Since the end of the 1990s, contemporary astronomy has revealed an unsuspected facet of our cosmos: a mysterious force, dubbed dark energy, which represents about 70% of the total energy density of the Universe and is responsible for the acceleration of its expansion. This phenomenon is intimately linked to the cosmological constant, a notion introduced by Albert Einstein in the early 20th century within the framework of general relativity theory. Dark energy and the cosmological constant thus reveal a complex cosmic dynamic, challenging our classical understanding of gravitation and cosmological evolution. Significant observations, particularly those from the Planck telescope or recent spectroscopic instruments like DESI, are renewing debates about the nature, constancy, and even the existence of this mysterious energy. It is a dive into the depths of space-time, rich in astrophysical stakes and theoretical potential that opens up to us today.
In short :
- The cosmological constant was added by Einstein to stabilize a supposedly static universe, before being abandoned and then rehabilitated in the context of the Universe’s accelerated expansion.
- Dark energy exerts negative pressure and acts as a repulsive force, contributing to the currently observed cosmic acceleration.
- The exact nature of dark energy remains enigmatic, with hypotheses ranging from quantum vacuum energy to more exotic scenarios like antigravity associated with antimatter.
- Recent measurements, particularly those conducted with DESI, suggest that the cosmological constant may not be constant over time, challenging the classical ΛCDM model.
- Tensions between theoretical predictions and observations reinforce the need to precisely model the energy density of the Universe to understand its past, present, and future.
Origins and historical foundations of the cosmological constant in the framework of general relativity
The cosmological constant, denoted Λ, is an essential parameter introduced by Albert Einstein in 1917 in his equations of general relativity. The initial goal was to conceive a model of a static Universe, in line with the cosmological ideas of the time. Einstein found that his equations from 1915 did not allow for a homogeneous and isotropic static solution without this addition. Thus, the cosmological constant was introduced as an additional scalar term, having the same dimension as an inverse curvature of the square of a length, namely m⁻².
This parameter acted as an anti-gravitational force to counteract the gravitational attraction of matter. Unfortunately, this application required a precise value of Λ for stability, which quickly proved fragile when researchers like Alexander Friedmann (1922) and Georges Lemaître (1927) demonstrated that the Universe was expanding, thereby ending the hypothesis of an eternal static universe.
Einstein closed the chapter by calling this introduction his “greatest blunder,” but the cosmological constant was not destined to disappear. Cosmic dynamics, particularly discoveries in the 1990s, would revive it as a central concept representing the intrinsic energy density of spatial vacuum. The fact that this constant appears in Einstein’s equation:
Gμν + Λgμν = (8πG/c⁴) Tμν
shows its role in the overall curvature of space-time, with a direct influence on gravity and the evolution of cosmic expansion. The term Λgμν corresponds to a form of energy possessing a negative pressure, a fundamental concept for explaining the gravitational repulsion caused by what is now referred to as dark energy.
Accurate measurements of the cosmic microwave background by the Planck satellite have allowed for precise assessment of cosmological parameters, including the constant Λ, highlighting its significance in the energy composition of the Universe. This historical context lays the groundwork for an increased understanding of the forces at work in the expanding universe.
Dark energy: the mysterious force accelerating the current cosmic expansion
Dark energy is at the heart of contemporary cosmological research. It refers to an unknown form of energy, responsible for the overall acceleration of cosmic expansion, observed indirectly through the spectral redshifts of galaxies. This form of energy does not seem to be related to dark matter, but rather to a type of vacuum energy with negative pressure, capable of generating a repulsive force on a large scale.
The two major teams led by Saul Perlmutter, Brian Schmidt, and Adam Riess discovered in 1998, through observations of distant supernovae, that the expansion of the Universe is not slowing down as anticipated by gravity alone, but is instead accelerating. This observation earned these astrophysicists the Nobel Prize in Physics in 2011, marking a major conceptual turning point.
In 2025, the analysis of data from the DESI (Dark Energy Spectroscopic Instrument) telescope suggests that dark energy may evolve over time, with an acceleration that may not be constant but tend to decrease slightly. These results raise new fundamental questions, as they challenge the classical ΛCDM model, which associates a fixed cosmological constant with this form of energy.
The idea that the cosmological constant is truly a constant must then be nuanced. This perspective raises alternative hypotheses to static dark energy, such as evolving scalar fields or unexplored interactions between dark energy and dark matter. One of the current challenges is to model these mechanisms and refine the precise measurement of the Hubble parameter to decipher the exact nature of this energy that now dominates cosmic energy density.
This unknown remains the main enigma of modern cosmology, offering fertile ground for interdisciplinary research, between astrophysics, quantum physics, and field theory. To deepen the mathematical foundations and observational proofs related to this enigmatic facet of the cosmos, one might recommend, for example, the resource The mathematical foundations of cosmological models and the synthesis site on the expansion of the Universe.
Measurements and implications of the cosmological constant in the contemporary expanding universe
Currently, the cosmological constant is primarily quantified by its impact on the expansion rate of the Universe, often condensed in the Hubble constant (H₀). According to results from the Planck mission, published from 2018 to today, and supported by various surveys, Λ represents about 68% of the total energy density, dark matter about 25%, and ordinary baryonic matter about 5%.
