Dark Energy: The Biggest Mystery in the Universe.

The dominating form of energy in the cosmos, dark energy is driving the accelerating expansion of the universe, but its nature remains a complete mystery.

WHAT IS DARK ENERGY?

Dark energy is a hypothetical form of energy that is proposed by physicists to explain why the universe is not just expanding but is doing so at an accelerating rate.

Think of dark energy as the "evil counterpart" to gravity–an "anti-gravity" force providing a negative pressure that fills the universe and stretches the very fabric of spacetime. As it does so dark energy drives cosmic objects apart at an increasingly rapid rate rather than drawing them together as gravity does.

Dark energy is estimated to account for between around 68% to 72% of the universe's total energy and matter  —   its matter/energy budget  —  meaning it heavily dominates both dark matter and everyday matter.

The only real answer to the question "what is dark energy?" currently is "we don't know" as unsatisfactory as that may be. Scientists aren't completely in the dark, however. There are some leading candidates to account for dark energy.

These include the vacuum energy of space  —  particles literally popping in and out of existence in empty space — and a "fifth force" responsible for the negative pressure that might cause the accelerated expansion of the universe.

Other possibilities a range of different "flavors" of fields that could account for dark energy such as a low-energy field dubbed "quintessence," fields of tachyons  —   hypothetical particles that travel faster than light and thus back in time  —  among other concepts.

All of these remain purely hypothetical meaning that the only way we can really "know" dark energy currently is via its effect on the universe.

WHAT DARK ENERGY DOES (AND DOESN'T) DO?

If dark energy is causing the universe to expand at an accelerating rate, shouldn't we see our coffee mug shift away from us or notice our commute to work getting longer each day?

We don't see things like this happen (even though it may feel like we do sometimes) because objects that are gravitationally bound such as stars, planetary systems, star clusters, galaxies, clusters of galaxies, and even our coffee mug and table, don't seem to experience dark energy's effects. Gravity still beats dark energy at small scales.

Dark energy only seems to act on the largest scales of the universe, with the expansion of the universe a phenomenon that can only be measured by observing galaxies and other cosmic objects that are separated by massive gulfs of space in the order of millions, billions, and even tens of billions of light-years apart and away from us. And the greater the distance that separates these cosmic objects, the more rapidly they race away from each other.

As a simple analogy for this, imagine drawing three spots on a deflated balloon, two close together and the other further apart. In this analogy, dark energy is the breath blowing into the balloon overcoming gravity which is represented by the tension of the balloon's rubber skin. As the balloon is inflated all three points will move away from each other, but the furthest point will move away more rapidly.

This is like three galaxies, two close together and the other further apart, the latter of which is moving away faster because the space between it and the other galaxies, like the rubber of the balloon, is stretching, and more space means more expansion.

Currently, scientists estimate that galaxies are getting 0.007% further away from each other every million years. American theoretical astrophysicist Ethan Siegel explained in a column for Big Think that in "real terms" for a cosmic object 100 million light-years away astronomers infer it is receding at 1,336 miles per second (2,150 kilometers per second). Meanwhile, a galaxy 1 billion light-years away is receding 10 times faster at around 13,360 miles per second (21,500 km/s).

The rate of expansion has been measured for the galaxy (Galaxy GN-z11), one of the oldest galaxies ever discovered which we see as it was when the universe was just 400 million years old. An estimated 32 billion light-years away, dark energy is expanding the fabric of space at such a rate that GN-z11 is moving away from us at an estimated speed of 426,882 miles per second (687,000 km/s) — over twice the speed of light.

While it's true that nothing can travel through space faster than the speed of light in a vacuum, 186,282 miles per second (299,792 km/s), dark energy demonstrates that the fabric space itself is not bound by such speed limits.

As they separate out, galaxies retain their shape and don't spread apart internally thanks to another 'dark' aspect of the universe  —   dark matter.

Don't be fooled by the similar names, dark energy and dark matter — sometimes grouped together and described as the "dark universe"' — aren't believed to be related aside from a few superficial similarities.

WHAT EVIDENCE DO WE HAVE FOR DARK ENERGY?

The first detection of dark energy through the discovery that the expansion of the universe is accelerating was made by two teams of scientists working independently in the late 1990s.

These teams were conducting surveys of Type Ia supernovas, cosmic explosions that occur when massive stars die and that produce light emissions so uniformly that they are excellent for measuring cosmic distances.

This is because as the universe expands light from distant sources that takes a long time to travel to Earth has its wavelength "stretched out." As red is a color associated with long wave light, this results in a reddening of light that astronomers call "redshift."

The further away a light source is, the more its light is red shifted, with sources from extremely distant sources that existed when the universe was young shifted into the infrared region of the electromagnetic spectrum.

The astronomers were observing these so-called "standard candle" supernovas to attempt to measure the rate of universal expansion  —  called the Hubble constant.

What they found was more distant supernovas that had exploded when the universe was much younger were fainter than expected. This meant these supernovas were further away than they should be, implying the expansion of the universe was speeding up.

This discovery would be confirmed with follow-up observations and by measurements of a field of radiation left over from just after the Big Bang called the "Cosmic Microwave Background (CMB)."

WHY IS IT IMPORTANT TO UNDERSTAND DARK ENERGY?

Understanding dark energy is key to building an accurate model of how the universe has evolved over time, and this includes the shape it takes and how it will end.

Both the origin and fate of the universe are determined by its "critical density" which Swinburne Center for Astrophysics and Supercomputing defines as "the average density of matter required for the universe to just halt its expansion, but only after an infinite time."

If the matter/energy density of the universe is equal to the critical density then in terms of geometry the universe is flat like a sheet of paper. In a matter-dominated universe, the critical density is between the density required by a collapsing "heavy universe" and the density of a "light universe" that expands forever.

The total content of the universe without dark energy is only around 30% of what is needed for a flat universe, which is the type of geometry the universe should have if created by the Big Bang. This is because early inflation should have "smoothed out" the universe geometrically like a sheet of paper.

The addition of dark energy to the universe's mass-energy budget "tops up" it up enough for the universe to be flat and in the simplest models of cosmic inflation, it brings the density of the universe close to the critical density.

Before the introduction of dark energy cosmologists had assumed that eventually, the attractive influence of gravity would overwhelm the expansion of the universe. This could lead to a few possible "ends" for the universe, one of which "the Big Crunch" suggested the universe would begin to contract and draw in on itself.

The acceleration of the universe's expansion dispels this idea. If dark energy continues to accelerate the expansion of the universe then, rather than a Big Crunch, its fate could be a "Big Rip."

This is a scenario in which dark energy eventually becomes dominant over every one of the universe's fundamental forces  —  gravity, electromagnetism, and the strong and weak nuclear forces  —  breaking or ripping apart everything that is currently bound together by those forces, be it galaxies, planets, or people, even the protons and neutrons that make up atoms.

So keep an eye on that coffee mug.