What is Dark Energy?

And, where does it come from?

In the 1990s astronomers discovered a previously unknown “dark energy” that is causing the universe to expand faster as it ages. They calculated that this energy constitutes about 70 percent of all the matter and energy in the universe.

Theorists have developed several explanations for dark energy, including an energy born from space itself, new kinds of subatomic particles, and even a flaw in our understanding of gravity. The answer will bring about a fundamental change in our understanding of the basic properties of the universe.

A History of the Universe

The history of dark energy appears to begin with the Big Bang — the instant of creation for all matter and energy, space, and time.

Big Bang 13.8 Billion Years Ago

In the first hundred-billionth of a trillionth of a trillionth of a second, the newborn universe began a period of hyperactive growth that’s known as inflation. It “inflated” from smaller than an atom to bigger than a golf ball. That doesn’t sound very impressive, but it was enough to “flatten” the geometry of the universe. In essence, it was like blowing up a balloon so large that any portion of its surface looks like a flat tabletop, not a curve. This geometry, in fact, is important evidence of the existence of dark energy.

Dark Ages

Scientists aren’t sure what caused inflation, but it may be related to dark energy — and perhaps it was the same dark energy that we see in the modern universe.

First Stars 13.3 Billion Years Ago

After this initial tiny burst, inflation disappeared, and the universe resumed a more leisurely expansion rate. After about 400,000 years, the universe had cooled enough that particles could combine to form atoms. In this environment, the universe became transparent, like a cloud that has spread out enough to let you see through it. This era also left an “imprint” that still permeates the universe today — a background radiation that glows at a temperature of just 2.7 °C (4.9 °F) above absolute zero.

Stars, Galaxies, & Planets Develop

Over the next billion years or so, the universe continued to expand and cool, with enormous amounts of hydrogen and helium lumping together to give birth to galaxies and their individual stars. Observations indicate that this early universe was dominated by dark matter, a mysterious form of matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible stars and galaxies. In fact, dark matter “outweighs” all the normal matter and energy in the universe by about six to one.

And so, the universe continued to expand from the impetus of the Big Bang, but the gravity of the dark matter and visible matter was trying to pull everything back together, slowing the rate of expansion.

Dark Energy Expansion Accelerates 5 Billion Years Ago

But about five billion years ago — nine billion years after the Big Bang — the universe began to expand faster. Dark energy began to dominate the universe, causing space itself to expand faster, carrying the galaxies along with it. Since then, the expansion rate has continued to speed up.

Present

If this trend continues, over the eons the galaxies will grow so far apart, and their motion away from each other so fast, that each galaxy will become like a little “island universe.” Anyone living inside a galaxy won’t be able to see all the other galaxies, so the universe will become a landscape of perpetual darkness — each galaxy isolated from all the others in a runaway universe.

Discovery and Confirmation of Dark Energy

In 1998, two teams of astronomers discovered that, instead of slowing down, the universe is expanding faster today than it was in earlier epochs.

Both teams were examining a class of exploding stars known as Type Ia supernovae. These stars are white dwarfs — the hot, dense, dead cores of stars that were once like the Sun — with close stellar companions. The white dwarf ‘steals’ hot gas from its companion. The gas piles up, forming a superhot layer atop the white dwarf. When the gas pushes the star past a critical mass, it sets off a runaway nuclear explosion that blasts the star to bits. For a while, the supernova can outshine an entire galaxy of normal stars, so it’s easy to see these exploding stars even in distant galaxies.

The ability to see Type Ia supernovae far across the universe makes them good “standard candles” — a way to measure distances to other galaxies. All Type Ia supernovae brighten and fade in a predictable way. Measuring how long it takes a supernova to brighten and then fade reveals its true brightness. By comparing a star’s true brightness to how bright it looks in our sky, astronomers can find its distance.

One more step completes the picture. Astronomers measure how fast the star is moving away from Earth by measuring its redshift — a stretching of its light waves by the expansion of the universe itself. By putting together the distances and speeds of many supernovae, the two teams hoped to show how the expansion of the universe had changed over the eons.

Both teams found that the expansion was getting faster, not slower and have since conducted additional studies using larger numbers of supernovae.

Exploding stars, the shape of the universe, and the way galaxies are sprinkled about all helped confirm the existence of dark energy.

The geometry of the universe depends on how much total matter and energy it contains. If there is a lot of matter and energy, then the universe is curved at an angle that scientists describe as “closed.” In that case, the universe would eventually collapse in on itself. With relatively little matter and energy, though, the curves the other way, so it is “open” — it would expand forever, although at an ever-slower rate.

But if matter and energy add up to just the right amount, then the universe is neither open nor closed, but flat.

For years, astronomers added up all the galaxies, gas clouds, dark matter, and radiation in the universe, and came up with only about a third of what was needed to produce a flat universe. Shortly after the turn of the 21st century, though, that changed.

Using telescopes aboard balloons and satellites, astronomers probed the “afterglow” of the Big Bang. This glow, known as the cosmic microwave background (CMB), produced when the universe was less than 400,000 years old, contains the imprint of the first “clumps” of matter, which eventually gave birth to galaxies and galaxy clusters.

Measuring the structure of the CMB revealed that the geometry of the universe is flat. Since dark matter and normal matter and energy account for only about 30 percent of the total needed to create a flat universe, scientists concluded that a hidden component must account for the balance: dark energy.

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