Buzz
Humanity has gathered an immense wealth of knowledge about the cosmos and its workings. We pride ourselves on being the most advanced intellectual species on Earth and, so far, the most capable in the known universe.
However, our understanding of the universe's structure comes from only a tiny 4 percent of observable, measurable, and analyzable matter—what we refer to as ordinary matter. The other 96 percent consists of 'dark' matter. It's called dark because we remain unaware of its nature (and physicists, it seems, struggle with creativity when it comes to naming things).
Out of that 96 percent, about 68 percent is dark energy. This makes it the most dominant yet elusive component of the universe. Currently, thousands of scientists across the globe are striving to decode this mysterious force that seems to influence the creation of large cosmic structures.
Without dark energy, the universe would eventually face a 'Big Crunch'—a gravity-driven collapse where everything would rapidly fall back in on itself. So, even though the nature of dark energy remains a mystery, we should be grateful for its existence.
Here are the top 10 hypotheses regarding the nature of dark energy and what each possibility might reveal about the ultimate fate of our universe.
10. It Is A Characteristic Of Space
This idea stems from Einstein’s theory of gravitation, particularly the concept that 'empty space' can possess its own energy, known as 'the cosmological constant.' Einstein also proposed that space itself could emerge from nothing, and as more space is created, more energy can be contained within it.
This could account for the rapid expansion of the universe that we observe. A universe like this would keep expanding indefinitely, until every object is so distant from every other one that the universe would end in an eternal, cold darkness.
9. The Theory of Everything
Many astronomers argue that the search for dark energy is a lost cause. Instead, they suggest that discovering the elusive 'theory of everything' (not the Stephen Hawking movie) would naturally resolve the mystery of dark energy.
This theory would aim to explain the behavior of all objects in the universe, from the largest to the smallest. Currently, our understanding of the universe is divided between large-scale theories (like the theory of gravity) and small-scale theories (such as quantum mechanics).
While resolving the dark energy puzzle in this way seems logically plausible, discovering such a theory has proven elusive, even for the brightest minds in physics. The usual laws of physics appear to 'break down' when we reach the quantum realm. Yet, the search continues.
8. It Introduces A New Fundamental Force
The fundamental forces that we are familiar with (gravity, electromagnetism, weak force, and strong force) each operate over different ranges. Some act only on atomic-sized particles, while others influence the motions of planets and even drive the formation of galaxies.
According to this dark energy hypothesis, there could be a fundamental force yet to be discovered that operates on massive scales, only becoming noticeable once the universe reaches a certain size. This force would counteract gravity, causing objects throughout the universe to move apart from one another.
Scientists argue that because this force operates on such a vast scale, we haven't encountered it in our daily lives, and measurements taken on Earth cannot be influenced by it. It's unclear whether this force is temporary or permanent. Depending on this, the universe could either keep expanding until it freezes over, or it might expand and contract cyclically for eternity.
7. Einstein’s Gravitational Theory Is Incorrect
Imagine telling one of the greatest minds in history that his (arguably) most famous theory is wrong... yikes. Einstein's theory of relativity states that every object in the universe is attracted to every other object, with the strength of this attraction depending only on the masses of the objects and the distance between their centers.
However, some physicists have suggested that this theory might be flawed and have been working on new gravitational models to account for dark energy. In these models, they reverse gravity's effects on large scales, causing objects to repel each other instead of attracting them.
Although these theories lack significant experimental support (since Einstein’s gravitational model has worked well for us thus far), they could provide an explanation for the universe's expansion. With these new theories of gravitation, our universe would eventually reach a state of cold darkness after a period of rapid expansion.
6. Time Dilation
If you've ever watched the movie Interstellar, you've likely encountered the concept of time dilation. This is a phenomenon where objects moving near the speed of light experience time at a slower rate.
This concept is similar to the twin paradox, in which one twin travels in a spaceship near the speed of light, while the other twin stays on Earth. When they reunite after many years, the Earth-bound twin is significantly older than the one who traveled in space.
