Proxima Dreaming is now available. You can get it here:
hard-sf.com/links/705470
Due to the fact that dark matter plays an important role here, I have provided a section entitled A Guided Tour of Dark Matter following this note. 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.
A Guided Tour of Dark Matter
Introduction
In this exact moment a particle of dark matter might be passing through your field of view, but you will not see it. It’s not because you need glasses. Even the most powerful microscope—one that can display individual atoms—could not see dark matter, not even as a black dot. ‘Dark’ in this sense means that it does not interact with light, or electromagnetic waves in general. Light penetrates dark matter as if it were not even there. It actually could be called ‘invisible matter.’ Yet even that term would not do it justice, as it refuses interaction with other forces of nature as well. The only exception is gravitation. A more precise term might therefore be ‘matter that interacts exclusively with gravity.’
Why do scientists even assume that something so strange exists? What clues do physicists have, and how do researchers hunt for the particles it consists of?
Indications for the Existence of Dark Matter
Among physicists, the existence of dark matter is generally accepted. Cosmology, the branch of science that examines the origin and structure of the universe, has made the Lambda-CDM, aka ΛCDM, its standard model. CDM stands for cold dark matter. It is ‘cold’ because it does not—like light particles do, for instance—gain its energy from movement, which is equal to heat, but mostly from its resting mass. The models currently accepted by the majority of scientists who study the Big Bang, and the time soon after it, cannot do without this form of dark matter. This does not mean that there couldn’t be hot dark matter as well, only that it is not necessary to these models.
The ΛCDM model is far more than a speculation, it has become a mature theory. All the knowledge we have about the world consists of theories. Some are well-proven, like quantum physics and the theory of general relativity. We know that they contain gaps, but that does not mean they are wrong. In order to turn mere speculation into a theory, it must be verifiable and falsifiable. Both require that it generates measurable predictions about reality. All this applies to dark matter. We know how it works, and its effects can also be measured.
Yet there is a problem that makes some physicists uneasy, and it has given dark matter a generally bad reputation: We only know it exists, but not what it consists of. Basically, this is a completely normal state of things. Phenomena including electricity, X-rays, and radioactivity were discovered and employed before they were thoroughly understood. Some present-day researchers, though, have forgotten about those times and make the unreasonable demand that we should immediately know what something is made of. You should not let yourself be confused by this. While there are alternative theories regarding dark matter, that is also the case for quantum physics. The existence of multiple theories does not mean there are serious doubts about dark matter. In reality, there are always new experiments trying to disprove the standard model that often end up confirming it.
How did we get on the trail of dark matter? Through its effects! In 1932, the Dutch astronomer Jan Hendrik Oort examined the numerical density and velocity distribution of various star populations vertically aligned to the disk of the Milky Way, and at various distances from the disk. From this, he calculated a mass density much greater than what was then known. In 1933 the Swiss physicist and astronomer Fritz Zwicky noticed that the Coma galaxy cluster could not be held together by the gravitational effect of its visible components. It would take 400 times the visible mass to keep the cluster from disintegrating. Zwicky seems to have been the first one to come up with the idea that the missing mass might exist in the form of ‘dark matter.’ However, he was not exactly applauded for this proposal.
In 1960, though, there were new indications. The American astronomer Vera Rubin measured the orbital speed of stars in relation to the center of the galaxy. Due to the fact that the visible mass is concentrated in the center, the stars on the outside should move considerably slower, due to Kepler’s third law and the law of gravity. In reality, their velocities remain constant or even increase. Therefore, there has to be mass in the exterior sectors which is not visible in the form of stars, dust, or gas.
This observation made the idea much more convincing, and by now, dark matter has been proven for almost all large astronomical systems. Further indicators included:
•The gravitation lens effect: The mass of a heavy object can bend the space around it in such a way that it acts as a lens for light. The strength of the effect depends on the mass of the object. For many gravitation lenses, though, not enough mass was found to create the measured effect.
•The cosmic background radiation: This developed 380,000 years after the Big Bang, and is not completely isotropic, meaning that it is not distributed through space in an absolutely even manner. Special anisotropies measured by satellites represent clear indications for the existence of dark matter.
•The creation of structures in space: In a very early stage, space was evenly filled with energy. At some time there must have been random variations that, due to their gravitation, coalesced into stars and galaxies. If there were only normal matter in space, as we can calculate, these random deviations would have been too small to persist.
•The shape of the universe: The universe, as measurements have shown, is almost flat. Its total energy density, therefore, should be close to 1. From the expansion velocity of the cosmos we can calculate the density of dark energy, and through observation of visible matter, we can determine the density of normal matter. Whatever is missing compared to 1 must correspond to dark matter.
•Baryonic acoustic oscillation: The distance between normal galaxies is 147 megaparsecs 1 percent more often than it is 130 or 160 megaparsecs. This is a small but significant indication for the existence of periodic density fluctuations in normal (baryonic) matter, which occurs in the ΛCDM model.
