If you are a fan of cosmology, you probably would have read one thing or the other about the universe expanding. However, the big question is this: is the world expanding? For many years, theorists have continued to search further in their quest to understand the cosmos. As early as 1929, American astronomer Edwin Hubble was able to determine that the universe is expanding by studying exploding star, called Supernovae. Many years later, in 1998, the Hubble Space Telescope study of very distant Supernovae indicated that the universe was expanding more slowly nine billion years ago than it is today. The revelation countered the belief that the universe has been expanding slowly due to gravity. Cosmologists felt something must have been counteracting gravity for the universe to have been accelerating. After several suggestions, a solution dubbed “dark energy” was adopted to explain the concept.
What is dark energy? Dark energy remains a hugely strange term. However, it is best described as an unknown form of energy which is believed to permeate all spaces, tending to accelerate the expansion of the universe. Below are six things you should know about dark energy:
- Dark energy is the dominant energy in the universe: The universe is made up of three components. These are dark energy, dark matter and ordinary matter. Among these three, dark energy makes up nearly three-fourths of the universe’s total energy. According to the best current measurements in the standard model of Cosmology, Dark energy contributes about 68.3% of the total energy make-up of the universe. On the other hand, dark matter and ordinary matter contribute 26.8% and 4.9%, respectively.
- Dark energy is a property of space: One of the different explanations for dark energy is that it is a property of space. It is uniform across space. This unique characteristic verifies one of Albert Einstein’s theories which explains that space is nothing.
- Dark energy is homogeneous and not very dense: Its homogeneity is asserted by the fact that it does not get diluted when space expands. Additionally, it is not known to interact through any of the fundamental forces other than gravity. Dark energy has also been discovered to be persistent with constant density. However, it is very unlikely to be detected in lab experiments.
- Dark energy must have negative pressure: For it to explain the observed acceleration of the expansion of the universe accurately, Dark energy would need a very high negative pressure like radiation pressure in a meta-material.
- The relationship between Dark energy and Dark Matter: As explained earlier, these two concepts are two essential parts of the universe composition but are widely dissimilar. Apart from having different energy make-ups, they also have different densities. The density of dark matter in an expanding universe decreases more quickly than Dark energy. After some time, that of dark energy usually dominates. Furthermore, when the volume of the universe doubles, the density of dark matter is halved, while that of dark energy remains nearly unchanged. Another difference between the two is that while Dark Matter explains how groups of objects function together, dark energy is the force that accounts for the expanding universe.
- Forms of Dark energy: The two proposed forms of dark energy are the cosmological constant and the scalar fields. The former is a constant energy density filling space homogeneously. The latter, on the other hand, are dynamic quantities whose energy densities vary in time and space. Quintessence and moduli are examples of these scalar fields. However, you should know that scalar fields that do not change in space are often different to distinguish from a cosmological constant. This occurs especially when the change is extremely small.
Given the endless quests of scientists to study the expansion of the universe, the mysteriousness of dark energy remains rather astonishing. Many cosmologists are of the opinion that the deficiency in the knowledge about dark energy signals to us that cosmos might be configured differently to what we imagine. Thus, there is a pressing need for not just scientists but all of us to endeavor to fill the gap in our knowledge about the universe.
Photo Credit: NASA