Modified gravity v. dark energy
For many years now, scientists have wondered why the universe is expanding faster than it should be. Through conventional knowledge of physics, the universe should be expanding at a slower pace that observations show that it is. “There are two main theories for why the universe is expanding so fast,” Martin Kunz tells PhysOrg.com. “One is modified gravity and the other is the presence of dark energy. We want to figure out how to distinguish between the two.”
Detecting the difference between dark energy and modified gravity would provide physicists with a better understanding of how the universe works. And detecting modified gravity would add a further benefit by contributing to the understanding of one the fundamental forces in the universe.
Kunz, a scientist at the University of Geneva, and his colleague Domenico Sapone have published a Letter in Physical Review Letters addressing the difficulties of distinguishing between modified gravity and dark energy. The Letter is titled “Dark Energy versus Modified Gravity.”
“There are theoretical problems with dark energy,” explains Kunz, “and this had led people to modified Einstein’s general relativity in order to get modified gravity, which some think would explain the expansion of the universe.” The problem, he says, comes in when one tries to observe one of these phenomena. “We cannot observe either dark energy or modified gravity directly. We can only observe how galaxies behave.”
Kunz points out that in many models, the universe is shown as smooth, assuming that the energy is evenly distributed and homogenous. “This is not completely the case,” Kunz says. “There are small fluctuations. But many measurements only probe the smooth universe.” He continues: “In this simple model, you can make everything look like a component with negative pressure, there is no way to decide whether it is due to dark energy or a modification of gravity.”
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