Physics @ Berkeley
Physics in the News
Title: Space's Dark Matter Expands The Universe
Date: 03/01/2006
Publication: The Daily Californian
Editor: Sheena Chen
Extended Text:
The expansion of the universe is a cosmological theory that generations of physicists have been contemplating. This has led one physicist to speculate on the fascinating questions about the nature of "darkness" that involve concepts from both cosmology and particle physics.

At the beginning of the 20th century, the theory was established that everything was made of atoms, which were composed of an atomic nucleus with electrons orbiting around.

"That was a convenient picture of what we thought everything was made of," said Hitoshi Murayama, a UC Berkeley professor in the physics department. "The more recent realization is that it's not the case."

According to Murayama, the atoms in our bodies comprise only 5 percent of the universe. The remaining 95 percent is something different and unfathomed, consisting of at least two components-dark matter and dark energy.

Dark matter is a hypothetical set of particles that cannot be visualized or felt, but only exists by inference from the gravitational effects of stars and galaxies. The crucial characteristic of dark matter is that it reinforces the structure of the universe, contributing to 23 percent of the total of the world's existing matter.

Albert Einstein once suggested that how fast the universe expands depends on how much energy exists. As the universe expanded, ordinary matter like stars and galaxies thus became more dilute with the changing volume.

"Less energy in a given volume tells you that the universe should be expanding slower, but because it is not, what that means is that energy is somehow increasing," Murayama said.

Permeating all of space then, some scientists believe that dark energy causes the apparent acceleration of the universe expansion. Popular scientific methods also support the fact that dark energy makes up a significant portion of the universe's missing mass.

According to Murayama, it is probably made of new kinds of fundamental matter, which we have not seen yet.

"We didn't have enough energy to create them in a lab," Murayama said. "That means they are heavy particles, but we cannot create them due to limited amounts of resources."

In order to test the concept of dark energy, Murayama turned to the magnitude of the teraelectronvolt, or TeV, scale. High-energy particle accelerators, normally used in experimental particle physics, show promise in reaching a high enough energy scale to create particles of heavier mass.

"We hope that one of these new particles that may get created will be this dark energy particle," Murayama said.

One of the forms that dark energy takes is in the cosmological constant, or a vacuum energy. The term explains that any given volume of empty space possesses an essential, intrinsic value. To demonstrate the concept of the expanding universe, Murayama proposed his own theory.

"One idea that I toyed around with is that dark energy is a complicated network of rubber sheets," Murayama said. "If you stretch them, you have to provide force and because of it, the sheet gains energy. The universe expansion is much like stretching this network of sheets, which increases energy."

Accordingly, the universe would accelerate expansion faster with more energy.

Though current research does not anticipate having direct implications for applications, Murayama says that he still conducts his work with interest, for the conundrum pointing toward the ultimate fate of the universe may potentially be debunked.

"These questions used to be just the subjects of philosophy or metaphysics," Murayama said. "Now that we can try with scientific methods, we hope to continue the tradition."