Science1 publisher2 min readPublished
A negative-pressure fluid yields stable negative-mass regions in two cosmological models
Shin'ichi Nojiri and S.D. Odintsov argue in Physics of the Dark Universe that negative mass does not always make general relativity inconsistent, on assumptions that include a negative cosmological constant.
The Scientist · Science desk
What happened
- A paper in Physics of the Dark Universe by Shin'ichi Nojiri in Japan and S.D. Odintsov in Spain argues that negative mass objects may not be as exotic as physicists have assumed.
- Their setup puts a positive point mass, such as a compact star, in a fluid of negative pressure, where the mass pushes the fluid away and leaves stable regions of negative energy density.
- In a second model, where the fluid's pressure is proportional to its energy density, a negative mass object can appear once the constant of proportionality drops below -2/3.
- A separate route adds two scalar fields to Einstein's theory under conditions inspired by string theory, and in special circumstances the evolution of those fields produces negative mass objects.
- In those modified gravity models the spacetime acts as a convex lens, sending a light beam hyperbolically around the object rather than bending it toward the mass.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Any cosmology that borrows these solutions inherits a negative cosmological constant, and the measured one is positive, so the burden falls on showing negative mass survives with the observed sign.
- capability Light diverging around a mass is something a lensing survey can hunt for, which moves the question out of pure theory and into image data.
- contradiction The fluid route and the modified gravity route give opposite signs for the force between a positive mass and a negative mass object, so no single antigravity prediction is on the table for an experiment to settle.
A bubble in water gave the authors their starting image. Nojiri and Odintsov wrote that the bubble behaves as if it has a negative mass, because the water around it falls away under gravity [3]. Mass itself is mostly binding energy: a particle's mass comes from its interaction with the Higgs field, but most of it is the energy binding its constituents together [18]. A proton is 1,836 times heavier than an electron and is made of three quarks and gluons [19]. Lattice gauge calculations in quantum chromodynamics put the quark masses at about 9 percent of the proton's mass [20], which leaves roughly 91 percent in the field energy of the gluons [21].
In Newton's gravity, negative mass is a bookkeeping change, the same equation with the gravitational constant G replaced by -G [4]. General relativity has been less accommodating, and does not seem to consistently allow antigravity [5]. What the paper gets out of the fluid is a force. Using a version of Einstein's equations one step beyond Newton's law, the negative-mass object imparts a repulsive force on the positive mass [9]. The equivalence principle carries the sign into inertia, so the object's mass in F=ma is negative too [10].
The model departs from our universe in two ways. Its cosmological constant is negative, while the real one, proportional to the vacuum energy, is positive and very tiny, as David Appell notes in the phys.org account of the work [7][23]. The spacetime is not asymptotically flat, though Appell points out that ours is not asymptotically flat either, with matter and radiation everywhere and an expansion driven by dark energy [12].
The scalar fields in the modified gravity route are not invented for this purpose. Fields like them are already used to drive early cosmological inflation, to supply dark matter or dark energy, or to give neutron stars and black holes a different structure [14].
For anyone who wants to test any of this, the direction of the force depends on which construction you take. In the modified gravity models the negative inertial mass makes the force between a positive mass and a negative-mass object attractive, and two pure negative-mass objects repel each other [16]. The fluid model gives the opposite sign for the star [9].
General relativity has passed every experimental and observational test to date, and it still does not explain dark matter or dark energy, or give simple accounts of cosmic acceleration and galactic dynamics [17]. The paper works in that gap. Its claim is a consistency claim: negative mass, the authors write, "does not always lead to any inconsistency" [2].
What to watch
- Whether the same negative-mass solutions can be built with a positive cosmological constant, the sign the universe actually shows.
- Whether any lensing survey reports a diverging, convex-lens signature instead of the usual bending toward mass.
- Whether other groups reproduce the claim that the regions of negative energy density are stable.