Does Heavy Dark Matter Hide In The Shadows?
Scientists currently propose that the Universe was born almost 14 billion years ago in the wild exponential inflation of the Big Bang, when it emerged from a tiny Patch that was smaller than a proton to reach macroscopic size in the smallest fraction of a second. The original Patch is thought to have been so extremely hot and dense that everything we are and everything that we can ever know was born from it. The Universe has been expanding and cooling off ever since. We can now observe from where we are situated on our small rocky planet, the dying fires of cosmic birth. Most of the Universe is hidden in the shadows--a mysterious form of invisible matter that is called dark matter accounts for most of its matter content. The dark matter is transparent because it does not dance with light--although its gravitational influence on objects that can be seen reveals its ghostly presence. Scientists do not know what composes the bizarre non-atomic dark matter, and its identity has long eluded those who have tried to understand it. However, in August 2019, astronomers from the Max Planck Institute for Computational Physics in Potsdam, Germany, and the University of Warsaw in Poland, have proposed a new and unusual dark matter candidate--a superheavy gravitino.
The gravitino is a hypothetical fermion, associated with graviton theories of supergravity. The graviton is also hypothetical but, if it does exist, it is an elementary particle that mediates the force of gravity. Fermions are subatomic particles.
The Universe itself is almost completely composed of dark energy (~68%), dark matter (~27%), and so-called "ordinary" atomic matter (~5%). Other contents include electromagnetic radiation (~0.005%-0.01%) and antimatter. Dark matter is thought to be composed of unidentified non-atomic particles, and it is a substance whose powerful gravitational pull serves as the "glue" that holds galaxies together. The identity of the dark energy is also unknown, but it is considered to be the mysterious substance responsible for causing the Universe to accelerate in its expansion--and it is possibly a property of space itself. While "ordinary" atomic matter accounts for much less of the Universe than dark matter and dark energy, it is the part of the Universe that we are most familiar with. Extraordinary "ordinary" matter accounts for literally every element listed in the familiar Periodic Table, and without its presence, we would not be here.
Dr. Hermann Nicolai, Director of the Max Planck Institute for Gravitational Physics, and his colleague Dr. Krzysztrof Meissner from the University of Warsaw, note that the existence of the still-hypothetical superheavy gravitino follows from a hypothesis that seeks to explain how the observed spectrum of quarks and leptons in the standard model of particle physics might emerge from a fundamental theory. Furthermore, the two scientists describe a possible method for actually tracking down this elusive possible particle.
The standard model of particle physics includes the building blocks of matter and the forces that bind them together. It proposes that there are a half-dozen different quarks and a half-dozen leptons that are grouped into a trio of "families". Quarks are any one of numerous subatomic particles carrying a fractional electric charge, and are postulated as the building blocks of hadrons. The most stable hadrons are protons and neutrons (baryons), which are the components of atomic nuclei. The six types of quarks are: up, down, strange, charm, bottom and top. Leptons are elementary particles that do not experience strong interactions.
We are, ourselves--as well as the matter that surrounds us--made up of only three particles: the up and down quarks and electrons. The electron is a member of the lepton family.
Until now, the long-established standard model of particle physics has not changed. The Large Hadron Collider (LHC) at CERN in Switzerland began operating about a decade ago with the primary purpose of hunting for what may reside beyond. Alas, despite expectations to the contrary, after ten years of obtaining data scientists have not detected any new elementary particles--with the important exception of the Higgs boson, the so-called "god particle", responsible for providing particles with mass. Hence, until now, measurements with the LHC have failed to provide any evidence at all of the greatly anticipated "new physics" beyond the standard model. This new research provides a dramatic contrast.
In a previous paper published in Physical Review Letters, Dr. Nicolai and Dr. Meissner proposed a new theory seeking to explain why only the already-known elementary particles emerge as the basic building blocks of matter provided by Mother Nature--and, also, why no new particles should be expected to show up in the energy range accessible to current or conceivable future experiments. In order to provide a solution, the two scientists studied the possible existence of superheavy gravitinos--and their real existence in nature would make them fascinating, albeit unusual, candidates for dark matter.
Ghostly Matter
Scientists do not know what the dark matter is, and its unknown identity is one of the most elusive mysteries in modern physics. While little is known about its origins, astronomers have been able to demonstrate that the dark matter plays an important role in the formation of galaxies and galaxy clusters. Even though dark matter is invisible and, thus, not directly observable, its gravitational influence on the way visible matter moves, and is distributed in space, indicates that the ghostly material really is there.
We live in a bizarre Universe--most of which we are unable to see. On the largest scales, the Universe appears the same everywhere we look--and it displays a bubble-like, frothy pattern throughout Spacetime. Heavy filaments composed of invisible dark matter weave themselves around each other, creating a web-like structure--the Cosmic Web. The filaments themselves are traced out by the brilliant stellar fires of galaxies that are strung out along these transparent structures like glittering diamonds on a celestial bracelet. The invisible filaments are interrupted by very black, enormous, and almost empty cavernous Voids. The appearance of the large scale structure of the Universe has been likened to a natural sponge or a honeycomb. Also, some observers have noticed that this enormous web-like pattern is intriguingly (and somewhat disturbingly) similar to the network of neurons in the human brain. This marvelous web-like construction; this tapestry woven of enormous and heavy transparent filaments traced out by starlight, may be composed of only one filament intricately wrapped around a single black and almost-empty Void.
