The universe includes everything, from the smallest subatomic particles to super clusters of galaxies (the most significant structures we know of). No one knows how big the universe is. Astronomers estimate it contains about 100 billion galaxies, each containing an average of 100 billion stars. The Big Bang Theory is the most widely accepted theory of the origin of the universe, and it states that the universe began in a massive explosion—the Big Bang—some 10 to 20 billion years ago. In the beginning, the universe consisted of a hot, dense, glowing ball of expanding, gradually cooling gas. After a million years, the gas probably began to condense into isolated clumps called protogalaxies. The protogalaxies continued to condense over the next five billion years until galaxies formed, in which stars were born. Today, after billions of years, the universe is still expanding, although there are localized regions where objects are held together by gravity; galaxies, for example, form clusters. The Big Bang Theory is supported by discovering faint, cool background radiation scattered evenly in all directions. This radiation is thought to be a remnant (relic) of radiation that was created during the Big Bang. Tiny differences in the temperature of the relic radiation are evidence of weak fluctuations in the density of matter in the early universe, which led to the formation of galaxies. Astronomers still do not know whether the universe is “closed,” whether the expansion will stop and the universe will begin to contract, or whether it is “open” and will continue to expand forever.

Before that, there was nothing, an absolute nothingness that we humans cannot even imagine. A speck of super-dense and unimaginably hot matter exploded in a massive flash of energy that created space. Its expansion continues to this day.

The entire future development of the universe was decided in the first second of its existence. This period, negligibly short by conventional standards, was packed with critical cosmic events:

10 seconds: The process begins. After a brief prologue, the concepts of space and time begin to make sense. At a temperature of 10 degrees, the first significant event occurs in the universe, which is a tiny point measuring 10 centimeters and contains an exotic mixture of constantly appearing and disappearing particles and antiparticles: gravity separates and becomes a separate force. This separation is one of the “phase transitions” in which the forces in the universe gradually “freeze out” from their original unified interaction as the temperature decreases.

10 seconds: Inflation begins. The strong interaction begins to freeze, and quantum bubbles appear in the surrounding vacuum. One of them starts to expand at a tremendous speed. Our visible universe today has the shape of a tennis ball in it. All forces except gravity are unified until the symmetric vacuum suddenly “realizes” that it is unstable and removes excess energy. This creates new particles, and the strong interaction “freezes out.” (Inflation: A quantum bubble creates a unique region in the supercooled universe and expands millions of times faster than the speed of light. At the end of inflation, the excess energy is dissipated into space, which increases the temperature and allows new matter to form.)

10 seconds: Inflation stops. According to the original Big Bang Theory, the universe enters a much slower, unimaginably powerful expansion. There are two types of particles in it: quarks, which sense the strong interaction, and leptons (the lightest particles: electron, positron, neutrino, and antineutrino), which sense the previously discerned electroweak interaction.

10 seconds: electroweak interaction splits. The temperature has dropped to 10 degrees, representing another “freezing point.” The electroweak interaction splits into a separate electromagnetic force and a weak interaction in the process of symmetry breaking. The carriers of the weak interaction – the W and Z particles – become heavy, while the carrier of electromagnetism, the photon, has zero mass.

10 seconds: Quarks disappear. Quarks and antiquarks have been moving freely through space until this point, creating, annihilating, and interacting with other particles. After the universe has cooled to 10 degrees, there is no longer enough energy for quarks to form freely. The pairs that have existed so far continue to annihilate, and it looks like quarks will disappear forever.

10 seconds: Baryons are formed. The universe has expanded to about the size of our solar system. As the temperature drops, annihilation stops, and the remaining quarks combine to form protons and neutrons. (baryons: collective name for nucleons – the proton and neutron in the nucleus of an atom)

1 second: Neutrino escape. Neutrinos, which are only affected by the weak interaction, have been very active up to this point. However, at the end of the first second, the interaction is so weak that it has almost no power over the neutrinos, and the neutrinos fly freely. They are still in the universe today. (neutrino: an electrically uncharged elementary particle of matter with no magnetic moment)

100 seconds: The first elements. Protons and neutrons react together to form helium nuclei. Nothing interesting happens for the next 100,000 years or so. Hydrogen, helium, and a tiny amount of other light nuclei, mixed with electrons and radiation, gradually cool to the temperature of red-hot iron in a blast furnace.

300,000 years: The universe becomes brighter. Electrons begin to bind to nuclei. The first atoms are formed. The radiation no longer has enough power to break atoms apart and is not absorbed. The universe becomes transparent and filled with light.

1 billion years. The first galaxies form, and the universe begins to look familiar.

15 billion years. The universe today – as we know it on cosmic and atomic scales.