Dark energy and cosmic acceleration

Abstract: The discovery that we live in an accelerating universe changed drastically the paradigm of physics and introduced the concept of \textit{dark energy}. In this work, we present a brief historical description of the main events related to the discovery of cosmic acceleration and the basic elements of theoretical and observational aspects of dark energy. Regarding the historical perspective, we outline some of the key milestones for tracing the journey from Einstein's proposal of the cosmological constant to the type Ia supernovae results. Conversely, on the theoretical/observational side, we begin by analyzing cosmic acceleration within the context of the standard cosmological model, i.e., in terms of the cosmological constant. In this case, we show how a positive cosmological constant drives accelerated expansion and discuss the main observational aspects, such as updated results and current cosmological tensions. We also explore alternative descriptions of dark energy, encompassing dynamic and interacting dark energy models.

• The paper synthesizes observational and theoretical advances in understanding dark energy’s role in cosmic acceleration.
• It details challenges of the ΛCDM model, including the cosmological constant and cosmic coincidence problems, using SN Ia, CMB, and BAO data.
• The study evaluates alternative models like quintessence and interacting dark energy to address tensions such as the H₀ discrepancy and guide future surveys.

Dark Energy and Cosmic Acceleration: Insights and Implications

The paper "Dark Energy and Cosmic Acceleration" by Rodrigo von Marttens and Jailson Alcaniz presents a detailed survey of the theoretical and observational aspects of dark energy (DE), especially in relation to cosmic acceleration. This text synthesizes the complex landscape of modern cosmology, addressing key developments, current constraints, and the challenges facing standard cosmological models.

Historical Context and Discovery

The concept of an accelerating universe necessitating dark energy emerged from unexpected observations of distant Type Ia supernovae (SN Ia) in the late 1990s, which implied a cosmic acceleration inconsistent with the previously accepted matter-dominated universe models. The paper notes how this discovery overturned the foundational assumption that cosmic expansion rates were slowing due to gravitational attraction. Instead, these observations prompted the introduction of 'dark energy'—an unknown form of energy contributing approximately 70% of the universe's total energy density.

The Standard Cosmological Model and Its Challenges

Von Marttens and Alcaniz meticulously outline the current paradigmatic $\Lambda$CDM model, which combines Einstein's cosmological constant ($\Lambda$) and Cold Dark Matter (CDM). This model presupposes a universe with a flat geometry dominated by dark energy and dark matter. Despite the model's success in describing diverse observational data such as the Cosmic Microwave Background (CMB) anisotropies, Baryon Acoustic Oscillations (BAO), and SN Ia distances, it is not without its theoretical and empirical challenges.

Key theoretical issues in $\Lambda$CDM include the cosmological constant problem, which refers to the discrepancy between the observed value of the cosmological constant and the predicted quantum field theory vacuum energy density—differing by over 120 orders of magnitude. Another critical challenge is the cosmic coincidence problem, which ponders why the densities of dark energy and matter are of comparable magnitude today, given their distinct evolutionary paths.

Dynamical Views of Dark Energy

Given these challenges, alternative approaches to DE are examined. Quintessence models, where a dynamic scalar field replaces the cosmological constant, are explored. These models offer insights into variable dark energy density but introduce new parameters and complexities, such as the need for specific potentials to drive acceleration.

Furthermore, $k$-essence models introduce a non-canonical kinetic term to the scalar field, allowing for a non-luminal sound speed. These models can provide clustering properties for dark energy, potentially reconciling some discrepancies between theory and observation.

Interacting Models and Observational Tensions

Interacting DE models address some open questions by allowing energy exchange between dark energy and dark matter. These models can be tailored to alleviate the aforementioned cosmic coincidence problem and offer potential solutions to current observational tensions, such as the infamous $H_0$ tension—an inconsistency between local and Planck-based measurements of the Hubble constant.

The paper discusses recent results from observational probes such as the Planck satellite, SDSS, and DES, which have highlighted these tensions. For instance, the $H_0$ tension now stands at over 4$\sigma$ discrepancy between the Planck CMB results ($\sim$67 km s$^{-1}$ Mpc$^{-1}$) and SH0ES local measurements ($\sim$74 km s$^{-1}$ Mpc$^{-1}$).

Prospective Directions

The authors encourage continual examination of both theoretical frameworks and observational methodologies. They acknowledge contributions from new data releases from surveys like DESI and anticipate revolutionary insights from upcoming missions like Euclid and the Vera Rubin Observatory.

In conclusion, the study of dark energy and cosmic acceleration remains a dynamic field at the frontier of cosmology, with ongoing debates and evolving theories. As such, von Marttens and Alcaniz's work provides a robust framework for understanding the current state of cosmological research and the myriad approaches being pursued to unravel the enigmatic phenomena of dark energy and cosmic expansion.