New Theory on Dark Matter, Dark Energy

Grav
Fg indicates the force, a indicates the acceleration

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…

Link to scientific article

 

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Dark matter wind blowing past the Sun

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.

DMFlow
Artists impression: C. O’Hare; NASA/Jon Lomberg

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.

Phys. Rev. D 98, 103006 (2018)

Ultra-diffuse galaxy contains no dark matter

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.

FC6E8A02-714A-4B7E-AC67B18960E27326 (1)
Ultra-diffuse galaxy NGC1052-DF2 imaged by the Hubble Space Telescope

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.

Scientific American article

Space.com article

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.

Scientific American article

Dwarf Galaxy orbits defy Dark Matter Theory

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.

Scientific American article

Can Neutrons decay into Dark Matter?

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.

Wobbling galaxy contradicts dark-matter model

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.

Futurism article

The Still Silent Night of Dark Matter

Two major experiments are reporting (8th Nov 2017) that they have been unable to detect any signals at all from dark matter – XENON1T in Italy and PandaX in China.

WEB_P1190369_FC-XENON

These experiments were based on the favoured theory of SuperSymmetry in which dark matter consist of Weakly-Interacting Massive Particles (WIMPs) that would interact, but only rarely with ordinary matter, and carry large mass.

Such detectors will continue to be developed, but alternative theories are now being taken more seriously – axions (a type of massive exotic photon) are one candidate. But it is also possible that the dark matter (whose gravitational effects are not disputed) exists in a ‘sector’ of the universe completely hidden from us, or that the theory of gravity needs modification in the large scale.

Click for more detailed Nature article

Update 19th Nov 2017: Despite the above failure to detect WIMPs, the Astroparticle Physics European Consortium (APPEC), which coordinates funding plans, has proposed scaling up the same experiments – instead of using 3 tonnes of noble gas, they would use 30 tonnes! There has been criticism that following this same line of research “will turn the dark-matter field into a desert of ideas.”

Update 2nd Dec 2017: A Chinese spacecraft (DAMPE) which is designed to measure cosmic ray properties, has confirmed an anomolous disruption in the curve when plotting number of particles against energy of those particles. This cannot be explained within existing physics knowledge, and is possible a pointer to darm matter WIMP annihilation. Futurism article

Update 4th April 2018: The DAMA collaboration have just released results showing that they are still seeing the signal, allegedly from dark matter, which they announced some years back, before upgrading their equipment. They record flashes of light in sodium iodide crystals caused by particles colliding with crystal nuclei. Although there are background sources for these collisions, the dark matter signal should peak in June as the Earth plows slightly faster into the supposed dark matter headwind, in concert with the solar system’s orbit round the galaxy.
Update 9th Dec 2018: The COSINE-100 experiment, designed to use the same methodology as DAMA above, has reported finding no evidence of dark matter after allowing for background radiation.
Update 7th April 2018: A paper, published March 23 in the journal Physical Review Letters, describes a mechanism whereby during the inflation stage of the universe, after the big-bang, the Higgs field, in a quasi-stable state, could exhibit quantum fluctuations sufficient to cause the local ‘stuff’ to clump together sufficiently to form a black hole. These primordial small mini black holes would be formed in the weight region of 10 trillion kilograms, their calculations suggest, and the existence of these in great numbers, growing over time, could explain away dark matter. other commentators are more conservative primordial black holes might explain only a few percent of dark matter.
Update 4th July 2018:  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 might vary from Relativity in extremes, and therefore makes the theoretical existence of Dark Matter seem even more likely.