No Universe without Dark Matter
The following composition of the universe according to matter proportion can be derived from the ΛCDM model: 68.3 percent dark energy, 26.8 percent dark matter and about 4.9 percent normal matter. The ‘normal’ matter is roughly divided into luminous matter such as stars, and non-luminous matter such as planets and primarily cold gas. Purely in terms of mass, dark matter amounts for 84.5 percent.
This ratio has not been constant across the history of the cosmos, though. For instance, there was no such dominance of dark energy—which is still growing—in the early periods. Shortly after the Big Bang, dark matter fulfilled its most important function. Let’s just take a look back at that time.
Scientists today still don’t know with certainty what took place at the beginning of time, approximately 13.8 billion years ago. All the matter in the universe today, 1053 kilograms, was located at that moment within a point, a singularity, where none of today’s fundamental laws of nature had any effect. You can imagine a kind of infinitely-dense proto-soup consisting of particles no longer known today. A single force, the primordial force, prescribed the movements of these particles. The temperature of the proto-soup, if the term temperature even makes any sense in this context, must have been around 1032 degrees. There were neither electrons nor photons, so there was also no light. If there had been outside observers—but remember, all that was lurking around outside this soup was just nothing—they wouldn’t have noticed that anything at all was happening.
This ultra-hot something was under tremendous pressure—and the cosmos expanded. The length of this time period is given from known laws of nature: it is known as the Planck time, that is, the time that light needs to cover a distance equal to a Planck length. That equals 10-43 seconds. However, at this pinpoint of time
, time itself didn’t yet exist. And, words are unable—unavailable—to accurately portray the paradoxes.
So, 10-43 seconds after the Big Bang is the first chance we have to use physics for studying the universe. The minuscule bits of matter are still under an unbelievable amount of pressure. But it has grown a little bit colder, because of the expansion. The first fundamental force to break free from the primordial force is the gravitational force that acts, as a force of attraction, against the expansion of the universe.
However, it is much weaker than the pressure of the Big Bang—so the universe continues to expand at a rapid pace. Because the average energy density of the proto-soup continues to decrease, it contains fewer and fewer exotic particles from the very beginning. After 10-38 seconds has elapsed, the strong nuclear force and the electroweak force split off of the primordial force.
Then comes a phase, the so-called inflation phase, in which the universe expands by a factor between 1030 and 1050. At the start of this phase it is still the size of a proton, but at the end, it is about as large as a soccer ball. This inflation, which scientists place between 10-38 and 10-35 seconds after the Big Bang, needs so-called inflatons to form an explanation that is reasonable (that is, one that fits into the cosmological world view). These particles, which never appear again, are not attracted to each other by gravity, they are instead repelled.
Only in this way could the universe have grown so much in such a short time period. The argument that this explanation is not so far-fetched is that it also provides a good explanation for other phenomena observed in the universe today, for example, the homogeneity and low curvature of the universe.
The density of the universe—at an incredibly short 10-35 seconds after the Big Bang— has already expanded to the point where particles that are known today can be formed: electrons and positrons, quarks—which later combine to form protons and neutrons—and antiquarks, neutrinos—the precursors of photons—as well as gluons, which are responsible for transmitting the strong nuclear force.
Particles and antiparticles are present in equal numbers. When particles and antiparticles meet, they annihilate each other. There is a constant coming and going. Newly formed particles behave normally under the influence of gravity—they attract each other, which somewhat slows down the expansion of the universe. At this time, a quark-gluon plasma is dominant, which can be simulated today, at least in a computer.
Scientists have not yet reached a consensus, but if the laws of nature are subject to supersymmetry, this process is now broken. The theory of supersymmetry assumes that for each known particle, there is a super-partner that differs by a half spin. Supersymmetry would elegantly unify particles and force particles, which transmit the known interactions.
Such super-partners would have to be so heavy that they could only exist at the start of the universe. Today, however, we only observe conventional particles—supersymmetry is broken. At the moment supersymmetry was broken, it is conjectured that mass was imparted to the particles via the Higgs boson, which was confirmed in 2012.
Approximately 10-10 seconds after the Big Bang, the last two of the forces known today arise, the weak nuclear force, which plays a crucial role in nuclear fusion, and the electromagnetic force. Somewhat later, space has cooled to just two trillion degrees, so that quarks no longer have to be alone, so that protons, neutrons, antiprotons, and antineutrons coalesce. Gluons act as the bonding agent.
Matter and antimatter continue to be in balance. But this situation doesn’t last much longer. Particles and antiparticles annihilate each other, forming photons, and the universe noticeably empties. It is already 10 trillion kilometers wide, approximately one light-year, and has further cooled to one trillion degrees. The fact that we exist, nevertheless, we owe to a small excess of matter. For every billion particle-antiparticle pairs, there resulted in an excess of one particle that wasn’t annihilated. So far there are only theories for where this asymmetry came from. Obviously, the laws of nature do not act symmetrically in every respect.
