The usual idea of cosmology relies upon 4 issues: the construction of house and time, matter, darkish matter, and darkish power. Of those, darkish power is the one we presently perceive the least. Inside the usual mannequin, darkish power is a part of the construction of house and time as described by normal relativity. It’s uniform all through the cosmos and expressed as a parameter often called the cosmological fixed. However preliminary observations from the Darkish Power Spectroscopic Instrument (DESI) counsel the speed of comedian enlargement could differ over time. If additional observations reinforce this, it might open up cosmological fashions to alternate options to normal relativity often called modified gravity.
In a current paper on the arXiv, the authors have a look at one model of modified gravity often called Horndeski’s idea. The speculation relies upon a generalization of normal relativity. Einstein’s unique idea was primarily based upon the precept of equivalence, from which he derived a generalized description of spacetime via what is called 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 below bodily and gravitational forces.
Common relativity is the best mannequin with a metric tensor. Horndeski’s idea is essentially the most normal 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 mannequin and the mannequin of quintessence. Each of those fashions have been used to explain darkish power in a extra normal approach, in addition to darkish matter in some instances. Whereas observations of gravitational waves, galactic clustering and cosmic enlargement constrain these fashions to a point, they don’t completely rule them out. Thus far, our knowledge on darkish power isn’t wealthy sufficient to tell apart between alternate options.
This newest work appears to be like on the DESI ends in the context of Horndeski fashions, particularly the way it may handle the time-evolution of cosmic enlargement steered 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 research 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 have been used to clarify darkish matter.
General, the authors argue that the DESI observations make Horndeski’s idea a viable various to normal relativity. That’s, if the information holds up. The Darkish Power Spectroscopic Instrument is nonetheless in its early levels, and we don’t but know what the ultimate outcomes shall 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 darkish power in mild of DESI knowledge.” arXiv preprint arXiv:2407.02558 (2024).
