Research

From Early Universe Cosmology to Particle Physics

Cosmic Inflation

Cosmic inflation proposes that, in the early universe, a small region of space underwent a brief yet dramatic expansion. This theory explains why the universe appears so flat and why it seems homogeneous and isotropic on large scales. The inflationary paradigm is strongly supported by the current Planck measurements of the cosmic microwave background (CMB) radiation. Furthermore, inflation solves the horizon problem by explaining the uniform temperature of the CMB across the sky, despite the vast distances between regions. Future experiments might provide conclusive evidence for this theory, potentially unveiling new aspects of the early universe and shaping our understanding of cosmic origins.

In ``BICEP/Keck Constraints on Attractor Models of Inflation and Reheating," we examined the implications of the latest cosmic microwave background (CMB) data from BICEP/Keck, WMAP, and Planck on attractor models of inflation. The new data provide stringent constraints on the scalar perturbation spectral tilt and the tensor-to-scalar ratio. These insights advance our understanding of cosmic inflation and reheating within attractor models. Our analysis of the combined CMB data pointed to specific parameter ranges that are favored for these models, taking into account the new upper limits on the tensor-to-scalar ratio provided by BICEP/Keck data and the Planck collaboration results. We noted that these findings significantly narrow down the viable parameter space for attractor models of inflation and reheating, giving us a clearer picture of the possible mechanisms and dynamics at play during the earliest moments of the universe.

At the end of inflation, the scalar field decays, heating the universe and marking the transition to the hot big bang phase. This process, known as reheating, aligns the universe’s evolution with the standard Big Bang theory, resulting in ongoing expansion and cooling. Additionally, one can also consider the genesis of dark matter at the end of inflation. We discuss this subject in ``Freeze-in from Preheating,” where we study in detail the dark matter production during the epoch of reheating. Our work examines the influence of the energy density and distribution of the inflaton scattering or decay byproducts, which constitute the radiation bath, on the relic density of dark matter produced by the freeze-in mechanism. Employing both perturbative and non-perturbative methods to calculate the radiation energy density, we evaluate the resulting relic densities under different energy distribution assumptions, finding a wide spectrum of dark matter masses that align with the current estimates of the cold dark matter density in the universe today.

Gravitational Production of Dark Matter

In ``Gravitational Portals in the Early Universe," we explored the production of matter and radiation during the reheating phase following inflation and focused on gravitational interactions. The processes that we studied include the exchange of a graviton and establishing a lower bound on the maximal reheating temperature, crucial for any extensions of the Standard Model theory based on Einstein's gravity. The paper compares these processes to understand their relative significance in the generation of both radiation and dark matter.

We examine the non-equilibrium production of scalar dark matter (DM) from the inflaton condensate during inflation and reheating in ``Scalar Dark Matter Production from Preheating and Structure Formation Constraints". We consider a scalar that only directly couples to the inflaton via a quartic coupling and is minimally coupled to gravity. Various production regimes are explored: purely gravitational, weak direct coupling (perturbative), and strong direct coupling (non-perturbative). For each regime, different methods are employed to determine the dark matter phase space distribution and the corresponding relic abundance. In the purely gravitational regime, an abundance of scalar dark matter is excited during inflation, leading to an infrared-dominated distribution function. This results in an overabundance of the dark matter relic abundance if the reheating temperature is lower than the mass of the inflaton.

In ``A New Window into Gravitationally Produced Scalar Dark Matter", we focus on gravitationally produced scalar dark matter within the context of inflationary cosmology. The paper introduces a non-minimal coupling to gravity, which allows for the production of lighter scalar dark matter masses compared to minimal coupling predictions. This approach addresses the tension between conventional dark matter production scenarios and the constraints on the isocurvature power spectrum from cosmic microwave background measurements.