One-fifth of a second after the Big Bang, the universe is already 500 trillion kilometers wide, or roughly 50 light-years. It has cooled to 20 billion degrees. Now it’s time for neutrons and electrons to feel the heat. Unstable neutrons in a free state are torn apart by the weak nuclear force, releasing an electron, a proton, and a neutrino. Electrons and positrons annihilate each other. Here, the excess of matter wins the day again.
Due to the now large distances of the cosmos, approximately one second after the Big Bang, the weak interaction is no longer strong enough to produce interactions between neutrinos and conventional matter. Since then, the neutrinos released at that time have been rushing around the universe as a measurable, constant neutrino background with almost no interactions.
From now on, the universe’s development progresses considerably more slowly. The remaining neutrons are rescued when, approximately two to three minutes after T-zero, deuterium and finally helium nuclei are formed. Here, the strong nuclear force protects the neutrons from destruction. The universe grows and grows all the while, continuing to further cool down.
After approximately 17 minutes, it has become too cold to support any more nuclear fusion. At this point, all the still-available neutrons are then bound into atomic nuclei. Approximately three-fourths of the nuclei are hydrogen nuclei, the rest helium—heavier nuclei can be found only in traces.
As it grows colder and colder, electrons also bind electromagnetically to positively charged atomic nuclei. They are constantly being thrown out of their orbits, however, by photons from the still boiling proto-soup. The universe, whose composition is dominated by photons, would look like a glowing fog to outside observers at this time.
That doesn’t change for a relatively long time. Starting around 70,000 years after the Big Bang, the ratio of atomic mass and radiation is 1:1. According to the Lambda-CDM model, the universe is now dominated by dark matter, whose nature can only be described by theories at the time of this writing. It has the result, however, that inhomogeneous areas left over from the inflation phase contract even more strongly, a prerequisite for the later formation of stars.
The dark matter increases the inhomogeneities in the structure of the cosmos so that areas with a higher concentration of hydrogen appear. Gigantic bubbles—so-called halos—form, even before the creation of galaxies, and implement the structure that inflation and dark matter impress on the universe. While dark matter is invisible, its gravitation acts on the matter known to us. Therefore, interstellar hydrogen agglomerates in the halos of dark matter. The closer the particles get to each other, the more the pressure in the gas increases—until pressure and gravitation are balanced. The cloud cools off because of the radiation emitted after particle collisions. This lowers the pressure and the cloud can contract further. The more matter gathers in a small space, the stronger it attracts atoms in the vicinity.
Once again temperature and density increase inside the cloud and start radiating in the infrared range. Now the proto-star stage has been reached. The heavier the cloud becomes, the faster it contracts—and then temperature and pressure are so high, about 3 million degrees Kelvin, that a nuclear fusion process is triggered: Groups of two protons collide and change into a deuterium nucleus with one proton and one neutron, while giving off energy. Then the deuterium reacts with an additional proton and becomes a helium-3 (3He) nucleus, while once more releasing energy. If the interior of the star is hot enough, above 10 million degrees Kelvin, the 3He nuclei can later merge into 4He nuclei, releasing two protons. This has the following effect: About 100 million years after the Big Bang, the first stars begin to shine, one after the other. This would have never been possible without dark matter!
Yet dark matter is still needed. Compared to today, the early universe was a very lively place. Smaller galaxies merged into larger ones. There were collisions and hostile takeovers. Today we know that this development was not completely random. The formation of stars actually focuses on
fine structures created in the universe by dark matter, the so-called filaments. Where filaments intersect, galaxies and galaxy clusters develop. Today, the observable universe contains several million of these galaxy superclusters. One of them, the Virgo supercluster, for instance, combines several thousand galaxies in a diameter of 110 million light years, among them the Milky Way as part of the ‘Local Group.’
It is not yet definitively known how the different types of galaxies form. It is assumed that dark matter plays an important role in the common spiral galaxies, like our Milky Way. After all, it only interacts via gravitation, while normal matter is also influenced by the radiation pressure of stars. Therefore dark matter should be concentrated in the exterior regions of the galaxy, where it formed the halo. Accordingly, the interstellar gas, which is strongly attracted by the core of the galaxy, and weakly by its outside, moves in a spiral course toward the core. Imagine someone holding a cat by its tail and swinging it in a circle. The force pulling the cat inward is greater than the outward push of the cat's bodyweight.
This model has one problem: The contraction stops eventually, and cosmologists still don’t know why. Furthermore, spiral galaxies are very fragile structures. Long ago, galaxy collisions should have deformed them more strongly than seems to be the case. At this time, scientists consider the elliptical galaxy as the end point of this development. This type of galaxy most commonly develops from a merger of smaller galaxies. This, according to a simulation, might also be the fate of the Milky Way and our neighbor M31, Andromeda: In 3 to 4.5 billion years there will be a collision between these two systems of approximately similar weight. The consequence should be the creation of a giant elliptical galaxy.
Proxima Trilogy: Part 1-3: Hard Science Fiction Page 49