In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain since the 1990s the observations indicating that the universe is expanding at an accelerating rate.
Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68.3% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount. Again on a mass–energy equivalence basis, the density of dark energy (~ 7 × 10−30 g/cm3) is very low, much less than the density of ordinary matter or dark matter within galaxies. However, it comes to dominate the mass–energy of the universe because it is uniform across space.
Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields that do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.
High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time and space. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today. Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model of cosmology" because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.
So what is dark energy? Well, the simple answer is that we don't know. It seems to contradict many of our understandings about the way the universe works.
We all know that light waves, also called radiation, carry energy. You feel that energy the moment you step outside on a hot summer day.
Einstein's famous equation, E = mc2, teaches us that matter and energy are interchangeable, merely different forms of the same thing. We have a giant example of that in our sky: the Sun. The Sun is powered by the conversion of mass to energy.
SOMETHING FROM NOTHING
Could dark energy show a link between the physics of the very small and the physics of the large?
But energy is supposed to have a source — either matter or radiation. The notion here is that space, even when devoid of all matter and radiation, has a residual energy. That "energy of space," when considered on a cosmic scale, leads to a force that increases the expansion of the universe.
Perhaps dark energy results from weird behavior on scales smaller than atoms. The physics of the very small, called quantum mechanics, allows energy and matter to appear out of nothingness, although only for the tiniest instant. The constant brief appearance and disappearance of matter could be giving energy to otherwise empty space.
It could be that dark energy creates a new, fundamental force in the universe, something that only starts to show an effect when the universe reaches a certain size. Scientific theories allow for the possibility of such forces. The force might even be temporary, causing the universe to accelerate for some billions of years before it weakens and essentially disappears.
Or perhaps the answer lies within another long-standing unsolved problem, how to reconcile the physics of the large with the physics of the very small. Einstein's theory of gravity, called general relativity, can explain everything from the movements of planets to the physics of black holes, but it simply doesn't seem to apply on the scale of the particles that make up atoms. To predict how particles will behave, we need the theory of quantum mechanics. Quantum mechanics explains the way particles function, but it simply doesn't apply on any scale larger than an atom. The elusive solution for combining the two theories might yield a natural explanation for dark energy.
STRANGER AND STRANGER
Most of the universe seems to consist of nothing we can see. Dark energy and dark matter, detectable only because of their effect on the visible matter around them, make up most of the universe.
We do know this: Since space is everywhere, this dark energy force is everywhere, and its effects increase as space expands. In contrast, gravity's force is stronger when things are close together and weaker when they are far apart. Because gravity is weakening with the expansion of space, dark energy now makes up over 2/3 of all the energy in the universe.
It sounds rather strange that we have no firm idea about what makes up 74% of the universe. It's as though we had explored all the land on the planet Earth and never in all our travels encountered an ocean. But now that we've caught sight of the waves, we want to know what this huge, strange, powerful entity really is.
The strangeness of dark energy is thrilling.
It shows scientists that there is a gap in our knowledge that needs to be filled, beckoning the way toward an unexplored realm of physics. We have before us the evidence that the cosmos may be configured vastly differently than we imagine. Dark energy both signals that we still have a great deal to learn, and shows us that we stand poised for another great leap in our understanding of the universe.
A mysterious quantity known as dark energy makes up nearly three-fourths of the universe, yet scientists are unsure not only what it is but how it operates. How, then, can they know this strange source exists?
The expanding universe
In 1929, American astronomer Edwin Hubble studied exploding stars known as supernovae to determine that the universe is expanding. Since then, scientists have sought to determine just how fast. It seemed obvious that gravity, the force which draws everything together, would put the brakes on the spreading cosmos, so the question many asked was, just how much was the expansion slowing?
In the 1990s, two independent teams of astrophysicists again turned their eyes to distant supernovae to calculate the deceleration. To their surprise, they found that the expansion of the universe wasn't slowing down, it was speeding up! Something must be counteracting gravity, something which the scientists dubbed "dark energy."
Calculating the energy needed to overcome gravity, scientists determined that dark energy makes up roughly 68 percent of the universe. Dark matter makes up another 27 percent, leaving the "normal" matter that we are familiar with to make up less than 5 percent of the cosmos around us.
Knowing how dark energy affects the spreading universe only tells scientists so much. The properties of the unknown quantity are still up for grabs. Recent observations have indicated that dark energy has behaved constantly over the universe's history, which provides some insight into the unseen material.
One possible solution for dark energy is that the universe is filled with a changing energy field, known as "quintessence." Another is that scientists do not correctly understand how gravity works.
The leading theory, however, considers dark energy a property of space.Albert Einstein was the first to understand that space was not simply empty. He also understood that more space could continue to come into existence. In his theory of general relativity, Einstein included a cosmological constant to account for the stationary universe scientists thought existed. After Hubble announced the expanding universe, Einstein called his constant his "biggest blunder."
But Einstein's blunder may be the best fit for dark energy. Predicting that empty space can have its own energy, the constant indicates that as more space emerges, more energy would be added to the universe, increasing its expansion.
Although the cosmological constant matches up with observations, scientists still aren't certain just why it fits.
Dark energy versus dark matter
Dark energy makes up most of the universe, but dark matter also covers a sizeable chunk. Comprising nearly 27 percent of the universe, and 80 percent of the matter, dark matter also plays a dominant role.
Like dark energy, dark matter continues to confound scientists. While dark energy is a force that accounts for the expanding universe, dark matter explains how groups of objects function together.
