A climber on a ridge notices something strange: the horizon around them seems to freeze. No forward, no backward — only emptiness and silence. This is roughly the picture painted by new calculations for our Universe. At some point in its expansion, the invisible boundary, the future event horizon, stopped growing. And precisely then, the key parameter governing dark energy crossed a critical threshold.
This parameter is w, the ratio of pressure to density. Above -1 is quintessence — expansion accelerates, but gently. Below -1 is phantom energy, tearing the world to shreds. Data from the DESI telescope, which collected light from millions of galaxies, hints that the transition has already occurred. The question remained — why now?
The answer came from the holographic principle: the physics of a volume is encoded on its boundary. For an expanding Universe, that boundary is the event horizon itself. Its entropy, a measure of hidden information, does not have to obey the old formula of Bekenstein and Hawking; it can include quantum corrections. In the new model, dynamics are governed by the slope of entropy: its local 'dimensionality'.
Calculations showed that the crossing of the phantom divide occurs on a specific 'kinematic branch' when the product HL = 1. Geometrically, this condition means that the radius L stops growing. The Universe balances on a peak. And which way it will roll, toward the phantom or back, depends on the entropy dimensionality dS. If dS < 4 (entropy grows slower than the fourth power of the radius), the path leads downward to a singularity. DESI data points to dS ≈ 1.62—far from the usual 4.
Thus an abstract model becomes a measurable prediction. By measuring the slope of w with redshift, astronomers can directly see the 'topography' of the horizon. If telescopes confirm that w crossed -1 at a redshift of ~0.4–0.5 with a local parameter $w_a$ ≈ 0.33, then the horizon is indeed fractal. Negative slope is ruled out.
The peak is behind us. We know that the moment when the horizon froze left its imprint in the light of distant supernovae and the cosmic microwave background. Now it remains to compare the map with the landscape, and perhaps we will see the outlines of the quantum structure of spacetime. This is a test for the very concept of gravity.
At the transition point, the horizon entropy is maximal and the dark energy density is minimal — it’s as if the Universe lets out a sigh before a powerful tug.
The Big Rip scenario, in which accelerating expansion tears everything apart, appears in Charles Stross’s novel ‘Accelerando’.