The Encyclopedia of Cosmology

The following sections are included:

• Preface
• Contents
• List of Videos

This chapter presents an overview of cosmological framework that is necessary to perform cosmological simulations. First, we start with a brief history of cosmological studies of the Universe, such as the discovery of Hubble’s law and cosmic microwave background radiation which constitute the major observational evidence of expanding Big Bang cosmology. Second, we present the basics of General Relativity theory and Friedmann models that describe the expanding universe. Under this theoretical framework, we introduce various cosmological parameters and current best-fit Λ cold dark matter (CDM) model. Third, we discuss the history and development of computational cosmology which was achieved concurrently with the evolution of supercomputers.

Cosmological N-body simulations play an important role in modern cosmology by providing vital information regarding the evolution of the dark matter: its clustering and motion, and properties of dark matter halos. The simulations are instrumental for the transition of the theoretical cosmology from an inspiring but speculative part of astronomy to the modern precision cosmology. In spite of more than 50 yrs of development, N-body methods are still a thriving field with the invention of more powerful methods providing more accurate theoretical predictions. Here, we review different numerical methods (PM, Tree, AMR) and ideas used in this field.

Currently, various cosmological and astrophysical experiments are planned or under way mapping a large number of cosmological objects from galaxies to galaxy clusters in unprecedented high precision to understand their dynamical structure in detail and to constrain their formation history and the involved physical processes. Exploiting these datasets is a challenging task. The physics at play, namely, the large-scale gravitational instability coupled to complex galaxy formation physics, is highly nonlinear and some aspects are still poorly understood. To capture the complexity of galaxy formation in a cosmological context, advanced computer simulations have to be performed. Therefore, it is needed to incorporate a variety of physical processes in the calculations, including three that are considered particularly important for the development of the visible universe: first, the condensation of matter into stars, second, their further evolution when the surrounding matter is heated by stellar winds and supernova explosions, and enriched with chemical elements, and third, the feedback of supermassive black holes that eject massive amounts of energy into the universe. In this chapter, we briefly describe the hydrodynamic methods used in cosmological simulations and the most common techniques used to include these processes.

First stars are the very first light of the Universe after the recombination. They illuminate, ionize, and heat up the surrounding material by the radiation. They also explode to put kinetic energy into the interstellar medium, thereby spreading the elements heavier than helium, such as carbon, oxygen, iron, etc., which are indispensable for the birth of lives. The formation process of these stars has been studied intensively to predict that the significant fraction of these stars is rather massive, ∼ 10–10³M_⊙, although still little evidence has been found observationally. In this chapter, the theoretical challenges and results in the last two decades on this issue, as well as the key observations which constrain the theory are described. In addition, future prospects in the field are discussed.

One of the exciting frontiers of modern cosmology is to understand the emergence of the first sources of light, galaxies and accreting supermassive black holes during the first billion years of cosmic history. To provide a heuristic map for the impressive array of upcoming next-generation observational facilities, among them the James Webb Space Telescope (JWST), ever more realistic numerical simulations play a key role. In this chapter, we will discuss some of the basic principles that govern first galaxy formation and the challenges they present for the simulation field. Prime among them is the proper treatment of the feedback effects exerted by the first (Population III) stars, including radiative and supernova feedback. A closely related problem is the formation of the first massive seed black holes, whose X-ray feedback could significantly impact the assembly process of the first galaxies. In conclusion, we will briefly survey the prospects for empirically testing our emerging theoretical framework.

In this chapter, we discuss basic physics of galaxy formation and evolution in cosmological hydrodynamic simulations. First, we describe some of the basic physical processes for galaxy formation, such as radiative cooling and heating of gas, star formation, and supernova (SN) feedback models. Second, we discuss basic output of simulations: star formation rate density (SFRD), galaxy stellar and luminosity functions as a function of time. Feedback processes by supernovae and supermassive black holes are considered to be the key for galaxy evolution, and various implementations have been attempted in cosmological hydrodynamic simulations over the past two decades.

In this chapter, we discuss evolution of disk galaxies, focusing on numerical simulations over dynamical and cosmological timescales. Being open systems, galaxies grow and redistribute their angular momentum and mass in response to various factors, internal and external ones, leading to the present state of the Hubble Fork. Galaxies can be studied as mechanical and statistical systems—due to long-range gravitational forces, they are strongly nonlinear systems with a healthy degree of chaotic motions, and capable of forgetting conditions at their origin. Despite their low cosmological average, baryons appear to decouple from the background dark matter on galactic and subgalactic scales, forming a rich variety of objects that can provide feedback to modify their own evolution. This complex interplay between various physical processes is currently at the forefront of astrophysical research using diverse observational methods in tandem with high-resolution numerical simulations.

Diffuse gas outside of galaxies plays an enormously important role in the evolution of the universe. In this chapter, we describe the physical processes and numerical models of cosmic gas from the CMB to the present day. We begin by outlining the basic theory for the thermal evolution of the gas, before taking an approximately chronological ride through a number of important phases: the evolution of gas before reionization, the process of reionization itself, the intergalactic medium and the Lyman-α forest, and finally ending with a discussion of the circumgalactic medium. We highlight both the key numerical results and outstanding questions throughout.

Galaxy clusters are the largest gravitationally bound objects in the universe, whose formation and evolution are driven by dark matter and dark energy. Recent multiwavelength observations have provided unprecedented insights into the structure and evolution of dark matter, gas, and stars from their cores to the virialization region in the outskirts of galaxy clusters. In order to realize the statistical power of upcoming cluster surveys, it is critical to improve our understanding of still uncertain cluster astrophysics. Given that clusters are nonlinear collapsed systems, N-body and hydrodynamic simulation is the method of choice for modeling the dynamics of dark matter, gas, and stars in their full complexity during the hierarchical build-up of structures. In this chapter, we will review recent advances in computational modeling of galaxy clusters formation with highlights on both successes and challenges.

The following section is included:

• Index