Dark Energy: The Most Mysterious Force in the Universe

A Discovery That Revolutionized Cosmology

At the end of the 20th century, cosmologists believed that the expansion of the Universe, initiated by the Big Bang, should slow down due to gravity. Two research teams, one led by Saul Perlmutter (1959-), the other by Brian Schmidt (1967-) and Adam Riess (1969-), made a stunning discovery in 1998. By observing Type Ia supernovae (very bright star explosions used as 'standard candles' to measure cosmic distances), they found that the light from these explosions was fainter than expected. This faintness meant they were farther away than predicted by models of a slowing expansion. The conclusion was revolutionary: the expansion of the Universe is not slowing down, it is accelerating.

This acceleration implies the existence of an unknown repulsive force, acting on a large scale against gravity. Scientists named this mysterious force "dark energy." Although counterintuitive, this concept is now the cornerstone of the standard model of cosmology, the \(\Lambda\)CDM (Lambda Cold Dark Matter) model, where Lambda (\(\Lambda\)) represents dark energy.

What Does Dark Energy Represent in the Cosmos?

According to the most precise data from the European Space Agency's (ESA) Planck satellite, published in 2018, the content of the Universe breaks down as follows:

• Dark energy: 68.3%, The mysterious force accelerating expansion.
• Dark matter: 26.8%, An invisible form of matter that does not interact with light but whose gravitational influence is detected.
• Ordinary (baryonic) matter: 4.9%, The matter that makes up stars, planets, and everything we know.

The dominant share of dark energy remains one of the greatest mysteries of modern physics. The most accepted current theory, though unconfirmed, is that it is a cosmological constant, a fixed property of empty space. But for many scientists, the idea that 68% of the Universe is a mysterious "constant" is deeply unsatisfying. The idea of constant dark energy is increasingly challenged by recent observations suggesting possible evolution.

How Do We Measure the Content of the Universe?

It is impossible to "weigh" the Universe directly. So how do scientists know that about 68% of its content is dark energy?

The key lies in dark matter. Although invisible, it exerts a measurable gravitational influence: it acts as a cosmic glue that keeps galaxies coherent. By observing the rotational speed of stars around their galaxy, astronomers can estimate the total mass-energy present. The result is striking: the detected mass-energy far exceeds that of visible matter. Thus, observable baryonic matter represents only a tiny fraction of the total energy contained in the Universe.

To complete the picture, scientists turn to the expansion of the Universe, measured using supernovae, and the study of the cosmic microwave background observed by the Planck satellite. Even by combining all matter, visible and dark, we only reach about 32% of the energy needed to explain cosmic acceleration. The remaining 68% corresponds to dark energy, which contributes to the acceleration of expansion as a gravitationally active form of energy.

How to Explain This Repulsive Force?

The acceleration of the Universe's expansion suggests the existence of a repulsive force that partially counteracts gravity. This "cosmic force" is not directly observable, but its effects are seen in the movement of galaxies and the cosmic microwave background. Scientists have developed several hypotheses to understand it, each based on different physical principles.

N.B.:
All these theories seek to account for the same observation: the accelerated expansion of the Universe. None is yet definitively confirmed, but the cosmological constant Λ remains the simplest and most used in current models.

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