Of what does Dark Matter consist?
Dark matter is not something completely unknown. It has to fulfill one single condition—it may only interact with other matter through the attracting force of gravitation. This applies to black holes, for example. Neutron stars, almost extinguished white dwarfs, or brown dwarfs—objects between a star and a planet—might represent the part of the galaxies’ mass that astronomers are missing. These are covered by the term MACHO, or Massive Compact Halo Objects. However, there are several arguments against the idea that MACHOs would be sufficient to explain dark matter. For instance, measurements have shown that five-sixths of normal matter interact in other ways than just through gravitation, which rules it out as dark matter.
Accordingly, dark matter has to consist of hitherto unknown particles. Here the candidates have been divided into cold, warm, and hot matter. As described above, this does not refer to a measurable temperature but to velocity. ‘Cold’ dark matter could not cover long distances in the early universe, ‘warm’ dark matter had medium speed, while ‘hot’ dark matter was fast and energy-rich. The standard model, which after all is named ‘CDM,’ only includes cold dark matter.
What could be candidates for cold dark matter? First we have the axion, a hypothetical elementary particle. Its existence would solve a problem faced by quantum chromodynamics—the theory that describes the strong interaction of quarks and gluons. It predicts that the electrically-neutral neutron should possess a measurable electric dipole moment, meaning that its charge should not be distributed quite homogeneously. However, no dipole moment has been found, not even a much weaker one than the theory predicts. The axion would solve this and explain the lack. It would have to be a very lightweight particle, up to ten billion times lighter than the electron, which is already quite lightweight. Yet due to sheer numbers, they still could account for the overwhelming part of the mass in the universe. If an axion gets in contact with strong magnetic fields it might turn into a photon. This could be happening inside the sun. Therefore scientists seek to detect axions of the sun based on their characteristic radiation in the X-ray range. So far, they have been unsuccessful.
The second candidate for dark matter is the complete opposite of the axion. The WIMP, or Weakly Interacting Massive Particle, certainly does not act like a wimp, but is especially heavy, heavier than most known elementary particles and about as heavy as two gold atoms. Nevertheless, it might traverse entire planets unhindered, which makes it complicated to prove its existence. On the other hand, they move leisurely, which makes it easier for them to clump together in clusters, as dark matter does.
As the WIMPs are heavy, the theory would not need nearly as many of them as in the case of axions. The theory of supersymmetry includes such particles, but none of them have been found so far, and supersymmetry itself still waits for confirmation.
For a long time, neutrinos were considered promising candidates for dark matter. One huge advantage is that they are already known. On the other hand, we know too much about them by now to assume that they make up the majority of dark matter, since they are too lightweight. What if a so-called sterile neutrino exists? While normal neutrinos are susceptible to the weak interaction—one of the four fundamental forces—the sterile neutrino would be immune to it. It could be relatively heavy and would, as is required for dark matter, only exchange forces via gravitation. Researchers have been trying to detect sterile neutrinos for some time, but so far without success.
How to Search for Dark Matter
Due to their extremely rare interaction with matter, WIMPs could be most easily proven indirectly, by their decay. To do so, we have to observe them in areas where our view is blurred as little as possible by normal matter, for instance in far-away galaxies. The decay of a WIMP should generate energy-rich gamma radiation. If we manage to measure more gamma radiation in the area under study than could be produced by the normal matter found there, WIMPs would have been proven indirectly.
The sun might be another source of information about WIMPs. Due to its enormous mass, it also possibly attracts WIMPs. Some of them might become so slow that they are caught by the sun. Accordingly, a pool of WIMPs should have collected in the sun over the course of millions of years. There they would collide and destroy each other. Researchers hope to be able to record decay products such as neutrinos.
Yet they also attempt to find a direct proof. In very rare cases a WIMP might collide with a nucleus that then becomes unstable and radioactive. The detectors necessary for this are usually deep underground in order to avoid the influence of radioactive background radiation and secondary cosmic radiation.
Alternatives to Dark Matter?
The existence of dark matter is derived from Einstein’s theory of general relativity, or GR. That theory has been experimentally proven in many ways. Yet there could be a theory that might explain reality better or more completely. In the case of GR, for example, we know that it fails at very small scales and in the case of very, very high energies, such as shortly after the Big Bang.
Researchers have suggested alternatives in order to explain dark matter. These are not yet generally accepted, because they currently have no proofs for certain predictions.
Physicists developed Modified Newtonian Dynamics, or MOND, in which at very low accelerations, inertial mass, i.e. acceleration, and gravitational mass, i.e. gravitation, are no longer equivalent. In this case, Newton’s law as you learned it in school does not exactly apply. Such low accelerations occur at an astronomical scale. They don’t play a role in everyday life, and we wouldn’t notice any of this. Yet for the rotation of galaxies, the equations of motion have to be modified. This leads to the observed measurements without assuming the existence of dark matter. However, in the standard model of cosmic evolution, this theory so far cannot replace dark matter, which would still be needed there. Furthermore, it describes the movement of galaxy clusters less precisely than the good old Newtonian law does.
