The universe we see is only the very tip of the vast cosmic iceberg.
The hundreds of billions of galaxies it contains, each of them home to billions of stars, planets and moons as well as massive star-and-planet-forming clouds of gas and dust, and all of the visible light and other energy we can detect in the form of electromagnetic radiation, such as radio waves, gamma rays and X-rays — in short, everything we’ve ever seen with our telescopes — only amounts to about 5% of all the mass and energy in the universe.
Along with this so-called normal matter there is also dark matter, which can’t be seen, but can be observed by its gravitational effect on normal, visible matter, and makes up another 27% of the universe. Add them together, and they only total 32% of the mass of the universe — so where’s the other 68%?
So what exactly is dark energy? Put simply, it’s a mysterious force that’s pushing the universe outward and causing it to expand faster as it ages, engaged in a cosmic tug-of-war with dark matter, which is trying to pull the universe together. Beyond that, we don’t yet understand what dark energy is, but Penn State astronomers are at the core of a group that’s aiming to find out through a unique and ambitious project 16 years in the making: HETDEX, the Hobby-Eberly Telescope Dark Energy Experiment.
“HETDEX has the potential to change the game,” said Associate Professor of Astronomy and Astrophysics Donghui Jeong.
Dark energy and the expanding universe
Today there is consensus among astronomers that the universe we inhabit is expanding, and that its expansion is accelerating, but the idea of an expanding universe is less than a century old, and the notion of dark energy (or anything else) accelerating that expansion has only been around for a little more than 20 years.
In 1917 when Albert Einstein applied his general theory of relativity to describe the universe as a whole, laying the foundations for the big bang theory, he and other leading scientists at that time conceived of the cosmos as static and nonexpanding. But in order to keep that universe from collapsing under the attractive force of gravity, he needed to introduce a repulsive force to counteract it: the cosmological constant.
It wasn’t until 1929 when Edwin Hubble discovered that the universe is in fact expanding, and that galaxies farther from Earth are moving away faster than those that are closer, that the model of a static universe was finally abandoned. Even Einstein was quick to modify his theories, by the early 1930s publishing two new and distinct models of the expanding universe, both of them without the cosmological constant.
But although astronomers had finally come to understand that the universe was expanding, and had more or less abandoned the concept of the cosmological constant, they also presumed that the universe was dominated by matter and that gravity would eventually cause its expansion to slow; the universe would either continue to expand forever, but ever-increasingly slowly, or it would at some point cease its expansion and then collapse, ending in a “big crunch.”
“That’s the way we thought the universe worked, up until 1998,” said Professor of Astronomy and Astrophysics Robin Ciardullo, a founding member of HETDEX.
That year, two independent teams — one led by Saul Perlmutter at Lawrence Berkeley National Laboratory, and the other led by Brian Schmidt of the Australian National University and Adam Riess of the Space Telescope Science Institute — would nearly simultaneously publish astounding results showing that the expansion of the universe was in fact accelerating, driven by some mysterious antigravity force. Later that year, cosmologist Michael Turner of the University of Chicago and Fermilab coined the term “dark energy” to describe this mysterious force.
The discovery would be named Science magazine’s “Breakthrough of the Year” for 1998, and in 2011 Perlmutter, Schmidt and Reiss would be awarded the Nobel Prize in physics.
More than 20 years after the discovery of dark energy, astronomers still don’t know what, exactly, it is.
“Whenever astronomers say ‘dark,’ that means we don’t have any clue about it,” Jeong said with a wry grin….