Since the discovery of cosmic acceleration in 1998, a widespread intuition has heldthat dark energy must provide a repulsive force strong enough to “counteract”gravity and prevent the universe from collapsing under its own weight. Yet this picture faces a sharp contradiction with observational facts: the dark energy density ismerely ρΛ ≈ 10−29 g/cm3, and on the scale of galaxy clusters its equivalent repulsiveacceleration is about six orders of magnitude weaker than the gravitational acceleration. How can such a feeble dark energy “overcome” gravity? This paper identifiesthe root of this paradox as a fundamental misunderstanding of the role that gravity actually plays in large-scale cosmic structure: galaxy clusters are not in astatic, radially aligned arrangement awaiting collapse; rather, they haveacquired orbital angular momentum through primordial tidal torques,and are engaged in complex orbital motions around one another. In thisdynamical picture, the primary role of gravity is to provide the centripetal forcerequired to sustain orbital motion, not to cause linear collapse. Dark energytherefore does not need to “overwhelm” gravity—it need only provide a weak, persistent radial stretching, sufficient to disrupt the delicately balanced orbital equilibriumbetween clusters and cause them to drift apart. This dynamical clarification impliesthat the actual dark energy density can be even smaller than the value conventionally inferred from the assumption of collapse resistance. This paper demonstrates1the self-consistency of this picture through explicit calculation, rules out alternativemechanisms such as radiation pressure and mass loss by quantitative estimation,and presents three quantitative predictions testable by forthcoming surveys. Weemphasize that this work does not claim to solve the fine-tuning problem—the 120order-of-magnitude discrepancy between the observed dark energy density and thequantum field theory prediction remains a fact. Rather, we provide a dynamicalunderstanding of why the dark energy required to drive cosmic acceleration can beso weak. This conceptual clarification offers a natural direction for interpreting the“coincidence problem” of dark energy, consistent with Newtonian mechanics andthe equivalence principle of general relativity.
Wang et al. (Wed,) studied this question.