An illustration of galaxies distorting the structure of space-time in the expanding universe. New theoretical work suggests that the cosmic expansion is not constant — it can dramatically change direction and intensity. (Image credit: NASA/JPL-Caltech)
Dark energy could have changed its properties at some point in the past, and this abrupt transition could explain why cosmological observations do not yield the expected results, researchers suggest in a new paper.
The current model of the evolution of the Universe is known as ΛCDM (or lambda-CDM), which stands for dark energy (symbolized by the Greek letter Λ) and cold dark matter. Dark energy is the mysterious force causing the accelerated expansion of the Universe, while cold dark matter refers to the invisible, mysterious substance that makes up the bulk of the mass of almost every galaxy.
This model explains a wide range of observations, such as the behavior of galaxies and clusters, the growth of large-scale structure, and the emergence of the cosmic microwave background. However, in recent years, two troubling disagreements have emerged.
One such problem, known as the Hubble tension, is a discrepancy in the measurement of the current rate of expansion of the universe, known as the Hubble constant. Probes exploring the distant, early universe seem to give estimates that are significantly lower than those examining the nearby, more recent universe.
Related to this problem is a second one, called the sigma-8 voltage. This is a measure of how unevenly distributed matter is in the universe, and again different probes give different results.
Cosmic slowdown
Something seems to be wrong with the ΛCDM model, but we can’t say exactly what. One hypothesis is that dark energy may be more dynamic than previously thought. In the standard ΛCDM model, dark energy is represented as a cosmological constant that remains unchanged throughout cosmic history.
But in a recent model gaining popularity, dark energy changes. And not just by a small amount; it goes through a complete phase transition from the universe's slowing expansion to its acceleration.
Now, adding a new dimension to this theory, a research team has explored the possibility that the phase change could be even more dramatic. In a paper posted to the preprint server arXiv in February but not yet peer-reviewed, dark energy doesn’t just change sign; it also changes intensity, so that it contributes to acceleration in a different way than it does to deceleration.
The researchers then tested their model on a wide range of observations and data. These included measurements by the Planck space observatory of the cosmic microwave background, the oldest light we can observe in the universe; measurements of a phenomenon called baryon acoustic oscillations, patterns in the arrangement of galaxies on very large scales; the Pantheon dataset of supernova distance measurements; and the weak lensing map, which provides details that account for the effects of dark matter.
They found that the new model helped resolve some of the discrepancies between Hubble and Sigma-8, and so argue that the approach could be a promising direction for further research.
However, the researchers noted that this model is not entirely based on known physical principles. It is simply a concept that allows one to explore the physical implications of the model without full confidence in the underlying physical principles. However, since it seems like a promising direction, this approach could encourage theorists to develop mechanisms that explain how dark energy could switch in this way.
Regardless, it appears that the Universe—especially dark energy—is much more complex than we thought.
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