Standard Cosmological Model Upheld
The heavyweight champion of modern cosmology, the standard cosmological model that includes dark matter and dark energy governed by Einstein's laws of gravity, remains undefeated, despite a rigorous new challenge by a team of scientists led by David Rapetti, a postdoc at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). The team used observations of supernovae and galaxy clusters to probe the expansion history of the universe.
Studies of the expansion of the universe are not new, but by using an approach called "kinematics," the current study is providing novel insight. The standard theory of how the universe expands uses Einstein's laws to describe cosmic motion, but scientists are always looking for ways to bolster the certainty of the calculations underlying the theory. To that end, the KIPAC team removed assumptions about how gravity works from their models and used multiple sets of data to give different perspectives on the expansion of the Universe.
"Different methods have different uncertainties and different assumptions," said coauthor Mustafa Amin, a Ph.D. student at Stanford. "When they result in the same answers, scientists are reassured that their theories and models are correct."
Kinematics is a simple way of modeling movement that ignores forces and focuses simply on where an object is, where it was, and where it is going. Using a similar approach in the early 17th century, Galileo repeatedly measured how objects fall. Based solely on observations of position and time, he created an equation that described their motion. But he did not have a clear idea about how gravity works and why objects fall.
Similarly, KIPAC scientists have modeled the expansion of the universe using only measurements of time and the position of distant celestial bodies, allowing them to plot how the Universe has expanded over the millennia.
"We took a step back and looked at the universe in the most primitive way," said Roger Blandford, director of KIPAC.
The KIPAC team used two sets of supernovae observations collected previously. By comparing how bright those exploding stars should appear with how bright they are, scientists can map out their distances and velocities and thereby model the expansion of the Universe.
Applying observations of supernovae is a common practice among cosmologists. However, because no technique is perfect, the KIPAC team also looked at x-ray observations of galaxy clusters. Similar to supernovae, scientists took observed data, such as density, size, and temperature of galaxy clusters to determine their distance and motion. This technique was pioneered by coauthor Steve Allen, an assistant professor at SLAC.
These new kinematic studies agree with predictions of the standard cosmological model that uses Einstein's theory of gravity. Previous studies have already shown that gravity slowed the expansion of the Universe for billions of years before a mysterious force, now referred to as "dark energy," overcame gravity and caused the expansion to accelerate, a threshold scientists have named the "cosmic jerk." This study reinforces this finding while removing some earlier assumptions.
Scientists have proven something is causing the Universe to expand faster and faster, though they don't know what it is or how it works. Studies such as this are small steps towards solving the cosmic riddle of dark energy.
"Over 300 years ago, Isaac Newton took Galileo's kinematic observations and formulated his theory of gravity," said Blandford. "In the future, scientists may be able to take these studies and formulate a complete theory for the expansion of the Universe and the dark energy that apparently drives it."
Ken Kingery, SLAC Today, June 28, 2007
Above image: David Rapetti (left), Mustafa Amin (center) and Steve Allen discuss their work.