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The Rigakubu News
The Rigakubu News Sep. 2026
Mysteries of Science >
Dark Energy or a New Theory of Gravity?
Shinji Mukohyama
(Professor, Research Center for the Early Universe)
It appears that the current universe is filled with a mysterious form of energy known as dark energy. This is not the stuff of science fiction, but rather the latest understanding of the universe based on Einstein’s general theory of relativity and precise observational data.
Observations of the cosmic microwave background—the afterglow of the early universe—and of supernovae, the massive explosions that occur when heavy stars reach the end of their lives, have revealed that the expansion of the current universe is accelerating.Here, let us introduce the ratio of pressure to energy density (denoted as w below) as a quantity characterizing the components of the universe, in a unit system where the speed of light equals 1. In general relativity, for dark energy to generate the universal repulsive force that accelerates the expansion of the universe, 3w + 1 must be negative (resulting in a negative w).In an expanding universe, the energy (energy density × volume) of ordinary matter, for which w is nearly 0, is largely conserved; the energy of radiation, which has a positive w, decreases; but dark energy, which has a negative w, increases as the universe expands.This can be understood as follows: when a system has negative pressure, the work done on or by the external environment due to volume changes has the opposite sign than usual, so the system actually gains energy as it expands.
Thus, when attempting to explain the accelerated expansion of the universe based on general relativity, the existence of dark energy—which generates a universal repulsive force due to negative pressure (more precisely, negative 3w+1)—is suggested. Dark energy is thought to account for about 70 percent of the current universe, but its true nature remains completely unknown.
Historically, a similar situation occurred in the 19th century. Precise observations of planetary orbits led to the discovery of a phenomenon known as the precession of Mercury’s perihelion, which could not be explained by Newton’s law of universal gravitation and equations of motion. Consequently, people attempted to explain it by introducing an invisible planet.This hypothetical planet was called Vulcan, and some researchers even claimed to have discovered it through observation. However, the true answer lay in “changing the theory of gravity.” General relativity brilliantly explained the precession of Mercury’s perihelion and thus established itself as a new theory of gravity, replacing Newtonian mechanics.
In light of this historical fact, it is understandable that quite a few researchers are asking, “Instead of introducing dark energy, couldn’t we modify general relativity?” Dark energy is necessary if we attempt to explain accelerated expansion using general relativity, but if gravity were to change over long distances, it might not be necessary after all.Furthermore, observations of the cosmic microwave background radiation mentioned earlier and recent baryon acoustic oscillations (a phenomenon in which cosmic fluctuations propagate as acoustic oscillations in a photon-baryon fluid) suggest that the dark energy parameter w is time-dependent and that w+1 may have been negative in the past.On the other hand, theoretically, assuming that general relativity and standard quantum field theory are correct, it can be concluded that w+1 is non-negative.
It is clear that further developments are needed in both observation and theory. At the Research Center for the Early Universe, we are tackling the fundamental question of “Dark energy or a new theory of gravity?” by mobilizing not only existing theories of gravity and cosmic observations but also effective field theories, black hole theories, and gravitational wave observations.
The VCDM, proposed by the author in collaboration with Antonio De Felice and Andreas Doll, is a theory of gravity that allows for arbitrary effective values of w while avoiding all fatal instabilities.Here, based on this theory, we present the results of analyzing observational data on the cosmic microwave background and baryon acoustic oscillations to reconstruct the effective w as a function of redshift z for five models. For all models, the results suggest that the effective w+1 becomes negative in the early universe at redshifts of approximately 0.5 or higher.[Excerpted from Phys. Rev. D 112 (2025) 12, 123518]