Dark Energy: Studying the Expansion and Fate of the Universe

Kaitlin McLean December 1, 2010 0

Since ancient times, people have stared into the heavens and pondered what was out there—what the fate of our universe might be. Dr. Priyamvada Natarajan’s, Yale Professor of Astronomy and Physics, research into dark energy has brought us closer than ever to answering these questions.

What is Dark Energy?

As Professor Natarajan admits, dark energy is currently one of the great embarrassments of astrophysics: it makes up roughly 72% of the universe, yet there is no consensus on what exactly dark energy is. As she described in a recent publication in Science, dark energy is an unknown material with negative pressure originally hypothesized to explain one of the most troubling conundrums of astrophysics.

Gravity, one of the four fundamental forces that act in our environment , causes objects with mass to attract each other. Beyond keeping the planets and their moons in orbit, gravity is also what makes people fall when they trip. Gravity should be pulling all the bodies in the universe towards each other and ultimately cause our universe to shrink and collapse inward upon itself. Yet despite this, astrophysicists have determined that the universe is expanding at an accelerating rate. For this to be possible, there must be a greater force that opposes gravity on the scale of the larger universe.

Dark energy was postulated to be a solution to this puzzle since dark energy, having negative pressure, is an expanding repulsive force that opposes the coalescing force of gravity. Though scientists aren’t precisely sure what dark energy is, they have been able to model its properties and behavior. To provide a more descriptive image of dark energy, it can be imagined as a fluid modeled by the equation P=w. This is the hypothesized equation of state for dark energy, yet definitively determining this equation of state has remained one of the main goals for cosmologists today.

Dark Energy vs. Dark Matter

Dark energy and dark matter are unrelated quantities and should not be confused, but both are not well understood by scientists. Dark matter is composed of exotic particles produced immediately following the Big Bang. The composition of these particles is unknown. Approximately 90% of the universe is composed of dark matter while the remainder consists of baryons—matter such as that from which objects on earth are composed.

Dark matter aggregates in the universe due to gravity, providing a scaffolding for galaxies. Our own Milky Way galaxy is constructed upon a base of dark matter, so from outer space, it appears as though our universe has a halo of dark matter surrounding it.

To detect dark matter, scientists measure the speed of stars. In 1933, the Swiss astrophysicist Fritz Zwicky of CalTech estimated the total mass of a galaxy cluster. He recorded the acceleration of the cluster’s orbits and found the force of gravity for the estimated mass to be much too small to account for such fast orbits. He determined that there must be about 400 times more estimated mass than was visibly observable. This invisible mass was deemed “dark matter.”

Professor Natarajan’s Groundbreaking Research

Priyamvada Natarajan was a member of an international team of cosmologists, astronomers, and physicists who used gravitational lensing to study dark energy. One of the current goals in observational cosmology is to characterize the mass-energy content of the universe. Natarajan’s team recently published their results from a geometric test based on strong lensing in galaxy clusters. They used data from images taken by the Hubble telescope and spectroscopic analysis of the galaxy cluster Abell 1689, a massive galaxy cluster 2.2 billion light years from Earth, to perform their geometric test. These results, combined with results from other experimental involving supernovae, x-ray galaxy clusters, and the Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft, have allowed the team to lower the uncertainty of their findings by about 30%.

What is Gravitational Lensing and How Does It Work?

Originally predicted by Einstein’s general theory of relativity, gravitational lensing occurs when light from a bright and distant source is bent around a massive object—say, a cluster of galaxies—between the source of light and the viewer. The gravity from an object as massive as a cluster of galaxies actually warps space-time, causing everything around it to bend. This has the effect of bending the path that light follows just as an optical lens refracts light. The light gets focused when passing massive objects and defocused when traveling through empty space. Gravitational lensing functions similar to optical lenses in which a lens transmits and refracts light, converging or diverging the beam.

Professor Natarajan’s research uses a special type of gravitational lensing called strong lensing. Strong lensing is so strong that multiple images of the same galaxy are produced. These copies provide many different light paths, which can then be reconstructed to show the geometry of the galaxy cluster of interest.

To make use of strong gravitational lensing, Professor Natarajan’s team took images of the galaxy cluster Abell 1689 with the Hubble Space Telescope, a telescope that produces the best resolution of any existing telescope. From these images, the team had to determine which galaxy clusters were in fact repeats of the same galaxy cluster of interest. To do this, the scientists analyzed the red shift of each galaxy. The ones with identical spectra were determined to be part of the same cluster.

Why This Research is Groundbreaking

Sometimes in science, the most exciting discoveries involve the discovery of a new application for an existing technique. Such is the case with Professor Natarajan’s recent research. The method of strong lensing has been used in several other cosmological applications, but never before has it been used to measure dark energy. Professor Natarajan’s development of this new application of strong lensing was the accumulated work of years of effort. The method had to be pieced together until all of its components were synthesized into a reliable and accurate technique.

This new method offers several different strengths when compared to other techniques used to study and measure dark energy—most remarkably its accuracy and wide applicability. Having been used since 1979, gravitational lensing is well understood. Additionally, there are numerous massive galaxy clusters that can produce the effect of strong lensing. Before, scientists would study dark energy through supernovae, a technique limited by the fact that cosmologists have to search for more supernovae to study, cosmologists at increasingly further distances; the farther cosmologists have to look, the weaker the supernovae. Another limitation of this approach is the assumption that all supernovae are a “standard candle”—that is, all supernovae are of the same magnitude. However, this assumption may not be accurate in all cases.

As Professor Natarajan states, “The content, geometry and fate of the universe are all linked, so by constraining two of those things, you can learn something about the third.” Like the dark energy that is its subject, Professor Natarajan believes this work continue expanding—not collapsing.

Extra Reading:

Amendola, Luca, and Shinji Tsujikawa. Dark Energy: Theory and Observations.

New York: Cambridge UP, 2010. Print.

Kirshner, Robert P. The Extravagant Universe: Exploding Stars, Dark Energy, and

the Accelerating Cosmos. Princeton, NJ: Princeton UP, 2002. Print.

Ruiz-Lapuente, P. Dark Energy: Observational and Theoretical Approaches.

Cambridge, UK: Cambridge UP, 2010. Print.

Wang, Yun. Dark Energy. Weinheim: Wiley-VCH, 2010. Print.

About the Author:

Kaitlin McLean is a sophomore in Johnathan Edwards college majoring in Molecular, Cellular and Developmental Biology. She is a writer for the Yale Scientific Magazine and works in Professor Crews’ lab studying the molecular mechanisms of limb regeneration in axolotls.

Acknowledgements:

The author would like to thank Professor Natarajan for her time and her advancements to the field of cosmology.

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