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In this chapter, galaxy formation theory is reviewed and confronted with recent observational issues. Galaxy formation is at the forefront of observation and theory in cosmology. An improved understanding is essential for improving knowledge of cosmological parameters, the contents of the universe, and its origins. In the first part of the chapter, the following topics are presented: star formation considerations, including the initial mass function (IMF), star formation efficiency, and star formation rate; the origin of the galaxy luminosity function; and feedback in dwarf galaxies. The second part of the chapter describes the formation of disks and massive spheroids, including the growth of supermassive black holes; negative feedback in spheroids; the active galactic nucleus (AGN)–star formation connection; star formation rates at high redshift; and the baryon fraction in galaxies.

This chapter studies radiative, mechanical, and chemical feedback in the earliest gaseous clouds, taking up the thread of discussion in the previous chapter to consider the influences placed on the formation of second-generation stars. While the feedback effects are sufficiently complex that a complete description of them is well beyond the capabilities of present-day computer simulations, the general principles that underlie them are well known. Therefore, the chapter focuses on these principles and then briefly sketches the global picture. Feedback is important in all galaxies, and many of the principles that are discussed in this chapter apply on a much wider scale than just the first stars and galaxies.

This chapter considers the emergence of the complex chemical and radiative processes during the first stages of galaxy formation. It studies the appearance of the first stars, their feedback processes, and the resulting ionization structures that emerged during and shortly after the cosmic dawn. The formation of the first stars tens or hundreds of millions of years after the Big Bang had marked a crucial transition in the early Universe. Before this point, the Universe was elegantly described by a small number of parameters. But as soon as the first stars formed, more complex processes entered the scene. To illustrate this, the chapter provides a brief outline of the prevailing (though observationally untested) theory for this cosmological phase transition.

Results from the LHC can expand our knowledge of the early stages of the universe. The connection between the world of particle physics and the structure of our universe is probably one of the most profound results of modern science, which captivates the imagination of anyone who is confronted with its wonders. This chapter describes this connection, explaining how the process of inflation could be at the origin of the cosmological properties we observe today. The empirical evidence in favour of dark matter is by now overwhelming. This still unknown form of matter could be made of some new kind of particle that can be produced at the LHC. This chapter presents the case of dark matter at the LHC and the ways in which the new form of matter could be discovered. It explains what dark energy is and its potential impact on the naturalness problem.

This chapter describes the essential aspects of the modern cosmological paradigm for understanding the formation of the first galaxies in the Universe. The basic question that cosmology attempts to answer is: What is the composition of the Universe and what initial conditions generated the observed structures in it? The first galaxies were shaped, more than any other class of astrophysical objects, by the pristine initial conditions and basic constituents of the Universe. This chapter shows how studying the formation process of the first galaxies could reveal unique evidence for new physics that has so far remained veiled in older galaxies by complex astrophysical processes.

This chapter examines the intergalactic medium (IGM). Although much of astronomy focuses on the luminous material inside galaxies, the majority of matter today—and the vast majority at z > 6—actually lies outside these structures, in the IGM. This material ultimately provides the fuel for galaxy and cluster formation and—because it is much less affected by the complex physics of galaxies—offers a cleaner view of the underlying physical processes of structure formation and of fundamental cosmology. The chapter thus takes up the study on the properties of the IGM, especially during the era of the first galaxies (when the IGM underwent major changes).

This chapter shows that, after cosmological recombination, the Universe had entered the “dark ages,” during which the relic cosmic microwave background (CMB) light from the Big Bang gradually faded away. During this “pregnancy” period (which lasted hundreds of millions of years), the seeds of small density fluctuations planted by inflation in the matter distribution grew until they eventually collapsed to make the first galaxies. In addition to the density evolution, the second key “initial condition” for galaxy formation is the temperature of the hydrogen and helium gas that had likewise collapsed into the first galaxies. Here, the chapter describes the first stages of these processes and introduces the methods conventionally used to describe the fluctuations. It follows the evolution of structure in the linear regime, when the perturbations are small.

The Standard Model works incredibly well but applies to a mere 5% of the content of the Universe; 27% of the Universe is made of a strange type of matter, something absolutely mysterious called dark matter. The remaining 68% of the Universe comes in the form of an enigmatic type of energy called dark energy. Evidence of the existence of dark matter abounds. We detect its presence through its gravitational effects and by means of gravitational lenses. It also plays an essential role in cosmology, acting as a catalyst for the formation of galaxies. Without it, the Universe would not appear as it does today. But dark matter remains elusive. Several experiments are currently in progress deep underground, aboard the International Space Station and at the Large Hadron Collider (LHC) at CERN hoping to catch or produce dark matter particles for the first time.