What role do dark matter and dark energy play in the evolution and fate of the universe, and how might our understanding of these mysterious components reshape our current cosmological theories

What role do dark matter and dark energy play in the evolution and fate of the universe, and how might our understanding of these mysterious components reshape our current cosmological theories?

Image Credit: Nasa Space Center hubble telescope 

What exactly is dark energy? 

There is still so much that we don't know about it. However, we do have some understanding of its effects on the expansion of the universe. Dark energy makes up about 68% of the universe, while dark matter accounts for roughly 27%. Surprisingly, all the matter we are familiar with, including everything on Earth and everything we have observed, only makes up less than 5% of the universe. It's quite remarkable to think about how small a fraction that is.

One idea proposes that dark energy is an inherent characteristic of space.

Albert Einstein was the first to realize that empty space is not truly empty, but rather has its own unique properties. One of these properties is the ability for more space to come into existence. According to a version of Einstein's gravity theory, known as the cosmological constant, empty space can possess its own energy. This energy would not be diluted as space expands, meaning that as more space is created, more of this energy-of-space would appear. Consequently, this form of energy would cause the universe to expand at an increasingly faster rate.

However, the mystery remains as to why the cosmological constant exists and why it has the precise value needed to explain the observed acceleration of the universe. It's a puzzle that scientists are still working to unravel.

In the early 1990s, there was a general understanding about the expansion of the universe. It could either have enough energy density to eventually stop expanding and collapse, or it could have such little energy density that it would continue expanding forever. However, gravity was expected to slow down the expansion over time, even though this hadn't been observed yet. The universe is filled with matter, and gravity pulls all matter together, so it was believed that the expansion would eventually slow down.

But then in 1998, the Hubble Space Telescope made some remarkable observations of distant supernovae. These observations showed that the universe was actually expanding at a slower rate in the past compared to today. This meant that the expansion of the universe wasn't slowing down due to gravity as previously thought, but instead, it was accelerating. This discovery took everyone by surprise, and no one knew how to explain it.

As a result, theorists came up with three possible explanations. One was that it could be a result of an old version of Einstein's theory of gravity, which included something called a "cosmological constant." Another possibility was that there might be some strange type of energy-fluid filling the space. Lastly, it was also considered that there could be something fundamentally wrong with Einstein's theory of gravity, and a new theory might be needed to explain this cosmic acceleration.

Even though the correct explanation is still unknown, theorists have given it a name - dark energy.

What exactly is dark matter? 

Scientists have been able to determine the composition of the universe through various observations, and they have found that dark matter makes up about 27% of it. However, we still have more knowledge about what dark matter is not rather than what it actually is. Firstly, dark matter is called "dark" because it cannot be seen in the form of stars or planets like the ones we observe. The amount of visible matter in the universe is not enough to account for the 27% of dark matter that we have observed. Secondly, dark matter is not made up of dark clouds of normal matter, which are composed of particles called baryons. We know this because if it were, we would be able to detect these baryonic clouds through the absorption of radiation. Thirdly, dark matter is not antimatter, as we do not observe the specific gamma rays that are produced when antimatter annihilates with matter. Lastly, we can rule out the possibility of large galaxy-sized black holes contributing to dark matter because we do not observe enough gravitational lensing events. These events occur when the concentration of matter bends light passing near them from objects that are farther away. The number of lensing events we observe does not align with the amount of dark matter required.

The core of the merging galaxy cluster Abell 520 is a fascinating sight, revealing the distribution of dark matter, galaxies, and hot gas. However, this discovery poses a challenge to our current understanding of dark matter. It seems that there is more to this mysterious substance than meets the eye.

Another intriguing explanation for the energy in space comes from the quantum theory of matter. According to this theory, "empty space" is not truly empty but filled with temporary particles that constantly appear and vanish. However, when scientists attempted to calculate the energy of empty space using this theory, the result was way off the mark. The number came out to be 10120 times larger than expected, a truly staggering difference. This discrepancy leaves us with more questions than answers, deepening the enigma.

Another possibility is that dark energy is a unique form of dynamic energy fluid or field, distinct from matter and normal energy. This hypothetical substance, sometimes referred to as "quintessence," fills the vast expanse of space. Yet, despite this idea, we still lack a clear understanding of what quintessence is, how it interacts with other elements, and why it exists. The mystery persists, leaving us searching for answers.

Lastly, there is the chance that Einstein's theory of gravity may not be entirely accurate. If this were the case, it would not only impact the expansion of the universe but also the behavior of normal matter in galaxies and clusters. By observing how galaxies come together in clusters, we could potentially determine if a new theory of gravity is necessary to solve the dark energy puzzle. However, finding such a theory poses its own challenges. It must be able to accurately describe the motion of celestial bodies in the Solar System, as Einstein's theory does, while also providing the different predictions we need for the universe. Although there are some candidate theories, none of them have proven to be truly convincing. Thus, the mystery remains unsolved, urging us to delve deeper into the unknown.