalignment mechanism for ultralight scalars and axions, and production of primordial black holes during inflation. Problem sets are provided.

• The paper offers a comprehensive overview of early universe processes by methodically analyzing inflation, baryogenesis, and dark matter frameworks.
• It demonstrates how slow-roll inflation and quantum fluctuations give rise to CMB anisotropies, aligning theoretical models with observational data.
• It examines mechanisms for matter-antimatter asymmetry and explores diverse dark matter production models, including freeze-out, freeze-in, and self-interaction scenarios.

Overview of Early Universe Cosmology: Inflation, Baryogenesis, and Dark Matter

Introduction

The paper authored by James M. Cline and presented at TASI 2018 provides a comprehensive exploration into pivotal concepts of early universe cosmology, namely inflation, baryogenesis, and dark matter. This manuscript serves as a primer on these essential phenomena, emphasizing intuitive understanding and offering techniques to analyze inflationary models, explore mechanisms behind baryogenesis, and categorize various models of dark matter. The lectures situate these cosmological phenomena within the broader timeline of the universe, contemplating events from cosmic inflation to the formation of structure.

Inflation

The paper begins with an analysis of the inflationary epoch. Recognized for solving the horizon and flatness problems, inflation is discussed in terms of scalar fields with suitable potentials that drive exponential expansion. The author elucidates the slow-roll conditions, characterized by the potential slow-roll parameters $\epsilon$ and $\eta$, which ensure sustained inflationary dynamics. Quantum fluctuations in these fields are addressed, providing a foundation for the anisotropies observed in the Cosmic Microwave Background (CMB). The paper aligns its treatment with leading observational data, implicitly integrating findings like those of Planck, which constrain parameters such as the spectral index $n_s$ and the tensor-to-scalar ratio $r$. Cline ventures into specific model predictions, such as those from chaotic inflation and Starobinsky's model, comparing them against empirical constraints to highlight their viability.

Baryogenesis

In exploring baryogenesis, the paper emphasizes the generation of matter-antimatter asymmetry in the universe. The necessity for baryon number violation, departure from thermal equilibrium, and CP violation are discussed under the Sakharov conditions. Two mechanisms receive particular attention: leptogenesis and electroweak baryogenesis. Leptogenesis is presented through the lens of heavy sterile neutrino decay within the framework of the Standard Model extended by right-handed neutrinos, segueing into discussions on the Seesaw mechanism for neutrino masses. The derivations of CP asymmetry and considerations of washout effects are outlined. Meanwhile, electroweak baryogenesis is reliant on first-order phase transitions during the electroweak symmetry breaking, a scenario augmented by physics beyond the Standard Model to provide requisite CP violation and a strongly first-order transition.

Dark Matter

The manuscript categorizes dark matter models based on their production mechanisms, shifting focus from the canonical WIMP paradigm encouraged by thermal freeze-out, to other avenues like asymmetric dark matter and freeze-in production. Cline delineates several contemporary models, including primordial black holes and ultralight axions, underscoring their contributions and limitations in accounting for the extant dark matter density. The paper highlights the prospects of dark matter as a relic density from freeze-out, requiring cross sections of about $10^{-26} \text{cm}^3/\text{s}$, and discusses the unitarity limits posited by Griest and Kamionkowski for thermal relics. The implications of self-interacting dark matter (SIDM) as a compelling solution to small-scale structure issues are also explored.

Implications and Future Directions

The discussions presented integrate theoretical constructs with observational data, highlighting an interplay that guides current cosmological inquiries. The constraints derived from precision measurements, such as those from the Planck mission, shape the evolution of theoretical models in cosmology. Future observational advancements, potentially in gravitational wave astronomy or further refined cosmic surveys, will continue to test these cosmological paradigms, either reinforcing existing theories or compelling novel propositions.

In summation, this paper provides a structured overview of fundamental processes and phenomena that have shaped the universe from its inception to its current state. It serves not only as a knowledge base but also as a vantage point for prospective research endeavors in early universe cosmology.