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   History of the Universe according to the Big Bang model

Published on 18 October 2017
					Today, the Universe is composed of stars, galaxies, clusters of galaxies, and immense voids. Yet, when it was born, about 13 billion years ago, matter was homogeneously distributed. The Big Bang model is used to describe the origin and evolution of the Universe. According to this model, one of the main forces behind the structure of our cosmos is due to dark matter.

About 13 billion years ago, the Big Bang shocks the Universe, transforming energy into matter. This was just the start of its expansion and its temperature is constantly decreasing from that time on.

During its early stages, the Universe looked like an extremely hot and dense soup of light, matter and dark matter particles.  These particles interacted with each other, colliding and creating new particles. No planets, stars or galaxies were formed at this time, not yet. 

The temperature drops down quickly and particles lose energy. While the matter and light particles continue to collide, dark matter particles stop colliding with matter and stop interacting with each other. This phase is referred to as dark matter freeze-out. 

No dark matter particles were created from now on. The gravitational forces of these particles have structured the Universe up to present days.
In terms of matter, quarks, some of the smallest known elementary particles, combined in twos or threes to form mostly protons or neutrons.

Three to 20 minutes after the Big Bang, the temperature continued to drop down so that protons and neutrons could combine to form the first nuclei of hydrogen, helium and lithium. Light nuclei ceased to form when the temperature decreases below a billionth of a degree: the primordial nucleosynthesis was over.

380,000 years later, electrons are slow enough to combine with the nuclei and form the first atoms. Once captured, the electrons no longer interact with the light, which could then travel freely. These light particles are the oldest markers of the Universe detectable today. They  are known as the Cosmic Microwave Background. The different colours on the image represent very small fluctuations of the density of matter. Dark matter could provide an explanation for these contrasts.  The quantity of dark matter in the Universe has been calculated using such images.

The creation of the first atoms was followed by a long period referred as to the Dark Ages. Dark matter dominate during this period and distort space by gravitation, creating holes. Atoms and light were drawn into these holes. Large structures started to form at this time.

100 million years after the Big Bang, the required conditions are met to allow stars to form. The birth of the first stars marked the end of the Dark Ages. The successive fusion of light nuclei in the cores of stars led to the formation of heavier nuclei such as carbon, nitrogen and oxygen: this process is labelled as stellar nucleosynthesis. 

The stellar population of the Universe gradually increases and the stars moved towards each other by gravitation. Galaxies then combined to form the largest known structures of the Universe: galaxy clusters and superclusters. The large structures found in our Universe were shaped by dark matter. The expansion of the Universe has been accelerating since the last 6 billion years. This effect is caused by a new and unknown force termed dark energy. 

Dark energy and dark matter are currently key ingredients in the Big Bang model: they are believed to represent 95% of our Universe. Research missions such as Euclid aim to further characterise these ingredients. This research is at the heart of fundamental physics and could revolutionise our understanding of the Universe.

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