Supersymmetry and Supergravity

Although the Standard Model is an extremely successful theory, it does not explain all observed physical phenomena and has its shortcomings. In supersymmetric theory, the three fundamental forces unify at high energy, while supergravity combines the principles of supersymmetry with classical gravity. In ``A General Classification of Starobinsky-like Inflationary Avatars of SU(2,1)/SU(2)×U(1) No-Scale Supergravity," we discuss a general classification of inflationary models within no-scale supergravity, focusing on consistency with cosmic microwave background (CMB) measurements. These models exhibit low tensor-to-scalar ratios, similar to the original Starobinsky model. The paper provides a thorough analysis of no-scale supergravity symmetries and explores their implications for cosmic inflation, dark matter, and connections to the Standard Model of particle physics.

We extended our previous work by focusing on no-scale supergravity's potential to describe inflation, supersymmetry breaking, and dark energy within a unified model. In ``Unified No-Scale Attractors," we examine no-scale inflationary models based on a simple Kähler potential and generalize these models to include multiple moduli and matter fields. The paper also explores the implications for the upper limit of the tensor-to-scalar ratio, supporting the consistency with the Starobinsky model of inflation.

Recently, in ``Supergravity Scattering Amplitudes," we investigated the calculation of scattering amplitudes in supergravity theories with non-minimal Kähler potentials. We presented a method of calculating these amplitudes using Riemann normal coordinates and a more compact method using Kähler normal coordinates. We applied these methods to no-scale supergravity with one and two chiral superfields and demonstrated that the scattering amplitudes are equivalent using both coordinate systems only at extremal points of the scalar potential. This work contributes to the broader goal of seeking viable theories beyond the Standard Model and may have implications for understanding the full set of scalar field interactions in theories derived from string theory.

Together, this research contributes to the effort to bridge the gap between the Standard Model, early universe cosmology, and potential theories of quantum gravity like string theory, providing a deeper theoretical framework for understanding the early universe, particle physics, and the fundamental forces that govern them.

Axion Dark Matter

The paper ``Axion Kinetic Misalignment and Parametric Resonance from Inflation" investigates the production of axion cold dark matter, challenging traditional models by proposing that significant axion production can occur even with decay constants below standard levels. It explores two mechanisms, the Kinetic Misalignment Mechanism (KMM) and Parametric Resonance (PR), which could explain the density of dark matter with a smaller axion decay constant. The study probes the impact of initial saxion field values on dark matter abundance and examines the potential of axion kinetic energy to address the observed baryon asymmetry.

Linking the behavior of saxion and axion fields to the early inflationary period, we consider scenarios where saxion field oscillations begin relative to the reheating process post-inflation. This timing is critical as it affects the resulting axion abundance, influencing their role as dark matter. By using cosmological parameters like the Hubble constant and inflaton mass, the paper provides insights into axion production conditions during inflation, contributing to our understanding of dark matter distribution.

Machine Learning Applications in Physics

Machine Learning has been making significant strides in particle physics and cosmology due to its ability to efficiently process and analyze large datasets. In our paper ``Deep Learning Symmetries and Their Lie Groups, Algebras, and Subalgebras from First Principles," we developed a deep-learning algorithm designed to uncover and identify continuous symmetry groups present in a labeled dataset. We employed fully connected neural networks to model the symmetry transformations and the associated generators. We also formulated loss functions to ensure that the applied transformations were indeed symmetries and that the set of generators formed a closed (sub)algebra. This method allows for a principled, data-driven approach to uncovering symmetries that may be present in high-dimensional physics data.

Developing new theoretical physics models requires adherence to both experimental evidence and the more subjective ``aesthetic" principles valued by physicists, such as simplicity and naturalness. In the paper ``Seeking Truth and Beauty in Flavor Physics with Machine Learning," we use machine learning to design models that satisfy these criteria, addressing issues in the Standard Model Yukawa sector. By applying machine learning with carefully crafted loss functions, we demonstrate, with toy examples, that this method can produce models that align with both experimental data and possess an element of aesthetic appeal as measured by a specific standard.

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