Astronomers determined in 1929 that the universe is expanding. This was one of the most important findings in science. In 1998, two separate groups of astrophysicists discovered that the universe’s expansion is speeding up: So, something is accelerating the universe’s expansion. Astronomers termed this something “dark energy,” but they have no idea what it actually is.
So how did scientists discover dark energy? And what (if anything) do they know about it? These findings rely on observational tools called “standard candles.”
A standard candle is a type of object that has a certain intrinsic brightness. Therefore, how bright the object appears depends on your distance from it. Think of automobile headlights. You can estimate how far away you are from a car based on how bright the headlights appear.
The two groups to unearth dark energy’s existence used type Ia supernovae for their observations. A type Ia supernova originates from a white dwarf star that is part of a binary star system. What’s a white dwarf star? It’s a stellar cinder leftover after a star similar to the Sun dies. The white dwarf pulls material from its binary companion. Once the white dwarf reaches a critical mass of 1.4 times that of the Sun, it begins to collapse. The star doesn’t collapse much before the remaining material ignites, and then the star explodes with a fantastically bright blast. Because all type Ia supernovae originate from white dwarfs of the same mass, they all have a similar luminosity.
Both groups tracked how a supernova’s light changes over time; this is called a “light curve.” They found that the more distant supernovae (which are from an earlier time) were dimmer than expected if the universe was expanding at a constant rate. This means that the distances between those supernovae and the telescopes that observed them are greater than predicted. The universe’s expansion has accelerated over time!
More recently (in December 2008) astronomers used a different observational method to confirm dark energy’s existence. This was a big deal because in science if two tests confirm a result, it eliminates many types of experimental uncertainty.
In this search, astronomers looked for dark energy’s effects on the growth of galaxy clusters. Over time, dark energy’s repulsive effect should put the brakes on the growth of such clusters by counteracting gravity’s pull. (This simulation shows how dark energy does that over time.) Astronomers found that structure growth has slowed in the past 5 billion years. And 5 billion years ago is about when astronomers believe dark energy’s effects kicked in. Both tests — cluster growth and type Ia supernova — give consistent results.
So what do scientists know about dark energy? A plethora of astronomical observations has led cosmologists to more accurately refine the amount of “stuff” in the universe. Everything that we directly observe — stars, gas, and dust — composes only about 5 percent of our universe. What about the other 95 percent? Astronomers have determined that roughly 23 percent of the universe is something called dark matter, and about 72 percent is dark energy. Dark matter interacts via gravity but not the electromagnetic force, meaning scientists know the matter exists, but there’s no way to see it directly. Dark energy is even more bizarre. Most of what astronomers know about dark energy is that it can be any type of “anti-gravity” and that it accelerates the universe’s expansion.
Even though cosmologists aren’t sure what dark energy is, they do have a few ideas. Scientists have three possible dark energy candidates: quintessence, phantom energy, and vacuum energy.
Quintessence is a dynamic field, meaning its density could change over time or from one place to another in the universe.
If dark energy is phantom energy, the universe would become progressively more dark-energy-dominated, and acceleration therefore would increase dramatically.
Vacuum energy gets its name from its role as the energy of “empty” space. Space is filled with a smooth density of something called “virtual particles” that pop in and out of existence. (Virtual particles are particle-antiparticle pairs.) Vacuum energy can also be represented by the cosmological constant term in Albert Einstein’s general theory of relativity — both have constant density. So if you hear people refer to the cosmological constant, they’re referring to vacuum energy. While the cosmological constant looks promising as a result of its density, the problem arises when physicists calculate how much vacuum energy is expected in the universe. Particle physics predicts 10120 times more vacuum energy than what scientists observe. (That’s a “1” followed by 120 zeroes.)
Clearly, astronomers do not yet understand dark energy. They need more data to learn about this mysterious stuff. Luckily, there are a number of detectors (both ground-based and space-based) that are currently being built or that are at least in the planning stages.
I hope this overview helped you understand dark energy a little better. I’ll be back later to talk more about cosmology. And for in-depth articles about cosmology, make sure to check out our special cosmology issue Cosmology’s Greatest Discoveries. You can also order this issue online at www.Astronomy.com.