Cosmologists are facing an unexpected puzzle. The latest observations do not perfectly match the mathematical model used for decades to describe the evolution of the Universe. This model, called ΛCDM (pronounced Lambda-CDM), is based on the idea that a dark energy constantly and uniformly accelerates the expansion of the cosmos. However, recent measurements suggest a slightly different behavior, forcing scientists to reconsider their work.
To explain this discrepancy, the results of recent research put forward a rather surprising idea: empty space might possess a property similar to the viscosity of a fluid. As an illustration, it's like moving from analyzing a substance where everything flows without resistance, like water for example, to a more resistant substance like honey.
Thus, in this view, the expansion of the Universe would encounter a tiny resistance, comparable to internal friction. Although it still requires much verification, this hypothesis offers a concrete lead to reconcile theoretical calculations with observation data, without however challenging the entire current cosmological framework.
The anomaly shaking the standard model
For decades, cosmologists have been using the ΛCDM model, a robust mathematical framework describing a Universe composed of cold dark matter and a constant dark energy. The latter, symbolized by the Greek letter Lambda, is supposed to be a uniform and unchanging repulsive force, responsible for the acceleration of cosmic expansion. This model has successfully explained many observations.
However, a recent publication sows doubt. The Dark Energy Spectroscopic Instrument (DESI), based in Arizona, maps the position and velocity of millions of galaxies with unmatched precision. Recent data from these maps reveal a slight (but significant) discrepancy when compared with the predictions of the standard model regarding the rate of expansion at several epochs of the Universe.
This discrepancy is not just a statistical margin of error. It indicates that the measured expansion rate today does not perfectly fit the expected trajectory if dark energy were a perfect constant. This tension, known as the Hubble tension when confronted with other measurement methods, signals that our fundamental description of the dominant component of the Universe might be incomplete or require a profound conceptual adjustment.
Viscosity, a new property of the vacuum
To explain this discrepancy, researcher Muhammad Ghulam Khuwajah Khan proposes a radical hypothesis in his article on arXiv. He envisions that the vacuum of spacetime possesses a "bulk viscosity." Viscosity, in fluid mechanics, measures the internal resistance of a substance to flow. Transposed to the cosmological scale, this property would imply that the expansion of the Universe encounters a slight resistance, an infinitesimal but cumulative drag effect over billions of light-years.
The proposed mechanism for generating this viscosity draws inspiration from condensed matter physics. The researcher introduces the notion of "spatial phonons." In a solid, phonons are quasiparticles representing the collective vibrations of atoms. Within this theory, the fabric of space itself would be the seat of analogous vibrations. These longitudinal waves, propagating through the vacuum, would create an internal pressure that opposes the dilation caused by dark energy.
The major interest of this approach lies in its fit to the data. When the researcher integrates this viscosity parameter into the Friedmann equations, which govern cosmic expansion, the resulting theoretical model agrees remarkably well with the problematic observations from DESI. It shows that a specific viscosity value can reproduce the measured expansion curve, offering an alternative solution to the hypothesis of a time-varying dark energy, all without invoking entirely new physics.
Article author: Cédric DEPOND