Abstract
The development of methods and algorithms to solve the N-body problem for classical, collisionless, non-relativistic particles has made it possible to follow the growth and evolution of cosmic dark matter structures over most of the universe’s history. In the best-studied case—the cold dark matter or CDM model—the dark matter is assumed to consist of elementary particles that had negligible thermal velocities at early times. Progress over the past three decades has led to a nearly complete description of the assembly, structure, and spatial distribution of dark matter haloes, and their substructure in this model, over almost the entire mass range of astronomical objects. On scales of galaxies and above, predictions from this standard CDM model have been shown to provide a remarkably good match to a wide variety of astronomical data over a large range of epochs, from the temperature structure of the cosmic background radiation to the large-scale distribution of galaxies. The frontier in this field has shifted to the relatively unexplored subgalactic scales, the domain of the central regions of massive haloes, and that of low-mass haloes and subhaloes, where potentially fundamental questions remain. Answering them may require: (i) the effect of known but uncertain baryonic processes (involving gas and stars), and/or (ii) alternative models with new dark matter physics. Here we present a review of the field, focusing on our current understanding of dark matter structure from N-body simulations and on the challenges ahead.
Highlights
No account of the properties of haloes based purely on gravitational dynamics can be complete since baryonic processes play a significant role in galaxy formation, and new dark matter physics could do so, we focus on the standard cold dark matter” (CDM) paradigm of structure formation in part because the subfield of cosmological N-body simulations has historically been developed in this context, and because the emergence and properties of dark matter structures are most understood in the context of CDM
A theory of structure formation aims to explain the evolution of the universe from a nearly homogeneous initial state, with tiny matter density perturbations, δρ/ρ, seeded by inflation, which grow to leave an imprint on the cosmic background radiation (CMB), through the emergence of the self-gravitating dark matter haloes where galaxies form, to the universe we observe today characterized by a web of filamentary large-scale structure
In the case of classical, non-relativistic, collisionless particles, i.e., CDM, N-body simulations follow the evolution of the dark matter phase-space distribution function, f (x, v; t), which in principle is given by the collisionless Boltzmann equation coupled with the Poisson equation for the gravitational field, Φ(x): df dt
Summary
The current theory of the formation and evolution of cosmic structure in the universe is based on the dark matter hypothesis in which ∼84% of the mass-energy density of the universe [1] is in the form of a new type of particle, or particles, with negligible electromagnetic interactions. In addition to the dark matter hypothesis, the standard theory of structure formation makes a specific assumption about the nature of dark matter, which is only partially supported by observations This is that the dark matter consists of classical, non-relativistic, collisionless particles which had negligible thermal velocities at early times. The combination of the WIMP miracle with the success of the CDM model in explaining the observed large-scale structure of the universe in the mid-1980s [14] established the current paradigm of structure formation in which gravity is the only dark matter interaction This model has been widely adopted by the community working on galaxy formation and evolution and, as a result, most of our understanding on how cosmic structure emerges comes from studies that assume the CDM model. In various parts of this review, we will explore how different assumptions for the nature of dark matter can lead to different predictions from CDM
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