The search for an explanation for the nature of dark energy received a boost last week, when the National Science Foundation announced it was awarding an $8 million grant to a consortium of universities dedicated to building the Hobby-Eberly Telescope Dark Energy Experiment - affectionately known as HETDEX.
The University of Texas at Austin, Texas A&M university and Penn State University will split the funds, each carrying out their respective roles in the HETDEX mission. Once completed, the project will conduct a three-year survey to map the positions of a million galaxies some 10 billion light years away, providing precise measurements for how the universe expands over time. That, in turn, will help physicists answer a central question about dark energy: is it constant, or does it vary over time?
The mystery dates back nearly 100 years. Physicists once believed the cosmos was static and unchanging. When Albert Einstein was forming his theory of general relativity in 1917, his calculations indicated that the universe should be expanding. But this didn't fit with prevailing scientific opinion.
So he introduced a mathematical "fudge factor" into his equations, known as the cosmological constant, or lambda. It implied the existence of a repulsive force pervading space that counteracts the gravitational attraction holding the galaxies together. This balanced out the "push" and "pull" so that the universe would indeed be static.
Maybe Einstein should have trusted his initial calculations. A few years later, Edwin Hubble discovered that the universe wasn't static - it was, indeed, expanding. Fast forward to 1998, and two separate teams of physicists - analyzing data from supernovae - concluded that not only is the universe expanding, but it's doing so at an accelerating rate. That's actually pretty weird: scientists have compared it to "throwing a ball into the air and realizing it is speeding up as it flies into the sky rather than slowing down and returning."
Enter dark energy, a repulsive force that pervades the cosmos, pushing the universe apart. The current understanding is this: very early in the universe's existence, dark matter dominated. Everything was closer together, so its density was higher than that of the dark energy, and its gravitational pull was stronger. This led to the clumping that formed early galaxies. But as the universe continued to expand, the dark matter density, and hence the gravitational pull, decreased until it was less than that of the dark energy. So instead of the expected slow-down in the expansion rate, the now-dominant dark energy began pushing the universe apart at ever-faster rates.