A recent paper by Edward Kipreos, a professor at the University of Georgia, suggests that only the object in motion experiences time dilation. (Typically, the observer of the fast-moving object also feels the effects.)
This would suggest that time passed more quickly in the past. This negates the necessity for a repulsive force or substance, as the apparent expansion of the universe would simply be a miscalculation of distances altered by time dilation.
If this theory holds true, it would not only challenge another of Einstein’s renowned theories (his theory of special relativity) but also imply that our universe would keep expanding due to the influence of the Hubble Constant.
5. An Exotic New Particle
The concept of particles and fields has been around for centuries. We know that an electron generates an electric field, and more recently, the gravitational field has been linked to the 'graviton'—the 'force particle' of gravity. Particle physicists and theorists have become accustomed to the idea that the energy of a given field is transmitted by its force particle, not by the field itself.
This concept can be applied to dark energy, with dark matter (the other 27 percent of the universe) serving as its force particle. This idea seems plausible, especially since some force particles, like the graviton, remain undetectable. However, there is little evidence to support this theory, as we have not yet discovered a method to measure any properties associated with dark energy or dark matter.
4. f(R) Theories
f(R) theories are models based on the current curvature of the universe (denoted by R). In 2007, researchers from the University of Chicago demonstrated that with a specific value of R, a model of the universe could be created in which dark energy is unnecessary to explain the universe's expansion.
This type of universe bends in such a way that minimizes its overall curvature while generating an additional gravity-like force that can either attract or repel objects, depending on certain conditions.
The theorists at the University of Chicago agree that for this theory to be valid, the additional force must vanish where gravity is relatively strong (such as on the scale of planets and galaxies) and appear only at the largest scales. A team of astronomers from Peking University has started taking measurements of clusters to determine if this f(R) theory could accurately describe our universe.
3. Quintessence
The variety of theories on this list illustrates just how far we are from understanding two-thirds of our universe. So far, each theory has profound consequences for the universe’s fate or for the legacy of Einstein.
Unlocking the mystery of dark energy could pave the way for an entirely new field of physics or could radically alter existing ones. This is why so many physicists and astronomers are intensely exploring this vast, enigmatic “dark stuff” that shapes the evolution of our universe.
The final theory of dark energy is arguably the most bizarre. A universe governed by 'quintessence' is one filled with an 'energy fluid.' Some physicists prefer to refer to this energy as 'phantom energy.'
The concept behind this is that quintessence changes with time and location, with its energy density increasing as time passes. Such a universe would meet a catastrophic end known as the 'Big Rip' (with its typically literal name), where the universe would literally tear apart as atoms can no longer withstand the forces pulling them apart and stretching them.
Everything, visible and hidden, would be destroyed. The universe would end with a tremendous bang.
2. Virtual Particles
Quantum mechanics is truly bizarre. It allows for phenomena where things can materialize and vanish, defying the basic principles we were taught in high school physics. ('Matter cannot be created or destroyed,' we once said.)
This theory incorporates the concept of virtual particles—tiny particles of matter that appear momentarily and then vanish. The constant emergence and disappearance of these particles results in energy being released, as matter is transformed into energy when the particles disappear.
Physicists propose that this is how space itself can accumulate enough continuous energy to generate a 'negative pressure,' which in turn drives the expansion of the universe. If this theory holds, the energy space gains from these virtual particles could account for dark energy, allowing our universe to keep expanding indefinitely as long as this process continues.
1. Multiverses And The Anthropic Principle
One of modern physics' greatest shortcomings is its failure to accurately predict the true value of dark energy. Quantum theory suggests a very small value, yet physicists have calculated a number more than 10 times larger! (This calculated value of dark energy is known as the cosmological constant, the same one mentioned in item 10 on this list.)
This is where the anthropic principle comes into play—the notion that fundamental constants of physics and chemistry (like the speed of light, the gravitational constant, etc.) are 'just right' to enable life in our universe, but could vary in other universes. In a vast set of parallel universes, it doesn’t seem too far-fetched that ours would be the one with the perfect value of dark energy for life to form.