The Universe may be weirder than we are even capable of imagining it to be. At the beginning of the previous century, scientists thought that our Milky Way was the entire Cosmos, and that it was static and unchanging. We now know that our Galaxy has plenty of company, and that the Universe evolves as time passes. Georges Henri Joseph Edouard Lemaitre (1894-1966), a Belgian astronomer and priest, was a professor at the Catholic University of Louvain. He was also the first to propose that the Universe is not unchanging--that it is, in fact, expanding, as it grows ever colder and colder. In addition, Lemaitre was the first to develop the theory of what would later be called the "Big Bang Universe." Once he commented that "The evolution of the world may be compared to a display of fireworks that has just ended... Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origins of worlds."
The observable Universe is the relatively tiny domain of the entire Universe that is visible to us. Most of the unimaginably enormous Cosmos is situated beyond what we can observe. This is because the light wandering to us from those remote domains hasn't had enough time to travel to us since the birth of the Universe, about 13.8 billion years ago, because of the accelerating expansion of Space. There is no known signal that can travel faster than light in a vacuum, and so light sets a kind of speed limit. This limit has rendered it impossible for beings on Earth to observe those remote regions of Space that are beyond the horizon of our visibility--set by the finite speed of light. Although no known signal can travel faster than light in a vacuum, Space itself can. It is thought that at the instant of our Universe's inflationary birth in the Big Bang, Space expanded exponentially--inflating much faster than the speed of light. For this reason, we cannot directly observe extremely remote domains of Spacetime. The secret of our very existence may well reside far beyond the horizon of our visibility.
Hidden In The Shadows
For their research, Drs. Nicolai and Meissner used an old theory devised from the work of the American physicist Murray Gell-Mann (1929-2019), who was awarded the 1969 Nobel Prize in Physics. Gell-Mann's idea is based on what is called the "N=g Supergravity" theory. One key element of this theory involves a new type of infinite-dimensional symmetry that serves to explain the observed spectrum of the known quarks and leptons in three families. "Our hypothesis actually produces no additional particles for ordinary matter that would then need to be argued away because they do not show up in accelerator experiments. By contrast, our hypothesis can in principle explain precisely what we see, in particular the replication of quarks and leptons in three families," Dr. Nicolai noted in an August 2, 2019 Max Planck Institute Press Release.
But the processes that occur in the Universe cannot be neatly explained by the influence of the so-called "ordinary" atomic matter that we are familiar with. One important indication of this is the way that galaxies move. Galaxies rotate at a high speed, and the relatively small amount of visible atomic matter in the Universe--the runt of the cosmic litter of three--would not exert a sufficient gravitational influence to keep them from falling apart. However, no one knows what the rest of the matter is made of. The elusive identity of the dark matter is therefore one of the most important unanswered mysteries in cosmology.
"The common expectation is that dark matter is made up of an elementary particle, and that it hasn't been possible to detect this particle yet because it interacts with ordinary matter almost exclusively by the gravitational force," Dr. Nicolai continued to explain. The model that he developed in collaboration with Dr. Meissner offers a new dark-matter-particle candidate, albeit one that sports completely different properties from all of the other candidates proposed so far--such as axions or WIMPS, that dance only very weakly with ordinary atomic matter. The same is true for very light gravitinos that have also been proposed as candidates for dark matter.
The proposal of a superheavy gravitino as a dark matter candidate goes in the opposite direction from previously proposed candidates. "In particular, our scheme predicts the existence of superheavy gravitinos, which--unlike the usual candidates and unlike the previously considered light gravitinos--would also interact strongly and electromagnetically with ordinary matter," Dr, Nicolai explained in the August 2, 2019 Max Planck Institute Press Release.
The hefty mass of the superheavy gravitino means that these very dense particles could only exist in a very dilute form in the Universe. This is because, otherwise, they would 'overclose' the Universe, thus resulting in its early collapse. According to Dr. Nicolai, it wouldn't require very many of them to explain the dark matter content of the Cosmos, as well as within our own Milky Way Galaxy. Indeed, one particle per 10,000 cubic kilometers would be quite sufficient.
The mass of the possible particle proposed by Drs. Nicolai and Meissner resides in the region of the Planck mass--approximately one milliionth of a kilogram. By comparison, protons and neutrons--the components of the atomic nucleus--are about ten trillion trillion times lighter. In the space between galaxies, the density would be considerably lower.
"The stability of these heavy gravitinos hinges on their unusual quantum numbers (charges). Specifically, there are quite simply no final states with the corresponding charges in the standard model into which these gravitinos could decay--otherwise, they would have disappeared shortly after the Big Bang," explained Dr. Nicolai in the August 2, 2019 Max Planck Institute Press Release.
Because of their strong and electromagnetic interactions with visible atomic matter, heavy gravitinos are relatively easy to track in spite of the fact that they are extremely rare. One potential avenue of investigation is for scientists to hunt for them with "dedicated time-of-flight" measurements conducted deep underground. That is because these particles travel much slower than the speed of light, unlike ordinary elementary particles originating from cosmic radiation. In spite of this, heavy gravitinos would be able to penetrate the Earth rather easily because of their large mass. This has been compared to the way a cannon ball cannot be stopped by a swarm of mosquitoes.
For this reason, researchers are thinking of using the Earth itself as a "paleo-detector". Our planet has been orbiting our Sun through interplanetary space for about 4.5 billion years. During this long stretch of time, it must have been penetrated by a great number of heavy gravitinos. As a result, the particles should have left behind straight, long ionization tracks etched into rock. However, it may not be a simple task to distinguish these tracks from other tracks carved out by known particles.
Dr. Nicolai explained in the August 2, 2019 Max Planck Institute Press Release that "Ionizing radiation is known to cause lattice defects in crystal structures. It may be possible to detect relics of such ionization tracks in crystals that remain stable over millions of years."
Because of its long "exposure time" this type of search strategy could also be successful if the dark matter is not homogeneously distributed within galaxies and is subject to local density fluctuations--which could also explain why previous hunts for more conventional types of dark matter candidates have come up empty handed, so far.
Gloss