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Most of the mysterious Universe is hiding in the shadows. The so-called “ordinary” atomic matter, that makes up the world we are most familiar with, is the runt of the cosmic litter of three. An unidentified exotic form of material, that scientists call dark matter, is thought to account for 25% of the Cosmos. But what is this strange form of non-atomic matter, thought to be the substance responsible for giving rise to the first galaxies to dance in the ancient Universe? Several theories have been proposed over the years, but the identity of this shadowy exotic material has not been determined. In October 2019, a team of astronomers offered a new explanation–that the dark matter is really “fuzzy”.
Soon after the Big Bang birth of the Universe, about 13.8 billion years ago, particles of the dark matter would have merged together to create clumps within gravitational “halos”. The clumps pulled in surrounding clouds of gas into their cores, which gradually cooled off and condensed into the first galaxies. Even though dark matter is considered to be the “backbone” of the large scale structure of the Universe, scientists know very little about its true identity. This shadowy substance has kept its secrets well.
However, a team of scientists from MIT, Princeton, and the University of Cambridge has now proposed their new findings that the primordial Universe, and the very first galaxies, would have appeared very different depending on the true nature of the mysterious ghostly and invisible material. The dark stuff is invisible, or transparent, because it does not interact with “ordinary” atomic matter except through the force of gravity. For the first time, the team has simulated what ancient galaxy formation would have looked like if dark matter were “fuzzy”–rather than “cold” or “warm”.
According to the most widely accepted model, the ghostly matter is “cold”–that is, it is composed of slow-moving particles that, with the exception of gravitational effects, do not dance with “ordinary” atomic matter. In contrast, “warm” dark matter is believed to be slightly lighter than if it were “cold”–and, as a result, would also zip around more quickly.
Fuzzy dark matter is a relatively new concept. It is something entirely different, and if the fuzzy stuff exists, it is thought to be composed of ultralight particles, each only approximately 1 octillionth the mass of an electron. In contrast, the mass of a “cold” dark matter particle would be considerably heavier, weighing in at about 10 to the fifth power times more massive than an electron.
In their supercomputer simulations, the scientists discovered that if dark matter particles are “cold”, then the primeval galaxies that were born in the early Universe would have taken shape in nearly spherical halos. In contrast, if the nature of the exotic material is really “fuzzy” or “warm”, the ancient Universe would have looked very different. In this case, the galaxies would be born first in extended, tail-like filaments. In a “fuzzy” dark matter Cosmos, these filaments would have appeared striated–like the strings of a harp on fire with starlight.
As new telescopes come online, with the capacity to peer further back in time to the ancient Cosmos, astronomers may be able to determine–based on the pattern of galaxy formation–whether the nature of the dark stuff, which composes nearly 85% of the matter in the Cosmos, is “fuzzy” instead of either “warm” or “cold”.
“The first galaxies in the early Universe may illuminate what type of dark matter we have today. Either we see this filament pattern, and fuzzy dark matter is plausible, or we don’t, and we can rule that model out. We now have a blueprint for how to do this,” explained Dr. Mark Vogelsberger in an October 3, 2019, MIT Press Release. He is an associate professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research.
Dr. Vogelsberger is also co-author of a paper appearing in the October 3, 2019 issue of the journal Physical Review Letters, along with the paper’s lead author, Dr. Philip Mocz of Princeton University, and Dr. Anastasia Fialkov of Cambridge University (previously of the University of Sussex).
Seeking Our Origins
Even though very little is known about its origins, astronomers have been able to demonstrate that dark matter played a major role in the birth of galaxies and galaxy clusters in the ancient Universe. Though not directly observable, scientists have been able to detect dark matter because of its gravitational influence on the way visible “ordinary” matter is distributed, and how it moves through space.
Almost 14 billion years ago, the Universe was born as an exquisitely tiny soup of searing-hot and very tightly packed particles–generally referred to as the primordial: “fireball”. The Cosmos has been growing larger and larger–and colder and colder–ever since. Astronomers frequently say that most of our Universe have gone missing, mainly composed as it is of a bizarre substance referred to as dark energy, which is even more puzzling than the dark matter. It is generally thought that dark energy is a property of space itself that is causing the Universe to accelerate in its expansion.