In the 1950s, scientists studying other galaxies expected gravity to cause the centers to rotate faster than the outer edges, based on the distribution of the objects inside of them. To their surprise, both regions rotated at the same rate, indicating that the spiral galaxies contained significantly more mass than they appeared to. Studies of gas inside elliptical galaxies and of clusters of galaxies revealed that this hidden matter was spread throughout the universe.
Scientists have a number of potential candidates for dark matter, ranging to incredibly dim objects to strange particles. But whatever the source of both dark matter and dark energy, it is clear that the universe is affected by things that scientists can't conventionally observe.
What is dark energy?
We need dark energy to explain why gravity seems to be driving galaxies in our universe apart instead of pulling them together.
The nature of dark energy is one of the most important unsolved problems in all of science. But what, exactly, is dark energy, and why do we even believe that it exists?
What goes up must come down… right? Ball image via www.shutterstock.com
Step back a minute and consider a more familiar experience: what happens when you toss a ball straight up into the air? It gradually slows down as gravity tugs on it, finally stopping in mid-air and falling back to the ground. Of course, if you threw the ball hard enough (about 25,000 miles per hour) it would actually escape from the Earth entirely and shoot into space, never to return. But even in that case, gravity would continue to pull feebly on the ball, slowing its speed as it escaped the clutches of the Earth.
But now imagine something completely different. Suppose that you tossed a ball into the air, and instead of being attracted back to the ground, the ball was repelled by the Earth and blasted faster and faster into the sky. This would be an astonishing event, but it’s exactly what astronomers have observed happening to the entire universe!
Scientists have known for almost a century that the universe is expanding, with all of the galaxies flying apart from each other. And until recently, scientists believed that there were only two possible options for the universe in the future. It could expand forever (like the ball that you tossed upward at 25,000 miles an hour), but with the expansion slowing down as gravity pulled all of the galaxies toward each other. Or gravity might win out in the end and bring the expansion of the universe to a halt, finally collapsing it back down in a “big crunch,” just like your ball plunging back to the ground.
This illustration shows abstracted ‘slices’ of space at different points in time as the universe expands.
So imagine scientists’ surprise when two different teams of astronomers discovered, back in 1998, that neither of these behaviors was correct. These astronomers were measuring how fast the universe was expanding when it was much younger than today. But how could they do this without building a time machine?
Luckily, a telescope is a time machine. When you look up at the stars at night, you aren’t seeing what they look like today – you’re seeing light that left the stars a long time ago – often many hundreds of years. By looking at distant supernovae, which are tremendously bright exploding stars, astronomers can look back hundreds of millions of years. They can then measure the expansion rate back then by comparing the distance to these far-off supernovae with the speed at which they are flying away from us. And by comparing how fast the universe was expanding hundreds of millions of years ago to its rate of expansion today, these astronomers discovered that the expansion is actually speeding up instead of slowing down as everyone had expected.
What pushes galaxies like these in the Hubble deep field apart? Image credit: NASA and A. Riess (STScI)
Instead of pulling the galaxies in the universe together, gravity seems to be driving them apart. But how can gravity be repulsive, when our everyday experience shows that it’s attractive? Einstein’s theory of gravity in fact predicts that gravity can repel as well as attract, but only under very special circumstances.
Repulsive gravity requires a new form of energy, dubbed “dark energy,” with very weird properties. Unlike ordinary matter, dark energy has negative pressure, and it’s this negative pressure that makes gravity repulsive. (For ordinary matter, gravity is always attractive). Dark energy appears to be smoothly smeared out through the entire universe, and it interacts with ordinary matter only through the action of gravity, making it nearly impossible to test in the laboratory.
Scientists used to think that the expansion of the universe was described by the yellow, green, or blue curves. But surprise, it’s actually the red curve instead.
The simplest form of dark energy goes by two different names: a cosmological constant or vacuum energy. Vacuum energy has another strange property. Imagine a box that expands as the universe expands. The amount of matter in the box stays the same as the box expands, but the volume of the box goes up, so the density of matter in the box goes down. In fact, the density of everything goes down as the universe expands. Except for vacuum energy – its density stays exactly the same. (Yes, that’s as bizarre as it sounds. It’s like stretching a string of taffy and discovering that it never gets any thinner).
Astronomers continue to probe the skies, looking for finer details that can build on what we suspect about dark energy. Photo credit: Reidar Hahn
Since dark energy can’t be isolated or probed in the laboratory, how can we hope to understand exactly what it’s made of? Different theories for dark energy predict small differences in the way that the expansion of the universe changes with time, so our best hope of probing dark energy seems to come from ever more accurate measurements of the acceleration of the universe, building on that first discovery 17 years ago. Different groups of scientists are currently undertaking a wide range of these measurements. For example, the Dark Energy Survey is mapping out the distribution of galaxies in the universe to help resolve this puzzle.
There is one other possibility: maybe scientists have been barking up the wrong tree. Maybe there is no dark energy, and our measurements actually mean that Einstein’s theory of gravity is wrong and needs to be fixed. This would be a daunting undertaking, since Einstein’s theory works exceptionally well when we test it in the solar system. (Let’s face it, Einstein really knew what he was doing). So far, no one has produced a convincing improvement on Einstein’s theory that predicts the correct expansion for the universe and yet agrees with Einstein’s theory inside the solar system. I’ll leave that as a homework problem for the reader.