The tensor–vector–scalar gravity, TeVeS, is derived from MOND and also includes the special theory of relativity. It modifies the relation of the strength of the gravitational field to the distance of the mass causing the gravitation. In doing so, TeVeS can avoid some of the problems of MOND and explain the strength of the gravitation lens effect. However, it also lacks applicability to the early phase of the universe. Furthermore, a researcher showed that under the influence of TeVeS, stars would become very unstable and collapse after a few weeks.
The scalar–tensor–vector gravity, or STVG, assumes that across large distances gravitation becomes stronger than Einstein calculated, but that over short distances it is counteracted by a fifth, repulsive force. On this basis, many of the phenomena mentioned above can be explained without dark matter. This also applies to the accelerating expansion of the universe, that otherwise would require dark energy. However, there are no differences to usual gravitation across the distance scales we can measure with today’s technology. This makes it hard to actually prove or disprove STVG. It also has difficulties in explaining the structure of the cosmic background radiation.
Overall, today it looks like dark matter really exists. The theory of general relativity, GR, supports this, one of the physical theories with the best proofs. Of course, this should not keep physicists from suggesting changes, though so far, Einstein has always been proven correct in the end. The theory of general relativity reaches its limits in very small dimensions, such as the singularities of black holes. It is there where dark matter plays no role, as it is a phenomenon defining the large structure of the cosmos.
If you register at hard-sf.com/subscribe/ you will be notified of any new Hard Science Fiction titles. In addition you will receive a color PDF version of A Guided Tour of Dark Matter.
Proxima Dreaming
Brightnight 1, 3307
He presses his four arms against his body, pushes off with both legs, and swims for dear life. He instinctively keeps the lids of his four eyes closed, as if he knows that the two suns
would otherwise blind him. He forces himself down into the water with all his strength. His smell-skins tell him the way. He only has to follow the increasing salt concentration to reach the depths of the ocean. He won’t be safe there, but definitely safer than here near the shore.
Gronolf can tell from how his skin feels that the water behind him is churning. Those are his brothers and sisters. Today they emerged from their eggs as if on command, and now they are fighting for survival. He only stands a chance if he stays in front. He can smell the blood spreading in the water. Some of his siblings failed to control their instincts. They try to fight. They tear apart the bodies of others with the claws on their swimming feet, and they in turn are ripped apart. It is a massacre one can only escape by fleeing.
How many of the seven times seven times seven will succeed? Gronolf suppresses the thought. He cannot allow himself to lose his concentration, not for a single swimming stroke! That is the only way he can be among the first to reach the depths of the ocean, where the oxygen content of the water increases and he can breathe more freely than now. He will have to survive out there for one brightnight and one darknight before he is allowed to return and become a part of the community.
Swim, swim, swim, he thinks. He pushes himself and soon falls into the magical rhythm of seven his mother taught him. He has never seen her, and so far his eyes only know the dim light that penetrated the membrane of his egg, but he remembers her voice well. Sometimes it came from nearby, and sometimes from far away, but it always seemed warm, kind, and concerned. Gronolf occasionally imagined that she only talked to him and not to the many siblings of his plex. When he returns, when he has finished his ‘draght’ and become an adult, taking on a new name, he will ask her whether she ever thought of him personally while she taught the secrets of life to her entire plex.
The darkness in front of his lids intensifies. Mother Sun must have just set, so Gronolf can take his first rest. He is not yet safe, but he has left the others far behind. He can only smell a few blood molecules now, but the numerous salts of the core ocean lure him ever more strongly. His gills filter breathable air from the water. He fills his swim bladder and lets himself drift to the surface. He turns on his back. His mother warned him not to open his eyes for the first time during the day, as the light of Mother Sun would have inevitably blinded him. Now he slowly raises his lids. A burning pain jolts his body. It is supposed to warn him against opening his eyes too soon, but he knows the time has come.
Then the blueness appears and practically fills his thoughts. Gronolf is surprised how intense the color seems to him, very different from the weeks inside the egg. The blue seems to be all-encompassing, but also profound. It stirs a craving in him to rise into the sky above. In his mind he suppresses his left eye and concentrates on what his right eye shows him. Father Sun is still low above the horizon. It emits a pure white that is reflected in the long waves of the ocean.
Next Gronolf calls up the image of his left eye, which looks at the other side. The sky is only slightly darker there. The blue is more impressive than any description his mother gave. On the horizon he detects black silhouettes. Are those the ‘Mountains of Legends’ his mother talked about? His father is supposed to be there. He is grateful to him, even though he has never seen him. His mother always praised his father profusely. Nobody, she swore, exhibited such virility, and only because of this, his plex became the largest one on the entire Birth Coast.
Gronolf holds his breath and lets his torso sink into the water. This way he can use his rear eyes to look into the direction from which he came. The flat coast is no longer visible, as he has moved too far away. Now and then he notices something flashing. That must be one of his siblings. He must not stay here too long. He uses his right touch hand to feel the muscles of his thighs. They seem to have grown noticeably since he left the egg. For the seven bubble periods of the night the nutrients from the egg will suffice, but then he has to search for fresh food—and watch out that he himself is not eaten by the creatures of the deep.
