The Hunt for 70% of the Universe
CARSON SCHUBERT For millennia, humans have looked to the sky for guidance and inspiration. In the past five hundred years we have come to understand our place in the cosmos with bigger and better telescopes, space probes, and new ways of seeing the universe. Now, astronomers at the University of Texas and the McDonald Observatory hope to revolutionize astronomy by providing the first direct data on a mysterious part of the universe: dark energy.
One of the great triumphs of modern astronomy is the discovery that what we see around us, from Earth to the Sun to distant galaxies and stars, makes up a mere 5% of our universe. The other 95% is composed of so called “dark” matter and energy. Dark, in this case, is both a description of the “stuff” itself (zero interaction with the electromagnetic spectrum, or light) and our understanding of it. The truth is, we know almost nothing about dark matter or dark energy, except for the fact that they make up a majority of our universe.
So what makes dark energy so interesting? It all started in 1929, when Edwin Hubble determined that the universe was expanding. This was huge for astronomers and science in general; until then, the universe had always been assumed to be static, unchanging over the eons. Hubble’s discovery shocked the world and marked a significant change in our understanding of the universe.
Fast forward to 1998. Saul Perlmutter and Adam Riess, along with Australian Brian Schmidt, are studying distant supernovae. In doing so, they make a groundbreaking (and 2011 Nobel Prize winning) discovery: the universe is not only expanding, but also accelerating. This went against everything physicists had theorized since Hubble’s discovery in 1929. “If you had a universe with no dark energy, there was just matter, we would expect the expansion rate to be slowing because [gravity] is pulling everything closer together. But it’s not, it’s accelerating!” says Dr. Steven Finkelstein, an astronomy professor here at UT. “So we inferred the existence of dark energy.” It was as if you threw an apple in the air, and instead of slowing and falling back to your hand, it continued to rise faster than you had thrown it.
Astronomers were baffled. Some mysterious, unseen force was causing the universe to speed up. In the nearly two decades since its discovery, dark energy has been revealed to comprise over 70% of the known universe. Some say it is a new, undiscovered force carrying particle. Some say it is simply a product of the energy of empty space. Yet for well over a decade now, science has been unable to provide anything more than theories. “There are essentially no observational constraints… We can measure how strong it is today, one data point. You can fit a lot of lines through one data point.” says Finkelstein.
Gary Hill (Principal Investigator) and Karl Gebhardt (Project Scientist), along with a consortium of other researchers and the McDonald observatory, aim to change that. The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), conceived in 2008 and slated to begin full operations in the fall of 2017, will use spectroscopy to obtain a three-dimensional map of galaxies in a portion of the sky between 9 and 11 billion light years away. “What that 3D map essentially tells us is, how fast is the universe expanding then, at that point.” Finkelstein explains. Using spectroscopy rather than traditional imaging will allow the team to calculate precise distances to each galaxy, which are vital for producing the three-dimensional map. The project will map nearly a million Lyman-α emitting galaxies, the same type of galaxy Finkelstein studied in his doctoral thesis. “They’re easiest to detect [in spectrographs].” he says of the galaxies, fourteen of which were the subject of his thesis. Such a large mapping project has never been done before and will yield unheard of scientific data with applications far beyond the slated goals of the experiment.
For cosmologists, the hope is that HETDEX can finally grant them a glimpse into the true nature of dark energy. Currently accepted theory holds that dark energy is merely a cosmological constant. This constant represents what astronomers call the vacuum energy of space. As the universe expands and more empty space is created, it creates a cyclical effect that continually increases the rate of expansion. “If that’s what we see, that’s great. But it’s not Nobel prize winning. If we prove dark energy does evolve over time, that might be Nobel prize winning.” says Finkelstein when asked about what HETDEX expects to see. Whatever the data reveals, one thing is for sure: we’re one step closer to the answer.
While dark energy is the main target for HETDEX, Finkelstein is excited for a different reason: “I study galaxies and how galaxies evolve… so I’m excited to have this sample of one million galaxies where we know the distance to all of them. Distance is always a major uncertainty.” Finkelstein says with a smile. “My main role is being able to take advantage of the huge data set we have to do lots and lots of awesome stuff to figure out how galaxies work.” For Finkelstein and other astronomers who study galaxy formation, such a large data set presents unbounded opportunities for new research. By analyzing the emission lines in the galaxies, “we should discover a million stars and [around] one hundred thousand accreting supermassive black holes. All of that science together can be at least as important, if not more, than the cosmology.” Finkelstein explains.
The project is on track to begin operations next fall and observe most nights for about three years. The data sent back may yield answers to some of our most pressing questions, including how our galaxy came to be and what the future of the universe will look like. Although things have not necessarily been problem-free, Finkelstein remains optimistic about the future of the project. “I would say HETDEX is definitely underrated. Assuming everything works, and I think it will, it’s going to be very exciting.”