The table below summarizes these key cosmological parameters determined through the analysis of the cosmic microwave background and large structures:
| Component | Ridiculous density (Ω) | Cosmological role |
|---|---|---|
| Dark energy (Λ) | ~0.6847 | Repulsive force, acceleration of expansion |
| Dark matter | ~0.3153 | Guides galaxy formation, gravitational attraction |
| Ordinary matter | ~0.05 | Visible constituents, atoms, stars, planets |
These figures are consistent with the values defined in the ΛCDM model, considered the standard framework for describing the expanding universe and its components. This model includes the cosmological constant as a fundamental parameter associated with dark energy, an intrinsic energy of empty space.
Moreover, the negative pressure associated with the cosmological constant is interpreted as a form of antigravity, responsible for an observable cosmic acceleration. This unusual, counterintuitive pressure reverses the natural tendency towards contraction described by classical gravitation, which has led to a major reform of cosmological evolution models.
Theoretical challenges and alternatives surrounding the cosmological constant and dark energy
The main theoretical puzzle associated with the cosmological constant is the gigantic discordance between the value predicted by quantum physics for vacuum energy and the value actually measured by astronomical observations. This problem, known as the “vacuum catastrophe,” consists of a gap of about 10¹²⁰ between the theoretically calculated energy density of the vacuum and the observed constant Λ.
To understand this divergence, innovative approaches have been proposed, notably in quantum general relativity, where the choice of reference frame and the quantization method (such as quantization on the light front) challenge the interpretation of these quantum fluctuations. These methods aim to eliminate artifacts related to methodological choices and suggest that most of the quantum contributions to the vacuum could cancel each other out.
Furthermore, alternative cosmogonic models are emerging to explain observations without resorting to a fixed cosmological constant. A notable example is the Dirac-Milne universe, a scenario in which matter and antimatter coexist with opposite gravitational properties, making conventional dark energy superfluous. This model would imply a very different cosmic structure and open the way to a radical understanding of cosmic dynamics.
Finally, the possible temporal evolution of the constant realized by DESI prompts reflection on dynamic nucleation mechanisms or different quantum vacuum landscapes, opening cosmology to models where the cosmological constant would be a variable quantity rather than an immutable fixed parameter.
Cosmological implications and future perspectives on the cosmological constant and dark energy
The cosmological constant and dark energy play a determining role in projecting the cosmic future. The ΛCDM model, with a positive cosmological constant, predicts that the Universe will continue to expand in an accelerated manner. The Hubble horizon, which defines the limit beyond which objects recede at a speed greater than that of light, is itself expected to grow towards an asymptotic value determined by Λ.
This scenario implies that in a few tens of billions of years, our cosmological horizon will only contain the local galactic group, making all distant galaxies invisible. The negative pressure of dark energy will have emptied the rest of the observable Universe, an effect perfectly compatible with the predictions derived from the ΛCDM model and confirmed by precise measurements.
However, recent evolutions of the parameter Λ suggest that dark energy may only be a temporary or dynamic component in the Hubble constant, raising fundamental questions about the ultimate nature of the Universe: will it eventually slow its expansion, stagnate, or even contract in the distant future?
Here is a list of key points about these perspectives:
- Cosmic acceleration supported by dark energy, ensures a future of gradual dilution of galactic structures.
- Hubble radius increasing towards approximately 17.6 billion light-years, ultimate limit of cosmic visibility.
- Negative pressure and antigravitational effect, exacerbated with the dominance of Λ over matter.
- Alternative models explore the temporal variability of Λ and the implications of a multi-component universe.
- Impact on fundamental physics calling for a potential revision of quantum and gravitational theories.
Future observations, particularly through improved spectroscopic probes and space missions, will provide essential insights into cosmic dynamics and the Hubble constant. The challenge is more than ever to integrate these phenomena within a unified theory, combining quantum physics, general relativity, and astrophysical observations. The still enigmatic nature of dark energy and the doubtful nature of the constancy of Λ thus pose, in 2025, one of the greatest challenges of modern cosmology.
What is the cosmological constant?
The cosmological constant is a parameter introduced by Einstein in his equations of general relativity to allow for a static universe. Today, it is associated with dark energy and the acceleration of the expansion of the Universe.
What is the relationship between dark energy and negative pressure?
Dark energy is accompanied by a negative pressure that acts as a repulsive force on a large scale, responsible for the observed cosmic acceleration.
What does the possible variability of the cosmological constant mean?
This implies that dark energy could evolve over time, which would question the standard model ΛCDM based on a fixed constant Λ.
What is the ΛCDM model?
The ΛCDM model is the standard cosmological framework that describes a Universe dominated 70% by dark energy (modeled by the cosmological constant Λ) and 25% by dark matter.
Why do we talk about the vacuum catastrophe?
The vacuum catastrophe designates the enormous discrepancy between the quantum theoretical prediction of vacuum energy density and its measured value in the Universe, a gap of about 10¹²⁰.