In the ordinary world without dark energy, the mass of any object is always positive. That seems obvious: positive mass attracts, creating gravity. But the real universe has dark energy — a mysterious force making the cosmos expand faster and faster. Physicist Albert Einstein once introduced it into his equations under the name cosmological constant (denoted by the Greek letter Λ). For a long time, it was believed that even with Λ, a star's mass must be positive.
This new work shatters that belief. The authors built a mathematical model of a star — regular, without discontinuities, made of ordinary matter that obeys reasonable conditions (energy is never negative, pressure does not exceed density). And it turns out that the mass of such a star, measured far away from the outside, can be positive, zero, or even negative. The most surprising case is the zero-mass configuration: in that case, the external spacetime becomes pure de Sitter, that is, a world filled only with dark energy, without any matter. Meanwhile, inside, ordinary matter quietly exists.
This result is a clear counterexample to the principle of mass "rigidity": in a world with Λ>0, mass is no longer required to be positive. It forces us to reconsider fundamental ideas about how gravity works in an accelerating universe. Perhaps we have been misinterpreting the very concept of mass in cosmology.
The cosmological constant Λ is incredibly small: its value is about 10⁻⁵² m⁻². That's like stretching vacuum energy across the entire universe: you'd need less than a billionth of a joule per cubic kilometer.
The idea of negative mass has appeared in science fiction: for example, in Arthur C. Clarke's novel "2010: Odyssey Two," there is a mention of "negative space," and in "Star Trek," the warp drive requires exotic matter with negative energy.