There are many alternative explanations for the accelerating universe. Some examples are quintessence, a proposed form of dark energy with a non-constant state equation, whose density decreases with time. A negative mass cosmology does not assume that the mass density of the universe is positive (as is done in supernova observations), and instead finds a negative cosmological constant. Occam's razor also suggests that this is the 'more parsimonious hypothesis'. Dark fluid is an alternative explanation for accelerating expansion which attempts to unite dark matter and dark energy into a single framework. Alternatively, some authors have argued that the accelerated expansion of the universe could be due to a repulsive gravitational interaction of antimatter or a deviation of the gravitational laws from general relativity, such as massive gravity, meaning that gravitons themselves have mass. The measurement of the speed of gravity with the gravitational wave event GW170817 ruled out many modified gravity theories as alternative explanations to dark energy. Another type of model, the backreaction conjecture, was proposed by cosmologist Syksy Räsänen: the rate of expansion is not homogenous, but Earth is in a region where expansion is faster than the background. Inhomogeneities in the early universe cause the formation of walls and bubbles, where the inside of a bubble has less matter than on average. According to general relativity, space is less curved than on the walls, and thus appears to have more volume and a higher expansion rate. In the denser regions, the expansion is slowed by a higher gravitational attraction. Therefore, the inward collapse of the denser regions looks the same as an accelerating expansion of the bubbles, leading us to conclude that the universe is undergoing an accelerated expansion. The benefit is that it does not require any new physics such as dark energy. Räsänen does not consider the model likely, but without any falsification, it must remain a possibility. It would require rather large density fluctuations (20%) to work.
There are many alternative explanations for the accelerating universe Some examples are quintessence a proposed form of dark energy with a non-constant state equation whose density decreases with time A negative mass cosmology does not assume that the mass density of the universe is positive (as is done in supernova observations) and instead finds a negative cosmological constant Occam's razor also suggests that this is the 'more parsimonious hypothesis' Dark fluid is an alternative explanation for accelerating expansion which attempts to unite dark matter and dark energy into a single framework Alternatively some authors have argued that the accelerated expansion of the universe could be due to a repulsive gravitational interaction of antimatter or a deviation of the gravitational laws from general relativity such as massive gravity meaning that gravitons themselves have mass The measurement of the speed of gravity with the gravitational wave event GW170817 ruled out many modified gravity theories as alternative explanations to dark energy Another type of model the backreaction conjecture was proposed by cosmologist Syksy Räsänen the rate of expansion is not homogenous but Earth is in a region where expansion is faster than the background Inhomogeneities in the early universe cause the formation of walls and bubbles where the inside of a bubble has less matter than on average According to general relativity space is less curved than on the walls and thus appears to have more volume and a higher expansion rate In the denser regions the expansion is slowed by a higher gravitational attraction Therefore the inward collapse of the denser regions looks the same as an accelerating expansion of the bubbles leading us to conclude that the universe is undergoing an accelerated expansion The benefit is that it does not require any new physics such as dark energy Räsänen does not consider the model likely but without any falsification it must remain a possibility It would require rather large density fluctuations (20%) to work