The Invisible Pattern of Dark Energy Across the Sky express
Cosmologists are like cartographers — only instead of mountains and rivers, they map invisible forces. Dark energy, which drives the accelerating expansion of the universe, remains a mystery. One idea is the 'kinetic gravity braiding' (KGB) model. In it, dark energy interlocks with gravity. The celestial map's paper becomes elastic. It alters the path of light rays.
To spot the differences, physicists simulated the whole sky. Light traveled through the elastic medium on its way from distant galaxies. They accounted for the bending of rays (spacetime curvature), the shift in frequency (caused by expansion of the universe), and time delays. It turned out that on large scales the difference is small, just a few percent. On finer details, comparable to individual constellations, the deviations reach 10–12%. The Sachs–Wolfe effect is especially noticeable (here dark energy plays the leading role). This occurs when light, passing through fading gravitational wells, loses a bit of its redness. In such regions, it ends up slightly less red than expected.
That level of precision is no longer abstract theory. Future sky surveys by Euclid and Roman will pick up these fingerprints. They will decode the invisible pattern dark energy leaves on the map of the universe.
Original abstract
as written by the authors arXiv:2607.28207v1
We study the observational signatures of kinetic gravity braiding (KGB) models in relativistic cosmological probes constructed along the past light cone. Using the relativistic $$N$$-body code KGB-evolution, we generate light-cone outputs and compute several observables that directly probe the gravitational field, including weak gravitational lensing convergence, Shapiro time delay, the integrated Sachs-Wolfe and Rees-Sciama (ISW-RS) effects, and gravitational redshift. Full-sky maps and angular power spectra of these quantities are constructed and compared with $$k$$-essence models and predictions from linear perturbation theory. We find that the derivative coupling between the scalar field and the metric modifies both the amplitude and the time evolution of the gravitational potentials, producing scale-dependent deviations ranging from a few percent to tens of percent. In particular, the ISW-RS signal exhibits the largest fractional response, as the slower decay of the Weyl potential suppresses the KGB signal in the ISW-dominated regime, whereas nonlinear evolution reverses this trend at higher multipoles, producing differences of tens of percent relative to $$k$$-essence. Weak gravitational lensing also provides a strong complementary probe and, for the model considered here, exhibits clear deviations from the $$k$$-essence prediction at small scales with enhancements up to $$\sim 10$$-$$12\%$$ at multipoles $$\ell \sim 10^2$$-$$10^3$$. Our results show that linear perturbation theory accurately describes the large-scale behaviour, while nonlinear effects become important at smaller scales, particularly for the ISW-RS signal and, more moderately, for the convergence, and must therefore be included for reliable theoretical predictions.
The Sachs–Wolfe effect is like an echo of the Big Bang: light passing through fading gravitational wells loses a bit of its redness.