Karl Gebhardt: Mapping the Unknown
“Dark energy may not be dark and it may not be energy — it’s the phrase we use to explain our ignorance.”
Karl Gebhardt is an expert in finding a way to measure the “un-measureable”. He has spent most of his career focused on understanding the role that black holes play in the formation of a galaxy. Now the UT Austin astrophysicist is exploring another elusive part of space — dark energy, a mysterious unknown force that makes up 70 percent of the matter and energy in the universe.
What do we know about dark energy?
Karl Gebhardt: We know nothing. We believe we understand the laws of gravity. We believe we understand how light evolves over time. But when we go out and try and measure how the universe expands, we find that it is expanding significantly faster than we thought.
The phrase “dark energy” is indicative of our misunderstanding of how the universe expands over time. Dark energy may not be dark and it may not be energy — it’s the phrase we use to explain our ignorance.
How do you research something we know nothing or very little about?
KG: Throughout my career I focused on black holes, dark matter and dark energy. These are all things we cannot see. The only way to measure them is to see their effect on things we can see, like stars or galaxies. With dark energy, for example, we see how the galaxies are distributed and move. From their location and motions we infer what caused them to be in that pattern. You use what you know to infer how it came into that configuration.
What is the cocktail-party explanation for how you measure the expansion of the universe?
KG: When the galaxies are expanding they have a “fingerprint.” Similar to a person, as galaxies grow the space between each line in the fingerprint expands. To measure the expansion of the universe, you look at how galaxies are distributed over time. But, to capture that pattern we need to make a giant map of the universe.
How do you make a map of the universe?
KG: My idea is a project called HETDEX — Hobby-Eberly Telescope Dark Energy Experiment. HETDEX is going to measure the position and velocities of about a million galaxies about 10 billion years ago. No one in astronomy has done this before, and the galaxies we are looking at are very faint. So we took one of the biggest telescopes in the world and put on a suite of 150 spectragraphs. And we are going to just sit there and take shots over shots of sky to make a map of the cosmos. It will take about three or four years.
How do you gauge success in a theoretical field? What would it mean to “understand” dark energy?
A major problem in science is knowing when you are done. Science is really an effort to exclude possibilities. You never really get to the answer.
KG: For example, we don’t understand how gravity works yet! That blows my mind. Newton had his ideas. Einstein came in and modified that. And we know Einstein needs modification as well. Dark energy may be yet another kind of hook towards understanding what gravity is doing. So we don’t ever get to a final answer. It just builds up over time. I hope HETDEX is going to be a big advance. We have designed it that way, but you just don’t know what you are going to get.
What is your ideal discovery?
KG: That we would discover a universe that is not conforming to our standard model. My expectation is we are going to find something that is unexpected.
Wait, your ideal discovery is not an answer, but more problems?
KG: [Laughing] Not exactly. I’m looking for something out of the box here. When we built HETDEX we thought long and hard about just trying to do better measurement in the nearby universe. Or, should we focus instead at a time that no one has looked at before. The current theory says you might not find a lot if you look that far back in time. And we said, we have to look. Let’s look at a new epoch and see if we can find a differential effect. I think that’s where you learn the most.
