Does the Universe Contain Negative-Mass Particles?
The mainstream of cosmology asserts that 84% of the matter in the Universe is invisible, labeled as “dark matter”. The total matter which accounts for attractive gravity amounts to 32% of the cosmic mass-energy budget, while the remaining 68% — in the form of “dark energy”- induces repulsive gravity. The ordinary matter that we are made of, makes only 5% of the cosmic budget. We are made of rare materials in the cosmic context!
Since the dark matter and dark energy components are invisible, we had not observed them directly but only inferred them indirectly through their gravitational influence. This is all fine as long as gravity is the curvature of spacetime, as formulated by Albert Einstein in 1916. Despite the overwhelming consensus of the mainstream, the nature of dark matter and dark energy remains unknown following a century of unsuccessful searches. Is it possible that these constituents are fictitious “ghosts” that do not actually exist, but were imagined because Einstein’s equations fail to describe gravity correctly on cosmic scales?
I spent the day today brainstorming through this possibility along the following lines.
The particles we know about have positive masses. If negative-mass particles existed and were coupled to positive-mass particles, then the known particles would have decayed to lower energy states and matter would have been unstable. But perhaps such coupling is sufficiently weak to allow for the stability of positive-mass matter on timescales longer than the age of the Universe. What would a cosmic sea of negative-mass particles do to the concentrations of positive mass particles in the form of galaxies and large-scale structures?
A hidden component of negative-mass matter could partially screen the inertia of positive-mass objects. When an electric charge is placed in a sea of positive and negative charges — called a `plasma’, it attracts opposite-sign charges and repels equal-sign charges. As a result, the electric potential of this charge is screened exponentially with distance, as first derived by the Nobel-laureate Peter Debye in 1923. At distances larger than the Debye length, the net charge approaches zero and the electric influence of the extra charge is diminished.
In contrast to electricity, gravity is attractive for equal sign masses. As a result, a positive mass concentration would be attractive to positive mass particles. At the same time, a positive mass cluster would also attract test particles with a negative mass by the equivalence principle. However, the resulting negative mass concentration would produce negative inertia and tend to cancel out the positive mass that clustered it. This would reduce the net inertial mass and mimic Debye screening in the electromagnetic plasma case.
Electric screening is effective as long as the extra charge is not accelerating faster than the response time of the surrounding plasma. By analogy, gravitational screening will only be effective at sufficiently low accelerations.
The most successful phenomenological theory of Modified Newtonian Dynamics (MOND) as a substitute for dark matter (as reviewed here by its discoverer, Moti Milgrom), asserts that the inertia or gravity of matter are modified at accelerations below some cosmological threshold. This threshold is coincidentally comparable to the speed of light divided by the age of the Universe, which in turn relates to the mass density of “dark energy”. Is there a fundamental physics explanation to this coincidence?
I spent the day brainstorming scenarios in which the low acceleration regime of MOND could be related to a cosmic sea of negative-mass particles which Debye-screen the effect of positive-mass matter. I was able to sketch some interesting scenarios in which a fictitious “dark matter” component would show up due to reduced inertia when the acceleration of matter is smaller than the MOND threshold while the negative-mass particles could also give rise to the repulsive gravity of dark energy. In such a context, the resultant negative pressure of negative-mass particles in halos around galaxies could explain the coincidence between the timing of structure formation and the dominance of the accelerated expansion of the Universe — which is observed only over the latest half of cosmic history.
However, negative-mass particles pose fundamental problems in quantum field theory and so the scenarios I examined are probably not viable. Nevertheless, it was fun to spend the day brainstorming on their existence.
Of course, the hypothetical existence of negative-mass particles is not only of academic significance as it bears practical benefits. For example, negative-mass matter can be used to cancel out the positive mass of a payload made of ordinary matter. The combined system would have a net zero mass, implying that no fuel would be needed to propel it out of the Earth’s gravitational potential well.
If alien engineers mastered the technology of negative mass production, they might have constructed spacecraft that can maneuver without fuel. Finding such a spacecraft in the crash site of Unidentified Anomalous Phenomena (UAP) would inform us also about the nature of dark matter and dark energy. Is there any hint of such vehicles in the UAP files to be released by the Pentagon in the coming months?
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, former director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
Social media:
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb
