Our Expanding Universe: Delving into Dark Energy

Image courtesy of NASA/WMAP Science Team

This diagram shows the timeline of the universe, from its beginnings in the Big Bang to today.

The universe is growing a little bigger, a little faster, every day.

And scientists don't know why.

If this continues, almost all other galaxies will be so far away from us that one day, we won't be able to spot them with even the most sophisticated equipment. In fact, we'll only be able to spot a few cosmic objects outside of the Milky Way. Fortunately, this won't happen for billions of years.

But it's not supposed to be this way – at least according to theory. Based on the fact that gravity pulls galaxies together, Albert Einstein's theory predicted that the universe should be expanding more slowly over time. But in 1998, astrophysicists were quite surprised when their observations showed that the universe was expanding ever faster. Astrophysicists call this phenomenon "cosmic acceleration."

"Whatever is driving cosmic acceleration is likely to dominate the future evolution of the universe," said Josh Frieman, a researcher at the Department of Energy's (DOE) Fermilab and director of the Dark Energy Survey.

While astrophysicists know little about it, they often use "dark energy" as shorthand for the cause of this expansion. Based on its effects, they estimate dark energy could make up 70 percent of the combined mass and energy of the universe. Something unknown that both lies outside our current understanding of the laws of physics and is the major influence on the growth of the universe adds up to one of the biggest mysteries in physics. DOE's Office of Science is supporting a number of projects to investigate dark energy to better understand this phenomenon.

The Start of the Universe

Before scientists can understand what is causing the universe to expand now, they need to know what happened in the past. The energy from the Big Bang drove the universe's early expansion. Since then, gravity and dark energy have engaged in a cosmic tug of war. Gravity pulls galaxies closer together; dark energy pushes them apart. Whether the universe is expanding or contracting depends on which force dominates, gravity or dark energy.

Just after the Big Bang, the universe was much smaller and composed of an extremely high-energy plasma. This plasma was vastly different from anything today. It was so dense that it trapped all energy, including light. Unlike the current universe, which has expanses of "empty" space dotted by dense galaxies of stars, this plasma was nearly evenly distributed across that ancient universe.

As the universe expanded and became less dense, it cooled. In a blip in cosmic time, protons and electrons combined to form neutral hydrogen atoms. When that happened, light was able to stream out into the universe to form what is now known as the "cosmic microwave background." Today's instruments that detect the cosmic microwave background provide scientists with a view of that early universe.

Back then, gravity was the major force that influenced the structure of the universe. It slowed the rate of expansion and made it possible for matter to coalesce. Eventually, the first stars appeared about 400 million years after the Big Bang. Over the next several billion years, larger and larger structures formed: galaxies and galaxy clusters, containing billions to quadrillions (a million billion) of stars. While these cosmic objects formed, the space between galaxies continued to expand, but at an ever slower rate thanks to gravitational attraction.

But somewhere between 3 and 7 billion years after the Big Bang, something happened: instead of the expansion slowing down, it sped up. Dark energy started to have a bigger influence than gravity. The expansion has been accelerating ever since.

Scientists used three different types of evidence to work out this history of the universe. The original evidence in 1998 came from observations of a specific type of supernova. Two other types of evidence in the early 2000s provided further support.

"It was this sudden avalanche of results through cosmology," said Eric Linder, a Berkeley Lab researcher and Office of Science Cosmic Frontier program manager.

Now, scientists estimate that galaxies are getting 0.007 percent further away from each other every million years. But they still don't know why.

What is Dark Energy?

"Cosmic acceleration really points to something fundamentally different about how the forces of the universe work," said Daniel Eisenstein, a Harvard University researcher and former director of the Sloan Digital Sky Survey. "We know of four major forces: gravity, electromagnetism, and the weak and strong forces. And none of those forces can explain cosmic acceleration."

So far, the evidence has spurred two competing theories.

The leading theory is that dark energy is the "cosmological constant," a concept Albert Einstein created in 1917 to balance his equations to describe a universe in equilibrium. Without this cosmological constant to offset gravity, a finite universe would collapse into itself.

Today, scientists think the constant may represent the energy of the vacuum of space. Instead of being "empty," this would mean space is actually exerting pressure on cosmic objects. If this idea is correct, the distribution of dark energy should be the same everywhere.

