“The slower growth of gravitational wells and the accelerating expansion might share the same underlying cause.” In so many words, this is what Isaac Tutusaus, author of a new study examining Dark Energy Survey (DES) data, summed up the nature of a burgeoning impasse in contemporary cosmology. Since more than a century ago, Einstein’s general relativity has been the foundation of gravity theory, but the accelerating expansion of the Universe, which was discovered in 1998, challenged physicists to question its completeness on the largest scales.
The center of the questioning is two parameters, μ and η, added to empirically generalize Einstein’s equations. μ figures in the Poisson equation, altering directly how matter sources gravity fields, whereas η measures the ratio of spatial to temporal metric perturbations, providing insight into potential anisotropies in gravity’s behavior. These values are not mere mathematical oddities; they are critical tools for deciphering the faint imprints of gravitational lensing and galaxy clustering, the very effects DES and other surveys such as BOSS and eBOSS have been monitoring with growing accuracy.
The newest DES findings, covering 100 million galaxies over four epochs of the cosmos between 3.5 and 7 billion years ago, tell a compelling story. At very ancient times 6 and 7 billion years ago grapes of gravity are as deep as theory says they should be. But since more recent times, at ages 3.5 and 5 billion, the wells seem shallower than predicted. This timing coincides with the dawn of cosmic acceleration, which could imply that gravity itself is changing or that new physics maybe a dynamical form of dark energy or a failure of general relativity may be occurring.
However, as Nastassia Grimm of the University of Geneva warns, “Such an incompatibility threshold arouses our interest and calls for further investigations. But this incompatibility is not large enough, at this stage, to invalidate Einstein’s theory.” The 3-sigma discrepancy found, while interesting, does not meet the 5-sigma gold standard for a discovery in physics. The redshift interdependence of μ and η and the need to rely on the use of Euler’s equation for dark matter add additional degeneracies, complicating matters for unraveling the real source of the anomaly.
To go beyond these fronts, cosmologists are looking to the Euclid space telescope, which was launched in July 2023 and is now in operation at the Sun-Earth L2 point. Euclid’s instrumentation is a technological wonder that in one instrument combines a 1.2-meter silicon carbide Korsch telescope with the specialized VIS and NISP instruments. The VIS camera, featuring 36 CCDs and 600-megapixel array, provides high-resolution images of 0.56 square degrees per exposure, optimized to pick up the very faint distortions of weak gravitational lensing. NISP, which ranges from 950 to 2000 nanometers, gives both photometry and spectroscopy information, which is important for estimating galaxy distances and clustering.
Euclid’s task is ambitious: to chart more than a third of the sky, counting more than a billion galaxies out to 10 billion light-years. Its thermal stability and sensitivity, obtained with the most stringent thermal control and high-precision pointing 75 milli-arcseconds in 700 seconds are designed to limit systematic errors that have affected ground surveys. Its silicon carbide structure maintains dimensions at cryogenic temperatures, while sophisticated calibration units and on-board processing shrink the daily 850 Gbit science data stream into downlinkable volumes.
The combination of weak lensing and galaxy clustering measurements enables Euclid to test the general relativity on cosmological scales in a rigorous way. By contrasting the evolution of the growth of structure (from clustering) and spacetime geometry (from lensing), researchers can constrain μ and η with unprecedented accuracy. This two-probe method is also sensitive to other theories of gravity, including Horndeski scalar-tensor models, that extend Einstein’s equations by the addition of new fields and interactions. These models are afflicted by parameter degeneracies, however, that can only a Euclid survey of this scale may hope to resolve, as demonstrated recently by DES analyses.
Euclid has already shown its potential to find a beautiful Einstein ring in the local galaxy NGC 6505 a case of strong gravitational lensing which had gone undetected for more than a century. “All strong lenses are special, because they’re so rare, and they’re incredibly useful scientifically. This one is particularly special, because it’s so close to Earth and the alignment makes it very beautiful.” These findings highlight the capabilities of Euclid’s technology and the potential of its six-year survey.
As Euclid presses on, cosmologists look forward to an inundation of high-fidelity data capable of allowing the strongest tests to date of gravity, dark energy, and the laws that govern the universe. Precision cosmology is moving into a new phase a one in which the future of Einstein’s theory will be decided not through thought experiments, but by the tiny warps and wells inscribed in the light of billions of galaxies.
