Revealing Dark Energy's Hold on the Universe

In early 2014, a project called SuMIRe, or Subaru Measurement of Images and Redshifts, will begin a five-year survey photographing a few billion galaxies and measuring the distances for a few million of them. Meanwhile, another project called ACTPol will use the Cosmic Microwave Background – the afterglow radiation of the Big Bang and the oldest light in the universe – to also detect galaxies and galaxy clusters deep in space and far back in time.

Together, data from both projects are expected to help reveal how dark energy has caused the universe to expand over cosmic time.

Three members of the collaboration spoke recently with The Kavli Foundation in a roundtable discussion about the ACTPol/SuMIRe partnership, dark energy and their future research plans.

The participants:

• Michael Niemack – an assistant professor of physics at Cornell University and a leading team member of the Atacama Cosmology Telescope (ACT) and ACTPol teams.
• David Spergel – a theoretical astrophysicist and professor at Princeton University, and leader of the ACTPol analysis team.
• Masahiro Takada – a professor at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) and a leading team member of the Institute’s research program, SuMIRe.

The following is an edited transcript of the discussion.

THE KAVLI FOUNDATION: To begin, when did each of you start paying serious attention to dark energy?

DAVID SPERGEL: In the 1980s when I first started working as a cosmologist, there was evidence of something missing in our cosmological model. We could not explain the distribution of galaxies, and we couldn’t reconcile the observed expansion rate of the universe with the age of the oldest stars. At conferences back then, people would often raise the possibility of dark energy, but it seemed a bizarre thing that the universe would be filled with energy filling all space. Then there were the observations of distant supernovae in 1998 that provided the intellectual tipping point to take dark energy very seriously. And that’s when I began really studying it.

MICHAEL NIEMACK: My interest in studying the early universe is closely related to my interest in dark energy. What might have happened in the first fraction of a second after the Big Bang, and how the universe has evolved since, is tied to the nature of dark energy and dark matter. We have the potential to understand cosmology from the most minute scales of particle physics, such as what dark matter might be made of, all the way to the grandest scales where dark energy is dominating the expansion today.

MASAHIRO TAKADA: I am kind of a newcomer to the study of dark energy. During the supernova observations that David referred to back in 1998, when dark energy was discovered, I was a graduate student. But I distinctly remember that time, and how excited I was to study the subject. The nature of dark energy is a huge question right now, and we hope we can use our galaxy survey at the Subaru telescope to probe it. A huge question is, “Why at this point in the history of the universe is there an upsurge in dark energy – in the expansion rate of the universe?” We don't know that yet, but here in Japan our project could help answer that question.

TKF: David, can you explain how ACTPol uses the Atacama Cosmology Telescope to detect galaxies and galaxy clusters?

SPERGEL: ACTPol observes the cosmic microwave background, the leftover heat from the Big Bang. As that light travels across the universe to reach us, it is affected by intervening matter. If the light interacts with hot gas, it gains energy. This increases the number of high-energy photons and decreases the number of low energy photons. This effect creates cold spots whose amplitude is proportional to the amount of gas. ACTPol can also measure the mass of galaxy clusters by studying how the cosmic microwave background’s pattern of polarization is changed by the gravitational effects of those galaxies and galaxy clusters. This effect is similar to the gravitational lensing effect measured by Hyper Suprime-Cam and provides another way of measuring the mass in clusters.

TKF: And how does this complement the science goals of the Hyper Suprime-Cam at the Subaru Telescope in Hawaii, which is the main instrument for the SuMIRe project?

TAKADA: The Hyper Suprime-Cam is designed to take direct pictures of galaxies and galaxy clusters. That said, it also studies how some galaxy clusters magnify the light from more distant clusters behind them – a phenomenon called gravitational lensing. By measuring this lensing effect, the Hyper Suprime-Cam allows us to measure the mass of the foreground galaxy cluster doing the lensing, revealing details about its constitution.

TKF: And how do these projects together give us insights into dark energy?

SPERGEL: Together, we can build a big picture for how fast galaxy clusters grew at different points in cosmic history. And that will tell us how fast the universe was expanding at different points in time – whether it changed and how it changed. Because dark energy drives the expansion, plotting the expansion rate over time will give us insights into dark energy.

TAKADA: Mapping galaxies and galaxy clusters throughout history tells us about the two dominant competing forces in the universe: the gravitational force of dark matter, which drives the growth of galaxies and galaxy clusters, and dark energy, which causes the universe to expand and pull everything apart. Our observations, from both groups, will allow us to map the distribution of dark matter, and how that changes over time. If we see galaxy clusters growing faster at one point in time than another, that tells us that the gravitational effects of dark matter are dominating the universe at that time. If we see galaxy clusters growing more slowly, that would suggest dark energy is dominating. So, mapping cosmic structure over time tells us the story about this ongoing competition between dark matter and dark energy.

TKF: At a certain point in cosmic history, it’s believed dark energy kicked in and began to accelerate the expansion of the universe. Will ACTPol and SuMIRe be able to tell us about that transition?

SPERGEL: Yes. Once the universe became dominated by dark energy, we expect that the growth of clusters slowed dramatically. Before that, when the universe was dominated by dark matter, the number of clusters grew quite rapidly. So by mapping the number of clusters we see as a function of time, we’ll be able to observationally constrain the properties of dark energy, and learn about this transition when it began to dominate.

TKF: At what point in cosmic history do we think that dark energy took over?

