The Eastern Sierra makes a person feel small. The highest peaks on either side of the Owens Valley measure more than 14,000 feet tall. The valley stretches north and south for hundreds of miles. In all of this vastness, a person fills just an iota of space. With that in mind, picture how the Owens Valley is a miniscule dot on the Earth, and the earth is a tiny spec orbiting around the sun within our solar system. Our galaxy is so large it takes our solar system about 250 million years to orbit around it. To make a person feel even smaller, our galaxy is just one of an unfathomable number of galaxies in the Universe. Imagine looking at the night sky through a hole the size of the little ball at the end of a pin held at arm’s length. The Hubble Ultra-Deep Field, the deepest image of the Universe ever taken, is about the size of that hole, and within that image, 10,000 galaxies are visible. And those are just the galaxies that Hubble is powerful enough to see—within a pin-sized patch of night sky.
Now, imagine that the Universe—as immense as it is already—is expanding. Because it is. Scientists know the Universe is getting bigger, and what’s more, it now appears to be accelerating. About 10 years ago scientists discovered that more than 70 percent of the Universe is made up of an unknown substance, dubbed “dark energy,” which they believe might be the driving force behind the accelerating expansion. Although it’s still unknown exactly what dark energy is, scientists are trying to narrow down the possible explanations. Some of these scientists, including Dr. Erik Leitch, are at the forefront of dark energy research at the Owens Valley Radio Observatory. Local astronomers, like Dr. Leitch and Dr. Stephen Muchovej, have recently been hosting the “Explore Your Universe” lecture series at Cerro Coso Community College’s Bishop Campus to share their findings with interested locals.
Last Thursday, Dr. Leitch took the audience of his “Cosmology with CARMA” (Combined Array for Millimeter-wave Astronomy) lecture on a wild trip through the Universe. And man, was it trippy. Audience members left with bulging brains and perhaps more questions than answers. But that’s the way science is; more questions arise the more scientists discover. Dr. Leitch has 20 years of astronomical research under his belt. He says he began asking questions as a kid while growing up on a farm in Maryland, in the midst of doing nerdy things like building telescopes and waiting up all night for some obscure galaxy to rise. The main question Dr. Leitch addressed on Thursday: What is dark energy?
The simple answer is that we don’t yet know. But we do know that normal matter—the stuff that we can see (that absorbs or reflects light) in the Universe like stars, planets, humans, plants, etc—is only about four percent of the Universe. Dark matter makes up 23 percent, and dark energy is 73 percent of the Universe. It’s this 73 percent of unknown substance that is responsible for the expanding acceleration of the Universe.
How do we know the Universe is accelerating?
“The Universe needs to be accelerating to explain the dimness of the very distant supernovae that we’ve observed,” Dr. Leitch said.
In brief, supernovae are bursts of radiation that briefly outshine galaxies. Leitch said you can think of a supernova as the taillight of a car. If every car’s taillight is exactly the same brightness, then the observed brightness tells you exactly how far away a car is.
“But let’s say that you agreed with a friend of yours that he’s going to drive away at 30 miles per hour,” Dr. Leitch said. “So he drives off and exactly one hour later, you observe his taillight and you say, ‘I know exactly how bright it should be because I know exactly how bright it is intrinsically, and he should be exactly 30 miles away from me.’ But then you look at it, and it’s actually four times dimmer.”
Leitch said this means your friend
is twice as far away as he said he was. From that, you can infer that in fact he wasn’t traveling at 30 mph; he was accelerating. He was traveling at a faster velocity at some point to get further away from you. In the same way, scientists have observed from the dimness of supernovae that the Universe is accelerating and creating more space.
Okay, so we know the Universe is accelerating and we know that dark energy is causing the acceleration. But we don’t know what dark energy is. Why do we care?
Dr. Leitch said that discovering what dark energy is could have huge implications that would redefine our worldview. In addition, by knowing what dark energy is, we might also be able to determine what the ultimate fate of the Universe will be.