Gronolf exhales and sinks below the surface. His lids close involuntarily. The light of Father Sun barely reaches into the water. At a depth of three leg lengths, it is already pitch black. But he still has his sense of smell, which guides him on his way and warns of enemies. Swim, swim, swim, he tells himself, and once more falls into the efficient seven-stroke rhythm his mother described.
Five bubble periods later he realizes he cannot wait until the end of the night. He expended all his strength to move away as fast as possible from his competing siblings, just as his mother told him again and again. However, in doing so he has consumed his reserves. His muscles need nutrients or they will stop working. He can no longer ignore the pain indicating this fact. He can find what he needs at the bottom of the ocean. As his mother told him, nowhere in this body of water that spans the entire planet is it deeper than 30 leg lengths.
Gronolf exhales and sinks down. He hopes he has not yet reached the area of maximal depth. Down there, he was told, breathable gas becomes scarce, and it is also the realm of the carrionteeth. Their name is only half true. These carnivorous fish don’t just eat carrion, but anything they can sink their teeth into. And their small brains need less oxygen, which makes them dangerous opponents in the depths. There they can swim even faster than an adult Grosnop. However, they would not dare attack a grown-up representative of his species. An adult Grosnop trained in the arts of combat can handle any animal on the planet, apart from the seacomb. Carrionteeth, it is said, only hunt at night, so he should be safe from them in the daytime.
However, if Gronolf waits until the rise of Mother Sun, his siblings will catch up and kill him. That is life. Few brothers and sisters of his plex will return to the birth beach after the end of the darknight. If there are seven of them, that is considered an especially auspicious sign—the mother who nurtured that plex is allowed to select the inseminating father for the next cycle. This is how his father—who has a high rank in the space fleet—became his father. A splendid career awaits Gronolf if he survives the draght. In the old times, his mother had explained, things were different. He cannot even imagine a time in which the Grosnops in their eggs possessed no spark of intelligence! How could the mothers explain to their children what they were going to face? It must have been terrible to be sent into the great massacre past the beach without any warning. Gronolf is grateful to his mother for preparing him so well.
She had also explained to him how to fill his stomach as fast as possible. In his mind he goes through the procedure: Empty swim bladder, paddle towards the bottom, scan continually with eyes in front, roll upper belly flap inward, let lower belly flap hang out. Then he can float closely above the ocean floor and use his lower belly flap to move the organic matter growing there directly into his feeding hole. He hopes he won’t encounter a carriontooth! Gronolf starts to implement his plan. He sniffs in all directions but doesn’t notice anything except for slowly-rising salt content. Then he uses strong leg strokes to swim toward the ocean bottom.
A cloud of organic molecules awaits him. He cannot miss this source of food. He cautiously loosens his chest muscles. He jerks up the skin flap over his belly. He only has to relax his belly muscles and his feeding hole will open. He sinks down a bit more, orienting himself by the increasing salt content, and starts feeding. He does not know what exactly is entering his stomach, but it does not matter. A kind of sieve filters the food and separates digestible parts from indigestible. Then the first stomach closes, squeezes the food into the second stomach and pumps the water and inedibles back outside, and the whole process starts anew. Whenever the peristaltic muscles pump food into the second stomach, Gronolf experiences a pleasant feeling. This comforting, warm emotion is a new experience for him.
He is no longer alone.
Carriontooth!
The sudden realization makes his body tremble. Both of his skin flaps close involuntarily. Gronolf has to keep himself from a wild flight. If he does that, he wil
l stand no chance against the carriontooth. The animal has tracked him down and is waiting for the best moment to strike. Gronolf has only one chance: He has to wait for the first attack of the carriontooth. The predator specializes in a rapid attack so it can spear prey with its big, sword-like tooth. Gronolf banishes all thoughts of danger from his brain. He has to concentrate on his sense of smell. If he can make himself wait until the last moment to evade the charge of the carriontooth, the attacker will shoot far past him, and he has a realistic chance of getting beyond the range where the predator can smell him. Then he would be safe.
Where is the carriontooth? Gronolf’s thought swirl through his olfactory centers, which are distributed just beneath the skin all over his body. Behind him, toward the coast, he can smell only the blood of his siblings. To the left there is... a trace of iron oxide. Very strange, but no carriontooth. To the right he smells nothing but the food, rich in calcium and potassium ions. The fine hairs on his skin register changes in the water pressure. This can only mean one thing: The predator has started to move. But where does the danger come from? In front of him the smell of food becomes even more intense. What did his mother say? The carriontooth sometimes uses camouflage... That’s it! The predator is coming from ahead!
Gronolf wants to flee immediately, but that would mean his death. The attacker can correct its course almost to the very last moment. He—the prey—must be patient... until the final instant, but not one instant longer. Gronolf trembles. The carriontooth can feel it when you tremble, his mother had told him, but he can’t help it. It was not even 12 bubble periods ago that he left his egg, and now his life might be over? Where should I swerve? Would the predatory fish expect that he will try to flee? Calm down, calm down, calm down, he tells himself, while checking his environment for the tell-tale sign.
Proxima Trilogy: Part 1-3: Hard Science Fiction Page 50