The usual idea of cosmology is predicated upon 4 issues: the construction of house and time, matter, darkish matter, and darkish power. Of those, darkish power is the one we at the moment perceive the least. Inside the usual mannequin, darkish power is a part of the construction of house and time as described by common relativity. It’s uniform all through the cosmos and expressed as a parameter referred to as the cosmological fixed. However preliminary observations from the Dark Energy Spectroscopic Instrument (DESI) counsel the speed of comedian growth might differ over time. If additional observations reinforce this, it might open up cosmological fashions to options to common relativity referred to as modified gravity.
In a current paper on the arXiv, the authors have a look at one model of modified gravity referred to as Horndeski’s idea. The idea is predicated upon a generalization of common relativity. Einstein’s authentic idea was based mostly upon the precept of equivalence, from which he derived a generalized description of spacetime by way of what is named a metric tensor. From this, you possibly can derive the equations of movement for objects in a gravitation area, simply as Newton’s legal guidelines result in equations of movement for objects underneath bodily and gravitational forces.
Basic relativity is the only mannequin with a metric tensor. Horndeski’s idea is essentially the most common mannequin with a metric tensor and permits for the presence of a uniform scalar area. There are particular instances of Horndeski’s idea, such because the Brans-Dicke model and the mannequin of quintessence. Each of those fashions have been used to explain darkish power in a extra common manner, in addition to darkish matter in some instances. Whereas observations of gravitational waves, galactic clustering and cosmic growth constrain these fashions to some extent, they don’t completely rule them out. Up to now, our knowledge on darkish power isn’t wealthy sufficient to differentiate between options.
This newest work seems on the DESI leads to the context of Horndeski fashions, particularly the way it may deal with the time-evolution of cosmic growth urged by the DESI knowledge. It discovered that if the time evolution is taken to be appropriate, then a modified gravity is a greater match than the usual mannequin. The examine goes on to point out that Horndeski fashions solely work the place the time evolution of the scalar area correlates to the proposed time evolution of darkish matter. This guidelines out some Horndeski fashions which were used to elucidate darkish matter.
Total, the authors argue that the DESI observations make Horndeski’s idea a viable various to common relativity. That’s, if the info holds up. The Darkish Power Spectroscopic Instrument is still in its early stages, and we don’t but know what the ultimate outcomes might be. However it’s clear that Einstein’s seat on the theoretical throne isn’t completely assured, and Horndeski’s idea may simply be the one to steal the crown.
Reference: Chudaykin, Anton, and Martin Kunz. “Modified gravity interpretation of the evolving dark energy in light of DESI data.” arXiv preprint arXiv:2407.02558 (2024).
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