Nicholas Suntzeff: Observer of the distant universe
One of the discoverers of the rapid expansion of the cosmos discusses the challenge of finding clues about what mysterious dark energy is
Nicholas Suntzeff, an American researcher at Texas A&M University, has been watching distant stars explode for more than 30 years. In 1986, he and his collaborators at the Cerro Tololo Inter-American Observatory (CTIO) in Chile showed how to use the brightness of a particular type of stellar explosion, type Ia supernovas, to accurately measure the distance of galaxies that are almost at the edge of the universe.
These measurements led to the 1998 discovery that the universe is expanding rapidly. This means that the galaxies are moving away from each other faster than ever. Up until that time, most astronomers had agreed that this expansion, which began just over 13 billion years ago after the Big Bang generated the cosmos, was slowing. The cause of this deceleration was assumed to be the gravitational attraction that clusters of galaxies, the largest structures found in the universe, have between each other.
Observations made by Suntzeff’s team (which also included the Australian astronomer Brian Schmidt), as well as those of the competing team led by the American Saul Perlmutter, suggested the opposite: over time, the space between galaxies expanded at an accelerated pace. For this discovery, Perlmutter, Schmidt, and Adam Riess received the 2011 Nobel Prize for Physics.
This finding was confirmed by later observations and transformed cosmology. Today, researchers in this area can only explain the current structure of the cosmos by considering accelerated expansion and attributing this fact to the existence of dark energy.
If dark energy—which is believed to repel matter, unlike gravitational force—in fact exists and is responsible for this rapid expansion, questions still remain. Nobody knows what it is; some theoretical models suggest it is a form of energy intrinsic to empty space called the cosmological constant. Others suggest that it is a fifth fundamental force, unlike the four others known to physics: gravitation, electromagnetism, strong nuclear force, and weak nuclear force.
To find out which of these proposals best explains the universe, physicists need more accurate measurements of the distances between the galaxies. The problem is that it is not yet known what degree of precision must be reached to eliminate some hypotheses. Suntzeff participates in the main project which is currently observing the universe on a large scale, the Dark Energy Survey (DES), as well as in the projects to build the NASA Wide Field Infrared Survey Telescope (WFIRST) and the Large Synoptic Survey Telescope (LSST) in Chile. In July, he presented seminars at the University of São Paulo (USP) in São Carlos and the South American Institute for Basic Research at the International Center for Theoretical Physics (ICTP-SAIFR), which operates in São Paulo within the Institute of Theoretical Physics at São Paulo State University (UNESP). The following is an interview Suntzeff gave to Pesquisa FAPESP.
What was it like to discover that the universe was rapidly expanding?
It was unexpected. The objective was to measure a deceleration in the expansion of the universe. We expected a positive value for this deceleration, but the value we measured was negative. We were excited. I was in charge of the observations. The other researchers came to me to find out if the telescope was working well and if the instruments were calibrated. My signing off on the data guaranteed their quality. The biggest concern was whether a mistake had been made that could have led to misleading data. Saul Perlmutter’s group was also at Cerro Tololo, taking the same measurements as us with the same telescope and the same instruments. I would help them one night, and the next night take data for my own team. It was a scientific competition, not a personal one. We liked them. And they reached the same results as we did.
What is this dark energy that causes the accelerated expansion of the universe?
Dark energy is a term we use for something we don’t understand. I don’t like the term. We don’t measure dark energy. Our measurements showed that supernovas, the intrinsic brightness of which we know, are much more distant than would be expected if the universe were made only of matter. The greater the amount of matter in the universe, the closer to us they should be. But their brightness is 20% weaker than it should be. This was the discovery.
What would explain this result?
The first interpretation is that if they are farther away than we would expect them to be if they were only influenced by gravitational force, then something must have pushed against gravity. A kind of antigravity could have pushed them that far. The only acceptable way of producing antigravity in the general theory of relativity is to add a constant term to Einstein’s equations, the so-called cosmological constant. What phenomenon would justify the existence of the cosmological constant? We don’t know. It may be a consequence of the same fluctuations in the energy of the vacuum that gives rise to the elementary particles. It was from this idea that the physicist Michael Turner, of the University of Chicago, coined the term dark energy. I don’t like the term because it introduces a bias in the way of considering an explanation for the phenomenon. We measured that distant galaxies are much farther away than they should be. The cosmological constant and fluctuations in the vacuum are interpretations for what we observed. I believe that my data is correct because other experiments using different techniques found the same thing.
What are the different techniques?
There are four. In addition to measuring the distance of supernovas, we can look for small distortions in the images of very distant galaxies. The light coming from these galaxies crosses the universe and is distorted by the mass of galaxy clusters on the way. It is called the gravitational lens. The gravitational lens allows you to measure how the cumulative effect of the universe’s mass and energy affects its expansion. Another technique is to examine the large-scale structure, in other words the size and shape of galaxy clusters, and measure how much the accelerated expansion of the universe impedes the formation of clusters by the gravitational attraction between galaxies. There is also the method that investigates baryon acoustic oscillations. These are waves in the ionized gas that filled the primordial universe and 300,000 years after the Big Bang produced cosmic background radiation, a form of radiation in the microwave range that permeates the universe. These oscillations left circular marks with a 400,000 light-year radius in the temperature distribution of the cosmic background radiation. These circles have expanded with the universe, affecting the formation of galaxies, and today they have a 500 million light-year radius. In today’s universe, we observed a small increase in the probability that a galaxy would have neighbors within a distance of 500 million light-years. Millions of galaxies must be observed to perceive this increased likelihood, which is very slight. Comparison between the distribution of the spots in the cosmic background radiation and the distribution of the galaxies provides an accurate estimate of the amount of dark energy. It is wonderful that all methods produce the same result. If we had made mistakes when measuring the distance of the supernovas, the measurements using the other techniques would not have led us to the same conclusion. When the results for the large-scale structures began to arrive, years after our measurements, I was sure that we had not made any mistakes.
