Last post was about Dark Matter. Now we’ll talk about Dark Energy; what is the evidence for Dark Energy, why is it needed, and what exactly is Dark Energy?

Our tale starts back around 1905, when Einstein was developing the theory of gravity called General Relativity. By 1915, he had worked out a system of equations that described the structure of spacetime, the famous Einstein field equations. Cosmologists began searching for solutions to these equations, and solutions were found in which spacetime was expanding or contracting, but not static. This was troubling, since before Edwin Hubble discovered evidence for the expansion of the universe, virtually everyone thought the universe was a eternal, static entity. So Einstein went back to the drawing board and added a term, the Cosmological Constant, **Λ** in the equation below, modifying the field equations so they allowed solutions giving a static universe. However, these solutions were unstable, and the slightest change would start the universe expanding or contracting forever.

In 1929, Hubble published experimental results showing the general expansion of the universe (which we call the *Hubble Flow*). When Einstein learned that the universe was not static, but expanding, he dropped the Cosmological Constant, referring to it as the biggest blunder of his career. Had he taken the equations seriously, he could have predicted the expansion of the universe years before it was discovered by observation. In fact, the cosmologist Georges Lemaître did propose an expanding universe, based on solutions of Einstein’s field equations, two years before Hubble’s discovery.

Cosmologists immediately realized that the gravitational attraction of all the mass in the universe would work to slow the rate of expansion of the universe. Astronomers tried to measure the rate of expansion, to determine if the universe would eventually stop and begin to contract, or continue to expand forever. The observations showed the rate of expansion was near the dividing line between expand forever and eventual collapse inward in a Big Crunch. But these measurements didn’t include objects in the distant (and therefore older) universe, because those objects were too faint to measure with the instruments of the day.

By the 1990s, the technology was available to measure the light from very distant exploding stars, or supernovae. It turns out that one class of supernovae, Type 1a, produces a consistent peak light output, therefore by measuring how bright the peak of a Type 1a supernova appears to us, astronomers can infer the distance to the supernova. The dimmer the peak luminosity, the farther the supernova is from us. In addition to distance, star’s motion directly toward or away from the Earth can be determined by measuring the color change of an object. For objects at cosmological distances, the line-of-sight velocity is so high, we know the expansion of the universe is responsible for virtually all of the line-of-sight velocity.

You are now ready for the last piece of the puzzle. Since light travels at a fixed, finite velocity, the more distant the star, the longer it takes for the light to reach the Earth. If light must travel 500 million years to reach the Earth, when we measure the object emitting that light, we are *looking back 500 million years in time*. If we measure the line-of-sight velocity (the object will be moving away from us at that distance) of a supernova so distant the light takes 500 million years to reach us, we are measuring the expansion speed of the universe 500 million years ago! By finding and measuring lots of faint supernovae, scientists can plot the line-of-sight velocity (speed of the universe’s expansion) against the peak brightness (which determines the distances to the supernovae, and therefore how far back in time we are seeing) of each supernovae. That plot shows how the expansion of the universe is changing over time. And that is exactly what these two teams did, measuring some stars so distant that they had exploded 4 billion years ago and whose light was just now reaching the earth. It was hard and exacting work.

The results were completely unexpected. *Stunningly* unexpected. Both teams found the expansion speed of the universe was speeding up, not slowing down, as the universe gets older. The expectation had been that gravity, always working to pull matter together, would gradually slow the speed of expansion as the universe got older. Indeed, the earlier measurements had not been able to see as far back in time, and therefore didn’t clearly show the speedup. Something in the fabric of spacetime was pushing outward, forcing the universe to expand faster and faster, more than overcoming the force of gravity that was working to slow the expansion.

Well, you can probably guess that mysterious force that is causing the universe to expand faster and faster we call Dark Energy. No one knew this force existed until measuring the distant supernovae revealed the accelerating expansion to the two teams. This discovery resulted in Nobel Prizes for the leaders of the two teams (3 prizes were given, the customary maximum number, to Saul Perlmutter, Brian Schmidt, and Adam Riess). Today, we don’t really know much about Dark Energy, except that it is stupendously powerful, that it pervades all of space, and it was completely unexpected. Well, maybe not to Einstein and Lemaître, because Dark Energy behaves very much like the hypothesized Cosmological Constant in Einstein’s field equations that produces a force counterbalancing the pull of gravity.

We don’t know much, but what we do know is this, the energy contained in the Dark Energy field is the dominant force in the universe today, representing a little less than three quarters of the total energy of the universe today. While it is too weak to be detected on a small scale, Dark Energy fills all of space and is the dominant force affecting the evolution of our universe. In the next post, we’ll talk a little about the various conjectures put forth to explain the origin and nature of Dark Energy.