Dark Matter Isn't Quite the Invisible Wallflower ✎ express

The cosmological standard model is like a recipe for a cake where everything is strictly measured: dark matter is like flour without a single lump (zero pressure), and dark energy is like the perfect leavening agent (equation of state exactly minus one). But what if the flour is a bit damp and the leavening agent has lost its oomph? That's precisely what the authors of this study set out to check, using the latest data on cosmic distances from the DESI project, supernovae, and the cosmic microwave background.

They allowed the parameters describing the pressure of dark matter and dark energy to vary simultaneously. The result was surprising: for dark matter, they obtained a tiny but nonzero pressure — about 0.00097 times its energy density. For comparison, the pressure of light inside a lightbulb is billions of times stronger. For dark energy, the value came out to about -0.938, not exactly -1. Both standard values were called into question at roughly the two-sigma level, like a coin landing on its edge twice in a row.

The authors went further, allowing dark energy to change over time, including crossing the 'phantom divide' (w=-1). It turned out that if such a crossing is permitted, dark matter pressure could be zero. But if you forbid that crossing (as in quintessence models, where dark energy is like a ball slowly rolling down a hill) then a small positive dark matter pressure becomes preferred again. Curiously, the model without a phantom crossing but with nonzero dark matter pressure fits the data even slightly better than the popular crossing models. This hints that the oddities in dark energy behavior might actually be a reflection of unaccounted-for properties of dark matter.