TAKADA: It’s something like seven billion years ago. But we don’t yet know for sure. Our theoretical predictions for dark energy are not yet advanced enough.

TKF: ACTPol and SuMIRe are both gearing up for major observation campaigns. When you begin, what are your initial mapping strategies?

NIEMACK: We’ve been running simulations of observations for around half a dozen different fields on the sky and we’ll focus on those. ACTPol and SuMIRe will both observe these particular patches. There are a lot of details to understand, and instrument systematics to work out as we optimize our observations and data pipelines. I think we have a solid plan.

TAKADA: This past spring we analyzed the commissioning data of Hyper Suprime-Cam sky images to make sure everything is functioning properly. This summer and fall we are finalizing our plan for data analysis and processing. We expect to begin our survey early next year, and it will last for five years. We’ll spend a total of 300 nights observing with the Subaru Telescope, and it will be the largest-ever survey carried out with it. Once we know we’re obtaining good data and can characterize what we’re seeing, then we’ll begin talking to the ACTPol team to discuss what kind of science we can do by combining our data sets.

SPERGEL: SuMIRe will actually cover about 1,500 square degrees on the sky, and ACTPol will cover a larger area but we will overlap with SuMIRe observations.

TKF: How does this partnership fit into the larger effort to learn about dark energy -- for example, the Dark Energy Survey, which is also a galaxy survey project?

SPERGEL: The Dark Energy Survey, which is also being conducted from Chile, is doing complementary work to the SuMIRe project. It's covering a wider area of the sky, but not as deeply, so it's going to tell us more about the nearby universe and less about the more distant universe. It's working together with our colleagues who built the South Pole Telescope, and those two projects will also cover overlapping areas of the sky.

TKF: Can you tell us about some of the new technology that distinguishes SuMIRe and ACTPol?

NIEMACK: The detectors we use for ACTPol are called superconducting polarization sensitive transition edge sensors. They are exquisitely sensitive thermometers that can detect tiny changes in temperature that reveal the polarization pattern in the CMB that we then analyze. The National Institute of Standards and Technology, where I used to work, and other institutions have done groundbreaking work in superconductor detector technologies. The new ACTPol camera has about 3,000 of these polarization sensitive detectors, and we also have new optics designs to focus the light from the telescope onto these detectors in a way that maximizes the transmission of light and minimizes spurious reflections that can corrupt the data.

To give you an idea of how quickly the field has advanced over the last decade: The WMAP satellite experiment in 2001 had around 20 detectors. ACTPol has more than 100 times as many detectors.

"In another decade or two, I do think the graduate students of today will look back at this time with nostalgia. I am very hopeful we can find answers to some of these biggest mysteries during my lifetime." —Michael Niemack

TAKADA: With SuMIRe, the Hyper Suprime-Cam will be taking images of a few billion galaxies. By 2018, it will also be equipped with a spectroscopic instrument called the Prime Focus Spectrograph to measure the distances to a few million galaxies. As we’ve discussed, this is critical for mapping the position of galaxies and clusters at different stages of the universe’s history.

TKF: As you build this picture of the universe’s expansion over cosmic time, how will the data give you insights into the nature of dark energy?

SPERGEL: One possibility would be that we see structure growing with time in a way that is not consistent with Einstein’s Theory of General Relativity, which is our current explanation for how gravity works. The whole notion that there's dark energy undermines the idea that relativity is valid on the largest scales in the universe. If we see a breakdown of General Relativity, it might tell us we need new physics to describe the universe. This seems like a radical idea but it's happened before. The way we got General Relativity was by studying the motion of Mercury and realizing Newtonian physics could not explain what we were seeing.

Another insight the data might give us is that dark energy is not just a static energy associated with empty space, but is actually evolving over time. We would see that through the way we measure how structure grows with time.

Either of those observational results would point toward a new way of thinking about what's causing cosmic acceleration, and it would open up a great deal of work for both theoretical cosmologists and observational cosmologists.

TKF: Can we ever know what dark energy actually is – what it’s made of?

SPERGEL: We’re probably missing at least two things to answer that question. We need both a clarifying observation that suggests a direction for understanding dark energy, and also a novel theoretical idea to explain what most of the energy in the universe is doing and what form it takes. We don’t have those yet.

"...[D]iscovering what dark matter is actually made from, will help us better understand the beginning of the universe and perhaps its ultimate fate. But for every question we answer, others will come up." -- David Spergel

TKF: Right now cosmology is dominated by the huge mysteries of dark matter and dark energy. Will today’s graduate students look back at 2013 with a sense of nostalgia, as a time when dark matter and dark energy were still undefined?

NIEMACK: In another decade or two, I do think the graduate students of today will look back at this time with nostalgia. I am very hopeful we can find answers to some of these biggest mysteries during my lifetime. We are already working on designs for a future major upgrade to ACTPol, called Advanced ACTPol, which will have much greater frequency coverage and many more detectors. I’m hopeful that projects like ACTPol continue progressing, and that coordinated observations enable major breakthroughs.

SPERGEL: I hope today’s grad students can look back at today with that sense of nostalgia. When I think about fundamental questions in cosmology, I hope we’ll make progress on how the universe began. Did dark energy dominate the first moments of the universe, when inflation exponentially expanded the size of the universe in a fraction of a second?

An answer to this question, as well as discovering what dark matter is actually made from, will help us better understand the beginning of the universe and perhaps its ultimate fate. But for every question we answer, others will come up. Today’s grad students will have plenty to work on.