Hiding In The Universe's Shadowland

 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 have 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 exquistely 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 has 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.