What is Dark Energy?


An image of the cosmic microwave background, the isotropic radiation field left over from the Big Bang. Scientists study it to better understand the Big Bang and the matter-energy content of the universe.
Credit: NASA

Dark energy is a concept in modern cosmology that is best defined as the intrinsic energy in empty space, or “vacuum energy.” This energy contributes to an outward pressure that causes space to expand. In turn, the expansion creates more space, which then creates more dark energy, and thus expands space further and faster. This paradoxical interaction in which energy is seemingly created out of nothing is what scientists theorize is responsible for the accelerating expansion of the universe. 

How did the idea of dark energy come to be?

The history of dark energy originates with Albert Einstein’s famous general theory of relativity. The theory is the most accurate descriptor of gravity and its functionality. It accomplishes this feat through what are known as the Einstein field equations. These equations explain that gravity is the effect of massive and energetic objects curving the fabric of space (and time).

In 1917, after Einstein first developed the equations, he included an additional term called the cosmological constant. The sole purpose of this constant was to ensure that the equations described a static universe. We now know that the universe is not static; it was created in a massive explosion, the Big Bang. This term now represents dark energy, as seen in the image below.

Einstein, however, formulated the field equations before the Big Bang theory became the prevalent theory of the origins of the universe. Therefore, the cosmological constant was initially meant to act as an outward pressure, or “negative gravitational force” in empty space, thus preventing massive objects from crushing space in on itself. Einstein calculated the constant in such a way so that this outward pressure would only be enough to keep the universe from changing size. 

A computer-generated depiction of a two dimensional version of gravity curving spacetime; this process actually occurs in all three. The Einstein field equations are beneath the image, where the cosmological constant is represented with the Greek letter Λ . The one equation actually represents 16 field equations.
Credit: NASA

In 1929, Edwin Hubble, an American astronomer for whom the famous space telescope is named after, discovered that galaxies outside our own were moving away from us. He noted that the farther away the galaxy was, the faster it was moving away. This indicated to Hubble and the scientific community at the time that the universe was expanding. To clarify, consider for a moment that the universe is a chocolate chip cake with chips representing galaxies. When the cake is baked in an oven, it expands. Each chocolate chip moves away from the others as the space between them expands.

This is exactly what happens with the universe; the space between galaxies expands, with the effect being that the farther away a galaxy is, the faster it is moving away from us. Hubble formalized this principle in what is now known as Hubble’s law. 

After Hubble’s discovery, Einstein retracted the idea of the cosmological constant and considered it one of his biggest blunders. From then on, it was accepted that the constant had a value of zero. However, in 1998, Einstein’s supposed blunder became relevant once again when scientists discovered that the rate at which the universe was expanding was accelerating. His idea of vacuum energy was the perfect explanation for an accelerating universe. Since then, scientists have calculated a positive, non-zero value for the cosmological constant to reflect the principle that empty space has energy. 

Why is understanding dark energy so important?

Why is solving the dark energy mystery so critical in modern science? Recent studies of the cosmic microwave background radiation have revealed the mass and energy content of the entire universe. Scientists believe that about 70% of the entire universe is made up of dark energy. Only around 30% of the universe is matter, and about 70% of that figure is dark matter.

Therefore, every galaxy, star, planet, and nebula only account for about 5% of the makeup of the universe. That alone should reveal why it is crucial to modern science that we understand what dark energy is and whether or not it actually exists. 

Dark energy will dictate the fate of the universe. If the cosmological constant has a positive value, then empty space has a positive energy density and therefore exhibits an “anti-gravitational” outward pressure on the universe, causing it to expand. As space expands, more space is created, and therefore the universe has more dark energy with which to push out on itself.

The cosmological constant may not be a constant; this type of dynamic dark energy is often referred to as quintessence. If this value increases over time, then the expansion will accelerate infinitely, and we are left with a “run-away” universe, which will expand until planetary systems and even subatomic particles rip themselves apart; this scenario is commonly known as the Big Rip. If the expansion slows to some constant rate, then objects will move away from each other until they cannot see each other and galaxies are left on their own, isolated from the rest of the universe; this is the Big Freeze.

If the expansion completely stops, then, depending on the critical density of the universe, everything may collapse in on itself in a “reverse Big Bang,” called the Big Crunch. Understanding the eventual fate of the universe depends greatly on our understanding of dark energy. 

An artist’s rendition of the possible fates of the universe, which depend on the type of dark energy that exists in our universe. Dark energy could lead to either the Big Freeze, Big Rip, or Big Crunch, depending on the density of the universe and type of dark energy.

Unfortunately, as was the case with dark matter, dark energy is not a perfect theory. While it works perfectly in the field equations, it does not match up with the value for vacuum energy predicted by quantum field theory (QFT). This theory is the modern model of quantum mechanics, a branch of physics that deals with atomic and subatomic interactions.

The Einstein field equations and general relativity as a whole are a classical theory; it does not take quantum theory into account. The discrepancy between a quantum and a classical theory can be seen in the difference of the prediction made by QFT and the cosmological constant. The quantum value is about 10120 times larger than the value given by Einstein’s equations. This extreme difference has been dubbed by physicists the “worst theoretical prediction in the history of physics.” 

So, the value is wrong, does this mean that dark energy is not real? Not necessarily. It is quite possible that the field equations need revision to include quantum mechanics, but this does not mean that the cosmological constant and the idea of dark energy would be removed entirely. Perhaps a greater problem for the existence of dark energy lies in our fundamental understanding of gravity. 

An illustration of the history of the universe, from the Big Bang to the present day. Note that the cosmic microwave background radiation is from 380,000 years after the Big Bang. It is only in the recent history of the cosmos that the aggregate dark energy content has accelerated the expansion of the universe.
Credit: NASA

General relativity, although the most accurate model of gravitation, does not explain what gravity is, only how it acts. The Standard Model of particle physics, one of the greatest achievements of quantum mechanics, dictates that there must be a force carrier particle, or boson, for each fundamental force in the universe. For the electromagnetic force, the boson is the photon, commonly known as a particle of light.

The boson for the strong nuclear force, the force that binds protons and neutrons together in atomic nuclei, is the gluon. For the weak nuclear force, the force that is responsible for the radioactive decay of atoms, the force carrier particles are the W and Z bosons. Scientists have not discovered the boson for gravity, but it is hypothetically known as the “graviton.” The discovery of this particle will provide valuable insight into the mysterious behavior of gravity. 

As was discussed regarding dark matter, gravity may work differently on large scales, perhaps due to the strange quantum behavior of the theorized graviton. It may be that some gravitational interaction on the distance scales as large as those seen between galaxies is what causes the accelerating expansion of the universe. In order to properly unveil the mysteries of gravity, it will likely require a revolution in quantum mechanics. Such a revolution would undoubtedly provide us with insight into the mystery of dark energy and unveil some of the deepest secrets of the cosmos.

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