Setting up the context for future related posts
Welcome to the first post in this blog! I hope to be posting about a variety of topics here, and some of them will be unconnected to each other. However, some posts will have some common link to cosmology as a context, because that is the field I work on. Therefore, I thought it would be appropriate to start by a brief descrition of cosmology as a field of study in physics. Later on, I will also introduce galaxy surveys, which is even closer to what I actually do, but let’s start by the basics and the big picture.
Cosmology is the branch of physics that studies the origin, evolution and structure of the Universe as a whole, and it was mainly developed in the twentieth century, definitely moving from philosophy to science. Modern cosmology is based on the belief that the place we occupy in the Universe is not special, statement that we know as the cosmological principle. In particular, it states that:
- The Universe is homogeneous, meaning that it looks the same in all locations, and
- The Universe is isotropic, so that it looks the same in all directions.
Needless to say, the cosmological principle is not exact, but an approximation that holds better and better the larger the length scales we consider.
Before 1900, the Universe was considered a static and inalterable system, due to the appearance of the dark sky and all far-away celestial objects occupying fixed angular positions on it. Together with the basis established by the cosmological principle, the evolution of the Universe as a whole should be governed by the laws of gravity, as the other fundamental forces do not play a role on such large scales. In 1915, Einstein presented his theory of General Relativity (GR), the most accurate description of gravity, represented in the theory as the consequence of the curvature of spacetime produced by the distribution of mass and energy in the Universe.
Shortly after these theoretical developments, in 1929, Edwin Hubble measured the motion of various galaxies along the line of sight using the 100-inch Hooker telescope at Mount Wilson Observatory and he found that most of these galaxies were indeed receding, the faster the further they were from us. It was therefore realized that the Universe was actually not static, but expanding. This observational breakthrough eventually led to the development of the Big Bang theory, which describes the Universe as expanding from an initial very high density and high temperature state. The Big Bang theory still remains the basis of today’s Standard Cosmological Model, supported by other crucial observations like the abundance of primordial elements and the measurements of the relic light from the early Universe, the Cosmic Microwave Background radiation.
Within the framework of GR, which naturally accomodates (or even predicts!) a dynamical Universe, the expansion rate of the Universe depends on its energy content in a way that, for instance, a universe containing only matter should eventually slow down due to the attractive force of gravity. However, in 1998, observations of type Ia supernovae (SNe) at distances up to about 6 bilion light years by two independent research groups, led by Saul Perlmutter and by Brian Schmidt and Adam Riess respectively, revealed that presently the expansion is instead accelerating (Perlmutter et al., 1999; Riess et al., 1998).
This acceleration of the expansion is attributed in the Standard Cosmological Model to the presence of an unknown kind of fluid, possibly related to a cosmological constant, that we call dark energy (DE). So far, we do not know what is the nature of this so-called dark energy that makes the expansion of the Universe to accelerate. Moreover, current measurements of the energy content of the Universe show that DE accounts for about 70% of the total energy density of the Universe. Of the remainder, more than 25% is due to an unknown form of matter (called dark matter, DM) and only less than 5% of the energy density corresponds to ordinary matter like protons, electrons and neutrinos (see e. g. Planck 2018 results).
In summary, the Standard Cosmological Model appears to be robust and simple, but only with the addition of two components of unknown nature and origin: dark matter and dark energy. Furthermore, these two components constitute the vast majority of the energy content in the Universe. For that reason, it is of capital importance to understand these components in depth, as they may be holding the key to the discovery of new physics beyond the standard cosmological model and the standard model of particle physics.