A new publication in Physics suggests that there may be a fifth force of nature – joining gravity, electromagnetic and strong and weak nuclear forces.
The possibility first emerged when observations of the decay of an isotope of Beryllium showed an unexplained excess of electron/positron pairs diverging from each other at a preferred angle (instead of the angle varying with the energy of the excitation of the Beryllium nucleus). The results suggested a new particle, a force-carrying boson, with a distinctive mass of 17 MeV, albeit with a tiny lifespan.
Now, experiments with excited Helium nuclei have yielded the same result.
The particle is provisionally named X17.
The new particle interacts with neutrons, which has physicists excited that the particle may be linked with the gravitational interactions of dark matter.
A fascinating new paper released today suggest that both dark matter and dark energy can be explained by a single phenomenon – particles of matter with negative mass. I give a link to the paper below, but I will attempt a quick layman’s explanation.
Fundamentally, orthodox positive-mass attracts other positive-mass, and negative-mass would repel other negative-mass. The interesting part is the interaction between positive-mass and negative-mass particles. The positive-mass particle is repelled, but the negative-mass particle is attracted i.e. the positive-mass ‘runs away’ but the negative-mass ‘follows’.
The scientist behind this new theory, has run computer simulations which show that a galaxy would indeed become surrounded by a halo of negative mass particles, and that this halo would exert a kind of pressure on the orthodox matter galaxy, maintaining, for example, its spiral shape in rotation. This is important because it is understood that without some kind of dark matter influence, the spiral arms of a galaxy would not rotate fast enough to maintain their shape, indeed not fast enough to be consistent with observations. This is one of the reasons dark matter is theorised in the first place.
The other aspect of this theory is that because the negative mass particles, outside of the halos, are repelling each other, they exert an outward pressure, like a gas, which can be interpreted as the cause of the expansion of the universe. However this would require continual creation of such particles to keep up the expansive pressure. My gut feeling is that this is the weakest proposal in the theory, particularly since the author points out the beauty of having negative mass particles to balance the orthodox positive mass particles – everything else of course has balance – positive and negative electrical charges, magnetic poles…
When galaxies collide, and several have merged into our own Milky Way, there are imprints that show as streams of stars, which are all moving in the same direction, but not yet equilibrated into the usual spiral arm rotation.
One such stream, the S1 stream, is of particular interest since it crosses the path of our sun. Thinking about the dark matter from the merged galaxy, this would imply that there is a dark matter “wind” blowing past us at about 500km/s. This could potentially help in our efforts to detect the components of dark matter in earth-bound experiments.
However it is possible that the dark matter from a merger would follow a different trajectory from the stars, since we know that the dark matter in a galaxy does not occupy the same geometric shape as the stars in that galaxy. Indeed dark matter may not have the same equivalence of gravitational and inertial mass.
Astronomers have made measurements on an “ultra-diffuse” galaxy – it is the same size as our galaxy but only contains about 1% of the number of stars in our milky way. Indeed the star density is so diffuse that it is relatively transparent and other galaxies can be observed through it.
Measurements of the movements of 10 globular star clusters within the galaxy show that those globular clusters are moving much slower than expected – in fact at a speed consistent with there being no dark matter in the galaxy – the movement of the clusters being entirely explained by the mass of the stars in the galaxy.
This unexpected result is not readily explained. It brings existing theories of galaxy formation into question, and also puts question marks around the behaviour or existence of dark matter as it is currently understood.
Update 4th July 2018:A new research paper, not yet peer reviewed, has challenged the above results, claiming that the galaxy is much closer than stated previously. I will update again when the peer review process clarifies the situation.
Coincidentally, published today in the journal Nature, is a study showing that the Equivalence principle, the idea that Gravitational Mass is equal to Inertial Mass, holds, even with very high mass objects like neutron stars and white dwarfs. This in turn rules out several theories in which gravity varies from Relativity in extremes, and therefore makes the theoretical existence of Dark Matter seem even more likely.
Update 25th January 2019: Another diffuse galaxy has now been found and the velocity-dispersion results in both confirmed by much more sophisticated telescope equipment. It now seems probable therefore that these two galaxies are indeed examples of galaxies lacking dark matter. Ironically this strengthens the dark matter theory, because it rules out modified gravity theories as an alternative.
A study published in “Science” today indicates that dwarf galaxies orbiting Centaurus A galaxy share the same orbital plane rather than having random orbits.
This, unless it is a fluke, challenges the current model of how dark matter interacts gravitationally. This comes only 3 weeks after the report of galactic centre offsets which also is not consistent with the standard dark matter theory.
The process of measuring dwarf galaxy orbits is, however, a time-consuming process, so we will not learn for some years whether other sets of dwarf galaxies share orbital planes, and therefore whether the dark matter model needs revision.
It is well known that neutrons (outside of atoms) decay with a half-life of around 15 minutes – traditionally it was thought, by beta-decay into a proton, an electron and an antimatter neutrino.
But the two methods of measuring the neutron lifetime have produced different results. One method simply bottles neutrons magnetically, and counts how many are left after certain times. The other method detects protons – the supposed decay product – to infer the lifetime.
A new theory to explain why these two methods give different results, is that neutrons might, less frequently, decay into dark matter particles, possibly emitting a gamma ray in the process.
Experiments are already underway to establish whether there is any gamma ray emission. The significance is that not only would it explain the lifetime measurement discrepancy, but would also give physicists their first real handle on dark matter.
European scientists using the Hubble telescope to measure the visible centre of a galaxy, and comparing it with the gravitational centre, measured using its gravitational lensing effect, have found the two centres substantially offset. This is interpreted as a wobbling in the galaxy, post-merger, but after the local cluster had relaxed.
If dark matter is causing the wobble then it is not following the current model of dark matter. Otherwise there may be some new physics to explain the wobble.