What’s beyond us in the Universe? What is what surrounds us made of? These are questions that are difficult to answer. Let’s find out why!

What is the Universe made of? The ancient Greek philosopher Democritus was among the first to ask himself this question and also to give himself an answer. According to Democritus, reality is composed of atoms, elementary particles, indivisible and marked by an eternal movement that characterizes them. This motion leads these infinitesimal entities to aggregate, giving rise to processes of creation, destruction and growth, which are the basis of everything that exists. The surrounding reality is therefore made of atoms and its variety is due to the different forms that these particles can take and the multiple positions that they occupy when they form the matter. Democritical atomism was successful and was resumed by both the Greek thinker Epicurus, founder of the Greek school that bears his name, and the Latin poet Lucretius.

Lithium atom: in red the protons, in blue the neutrons and in gray the orbiting electrons.

The atomic theory

The revolutionary intuition of Democritus has been confirmed and flipped at the same time by the modern atomic theory. Nowadays it is immediate to affirm that matter is made of atoms, but they are not elementary particles, as they have their own internal structure.

The study of the atom has led to assert that every atom has a nucleus, made of protons and neutrons, around which electrons orbit, in the same way that the Earth orbits the Sun in the Solar System. Although there is an analogy in the type of motion that describes a planetary system and an atomic system, these differ in the type of interaction that characterizes them. This is because the Earth and the Sun interact by gravitational attraction, while the electron interacts with the nucleus through forces of electromagnetic nature.

To explain this point, it is important to highlight that protons, neutrons and electrons are very different particles. Protons and neutrons have similar masses but much greater than the electron, which is a light particle. Neutrons are neutral, as the name suggests, while protons have a net positive charge, opposite to the one of the electron, which is negatively charged. Charge is exactly the physical quantity responsible for electromagnetic interaction between particles.

The discussion on the concept of elementarity could go on, because protons and neutrons are not elementary themselves. In fact, they are constituted by the so called quarks, even smaller particles that are part of the Standard Model of Elementary Particles, the finest description of reality obtained so far.

Stopping at the level of nuclei and electrons, without going into the quark model, we can answer more accurately the starting question. With various combinations of protons, neutrons and electrons we obtain a plethora of elements. Hydrogen consists of one proton and one electron. Helium consists of two protons, two neutrons and two electrons. Lithium consists of three protons, three neutrons, and three electrons. And so on we have heavier and heavier elements, such as oganesson, which has 118 protons! To give order to this zoo of chemical elements, the well known Periodic Table of Elements was created, in which the elements are arranged in order according to their number of protons, electrons and according to their chemical properties.

This table represents a formidable description of nature, as every existing material can be divided into fundamental constituents that belong to it, and that’s not all.

The matter we have discussed so far is called ordinary matter, or baryonic, and it represents only 4.6% of what constitutes the Universe. The remaining part is called dark material and is divided into Dark Energy and Dark Matter.

Pie chart of the Universe today (above) and 13.7 billion years ago (below). Credits: Wikipedia.

Dark Matter and Dark Energy

Dark matter constitutes 24% of our Universe and bears this name because it does not reflect or emit light or other forms of electromagnetic radiation. Its presence is experimentally verified because of the gravitational effects it exerts on visible matter. It was astronomers who verified its existence by studying the rotation speed of galaxies. In fact, in the late ’60s astronomer Vera Rubin showed that most of the stars of spiral galaxies orbit at about the same speed and this implies the existence of non-visible matter, namely dark. The question arises: what is dark matter made of? The hypothesized candidates have exotic names, including neutralinos, elementary particles theorized, but not yet discovered, similar in some sense to neutrinos, or gravitons, which are the particles responsible for gravity.

In addition to these new forms of matter, other plausible hypotheses present brown dwarfs and supermassive black holes as candidates. Brown dwarfs are stars with enough mass that they do not trigger the fusion reaction of the hydrogen inside them and thus cool indefinitely. If they were part of the dark matter, then baryonic matter would deserve a larger slice of the Universe’s pie. Supermassive black holes have a mass billions of times that of the Sun and are assumed to be at the center of many galaxies.

They are the result of powerful gravitational collapses in the final stage of the stars’ lives.

Dark energy constitutes 71.4% of our Universe. The proof of its existence came in 1998 during the observation of distant supernovae, or explosions of stars. Its presence is necessary to justify the increasingly rapid expansion of our Universe, but beyond that it is a real mystery. Some hypothesis argue that it will lead to the extinction of the Universe in the so-called Big Rip, which represents the ultimate fate of the Universe, in which atoms are disintegrated and the Universe is reduced to a series of elementary particles separated from each other, without the possibility of interaction. Despite this end may seem catastrophic, the British physicist Roger Penrose states that the dead Universe could give rise to a new Big Bang and that the current Universe is just one of many universes that follow one another.

Paraphrasing the words of Freeman and McNamara, we are not essential to the Universe and we are not even made of most of its own matter. In fact, we are made of the smallest part of what constitutes it. The Universe is made of darkness.

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