Dark matter and dark energy are two of the most important enigmas of modern cosmology. Dominating the Universe, these two cosmic components still elude scientists. If the latter wonder about their nature, their origin is also the subject of active research.
The various observation missions carried out in recent years have made it possible to estimate that dark matter represents around 27% of the total energy density of the universe, and dark energy 68%. However, these two cosmic components remain hypothetical and still elude scientists' instruments. If the question of their nature is in the foreground, that of their origin is also essential.
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Even though there is still very little information about these two elusive elements, that does not mean that cosmologists are completely ignorant of them. Theoretical models, supplemented by certain observations, show for example that the effect of dark energy on the expansion of the Universe only began to be significant between 6 and 9 billion years ago.
Dark energy appears to affect expansion in all directions, possess a constant energy density, and be homogeneous. Without the dark energy hypothesis, it seems difficult today to account for the acceleration of the expansion of the Universe observed in 1998.
Dark matter, on the other hand, seems to have been showing its effects for 13.8 billion years. The cosmic structure it forms requires five times the abundance of baryonic matter.
Unlike dark energy, dark matter interacts with gravity and plays a role in the formation of quasars, galaxies, and the large gas clouds that populate the cosmos. According to the standard model of cosmology (L-CDM model), it would be present in the form of halos and galactic filaments. The primitive gravitational imprints of dark matter can be observed in the structure of the cosmic microwave background.
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Although traces and clues of these two components can be found throughout the history of the Universe, this does not mean that they were necessarily created during the Big Bang. Searching for the exact origin of matter and dark energy is all the more difficult since, at certain times in the Universe, the effects of these may have been obscured by other phenomena of greater intensity.
This is especially true for dark energy. As the Universe expands under the effect of expansion, its volume increases but the number of particles remains constant; the density of baryonic matter and dark matter therefore gradually decreases, it is diluted by expansion. As for radiation, its density drops even faster because the wavelength of the photons is stretched by the dilation of space (redshift). The density of dark energy, on the other hand, remains constant.
Today, the Universe is dominated by dark energy. But in the past, the Universe was smaller and denser, so matter and radiation densities were greater. About 6 billion years ago, the densities of matter and dark energy were identical.
Going back further, around 9 billion years ago, the contribution of dark energy to the energy density of the Universe is very small, its effects are negligible and it therefore becomes difficult to reconstruct its dynamics.
Current observations reveal an absolutely constant dark energy density. The equation of state of dark energy, constrained by the observation of the acoustic oscillation of baryons, shows that the quantity associated with dark energy is constant. However, this does not mean that this energy density has always been constant. It could have changed over time, since this modification took place with the variation of the other cosmological parameters.
Dark energy could thus exist since the inflationary period that took place a few moments after the Big Bang, at the end of the Planck era. This is the hypothesis proposed by the quintessence model.
But dark energy could also just as easily have emerged much later in the history of the Universe. Currently, no observation allows us to decide on the presence or absence of dark energy during the first 4 billion years of the Universe.
However, the period of appearance of dark matter is better constrained. The pattern of fluctuations observed in the cosmic microwave background is the oldest evidence of the presence of dark matter, 380,000 years after the Big Bang.
The angular amplitude of these fluctuations shows a 5:1 ratio in favor of dark matter over ordinary matter. In the L-CDM model, dark matter constitutes the cosmic cement allowing the formation of large structures from the young Universe.
However, again, this does not necessarily indicate a creation of dark matter at the time of the Big Bang. Several post-Big Bang hypotheses exist on this subject. It could have been created by very high energy interactions after inflation; by the disintegration of ultra-energetic particles within the framework of a grand unification theory (GUT); by the spontaneous breaking of any symmetry as proposed by the Peccei-Quinn model; by the oscillation of massive sterile neutrinos.
The ignorance of the exact nature of dark matter makes it impossible to determine its origin with certainty. Nevertheless, the various observations show that it has existed since, at least, the very first moments of the Universe.
Dark energy may have appeared at the same time, or later. Some hypotheses hold that dark energy only appears when large cosmic structures have already formed, implying post-dark matter emergence.
Dark matter is therefore known to exist and act since the very first moments of the Universe, if not during the Big Bang itself. While dark energy is also said to have existed from the beginning but only started showing its effects later in the history of the Universe. In any case, these questions will be the next great challenges of cosmology.
