This week the very big and the very small collided at UCSB.
UCSB’s Kavli Institute for Theoretical Physics (KITP) hosted a conference for prominent cosmologists and string theorists from around the world, including Stephen Hawking from the University of Cambridge. Among the topics discussed were the origin of the universe, the basic composition of space itself and the methods used to support hypotheses in these areas. The conference was a rare chance for researchers who study the very large – cosmologists, who study the universe as a whole – to meet researchers who study the very small – particle and theoretical physicists who study subatomic particles and the math behind them.
The leading hypothesis among scientists like Sean Carroll, who was visiting from the Enrico Fermi Institute at the University of Chicago, is known as string theory.
The defining question of the last century of physics is whether there is a grand unified theory in which the three separate forces of classical physics are explained by a single phenomenon.
String theory could be described as a grand unified theory because it attempts to describe every force in the observable universe. In classical physics, there are three forces: the electroweak, the strong and gravity. Each of the forces explain, respectively, why a refrigerator magnet sticks, why atomic nuclei stay together and why apples fall toward the earth.
“String theory is pretty widely accepted,” Carroll said. “Mainly because there is a lack of an even better theory.”
The general premise of string theory is that everything in the universe is made of very small vibrating loops. Vibrating loops can be categorized into different constituents of space, much the way protons and neutrons are constituents of the atomic nucleus. Interactions between the vibrating loops explain the physical forces that the universe exhibits on a macroscopic scale.
“String theory says the universe is only meta-stable,” KITP permanent member Joe Polchinski said. “It would be possible that the universe could collapse.”
In addition to string theory, conference attendees discussed cosmological topics such as the origin of the universe. Polchinski explained the two main competing theories: an infinitely expanding universe and a non-singular universe.
The infinitely expanding universe theory states that the universe had no starting event. When thinking about the universe’s distant history, it becomes apparent that time did not have the same meaning as it does today. Instead the universe may have consisted of a series of expansions that had no beginning. Think of a directional arrow with no starting point.
The non-singular universe theory states that a quantum event occurred, and the universe’s expansion – and hence time – started afterward. In this theory, time looks more like an arrow with a starting point and another arrow attached to that point, pointing in the opposite direction.
While these theories may not seem that different, the mathematical equations involved are not the same, and they predict different futures.
In the case of the infinitely expanding universe, the collapse would be “catastrophic” and unpredictable. In the case of the non-singular universe, the collapse could be slower and more predictable, Polchinski said.
Despite the disagreement concerning the beginning of the universe, everyone at the conference agreed that it is expanding right now. In fact, it is expanding at an increasing rate.
“Two main pieces of evidence are the cosmic radio background and the distributions of galaxies,” Polchinski said.
The cosmic radio background is the radiation that is still being emitted from the time the universe started expanding. The universe was much hotter when it was younger and emitted vast amounts of radiation. This radiation can still be measured and gives insight into the age and expansion characteristics of the universe. The universe is like an electric stove burner that is cooling down but still warm to the touch. By measuring how warm the burner is, one can calculate how long ago the stove was heated.
The distribution of galaxies also shows how old the universe is and how fast it is expanding. Imagine a polka-dot balloon being inflated. As it inflates the spots become farther apart. Spots that started out farther apart would recede from each other faster than spots that started very close. Similarly, galaxies that are very far from earth are receding very quickly. This can be detected using the Doppler effect. Quickly receding galaxies have altered appearances that can be used to determine their speed. This is similar to the alteration that affects a sound when the source of the sound is moving. A car horn will sound lower in pitch when the car is moving away from the observer. This happens because the sound of the horn is composed of physically separated wave crests altered by the car’s motion.
To explain how the universe is expanding, Polchinski first described the generic composition of the universe.
“The universe is about 5 percent baryonic matter, about 25 percent dark matter and about 70 percent dark energy,” Polchinski said.
Baryonic matter is the stuff from which everything on this planet is composed. The sun and all other planets and every kind of known physical thing is baryonic. A baryon is a category of subatomic particle that include protons and neutrons. The startling idea is that the whole universe is only made of 5 percent baryons. The non-baryonic matter is called dark matter, which has mass but cannot be seen.
The curious thing about dark matter is that it has negative gravity. If there were two planets made of dark matter, they would repel each other, unlike normal matter, which would exhibit a strong gravitational attraction.
Dark energy is also called dark pressure because it has negative gravity but no mass. Dark pressure is partly responsible for the increasing rate of the expansion of the universe. It is evenly distributed, meaning that there is dark pressure of the same density inside every person and thing on this planet and at every other location in the universe – even in the vacuum of space. This negative pressure keeps the universe expanding against the normal pull of gravity between matter.
When formulating and investigating these theories, researchers have more guidelines than it may seem.
Rather than simply present what seems like a good idea, theoretical physicists must spend a great deal of work making sure theories are self-consistent and agree with the experimental results. Paul Steinhardt of Princeton University explained some of these difficulties in his Monday lecture.
“In theoretical physics, we look for good economy in explanation,” Steinhardt said. “Spectacular claims require spectacular verification.”