Dark Matter and Dark Energy— 95% of our Universe

  I don’t know about you, but whenever I come across the topic of astronomy, I always have a slight existential crisis over how vast our observable universe is. It doesn’t help that my family loves to go to planetariums to periodically remind me of our insignificance in the grand scheme of things. Everytime I go to these shows, I always admire how astronomers undertake the grand task of bringing sense and order to the contrasting mystery of space. A few months ago, I happened to watch one of these shows that focused on the broad forces of dark matter and dark energy. Because these were complex topics being presented to the general public, the show simplified their descriptions: Dark matter is the driving, pulling force that holds the universe together, while dark energy is the driving, pushing force of expansion that pushes the universe further apart.

The vague description of these two gigantic forces being at play caught my attention, and I immediately wanted to explore the topics further. So, any free moment I got in between my painful schedule as a junior in high school, I spent researching dark matter and dark energy. Long story short: it was A LOT of information to digest, which you can probably tell when you look at the monstrosity that is my citations. I would read an article to answer one question, and leave with 10 more questions. Thankfully, I’ve worked my magic to compile most of what is known about the mysterious dark forces, including numerous hypotheses to cover the wide area of uncertainty regarding this topic. I'm specifically going to explore what exactly these forces are, their functions on the grand scale of the universe, possible theories of what they are made of, and how the universe is expected to be impacted by them in the future. Let’s begin with dark matter, the more well-known of the two.

  There are several different hypotheses of what dark matter could be, but they are vague and prone to error because of our inability to observe dark matter directly. Knowing what dark matter is made of is essential in discovering more about its properties, and the mechanics of those properties as well. Luckily, there are several promising theories, with experiments running to gather evidence behind them. Most likely, dark matter is  composed of multiple, complex particles that we have not discovered yet. The candidates must have the characteristics of what we know about dark matter currently: it must not interact with normal matter or light, but still possess a gravitational pull.

One of the earliest theories for the composition of dark matter is the WIMP hypothesis– quite the name. WIMP stands for “Weakly Interacting Massive Particles,” which is pretty self-explanatory. Scientists theorized the existence of these particles, stating that it could be a new type of neutral, massive particle that only interacts with other particles through gravity and a weak nuclear force. To fully define what a weak nuclear force is would be dragging you through particle and nuclear physics, so I’ll oversimplify it. Think of it as a small, directional force that’s limited to only affecting particles on the subatomic level. While the description of a singular particle is somewhat underwhelming, this would make some more sense on a larger scale, and the mechanics of this particle would likely imitate the behavior of dark matter. 

  So far, nothing has proved that WIMP particles actually exist. Because they interact so weakly, they would be difficult to detect in the first place. Searches for WIMP particles include using sodium iodide detectors that emit radiation upon possible WIMP collisions, or watching for slight temperature increases as an indication of WIMP collisions. Theoretically, these particles can’t be too large either, otherwise it would be inconsistent with the observed density of matter in the universe. However, this hypothesis is especially promising because of the simplicity of the WIMP particle, which has no electric charge, is stable, and is allowed to have a wide range of masses.

   Most of the theories surrounding dark energy are based on the concept of “empty space.” This describes areas of space that are essentially rid of any matter, but still contain energy– otherwise known as a vacuum. The majority of our universe is empty space between known bodies of mass, and the amount of empty space continues to increase because of dark energy stretching the universe further apart. Dark energy is emitted from these large areas of empty space, so it must be considered in each prevailing theory on dark energy.

The leading theory for the explanation of dark energy is the cosmological constant theory. The cosmological constant is essentially a measure of the energy that empty space should have to explain the accelerating expansion of our universe. Think of the cosmological constant as the lowest energy state, like a “0” on the scale of energy. Although this form of energy would drive the accelerating expansion that is observed, it still remains a mystery. No one knows exactly why the constant exists in the first place, and the number was actually created by Einstein to fix an error in his theory of general relativity. Dubbed Einstein’s “biggest blunder,” as it turned out to be unnecessary in his calculations, the concept was picked back up again later and utilized for a different purpose when the universe was proven to be expanding at an accelerating rate. What’s even more of a mystery is how the constant would have exactly the right value to cause the observed expansion. The cosmological constant theory is dubbed one of the more “simpler” explanations for dark energy– which I scoff at because it took about an hour to understand the theory well enough to write about it– and has remained the most popular explanation for about two decades.

   This theory has also been backed up by evidence recently obtained by researchers at the Baryon Oscillation Spectroscopic Survey (BOSS), who measured galaxies up to 6 billion light-years away to a shocking accuracy of 1%. The measurements corresponded to a form of dark energy that is uniformly spread throughout the universe, which aligns well with the cosmological constant theory and the quintessence theory, which will be discussed later on. The idea of a cosmological constant may seem to contradict the observation of an accelerating universe, but I can assure you that it is consistent with current observations. This is because dark energy’s uniformity in space implies that its effect is constant in terms of time as well. In other words, dark energy’s effect isn’t weakened as time goes on, allowing a constant, gradual expansion of the universe even if its energy was theoretically constant. So, the cosmological constant theory in practice would lead to the acceleration of the universe, but just at a near-constant pace. The rate of acceleration isn't clear from current observations of our universe, and it's likely that the acceleration will occur at either a constant rate (cosmological constant theory) or exponentially (quintessence theory). 

   The Quintessence theory shares the idea that dark energy is uniform throughout space, but contradicts the cosmological constant theory by claiming that the energy assigned to it isn’t a constant. It assigns the properties of dark energy to an entirely new, fifth force in physics called “quintessence” that changes over time. It can either be an attractive or repulsive force based on the potential to kinetic energy ratio, making it a little bit more nuanced. Because of its ability to fluctuate, quintessence allows for a lot more hypotheticals and a wider range of behavior than the cosmological constant theory. It's possible that quintessence could allow for a very high rate of expansion, or it could potentially fluctuate and cause the universe to slow its expansion, even contracting back into itself at a certain point. I'll further expand on the possible effects of quintessence in the next section.

   Other theories range from claiming that dark energy comes from black holes to simply claiming that Einstein’s theory of gravity is untrue, allowing for a wider range of hypotheticals. Because Einstein's theories have been backed up by over a century of research, I didn’t look too deeply into any theories that weren’t grounded in his gravity theory. However, given the amount of limitations in directly studying dark energy, almost anything could be possible.

What is the future of dark matter and dark energy?