Recent measurements suggest that the Cosmos is composed of approximately 70% dark energy and 25% dark matter. A much smaller percentage of the Universe–only about 5%–is composed of so-called “ordinary” atomic matter, which is the material listed in the familiar Periodic Table. Even though it is unambiguously the runt of the litter, “ordinary” atomic matter is extraordinary because it is the stuff of stars and of life on Earth. Only hydrogen, helium, and traces of lithium were born in the Big Bang. The stars cooked up all of the rest of the atomic elements in their seething-hot, roiling, broiling nuclear-fusing furnaces. When stars died, they tossed these freshly-forged atomic elements out into space, where they became the material of our familiar world. The iron in your blood, the rocks beneath your feet, the iron in your blood, the calcium in your bones, the water that you drink, and the air that you breathe, we’re all created in the hot hearts of the Universe’s stars out of the relatively small amount of “ordinary” atomic matter.
Adding to the mystery, the Universe appears to be exactly the same wherever we look. It displays the same foamy, bubbly appearance in every direction, with extremely massive, enormous filaments weaving themselves around one another like the tendrils of a web spun by an invisible, giant spider. The immense filaments of this Cosmic Web are composed of the elusive dark matter, and the structure is brightly lit up by the stellar fires of billions upon billions of brilliant stars. The bright flames of star-light, as well as clouds of glowing gas, outline what we cannot see with our human eyes. This is because these brilliant objects trace out the otherwise invisible filaments composed of the ghostly dark matter. The filaments themselves are interrupted by huge, very black, and cavernous Voids. The Voids, in contrast to the filaments, are only sparsely populated by galaxies.
When scientists refer to the “observable” Universe, they are actually referring only to the relatively small expanse of Spacetime that can be observed–that is visible to us. Most of the Universe is located very far beyond what is called the cosmological horizon. The light that wanders to us, from those unimaginably remote domains of the Universe, has not had enough time to reach us since the Big Bang. This is because of the accelerating expansion of Spacetime. No known signal can travel faster than light in a vacuum, and this sets a universal speed limit that has made it impossible for us to directly observe most of the Universe–located beyond the horizon of our visibility. Although no known signal can travel faster than light, Space itself can.
The temperature of the original fireball–that eventually grew to become the Universe we know–was almost, but not precisely, the same everywhere. This very slight deviation from perfect uniformity is responsible for the formation of everything we are and know. Before a period termed inflation–when the newborn Universe expanded faster than the speed of light–it was completely homogeneous. At this ancient time, the Cosmos was smooth and featureless. It is thought that the exponential expansion of the period of inflation caused the smooth and homogeneous primordial Universe to start to ripple.
The primordial ripples in Spacetime–the extremely tiny fluctuations–occurred in the smallest units we can measure (quantum). That is, the theory of inflation explains how these very small quantum fluctuations eventually expanded to become the large-scale structures that we see in the Universe today.
The quantum world is a jittery arena. Nothing here can stay perfectly still. In this strange place that defies our Earth-evolved common sense, Time itself is meaningless. The originally smooth, isotropic Universe formed tiny hills and valleys. The valleys became emptier and emptier, as the hills grew heavier and taller, because of the pull of gravity. Gravity pulled the original material of the primordial Universe into the increasingly heavier and taller hills. The impoverished valleys, on the other hand, became increasingly barren because they lacked the same gravitational pull as the heavier hills. The large-scale structure of the Universe, as we now know it, resulted from the smallest variations of the density of matter in the most ancient era of cosmic time.
Fuzzy Harp Strings
Dark matter has not been detected directly. However, it has nevertheless revealed its ghostly presence by the way its gravity affects the motions of objects that can be observed. The theory that describes dark matter as “cold”–that is slow-moving–has so far proven successful at describing the large-scale structure of the observable Universe. Because of “cold” dark matter’s success, models of galaxy formation are based on the assumption that it is, indeed, “cold”.
“The problem is, there are some discrepancies between observations and predictions of cold dark matter. For example, if you look at very small galaxies, the inferred distribution of dark matter within these galaxies doesn’t perfectly agree with what theoretical models predict. So there is tension there,” Dr. Vogelsberger noted in the October 3, 2019, MIT Press Release.
This tension has opened the door for other theories that may be better able to explain the nature of the invisible material. Such alternative theories include both “warm” and “fuzzy” dark matter.
“The nature of dark matter is still a mystery. Fuzzy dark matter is motivated by fundamental physics, for instance, string theory, and this is an interesting dark matter candidate. Cosmic structures hold the key to validating or ruling out such dark matter models,” Dr. Fialkov noted in the MIT Press Release.
String theory basically proposes that elementary particles are not points. Instead, they are exquisitely tiny vibrating strings–and the vibration of a particular string determines its role as a particle.
