The origin of our Universe remains one of the greatest enigmas of science, a question that seems to defy the limits of our physical understanding. For decades, cosmologists have struggled with the impossibility of mathematically describing the extreme conditions that preceded the Big Bang, where the laws of physics as we know them break down.
A team of British researchers now proposes an innovative approach to explore these unknown territories. Their work, published in
Living Reviews in Relativity, suggests using advanced numerical simulations to solve Einstein's equations in environments where gravity becomes so intense that it surpasses our traditional computing capabilities. This method, called numerical relativity, makes it possible to study cosmological scenarios that were until now out of reach.
The analysis is divided into two parts: the pre-Big Bang phase, which covers the period up to the end of inflation on this diagram. The post-Big Bang phase covers the non-perturbative dynamics from the end of inflation to the emission of the CMB. The late phase of the Universe corresponds to the remainder of the diagram, which contains standard cosmological history. Numerical relativity is not a new idea: it emerged in the 1960s to study black hole collisions and gravitational waves. However, its application to cosmology represents a significant step forward. By abandoning the simplifying assumption of a homogeneous and isotropic Universe, researchers can model various initial conditions and test theories like cosmic inflation or cyclic universes.
Among the promising applications is the search for cosmic strings, hypothetical structures that could leave detectable signatures in the cosmic microwave background. Similarly, this approach could reveal traces of collisions between our Universe and others, providing tangible evidence for the multiverse theory. The simulations require colossal computing power, but technological progress is making these explorations increasingly feasible.
The implications of this work are profound. Not only could they illuminate the moments following the Big Bang, but they could also inform us about what might have existed before. The idea of a cyclic universe, alternating between expansions and contractions, thus becomes accessible to numerical analysis. This methodology paves the way for fruitful collaboration between cosmologists and numerical relativity specialists.
Computational methods could unlock cosmic mysteries.
Credit: Gabriel Fitzpatrick for FQxI, FQxI (2025)
This approach represents a paradigm shift in our quest for cosmic origins. By combining the power of supercomputers with the equations of general relativity, scientists hope to unravel some of the Universe's best-kept secrets, transforming philosophical questions into physical problems solvable through simulation.
Numerical relativity: when computers explore the Universe
Numerical relativity is a discipline at the interface between theoretical physics and computer science. It involves solving Einstein's general relativity equations using numerical methods rather than analytical ones. These equations describe how matter and energy curve spacetime, creating what we perceive as gravity.
Unlike exact solutions that require often unrealistic simplifications, numerical methods make it possible to address extreme physical situations. They break the problem down into small elements that can be calculated individually, then reconstruct the overall picture. This approach is particularly useful for studying singularities, those points where physical quantities become infinite.
The development of this discipline was motivated by concrete problems such as predicting gravitational waves emitted during collisions of compact objects. Today, it finds applications in cosmology for simulating the evolution of the Universe with various initial conditions. Recent advances in computing power are opening even more ambitious perspectives.
Technical challenges remain significant, particularly the management of numerical instabilities and the need to validate results through independent methods. Despite these difficulties, numerical relativity is establishing itself as an indispensable tool for exploring the frontiers of our cosmological knowledge.
Cosmic inflation: the breath of the Universe
Cosmic inflation is a major theory in cosmology that postulates an exponential expansion of the Universe in the first moments after the Big Bang. In a tiny fraction of a second, the Universe would have expanded by a considerable factor, homogenizing its structure and giving rise to the large structures we observe today.
This phase of ultra-rapid expansion solves several cosmological problems, such as the large-scale homogeneity of the cosmic microwave background. It also explains why the Universe appears flat on large scales and why we don't observe magnetic monopoles, particles predicted by some theories but never detected.
However, the precise mechanism of inflation remains poorly understood. Physicists consider that it could be triggered by a scalar field, a kind of energy present in the quantum vacuum. The transition between the inflationary phase and the standard expansion of the Universe constitutes another active research subject.
Observations of the cosmic microwave background, particularly by the Planck and WMAP missions, have provided indirect evidence for inflation. However, the direct detection of primordial gravitational waves, considered a direct signature of inflation, remains an unachieved goal that motivates many observational projects.