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What is Dark Energy?

Expansion Discovered

Astronomers in the early 20th Century got the shocks of their lives when they discovered that galaxies appeared to be rushing away from us. They did this by taking spectra of the galaxies, and then measuring the shift in their spectrum due to their motion.

diagram of red shift from

You're probably already familiar with this phenomenon, called the Doppler Shift - it's the same principle that makes a car horn change in pitch from high to low as it approaches and passes you. The sound waves are compressed as the car approaches you (resulting in a higher pitch) and are stretched as it recedes (which lowers the pitch). With light, an object approaching you has its light waves compressed, shortening the wavelength. This is called a blue shift ("blue" in this sense doesn't necessarily mean the object gets bluer; astronomers the word as a kind of shorthand, since in visible light the shorter wavelengths are blue). If the object is moving away, the wavelengths are stretched, resulting in a red shift of the spectrum.

A similar change in the wavelength can occur if distances are changing due to the expansion of space itself. While the light from a distant galaxy travels through the Universe, space is expanding, stretching the light with it, and increasing its wavelength. By carefully measuring the spectrum of an astronomical object, astronomers can tell how much the space between the object and observer has stretched while the light was traveling through it.

Astronomer Vesto Slipher was the first to use this technique on galaxies, and found that all the galaxies he measured (with very few exceptions) were redshifted. Moreover, the ones that were smaller and fainter - and therefore, presumably, farther away - had higher redshifts. Edwin Hubble expanded on this work, and determined that the Universe itself was expanding. Albert Einstein and other scientists were indeed able to explain this phenomenon as the result of space itself expanding, carrying galaxies along with it. This meant that in the past, the Universe was smaller. In fact, extending this idea backwards, the Universe was smaller, hotter and denser in the past.

This indicated that the Universe had a beginning, which at the time was a radical idea. There must have been some singular event in which space and time themselves were created, and subsequently the Universe expanded and cooled. This idea, called the Big Bang model, has since been confirmed by many experiments.

The Big Bang, Extended

Over the years astronomers have had to modify the Big Bang model. Inflation, for example - the idea that for the tiniest fraction of a second, the Universe underwent a super-expansion - was added to explain some problems that cropped up in some astronomical observations.

diagram of the Big Bang
Credit: NASA/WMAP Science Team

Observations of clusters of galaxies, as well as individual galaxies, made it clear that much of the matter in the Universe was not radiating light, and was invisible to our telescopes. You can feel the gravitational effects of dark matter, but you cannot see any light radiating from it. Teams of astronomers over many years have tried to identify what makes up dark matter - dust, gas, planets, burnt-out stars, black holes - and have come up empty. It's now thought that dark matter is a new kind of exotic matter that is not found in our current inventory of known particles. At least 80% of all matter must be of this exotic dark variety.

Dark Energy

But then, in 1998, astronomers got the biggest shock of them all. Until then, it was thought that the expansion of the Universe found by Slipher and Hubble was slowing, since the gravity of all the combined matter in the Universe pulled on that matter. But observations of distant supernovae (exploding stars) led to the inevitable conclusion that the expansion wasn't slowing down, it was speeding up! Two teams of astronomers independently came to this conclusion. They were reluctant at first to believe it, but over time the evidence became overwhelming. As two team leaders said in a joint paper, "Both samples [of supernovae] show that [supernovae] are, on average, fainter than would be expected, even for an empty Universe, indicating that the [expansion of the] Universe is accelerating."

different models of the expansion of the Universe
Credit: Adam Reiss,

Since the supernovae were too faint, they must be farther away than their simple redshift distances would indicate. After eliminating other possibilities, the only one left was that something was causing the cosmic expansion to step on the gas.

Astronomers called it dark energy. According to calculations, it accounted for nearly 3/4 of the Universe's matter and energy budget! At first (and to some extent even now) it was met with much skepticism, as any new hypothesis should be. But around that same time, observations were being made that would lend strong support to these findings.

pie diagram of dark energy, dark matter, and normal matter, courtesy  WMAP: For the first 400,000 years or so after the Big Bang, the Universe was a hot, dense plasma, and any beam of light couldn't get very far without being scattered by matter. But the Universe cooled as it expanded, and eventually became transparent. Light was able to freely travel for long distances. The "imprint" of the conditions in the early Universe are embedded in the light emitted at that time, and astronomers can use that information to determine what things were like back then.

For example, the temperature everywhere in the Universe was very close to being constant, but there were very slight differences, just hints at variations. The size of these "hot and cold spots" indicate that indeed, the Universe is filled with some sort of invisible energy. And, like dark matter, no one knows what it is.

Three leading theories are that

  1. dark energy is some sort of energy pervading space that doesn't change with time (usually called the cosmological constant),
  2. it is an energy field that changes in space and time (sometimes called quintessence), or
  3. it's some property of gravity we don't as yet understand; an extension of Einstein's theories, such as quantum gravity.
All three theories agree that dark energy will accelerate the expansion of the Universe, but predict differences in how the expansion rate should change with time. Finding out which theory is correct using astronomical observations, however, is very difficult.

Enter JDEM. It will precisely observe thousands of distant supernovae, measuring cosmic expasion over the last 10 billion years. Other methods, such as gravitational weak lensing and baryon acoustic oscillations, measure the shapes and clustering pattern of distant galaxies and search for the imprint of the cosmic acceleration in their distribution and evolution.

These observations by JDEM zero in on the "equation of state" of dark energy - the mathematical relationship between the pressure dark energy exerts and the amount of energy there is in the Universe. The first two types of dark energy theories predict very different equations of state, and a modification of Einstein's gravity could be found by contrasting results of the supernova and the weak gravitational lensing measurements.

JDEM's ability to distinguish between these three theories makes it our best tool yet to understand just what makes up the majority of the Universe in which we live.


Last updated: 04/30/2009