[Astrophysics/Astronomy] What is Dark Energy

──The invisible force accelerating the expansion of the universe

　Dark energy. It sounds like something straight out of science fiction, doesn't it? It is also called dark energy. If you don't usually keep up with science news, this might be a term you've never heard before, but it is a legitimate scientific term. In this article, I will explain what this sci-fi-sounding dark energy is.

1. The universe is expanding, and the expansion rate is accelerating

　In the early 20th century, when Einstein published his theory of general relativity, many scientists, including Einstein himself, believed that the universe was static and unchanging.
　However, in the 1920s, Edwin Hubble discovered from his observations that all galaxies are moving away from Earth. Furthermore, the farther a galaxy is from Earth, the faster it is moving away. This means that the entire universe is expanding.

The universe is expanding...!

　That alone was a huge surprise, but in 1998, there was another shocking discovery.

　Detailed measurements of the brightness of distant supernovae revealed that the expansion rate of the universe is accelerating.

　Specific supernovae in distant galaxies have a "standard brightness," and their distance can be measured from the intensity of their light. By comparing these distances with their redshift (an indicator of the speed at which a galaxy is moving away), it was found that the universe is expanding faster now than it was in the past.

　Since gravity is an attractive force, it was thought that the stars would gradually draw closer together due to the gravity of the matter in the universe (there are 20,000,000,000,000,000,000,000 stars alone!). It was also believed that because of this gravity, the universe itself would eventually lose the momentum of the Big Bang and shift from expansion to contraction. However, the facts obtained from observations were the exact opposite. Why is the expansion of the universe not stopping, but rather accelerating? This is where "dark energy" comes in.

2. What is dark energy?

　Dark energy is an unknown energy whose existence was predicted in order to explain the expansion of the universe without contradiction.

　The fact that the universe is accelerating in its expansion led to the idea that there must be some kind of "repulsive energy" that exists throughout the universe, pushing it outward against gravity. This unknown entity was named dark energy.

　Dark energy is invisible, and with current technology, it cannot be measured or observed. In other words, its true nature is unknown.
　If it cannot be seen or observed, common sense would suggest that it does not exist. However, scientific thinking suggests that "if its existence can be inferred from other observational results, then it certainly exists, and perhaps the technology or methods to observe it have simply not yet been established."

　It is believed that approximately 68% of the current composition of the universe is this dark energy.
　The rest consists of only 5% normal matter that we can perceive (elements, elementary particles, etc.) and about 27% dark matter.
　In other words, with current science, the vast majority of the universe is of unknown nature!

3. Candidates for its true nature

　The essence of dark energy remains a mystery, but there are several hypotheses regarding its true nature.

・The Cosmological Constant (Λ) Theory

　The "cosmological constant" that Einstein once introduced into his theory of relativity is being re-evaluated.
　The cosmological constant was something Einstein forcibly introduced because he wanted to believe that "the universe must be immutable!" It was later dubbed his "greatest blunder" by Einstein himself after the expansion of the universe was observed.
　However, once it was discovered that the expansion of the universe is accelerating, the possibility emerged that this cosmological constant might not have been a mistake after all.
　If we consider the cosmological constant to be the energy inherent in the vacuum itself, and that a repulsive force is generated simply by the existence of space, the existence of dark energy can be explained quite well.
It is a mathematically simple theory with many supporters.

・Scalar Field (Quintessence)

　This is the idea that an unknown energy field, whose strength changes depending on time and space, is spread throughout the universe.
It is a more flexible model than the cosmological constant and has the potential to explain observational results more precisely.

・Modification of the Laws of Gravity

　There is also the idea that general relativity itself is not perfect on the scale of the entire universe, and that the laws of gravity themselves should be reconsidered.
If this is correct, it would mean rewriting the foundations of physics.

4. An existence that determines the future of the universe

　Depending on the nature of dark energy, the future that the universe follows will change significantly.

　If the strength of dark energy remains constant as it is now, the universe will continue to accelerate, galaxies will drift further and further apart, and eventually, a dark and cold "Big Freeze" will arrive.

　If the power of dark energy increases over time, a scenario called the "Big Rip" awaits. Every structure in the universe could be torn apart, with galaxies, stars, and even atoms being ripped to pieces. Of course, we humans would be torn apart as well.

Conversely, if the power of dark energy were to weaken, the expansion could stop, and a future 'Big Crunch' where the universe contracts again due to gravity is also possible.

5. Connecting it to familiar sensations

While 'the accelerated expansion of the universe' may sound like a distant topic, it is actually related to the very background that allows us to exist. It has been pointed out that if the power of dark energy were stronger or weaker than it is now, galaxies might not have been born, and there might not have been an environment for life to exist.
When we consider that we can gaze at the night sky and imagine the past and future because the energy balance of the universe is miraculously maintained, it feels a little more familiar.

6. The forefront of science facing the unknown

Research into dark energy is one of the greatest challenges in cosmology. Many international projects are underway, including precise observations of the CMB (Cosmic Microwave Background), large-scale surveys of galaxy distribution, and analysis of gravitational lensing effects.
Cutting-edge observations, such as Japan's 'Subaru Telescope,' the European 'Euclid Space Telescope,' and NASA's 'Roman Space Telescope,' are searching for clues about dark energy.

◇ Summary

Dark energy is an invisible energy that causes the accelerated expansion of the universe and is estimated to account for about 70% of the universe. Its true nature is still unknown, but unraveling it is directly linked to understanding the future of the universe.

Dark energy, one of the greatest mysteries of the universe, is also the background that determines the conditions of our existence. If you imagine the accelerating universe spreading out beyond the night sky, you can realize just how deeply this invisible force shapes our world.