What is and what could be dark energy?

There is no doubt in the science community, the expansion of the universe is accelerating! The reason behind this acceleration is one of the most challenging contemporary puzzles, and this is where the concept of dark energy comes in. Accounting for 68% of the energy in the universe, dark energy is at the same time responsible for the acceleration of the expansion of the universe. However, the crucial question remains: What could dark energy be?

To understand dark energy, let’s first take a first look at the Universe’s history described by the Big Bang theory initially proposed by Georges Lemaître, and confirmed by Edwin Hubble. In its early stages, the universe was an extremely hot and dense mixture of particles and light. With its expansion, it finally cooled down and was then able to form atoms, stars and galaxies to become what we observe today. Early models of the universe’s mass content suggested that the expansion should slowing down due to the gravitational pull of matter. However, in 1998, two independent teams – the Supernova Cosmology project and the High-Z supernovae search team, in which 2011 Nobel Prize winners Saul Perlmutter, Brian Paul Schmidt, and Adam Riess are members – discovered that the expansion was not at all slowing down, but rather accelerating!

Courtesy NASA/JPL-Caltech.

Courtesy NASA/JPL-Caltech.

Something is pushing the universe to expand faster and faster but does not seem to be directly detectable by our telescopes or particle detectors. Its presence can only be inferred from the observation of the large-scale structure and its dynamics. This led scientists to propose the existence of “dark energy”. Since then the suspect has had a name, however its nature, origin and composition remain mysterious.

The most prevalent hypothesis is that of a cosmological constant, also referred to as vacuum energy. Initially proposed by Albert Einstein as part of its equations of general relativity, it never really fit in at the beginning. While at the time it was thought that the universe might be static, the cosmological constant was then abandoned with the discovery of the expansion of the universe. Today, the cosmological constant resurfaces to potentially explain dark energy. This hypothesis states that space is not really empty, but rather contains a constant energy density in every region of space that drives the accelerated expansion of the universe. This implies that space’s energy content remains the same over time and across different regions of the universe, and does not vary with the expansion of the universe nor with the presence of matter or radiation.

However, this concept just by itself cannot fully explain the observations. Indeed, the predicted value of the cosmological constant based on quantum field theory is around 120 orders of magnitude larger than the observed value! This large deviation between theory and observation is currently unexplained and constitutes a fine-tuning problem, raising questions about the underlying physical principles. In addition, the timing of the accelerated expansion raises a question. In the cosmological model – Lambda Cold dark matter model –  dark energy only dominates the energy content of the universe in recent cosmic history, which also happens to correspond to the epoch when matter density and dark energy density became comparable. Is this a coincidence? This definitely requires more investigations – scientists do not like coincidences.

Another hypothesis that could explain the nature of dark energy is related to a new type of field or particle that yet needs to be discovered. To compensate for the attractive force of gravitation and to bring the universe to expand at an accelerated rate, this new fundamental force or quintessence will have to be repulsive.

The third main hypothesis for dark energy is to question Einstein theory of general relativity. What if gravity behaved differently at large scale?

Despite its mysterious nature, dark energy is estimated to account for 68% of the total energy content of the universe, with dark matter accounting for roughly 27%, and ordinary matter making up just 5%. In other words, dark energy currently – and in the future – plays a dominant role in shaping the fate of the Universe!

Credits: ESA

The mysterious nature of dark energy leaves scientists with an exciting and open field of research. Some theories suggest that dark energy might not be constant over time, this would have profound implications for our understanding of the Universe’s past and future. New discoveries in fundamental physics could also shed light on the true nature of dark energy. Understanding dark energy is not only a fundamental puzzle in cosmology but also could help us understand the universe’s ultimate fate. Will the expansion continue indefinitely, or will dark energy cause a “Big Rip” where it becomes so dominant that it tears apart galaxies, stars, and even atoms?

So, what could dark energy be? Well, that’s a question that scientists are actively exploring with Euclid by looking at the large-scale structure of the universe. They will map the distribution of galaxies and measure their properties and directly the universe’s expansion history across cosmic history.

While we still have much to learn, the quest to unravel the mysteries of dark energy, its mysterious properties and its influence on the cosmos pose an exciting area of research for scientists of the Euclid Consortium.

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