On Friday, Steven Weinberg, Nobel laureate in physics from the University of Texas, delivered a talk titled “The Dark Side of the Universe” to a teeming Wege Auditorium. The lecture focused on dark matter and dark energy in the universe.
Weinberg began by explaining that much of the field of science through the 19th century involved finding the laws that apply to the universe as a whole and proving that the same forces could be applied on Earth. However, discoveries over the past century have taken science in a new direction. “This impression of a fairly home-like universe, a universe that mostly consists of things familiar to us, has been displaced recently with the discovery of dark matter and then dark energy,” Weinberg said.
“Dark matter is a kind of material that is not made of particles like you and I are made of but unfamiliar particles, particles that we do not observe on Earth and are different than the particles that make up our everyday life and stars,” Weinberg said. According to Weinberg, dark matter is not actually rare: It makes up five-sixths of the universe. “It is we – people, planets and stars – that are a contamination in the universe,” Weinberg said.
In 1998, it was discovered that the energy in the masses of all matter in the universe – regular matter and dark matter – only accounts for approximately one-fourth of the total energy in the universe. Dark energy, inherent to the universe, composes the other three-fourths. It is invisible to the human eye, as it does not react with light.
“We only know about it because of the gravitational effects it produces,” Weinberg said.
The first hint of the existence of dark matter was seen in the 1930s by astronomer Fritz Zwicky who noted that stars are not scattered at random, so there must exist a type of matter that could produce a gravitational field strong enough to hold together clusters of galaxies. “This was mysterious,” Weinberg said. “What was this matter?” In the 1970s, astronomer Vera Rubin applied a similar sort of analysis to individual galaxies.
Weinberg went on to talk about the synthesis of various elements in the early universe and the big bang theory. “Almost all of the elements on [the periodic table of elements] were synthesized in stars, most of them in supernova explosions,” Weinberg said. “Only the lightest elements couldn’t have been synthesized in stars.” These light elements, including isotopes of hydrogen, helium and lithium, were synthesized in the first three minutes of the creation of the universe, according to Weinberg.
Weinberg said that according to current theory, the universe started with particles found in nuclei – protons and neutrons. “Then, as the universe cooled, the neutrons tended to decay – they are unstable – and at a certain point, the temperature became cool enough for nuclei to hang together, and nuclear reactions occurred,” Weinberg said. He added that the reactions happened extremely fast and did not have time to finish. “There must have been one neutron or proton for every one or 10 billion photons,” Weinberg said, adding that such a ratio helped to determine the number of particles formed in the first few minutes of the universe’s creation.
“The dark matter necessary to hold galaxies and clusters of galaxies is much larger than what could possibly be provided by the number of protons and neutrons in the universe,” Weinberg said. This discrepancy between the amount of matter needed and the amount of matter we actually see gave rise to the idea that there must exist dark matter.
“Today we can measure the abundance of this dark matter much more precisely,” Weinberg said. “All of these observations agree that about one-sixth of the matter of the universe is ordinary matter and the other five-sixths is something that we have never observed. It is dark, it is cold … and it’s just there.
“As you can imagine, there are a lot of physicists who want to figure out what this dark matter consists of,” Weinberg said. However, experiments to observe dark matter directly have not yet yielded conclusive results.
“There is a theory of what the cold, dark particles are – they are called WIMPs – weakly interacting massive particles,” Weinberg said. They carry a charge that has been conserved since they were formed around the time of the big bang.
“There is a new experiment which is going up to the International Space Station next week called the Alpha Magnetic Spectrometer that has the capability of accurately studying charged particles coming from all kinds of astronomical particles,” Weinberg said. He added that even if dark matter is not discovered, the project is capable of observing cosmic rays above the Earth’s atmosphere.
Also discovered in 1998, the expansion of the universe is accelerating, not slowing down as originally thought. This phenomenon cannot be produced by ordinary or dark matter and therefore must be produced by an energy inherent in space itself that is independent of the existence of matter. With respect to dark energy, “we don’t see it. We don’t know anything about it,” Weinberg said.
The idea of dark energy did originate before 1998, Weinberg noted. “For many decades, for physicists, the problem has been not why is there dark energy but why is there not more of it,” Weinberg said. He added that only one explanation seems to fit the data: “It is highly speculative,” he said. “The whole universe that we see … may be only part of a much larger universe with many parts that resemble our big bang … but where many constants of nature take on different values.”
In many parts of this multiverse, Weinberg continued, it is impossible for life to exist because of expansion that is too rapid. “We now know that … there are billions of planets and there are billions of galaxies, so it is not surprising that occasionally you would find a planet where life can exist,” such as Earth, Weinberg said.
Weinberg said that he is not sure that the results of his work will hold up. “I hope the explanation is wrong because if it is right, we will never be able to calculate the precise amount of dark energy in space,” Weinberg said.
He noted that in order to delve further into the field of dark energy, it may be necessary for scientists to give up precise measurements of fundamental constants. “I find this a frightening possibility as a physicist because I like being able to calculate things,” Weinberg said.