All of the observations fit this idea - so far. But there's a major issue. The theoretical equations and the physical measurements don't match. When researchers calculate the cosmological constant using standard physics, they end up with a number that is off by a huge amount: 1 X 10¹²⁰ (1 with 120 zeroes following it).

"It's hard to make a math error that big," joked Frieman.

That major difference between observation and theory suggests that astrophysicists do not yet fully understand the origin of the cosmological constant, even if it is the cause of cosmic acceleration.

The other possibility is that "dark energy" is the wrong label altogether. A competing theory posits that the universe is expanding ever more rapidly because gravity acts differently at very large scales from what Einstein's theory predicts. While there's less evidence for this theory than that for the cosmological constant, it's still a possibility.

The Biggest Maps of the Universe

To collect evidence that can prove or disprove these theories, scientists are creating a visual history of the universe's expansion. These maps will allow astrophysicists to see dark energy's effects over time. Finding that the structure of the universe changed in a way that's consistent with the cosmological constant's influence would provide strong evidence for that theory.

There are two types of surveys: imaging and spectroscopic. The Dark Energy Survey and Large Synoptic Survey Telescope (LSST) are imaging surveys, while the Baryon Oscillation Spectroscopic Survey (part of the Sloan Digital Sky Survey), eBOSS, and the Dark Energy Spectroscopic Instrument are spectroscopic.

Imaging surveys use giant cameras – some the size of cars – to take photos of the night sky. The farther away the object, the longer the light has taken to reach us. Taking pictures of galaxies, galaxy clusters, and supernovae at various distances shows how the distribution of matter has changed over time. The Dark Energy Survey, which started collecting data in 2013, has already photographed more than 300 million galaxies. By the time it finishes in 2018, it will have taken pictures of about one-eighth of the entire night sky. The LSST will further expand what we know. When it starts in 2022, the LSST will use the world's largest digital camera to take pictures of 20 billion galaxies.

"That is an amazing number. It could be 10% of all of the galaxies in the observable universe," said Steve Kahn, a professor of physics at Stanford and LSST project director.

However, these imaging surveys miss a key data point – how fast the Milky Way and other galaxies are moving away from each other. But spectroscopic surveys that capture light outside the visual spectrum can provide that information. They can also more accurately estimate how far away galaxies are. Put together, this information allows astrophysicists to look back in time.

The Baryon Oscillation Spectroscopic Survey (BOSS), part of the larger Sloan Digital Sky Survey, was one of the biggest projects to take, as the name implies, a spectroscopic approach. It mapped more than 1.2 million galaxies and quasars.

However, there's a major gap in BOSS's data. It could measure what was going on 5 billion years ago using bright galaxies and 10 billion years ago using bright quasars. But it had nothing about what was going on in-between. Unfortunately, this time period is most likely when dark energy started dominating.

"Seven billion years ago, dark energy starts to really dominate and push the universe apart more rapidly. So we're making these maps now that span that whole distance. We start in the backyard of the Milky Way, our own galaxy, and we go out to 7 billion light years," said David Schlegel, a Berkeley Lab researcher who is the BOSS principal investigator. That 7 billion light years spans the time from when the light was originally emitted to it reaching our telescopes today.

Two new projects are filling that gap: the eBOSS survey and the Dark Energy Spectroscopic Instrument (DESI). eBOSS will target the missing time span from 5 to 7 billion years ago. DESI will go back even further – 11 billion light years. Even though the dark energy was weaker then relative to gravity, surveying a larger volume of space will allow scientists to make even more precise measurements. DESI will also collect 10 times more data than BOSS. When it starts taking observations in 2019, it will measure light from 35 million galaxies and quasars.

"We now realize that the majority of … the universe is stuff that we'll never be able to directly measure using experiments here on Earth. We have to infer their properties by looking to the cosmos," said Rachel Bean, a researcher at Cornell University who is the spokesperson for the LSST Dark Energy Science Collaboration. Solving the mystery of the galaxies rushing away from each other, "really does present a formidable challenge in physics. We have a lot of work to do."

The Office of Science is the single largest supporter of basic energy research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit https://science.energy.gov.

Shannon Brescher Shea is a Senior Writer/Editor in the Office of Science, [email protected].