What level of precision do the measurements have to reach in order to know which theory would best explain what dark energy is?
The energy of the fluctuations in the vacuum, as established by quantum field theory, does not match what we see. It predicts huge numbers, and we observed a very small number. Dark energy, whatever it is, has a negligible effect on small scales and is almost impossible to measure in a laboratory on Earth. It is the cumulative effect dark energy has throughout space that allows it to master the dynamics of the universe. Physicists have no idea what the value of the cosmological constant might be, so they cannot say what degree of precision we need to use to measure the expansion of the universe. Without a theoretical value, there is no way to compare the data from the observations and state whether a certain model is correct. The physicists ask us to take our measurements using the highest degree of precision possible. We do this, but what are we going to test? They need to tell us what level of accuracy needs to be achieved in measurement so that any deviations between the predicted values for the cosmological constant and the measurements can be seen. Boosting accuracy is increasingly difficult. I want to test a theory, not devote myself to an unlimited search for more and more accurate measurements. An interesting result would be to show that the increase in the universe’s rate of expansion is not constant over time. This would not indicate what dark energy is, but it would show that we need a new theory of physics to explain it. As long as there is a possibility that there is a cosmological constant, we cannot attribute this effect to an unknown power.
How did you become interested in cosmology?
After my doctorate, I went to work at the Palomar Observatory on Mount Wilson in California. I wanted to study the structure of the Milky Way and understand how our galaxy formed by observing its oldest stars. One of the astronomers at the observatory, Allan Sandage [1926–2010], was interested in the stars I was observing because he used them in his cosmological studies. Sandage had been an assistant to Edwin Hubble [1889–1953], who in 1929 was the first to measure the universe’s rate of expansion, known as Hubble’s constant. Sandage was the most prominent observational cosmologist at that time. I had never thought about working with cosmology, but as we got to know each other better he encouraged me, saying that ultimately, everything boiled down to cosmology, and in his opinion there were only two important numbers to measure: Hubble’s constant and the deceleration rate of the universe’s expansion. His purpose in life was to measure these two values. At the time, I didn’t think I was smart enough to do what Sandage was doing. I spent many nights with him in the Las Campanas observatory. On cloudy nights, I began to study literature and talk with Sandage about cosmology. I realized that it was not so difficult and that I could work in this area. We worked together when he began using supernovas to measure the deceleration of the universe. But it didn’t work. When I went to Cerro Tololo, he said: “Nick, you’re going to this other observatory where you will develop new digital detectors. You should use the supernovas to measure the deceleration of the universe.” A close friend, Mark Phillips, with whom I had studied in graduate school, worked at Cerro Tololo with supernovas and we began to collaborate. That was how it took off. When finally we measured the deceleration, which actually proved to be an acceleration, Sandage was upset.
Maybe he felt jealous. Sandage had obtained the value for Hubble’s constant, but had not been able to measure the rate of deceleration. When we published our data in 1998, he began to criticize us and to look for holes in our argument. I think that he believed in our results, but he wished that it had been him who got them. Around that same time, I published a new value for Hubble’s constant, which he also disagreed with. He wrote me a letter saying that he was disappointed, that the quality of my work had declined and that I had fallen in with the devil. He said it would be our last conversation. He did not speak to me for 10 years, until he realized that we were right. Later he wrote me another letter, saying: “Nick, that was the biggest mistake of my life, forgive me for writing that letter, I believe you did a great job and I am proud of you.” It is ridiculous that a relationship between two people can depend on the expansion rate of the universe, but these measurements were important to him.
Your family has an unusual history. Your grandfather owned an arms factory in czarist Russia and fled the country during the Russian Revolution.
I grew up in San Francisco, California. All my relatives spoke Russian. I had an uncle who wore a Cossack uniform that was totally inappropriate for San Francisco. It was a red jacket with brass buttons, boots up to the knees and spurs, as if he were going to ride a horse at any moment. It looked like he had just stepped off the scene of an opera. I was scared to death of being seen next to him on the street. I wanted to be a normal teenager. Only later did I realize how wonderful these people were and I began to appreciate my family’s history.
Were you always fascinated by observatories?
In the beginning, I wanted to be a mathematician. I graduated in mathematics, from Stanford University in California. Later I decided that I was not a good enough mathematician. But I have always been good at conducting experiments and always liked to build things. My classmate Michael Kast and I built the Stanford Student Observatory, which is still in operation. Then I did a post-graduate degree in experimental physics there. Stanford’s strong point was the particle accelerators, but I didn’t like the culture of the department, the people who thought they knew more than everyone else. When I decided to do my doctorate in astronomy, the head of the physics department was horrified. He said that Stanford physicists didn’t go into astronomy, which was an area for failed physicists. This ego was why I didn’t like the experimental physicists. Today I know I was wrong. I have many friends who are experimental physicists, very good people. The truth is that I didn’t get along very well with them and I went to do my doctorate at the University of California in Santa Cruz.