# Terminology

Here we describe briefly some concepts that non-cosmologists may not be familiar with.

Dark matter. All matter we know (atoms, molecules, quarks…etc) only represent about 15% of the matter present in the Universe. The other 85% is called dark matter, and it is believed to only be subject to, and interact with other types of matter through, the force of gravity but not other forces. The effects of dark matter has been observed in many different systems, but the nature and properties of it is still a mystery.

Total matter. We define total matter as the sum of mass in dark matter, gas, stars, and black holes.

Dark energy. The Universe is currently accelerating its expansion. The substance responsible for that behaviour is called dark energy, and its nature and properties are one of the biggest mysteries in modern physics. Furthermore, dark energy represents around 70% of the full mass-energy content of the Universe. Learning more about dark energy is one of the main goals of modern cosmology.

Cosmological parameters. Parameters describing fundamental properties of the Universe such as its age, geometry, composition…etc. In CMD, we only consider two of them: $$\Omega_{\rm m}$$ and $$\sigma_8$$.

$$\Omega_{\rm m}$$: The fraction of the mass-energy density in the Universe in the form of total matter. Higher values of this parameter indicate that the fraction of dark matter mass plus gas mass plus stars mass plus black holes mass is higher. Since we are considering the Universe to be flat, a higher value of $$\Omega_{\rm m}$$ will imply a lower fraction of the mass-energy content of the Universe in the form of dark energy.

$$\sigma_8$$: The variance of the total matter density field, or the amplitude of its fluctuations. This parameter quantifies how clustered the considered field is. For instance, a very homogeneous and isotropic field will have a low value of $$\sigma_8$$, while if the fields exhibit large variations (e.g. some regions with very high and others with very low concentration of matter), this value will be larger.

Astrophysical parameters. Parameters describing astrophysical effects such as supernova and active galactic nucleus (AGN) feedback.

Supernova feedback. This refers to the energy and momentum released by supernova explosions to their surroundings.

AGN feedback. This refers to the energy and momentum released by active galactic nuclei (powered by supermassive black holes) to their surroundings.

Astrophysical effects. Also called baryonic effects, they refer to astrophysical processes such as supernova explosions or feedback from black holes. The physics of these physical processes is poorly known, but it is known that they can affect the properties of gas, dark matter, stars, and black holes on small scales.

Megaparsec. 1 megaparsec correspond to 1 million parsec. 1 parsec is 3.26 light years, i.e. the distance traveled by light over 3.26 years: 30.9 trillion kilometers or 19.2 trillion miles.

Redshift. In cosmology, redshift is commonly used as a measure of time. Redshift 0 corresponds to current time, while higher redshifts correspond to more distant times in the past. For instance, redshift 1 corresponds to around 8 billion years ago, or the epoch when the Universe was approximately half as old as it is today.

Subgrid physics. Cosmological hydrodynamic simulations sample very large cosmological distances of tens or hundreds of millions of light years. Unfortunately, astrophysical processes such as the formation of stars and black holes, supernova explosions, feedback from black holes…etc happen on much smaller scales that cannot be resolved in these simulations due to the very large dynamic range. In this case, these astrophysical processes are modelled in a phenomenological manner aimed at mimic the physics of these processes; this is called subgrid physics.

IllustrisTNG. This denotes data from cosmological magneto-hydrodynamic simulations run with the AREPO code (a code used to solve the gravity plus magneto-hydrodynamics equations), employing the same subgrid physics as the IllustrisTNG subgrid model, up to variations of the astrophysical parameters.

SIMBA. This denotes data from cosmological hydrodynamic simulations run with the GIZMO code (a code used to solve the gravity plus hydrodynamics equations), employing the same subgrid physics as the SIMBA subgrid model, up to variations of the astrophysical parameters.

Metallicity. In astronomy, all elements other than hydrogen and helium are called metals. The metallicity of a gas cloud, a galaxy or a star is the fraction of mass in metals in that system.

Magnesium over iron. Cosmic gas, stars and galaxies contain a fraction of their total mass in different elements, such as carbon, oxigen, magnesium, iron…etc. The ratio between the magnesium and iron, Mg/Fe, is an interesting quantity from the point of view of astrophysics. In CMD, we use it to see if we can extract cosmological information from it.