Absolute magnitude: the magnitude that a star or any other astrophysical object would have when placed at the standard distance of 10 parsecs.

Annihilation: Physical process in which a subatomic particle collides with its respective antiparticle, producing energy that can be transformed in other particles.

Axion: Hypothetical particle postulated to solve a theoretical problem in particle physics known as the strong CP problem.

Baryon: A baryon is an elementary particle made up of three quarks. Typical examples of baryons are the neutrons and protons contained in atomic nuclei. In neutral atoms, the number of protons in the nucleus is equal to the number of electrons around it, but the electrons are 1,000 times lighter than protons, so most of the atomic mass comes from neutrons and protons. That’s why normal matter, made of atoms, is also referred to as ‘baryonic’.

Cherenkov radiation or Cherenkov light: electromagnetic radiation emitted when a charged particle passes through a medium at a speed greater than the speed of light in that medium.

Cluster of galaxies: an agglomeration of thousands of galaxies bound together by gravity.

Constellation: area of the celestial sphere that take the name from imaginary figures identified in ancient times in star patterns produced by the accidental alignment of stars.

Cold dark matter: a type of dark matter (see ‘dark matter’) that was moving at much less than the speed of light when structures in the Universe started to form, 100,000 years after the Big Bang.

CMB: Cosmic Microwave Background radiation, is the radiation left over from the Big Bang. Due to the expansion of the Universe, it has cooled to a temperature which today is slightly less than 3 degrees above absolute zero.

Cosmological constant: a quantity introduced by Einstein in his theory of General Relativity, originally to balance the expansion the Universe, and achieve a static universe. It was abandoned by Einstein when the expansion of the Universe was discovered, and then reintroduced in modern times, to explain the observed acceleration of the expansion of the Universe.

Dark energy: a hypothetical form of energy introduced in modern cosmology to explain the observed accelerated expansion of the universe. In its most simple form, its effects are identical to those produced by a cosmological constant, but it .

Dark matter: A mysterious form of matter that we have so far observed through its gravitational influence on visible matter. It is approximately 5 times more abundant than, and fundamentally different from, any form of matter we ever studied in our laboratories.

Electromagnetic force: one of the four fundamental forces in nature. Electromagnetic interactions are responsible for the interactions between the electrons and nuclei in atoms, and between atoms in molecules.

Electroweak interactions: a unified force that combines the electromagnetic and weak nuclear interactions, and is today part of the experimentally verified Standard Model of Particle Physics.

Fine structure constant: usually denoted as α, it is a dimensionless quantity that gives the strength of the electromagnetic interaction.

Energy: in particle physics, the energy of a massive particle is given by a kinetic energy, that depends on its speed, and by a rest energy, that depends only on the mass of the particle. Mass and energy are both but different manifestation of the same thing, following the famous discovery by Einstein, encoded in the equation E=mc^2.

Flux: number of particles or quantity of energy flowing through the unit area in the unit time. For example, we measure a flux of neutrinos from the Sun equal to 70 billion particles per centimeter square per second.

Galaxy: A collection of stars, gas, dust and dark matter, bound by gravity and having a mass ranging from 100,000 to 10 trillion times that of the Sun. The galaxy we live in is called Milky Way, and it is also referred to as the Galaxy (with a capital G).

Gamma-ray: a very high energy photon, with an energy larger than the mass of a proton.

Gravitational Potential: the gravitational energy, i.e. the energy arising from the gravitational influence of an astronomical body, per unit particle mass. For an object near the surface of the Earth, it is equivalent to the acceleration of gravity times the height.

Higgs Boson: elementary particle of the Standard Model of Particle physics and ‘quantum’, i.e. smallest excitation, of the Higgs field. The interaction of some other elementary particles with the Higgs field sets their mass.

Homogeneous: the same at all locations.

Horizon: in Cosmology, the edge of the visible Universe (not of the actual Universe, which might be even infinite). in Black Hole physics, the membrane that separates the inner region of the Black Hole, from which nothing, not even light, can escape, from the outside region.

Inflation: a hypothetical period of rapid accelerated expansion in the earliest instants of existence of the Universe.

Isotope: variant of a particular chemical element with the same number of protons and different number of neutrons in the atomic nucleus.

Isotropic: the same in all directions.

Kelvin degree: a unit of temperature, same size as the Celsius degree, but with the zero of its scale at the minimum possible temperature, also called Absolute Zero. The Kelvin temperature is 273.15 degrees bigger than the Celsius temperature.

Large Hadron Collider: the world’s largest and most powerful particle accelerator. It consists of a 27-kilometre ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way. Inside the accelerator, two high-energy particle beams of protons are brought close to the speed of light before they are made to collide.

Milky Way: the galaxy that contains the Solar System, also referred to as the Galaxy, with a capital ‘G’.

Muon: elementary particle similar to the electron, but with a 200 times greater mass.

Neutralino: a particle predicted by the theory of supersymmetry. It is in many realizations of supersymmetry the lightest among the predicted new particles, and a very good dark matter candidate. It is a prototypical example of Weakly Interacting Massive Particle, or WIMP.

Neutrino: electrically neutral, weakly interacting elementary particle with a mass more than a million times smaller than that of an electron.

Neutron: Composite particle found in the nucleus of atoms, made of one ‘up’ quark and two ‘down’ quarks (see ‘quarks’).

Non-baryonic: not made up of neutrons, protons and electrons, therefore different from any form of matter we are familiar with.

Nucleon: one of the particles inside an atomic nucleus, i.e. a neutron or a proton.

Parsec: unit of distance used by astronomers, corresponding to to 30,856 billion kilometers, or 3.26 light-years. One kiloparsec corresponds to 1000 parsecs; one Megaparsec to 1 million parsecs.

Proton: Composite particle found in the nucleus of atoms, made of two ‘up’ quarks and one ‘down’ quark (see below).

Quark: elementary particles that constitute in particular the fundamental building blocks of atomic nuclei. There are six types – called flavors – of quarks: up, down, charm, strange, top and bottom. The proton is made of two ‘up’ quarks and one ‘down’ quark: (uud), while the neutron is (udd).

Redshift: in Cosmology, redshift quantifies the decrease in energy of the light produced by distant object due to the expansion of the universe. Objects with high redshift are far away, and we see them as they were a long time ago.

Rotation Curve: plot of orbital velocities of visible stars or gas in a spiral galaxy as a function of their radial distance from that galaxy’s centre.

WIMP: a Weakly Interacting Massive Particle, a possible form for cold dark matter.