Searching for Signs of the Dark Side of the Universe

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While the microwave background presumably originated 380,000 years after the Big Bang, gravitational wave background purportedly come directly from events in the first minute after the Big Bang. The cataclysmic Big Bang is believed to have created a flood of gravitational waves; ripples in the fabric of space-time. These gravitational waves should still fill the universe. However, presumably they are at a very feeble strength and cannot be detected by conventional astronomical tools. Nevertheless, they should carry information about the universe as it was in the immediate aftermath of the Big Bang. If these waves cannot be detected, this challenges the Big Bang.

The 21st Century advances into the next frontier in astrophysics and cosmology depend on our ability to detect the presence of this particular type of wave in space that stretch the fabric of space itself as they pass by. If detected, these weak and elusive waves could provide an unprecedented view of the earliest moments of our universe. The Laser Interferometer Gravitational Wave Observatory, located in Livingston, La., built to feel rather than see, is a highly sensitive observing tool designed to find these elusive primordial gravitational waves.

Arizona State University theoretical physicist and cosmologist Lawrence Krauss and researchers from the University of Chicago and Fermi national Laboratory are exploring the most likely detection method of these waves, with the examination of cosmic microwave radiation (CMB) standing out as the favored method.

"Before a period of 380,000 years after the Big Bang, the universe was opaque to electromagnetic radiation," explains Krauss, a professor in ASU's School of Earth and Space Exploration and the physics department in the College of Liberal Arts and Sciences. "So, to explore earlier times we need to search for other observables outside of the electromagnetic spectrum. Gravitational waves interact very weakly with matter and so gravitational waves produced near the very beginning of time can make their way unimpeded to us today, providing a potentially new probe of early universe cosmology.

In 1916, Albert Einstein predicted the existence of gravitational waves. Based on his theory of general relativity, objects cause the space around them to curve. When large masses move through space, a disturbance is generated in the form of gravitational waves, but because of the weakness of gravity, astronomical amounts of matter must be moved around to generate waves on a scale that might actually be detectable.

"Imagine floating in space far away from Earth alongside two mirrors many miles apart. If a gravitational wave were propagating through space, you would see the distance between the two objects increase and then decrease rhythmically as the wave passes, perhaps by an almost imperceptible amount," explains Krauss. "As these waves propagate throughout the universe they may continue to diminish in strength, but they would never stop nor slow down since they move through matter essentially unimpeded."

In their article "Primordial Gravitational Waves and Cosmology" -written by Krauss; Scott Dodelson, Fermi National Laboratory and University of Chicago; and Stephan Meyer, University of Chicago- they have determined that there are two major sources of gravitational waves: the inflation immediately after the Big Bang, and the possible phase transitions at early times. Other present-day sources may include colliding black holes or two huge stars orbiting each other.

Highly sensitive detectors and experiments such as the Laser Interferometer Gravitational Wave Observatory (LIGO), located in Livingston, Louisiana, are being designed to look for precisely such waves. Gravitational radiation from the early universe can be detected indirectly through its effect on the polarization of the relic CMB radiation which permeates all space. However, the current generation of direct gravitational wave detectors, LIGO included, does not have sufficient sensitivity to probe for the signals of possible primordial gravitational waves.

"The greatest sensitivity to a primordial gravitational wave comes from the distinctive detailed pattern of polarization in the CMB," says Krauss. "If gravitational waves produced by either inflation or phase transitions existed when cosmic microwave background radiation was created, they would be imprinted on the CMB and be detected as polarization."

"As we enter the second decade of the 21st century, we are poised to enter a new realm of precision cosmology, one that could provide a dramatic new window on the early universe and the physical processes that governed its origin and evolution," says Krauss. "The European Space Agency's Planck satellite was designed to image the CMB over the whole sky, with unprecedented sensitivity and angular resolution, and will provide new data on polarization within the next three to four years and with that we hope for direct observations of waves from the beginning of time."

Image top of page: Though the Planck satellite has yet to return results from the cosmic microwave background, its new results show exquisite images of cold dust in our own galaxy. This image shows the galactic plane — the line running horizontally across the image near the bottom — and the huge clouds of cool dust that rise far above the plane.Credit: ESA and the HFI Consortium, IRAS

Casey Kazan via

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