One hypothesis is that dark energy is not real and is just a byproduct of a flawed theory of general relativity—proving that general relativity is essentially wrong. “That would have dramatic implications for everything else that general relativity has ever predicted,” Leitch said. “And general relativity has been an incredibly productive theory that explains a huge host of phenomena, and if it were wrong in some fundamental way, we would have to rethink a lot of things.”
But, if dark energy is real, and if it turns out to be vacuum energy, it means that the energy density is absolutely constant as a function of time. This means the Universe will continue to accelerate at the present rate and it will get bigger and bigger and objects will get further and further away and eventually it will dissipate, leading to the Universe becoming so cold that stars will burn out and new stars won’t be able to form, until eventually everything becomes a cold cinder.
“If [dark energy] turns out to be something other than vacuum energy,” Dr. Leitch said, “then it means whatever is driving the expansion of the Universe is probably time variable, meaning that in the far distant future the acceleration may coast to a stop and the Universe may re-collapse, in a sort of reverse picture of the Big Bang Theory.”
While these respective fates of the Universe could take billions to trillions of years, discovering what dark energy is could overturn certain scientific theories that we rely on today. For example, if dark energy isn’t vacuum energy, scientists may have to revise their current understanding of particle physics.
Dr. Leitch and his colleagues are narrowing down the search for dark energy explanations by assessing population statistics of galaxy clusters. From the cosmic microwave background (CMB), which is the oldest light we can see in Space, Dr. Leitch has been trying to piece together a population curve for galaxy clusters to try to infer information about dark energy.
“The number of galaxy clusters that formed in the early Universe is a direct measure of how much time there was for gravity to assemble objects,” Dr. Leitch said. “So the density of galaxy clusters in the sky today and how they formed as a function of time tells you something directly about how fast the Universe was expanding at early times. So indirectly you can infer something about dark energy from that.”
In his lecture, Leitch used an ecosystem analogy to explain the idea behind galaxy cluster population statistics. He said to imagine a food supply driving an ecosystem. However, you have no idea what that food supply is and therefore you have no idea how to observe it directly. But you can observe its effects on the deer population, for example. If there is a lot of food at early times then the deer population will boom, and if the food tails off at late times then the deer population will tail off in a very specific way.
“In the same way,” Dr. Leitch said, “if there’s a lot of dark energy that’s dominating the dynamics of the early Universe, you’ll form many fewer galaxy clusters. So depending on the characteristics of dark energy, you’ll get a different population curve.”
From its observed behavior, scientists think dark energy is uniformly distributed through space. So in a sense, scientists know where it is, they just don’t know what it is.
To narrow down the hypotheses for dark energy, scientists have to locate the galaxy clusters so they can do the population studies. But they also have to know when the galaxy cluster formed to determine the number of galaxy clusters forming as a function of time.
While instruments like the South Pole Telescope in Antarctica find galaxy clusters, CARMA research at the Owens Valley Radio Observatory conducts detailed studies of these images to determine how to convert the shadow the galaxy clusters cast on the CMB to something physically meaningful like mass. The mass of the galaxy cluster is ultimately what scientists need to know to do these population statistics.
“It’s like saying I use my binoculars to find the deer, but I need another instrument to tell me how heavy that deer is, and when he was born,” Dr. Leitch said. “That’s where CARMA comes in.”
Dr. Leitch ended his lecture on Thursday by saying that it’s a running joke among astronomers that, “We live in a preposterous Universe!” But Dr. Leitch said he wanted to revise the statement to clarify that, “It’s not the Universe that’s preposterous; it’s our theories that fall short of making perfect sense of it. But we’re working on it!”
Information about the upcoming talks in the “Explore Your Universe” lecture series is posted on the observatory’s website at: www.ovro.caltech.edu. The last two lectures will be hosted at Cerro Coso Community College’s Bishop Campus at 7 pm. On Nov. 10, Chat Hull (UC Berkeley) will give a talk titled “Star Formation Through Radio Eyes.” On Dec. 8, Andrea Isella (Caltech) will give a talk on “Origins of Solar Systems: From Dust to Planets.”