Fuzzy dark matter is thought to be composed of particles that are so light that they behave like quantum waves, instead of individual particles. According to Dr. Mocz, this quantum, fuzzy nature could have been responsible for forming the earliest galaxies. If so, those very ancient galaxies would look entirely different from what standard models predict for cold dark matter.
Wave-particle duality is a concept in quantum mechanics that suggests every particle–or quantum entity–can be described as either a wave or a particle. It expresses the inability of the classical concepts of particle or wave to completely describe the indisputably weird behavior of quantum-scale entities.
“Even though in the late Universe these different dark matter scenarios may predict similar shapes for galaxies, the first galaxies would be strikingly different, which will give us a clue about what dark matter is,” Dr. Mocz noted in the October 3, 2019, MIT Press Release.
In order to determine how different a “cold” and a “fuzzy” ancient Universe would appear, the scientists simulated a small, cubic space of the ancient Universe. This simulation measured an early Universe about 3 million light-years across, and they then ran it forward in time to see how galaxies would take shape according to each one of the three current dark matter scenarios.
The team started each simulation by assuming a particular distribution of dark matter, which scientists have some idea of, based on measurements of the cosmic microwave background (CMB). The CMB is the “relic radiation” of the Big Bang itself.
“Dark matter doesn’t have a constant density, even at these early times. There are tiny perturbations on top of a constant density field,” Dr. Vogelsberger explained to the press.
The scientists used already existing algorithms to simulate galaxy formation under the two scenarios of cold and warm dark matter. However, in order to simulate the dark matter of the fuzzy kind, along with its quantum attributes, they had to devise a new approach.
For this reason, the researchers modified their simulation of cold dark matter, enabling it to solve two extra equations. They did this in order to simulate galaxy formation in an ancient fuzzy dark matter cosmos. The first, termed Schrodinger’s equation, describes how a quantum particle behaves as a wave. The second equation, termed Poisson’s equation, describes how that wave would generate a density field or distribution of the exotic invisible matter, and how that particular distribution leads to gravity. Gravity is the force that ultimately pulls matter together in order to create galaxies. The scientists then coupled this simulation to a model that describes the way that gas in the Universe behaves, and the way that it finally condensed into the first galaxies to dance in the Cosmos–in response to the effects of gravity.
In all three of the simulations, galaxies were born wherever there existed over-densities–meaning large concentrations of gravitationally collapsed dark matter. However, the pattern of this dark matter would be different depending on whether it was cold, warm, or fuzzy.
In one scenario describing cold dark matter, galaxies formed within spherical halos, as well as smaller subhalos. In contrast, in a warm dark matter ancient Universe, the first galaxies would have been born in tail-like filaments–with no subhalos. This difference may be the consequence of warm dark matter’s speedier, lighter nature. The zippy warm dark matter particles would be less likely to stay still long enough to allow the first galaxies to form in smaller, subhalo clumps.
In a way similar to how warm dark matter behaves, fuzzy dark matter formed fiery baby stars along the filaments. However, after that, quantum wave effects began to dominate the evolution of the earliest galaxies–which went on to form more striated filaments that have been likened to invisible, ghostly harp strings. Dr. Vogelsberger explained to the press that this striated pattern resulted from interference. This is something that occurs when a duo of quantum waves overlap. When this happens–for example, in waves of light–the points of the crests and troughs of each wave align to form darker spots. This creates an alternating pattern of bright and dark areas.
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In contrast, in the case of fuzzy dark matter, instead of bright and dark points, it produces an alternating pattern of over-dense and under-dense regions of dark matter.
“You would get a lot of gravitational pull at these over-densities, and the gas would follow, and at some point would form galaxies along those over-densities, and not the under-densities. This picture would be replicated throughout the early Universe,” Dr. Vogelsberger explained in the October 3, 2019, MIT Press Release.
The team of scientists is currently developing more detailed predictions of what early galaxies may have looked like in a fuzzy dark matter-dominated Universe. The team hopes to provide a map for upcoming telescopes, such as the James Webb Space Telescope, that may have the ability to peer far enough back in time to detect the most ancient galaxies. If they observe filamentary galaxies such as those simulated by Drs. Moz, Fialkov, and Vogelsberger, and their colleagues, it could be the first indications that the invisible dark matter is fuzzy.
“It’s this observational test we can provide for the nature of dark matter, based on observations of the early Universe, which will become feasible in the next couple of years,” commented Dr. Vogelsberger to the press.
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