Persistent Mystery: Why Is the Universe Expanding Ever Faster? Could Einstein’s idea of gravity be wrong or incomplete? Astronomers have now investigated this with the help of 100 million galaxies for six alternative theories – models that modify Einstein’s description of gravitation or add as yet unknown particles. The result: Einstein and the current cosmological model are right – at least for the time being.
It’s one of the great mysteries of modern cosmology: our universe is expanding faster and faster because something seems to be counteracting gravity. A possible candidate for this is the Dark Energy, but whether it exists and what it consists of is unclear. But there are also some theories that see the reason for the accelerated expansion in as yet undiscovered particles or even look for it in gravitation itself. Unlike Einstein’s general theory of relativity, these alternative theories posit that gravity varies with matter densitymagnitude or period changes.
Gravitational lenses as a test tool
The international research collective of the Dark Energy Survey (DES) has now carried out the most comprehensive test to date of six such alternative theories. To do this, the researchers use DES measurement data on more than 100 million galaxies, which have been examined over the past three years by the Dark Energy Camera on the four-meter telescope of the Cerro Tololo Inter-American Observatory in Chile. What all these galaxies have in common is that they are part of gravitational lenses.
In this phenomenon, a foreground galaxy distorts the light of a more distant background galaxy. The extent of the distortion provides an indication of how strongly the foreground galaxy’s gravity is affecting the light. In the first step of their study, the astronomers used this gravitational lensing effect to test whether distant lensed galaxies have the same gravitational effect as near ones. From this it can be deduced whether gravity was similarly strong at different times.
Alternative theories don’t fit
In the next step, the scientists tested whether the current cosmological model – the so-called ΛCDM model – fits the observed data. In the same way, they then tested the “fit” of the six alternative theories. These included a time-dependent gravitational equation, non-flat curvature of space, additional relativistic degrees of freedom, sterile neutrinos, modifications of gravitational physics, and variable growth rates of cosmic structures.
The result: “Our analysis shows no significant deviations from the ΛCDM model,” reports the team. According to this, there is no evidence that the strength or behavior of gravity differs between distant and nearby galaxies. It also remained constant in relation to different orders of magnitude. To hypothetical gravity-affecting particles like the sterile neutrinos, the observational data also did not match. They also confirmed the flatness of the curvature of space.
mystery remains
“Ultimately, we find no significant preferences for any of the alternative models examined in our analysis,” the scientists state. “The current cosmological ΛCDM model remains the model favored for describing our measurement data.” Also in an additional check based on data from the Planck mission to cosmic background radiation as well as measurement data on supernovae and the galaxy distribution yielded the same result.
This means: Dark energy still seems to offer the best explanation for the cosmic expansion so far – although its existence has not been proven and is completely mysterious. Basically it is nothing more than a term that describes the cosmological constant in Einstein’s field equations. “Of course, there is still room to challenge Einstein’s theory of gravity,” explains co-author Agnès Ferté of NASA’s Jet Propulsion Laboratory.
“It is therefore essential to cooperate with researchers all over the world in solving this problem, as we did in the Dark Energy Survey,” Ferté continues. (International Conference on Particle Physics and Cosmology (COSMO’22); Preprint: arXiv:2207.05766)
Source: NASA/ Jet Propulsion Laboratory