The mystery of the universe, and the many riddles that remain open about it can be exemplified in a myriad ways. One of the most frustrating challenges involves the fact that science is familiar with less than five percent of the matter that makes up the universe. That paltry figure is what you get after adding up all the matter on Earth and outside it – including all the galaxies, solar systems, and such heavenly objects as asteroids, meteors, gas clouds and interstellar dust. Effectively, everything that’s made of atoms.
The problem was that science could not explain empirical observations of the universe based on the matter and energies we know about. The baffling conclusion of centuries of observations and calculations is that the matter and energies that have been identified constitute, to be precise, just 4.9 percent of everything in the universe. So what else is out there, that we haven’t seen?
Scientists call the mysterious 95.1 percent of whatever is making up the universe “dark matter” and “dark energy.” It’s called “dark” because it can’t be seen, not in the form of celestial bodies or even dust clouds. Nor does it appear to interact with the particles we do know about, including light particles.
It is on dark matter that Prof. Lisa Randall, a physicist at Harvard University, hopes to shed light.
A day before Randall arrived in Israel, this past December, to participate in the annual conference of the Israel Physical Society, U.S. President Donald Trump forbade the Centers for Disease Control and Prevention, a U.S. public health agency, from using the terms “evidence-based” and “science-based” in budgetary documents. Randall opened her talk at the conference, which was held at the Technion – Israel Institute of Technology, in Haifa, on December 17, by saying, “It’s nice to speak at a conference in a country where you can still use the word ‘science.’”
Her barbed remark reflects the apprehension of many of her colleagues about the future of scientific research in the United States in light of the Trump administration’s approach. In the meantime, though, Randall continues to occupy herself vigorously with the big questions of the universe, including, most recently, the nature of dark matter.
If less than five percent of what comprises the universe is even known to science, what can it say about the rest? Scientists estimate that some 26 percent of the universe is dark matter. The search for dark matter began in the 1970s. The idea of its very existence arose in an attempt to explain the speed at which galaxies rotate around their axes in the cosmos. The measurements, which didn’t correspond to the known effects of gravity, led scientists to postulate the existence of additional matter, not visible to the eye or measuring devices, that exercised gravitational influence, and could in part explain the speeds that were calculated.
(This week saw a dramatic development in the study of dark matter. In an article in the prestigious journal Nature, Prof. Rennan Barkana, head of the astronomy department at Tel Aviv University, described how the EDGES radio telescope in Australia had picked up a radio wave dating back to a much earlier period in the history of the universe. The signal serves as the first direct evidence of dark matter’s ability to interact with regular materials. It provides new testimony about the mysterious period shortly after the universe came into being, a period described as the era of “the cosmic dawn,” meaning the period when the first stars were formed, some 200 million years after the Big Bang, which, according to current understanding, took place about 13.8 billion years ago. The findings are still preliminary, and require extensive analysis, but if they can be confirmed, they can be seen as evidence that dark matter interacted in some way with hydrogen atoms, thus leading to the lowering of the temperature of the dark matter. Physicists responding to the article in nature suggested that the discovery could open a new direction into the study of dark matter and the universe).
The remaining 69 percent of the “explanation” for the observed behavior of the universe is the postulation that there is some sort of energy out there that researchers called “dark energy.”
This force did not enter the theoretical picture until the end of the 1990s, on the basis of surprising observations of supernova events (the explosion of stars), which revealed that the universe is not only expanding and spreading, but is constantly accelerating the speed with which it does so. (The assumption had been that the expansion was decelerating, because of the effects of gravity.) The basic argument for the existence of dark energy is that there must be a hidden force that is pushing the galaxies away from one another.
When I met Randall, the Frank B. Baird, Jr. Professor of Science at Harvard, I asked why she chose to focus her physics research on dark matter. Our interview took place at the Dan Carmel Hotel, in Haifa.
“I think that dark matter is just at a very interesting juncture right now, because we know it’s there, but we don’t know what it is. And, depending on what it is, there can be different ways to look for it. My research is being conducted in areas that others haven’t looked at.”
But are you certain that it exists?
“I would say yes. There are many different pieces of evidence that point to its existence. We see how stars move in our galaxy; we see how galaxies move in galaxy clusters. We also see evidence in cosmic microwave background radiation, left over from the earliest moments of the universe.” (Randall is referring to radiation that is a kind of remnant of the Big Bang, the event that brought the universe into being nearly 14 billion years ago. The radiation is an artifact that serves as something like a photograph of the state of the universe at its genesis, and its existence is one of the proofs that the Big Bang occurred.)
