Scientists using the BICEP2 Radio Telescope at the South Pole announced they have found the first direct evidence that gravitational waves – disturbances in the curvature of space-time – rippled through the infant Universe during an explosive period of growth called cosmic inflation.
The tiny temperature fluctuations of the cosmic microwave background, shown here as color, trace primordial density fluctuations in the early universe that seed the later growth of galaxies. Gravitational waves from inflation are expected to produce much a fainter pattern that includes twisting B-mode polarization, consistent with the B-mode polarization pattern observed by BICEP2, which is shown here as black lines. The line segments show the polarization strength and orientation at different spots on the sky. Image credit: BICEP2 Collaboration.
About 13.8 billion years ago, the Universe burst into existence in an extraordinary event that initiated the Big Bang.
In the first fleeting fraction of a second, it expanded exponentially, stretching far beyond the view of best telescopes. All this was just theory.
The new results confirming this theory came from observations of the cosmic microwave background – a faint glow left over from the Big Bang.
Tiny fluctuations in this afterglow provide clues to conditions in the infant Universe. For example, small differences in temperature across the sky show where parts of the Universe were denser, eventually condensing into galaxies and galactic clusters.
“Detecting this signal is one of the most important goals in cosmology today. A lot of work by a lot of people has led up to this point,” said Dr John Kovac of Harvard-Smithsonian Center for Astrophysics, who is the lead author of a paper accepted for publication in the journal Nature.
“Small, quantum fluctuations were amplified to enormous sizes by the inflationary expansion of the universe. We know this produces another type of waves called density waves, but we wanted to test if gravitational waves are also produced,” said co-author Dr Jamie Bock of NASA’s Jet Propulsion Laboratory in Pasadena.
Since the cosmic microwave background is a form of light, it exhibits all the properties of light, including polarization.
Light can become polarized by scattering off surfaces, such as a car or pond. Polarized sunglasses reject polarized light to reduce glare. In the case of the cosmic microwave background, light scattered off particles called electrons to become slightly polarized.
The gravitational waves produced a characteristic swirly pattern in polarized light, called B-mode polarization.
Gravitational waves from inflation generate a faint but distinctive twisting pattern in the polarization of the cosmic microwave background, known as B-mode pattern. For the density fluctuations that generate most of the polarization of the CMB, this part of the primordial pattern is exactly zero. Shown here is the actual B-mode pattern observed with the BICEP2 telescope, which is consistent with the pattern predicted for primordial gravitational waves. The line segments show the polarization strength and orientation at different spots on the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of this B-mode pattern. Image credit: BICEP2 Collaboration.
“Our team hunted for a special type of polarization called B-modes, which represents a twisting or curl pattern in the polarized orientations of the ancient light,” Dr Bock said.
Gravitational waves squeeze space as they travel, and this squeezing produces a distinct pattern in the cosmic microwave background. Gravitational waves have a handedness, much like light waves, and can have left- and right-handed polarizations.
“The swirly B-mode pattern is a unique signature of gravitational waves because of their handedness. This is the first direct image of gravitational waves across the primordial sky,” said co-author Dr Chao-Lin Kuo from the Stanford University’s SLAC National Accelerator Laboratory.
The B-mode signal is extremely faint. In order to gain the necessary sensitivity to detect the polarization signal, the scientists developed a unique array of multiple detectors, akin to the pixels in modern digital cameras but with the added ability to detect polarization. The whole detector system operates at a frosty 0.25 Kelvin, just 0.45 degrees Fahrenheit above the lowest temperature achievable, absolute zero.
The team examined spatial scales on the sky spanning about one to five degrees. To do this, they traveled to the South Pole to take advantage of its cold, dry, stable air.
“The South Pole is the closest you can get to space and still be on the ground. It’s one of the driest and clearest locations on Earth, perfect for observing the faint microwaves from the Big Bang,” Dr Kovac said.
The scientists were surprised to detect a B-mode polarization signal considerably stronger than many cosmologists expected.
They analyzed their data for more than 3 years in an effort to rule out any errors. They also considered whether dust in our galaxy could produce the observed pattern, but the data suggest this is highly unlikely.
“This has been like looking for a needle in a haystack, but instead we found a crowbar,” said co-author Dr Clem Pryke from the University of Minnesota.