“In contrast to dark energy, dark matter is, after all, matter, which means it can interact, it can clump differently, it could take on certain shapes and sizes and exist in different concentrations in galaxies. So there’s a lot of research to be done – to figure out what we can really predict from these models, and understand what we hope to see.
“People often ask me, ‘You haven’t seen it, so how do you know it’s there?’” Randall says, in response to which she usually compares the investigation of dark matter to an unidentified celebrity who comes to town for a visit. “You know that a celebrity is there because you see people gathering around, and there are cameras, but you don’t necessarily know who it is. It’s the same with dark matter: We see its gravitational effects on other matter, on visible matter, so we know it’s there, but we’re still trying to figure out what it is.”
In her Technion lecture, Randall explained that the existence of dark matter was first postulated on the basis of the rotational speed of stars in our galaxy. In certain cases, she said, the motion of the stars in their orbits is so rapid that, without hypothesizing the existence of dark matter – that is, without the existence of a gravitational force in addition to that of regular matter – we would expect these stars to lurch out of those orbits. Today it’s clear that without dark matter, galaxies and galaxy clusters would not even have come into being.
Over the years, studies have mapped the presence of dark matter in various places in the universe, including in the center of our galaxy, the Milky Way. In her most recent book, “Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe” (HarperCollins, 2015), Randall takes dark matter and its possible effects one step further, linking its presence in our solar system with the extinction of nearly all of the dinosaurs some 66 million years ago.
The unusual connection between dark matter and dinosaurs proved irresistibly intriguing on the lecture circuit and TV talk shows. Her research on this subject, first described in an article co-written with the Harvard physicist Matthew Reece, began with a survey of all the known craters on Earth with a diameter of more than 20 kilometers and which are no more than 250 million years old. There are 26 such monster craters, all created, according to the geological evidence, by large objects that slammed into the planet. Randall found a consistent cyclicality in the formation of the craters, with a new one having appeared approximately every 30 million years. The reason for this recurring cycle, she argues, is to be found in the cyclicality of the solar system’s circumnavigation of the Milky Way – once every 30 million years.
The cyclical blow to Earth is related to two phenomena, Randall believes. One is a familiar concept from space research known as the “Oort cloud.” The second is Randall’s theory of the existence of the “double-disk dark matter.”
The Oort cloud, named for the 20th-century Dutch astronomer Jan Oort, is thought to consist of billions of blocs of ice that are the source of many of the meteors, the long-tailed icy bodies that look like falling stars as they hurtle across the galaxy.
Randall’s double-disk dark matter is postulated to be a large expanse of dark matter in the center of the Milky Way galaxy that possesses immense gravitational attraction. According to her theory, once every 30 million years – that is, once during the solar system’s orbital cycle around the galaxy – a situation forms in which the gravitational pull of the dark matter disk causes large meteors to be “wrenched” from the Oort cloud, some of which then proceed to collide with Earth. That, Randall says, is the “season” in which the large craters are created.
One such crater, called Chicxulub, which was discovered in the 1970s by employees of the Mexican oil company Pemex, on the edge of the Yucatan Peninsula, was studied and dated. The crater was created by an impact with Earth very close to the time of the great dinosaur die-off, Randall says.
Lisa Randall was born in Queens, New York, in 1962, the second of three children, to a father who was a salesman and a mother who was a primary school teacher. Her impressive memory and interest in mathematics were evident from an early age. She attended the prestigious, public Stuyvesant High School in Manhattan, which specializes in the sciences. In a 2016 interview with The New Yorker, she related that in her teens she had frequent confrontations with her mother, who was reluctant to let her leave for school before dawn so she could arrive in time for the early-morning meetings of the school’s math team, of which she was captain (the first girl to serve in that capacity).
At age 17, Lisa finished first in the prestigious Westinghouse Science Talent Search (as it was then known), a nationwide science competition for high-school seniors.
She entered Harvard in 1980, and by 1987 had obtained a doctoral degree in particle physics there. She was the first woman to receive tenure in the physics department of Princeton University and the first female, tenured theoretical physicist at Harvard. She also served as a tenured professor at MIT.