Detection of gravitational waves

Scientists have found evidence for the existence of gravitational waves and support for cosmic inflation. Gravitational waves are a prediction of Einstein’s theory of general relativity, which states that mass and energy (which are equivalent, as we know from E=mc2) can distort space-time. Gravitational waves can be thought of as waves in the fabric of space-time, which simultaneously stretch and compress different regions of space-time.  Massive objects, such as black holes, bend space-time around them, and when objects move they create ripples in space-time. Imagine a duck swimming through a lake: as she moves forward she pushes the water away from her and creates a wake behind herself. This is what massive objects do when they move through space. Stars in a binary that orbit one another will create ripples in space-time that propagate outwards as gravitational waves.

Ripples in space-time are created by moving masses, in this case a black hole binary (Credit: T. Carnahan, NASA/GSFC)

Energy – being equivalent to mass – can also induce gravitational waves. So what created the gravitational waves that scientists claimed to have detected? The answer is inflation.  Right after the Big Bang, when our Universe was very young and very hot, matter could not exist and all that existed was energy. During this time, the four fundamental forces – gravity, electromagnetism, the strong and weak forces – were one unified force. However, the Universe quickly began to cool, which allowed the fundamental forces to separate into individual components. This kick-started a chain reaction that lead to our Universe inflating from 6 x 10-28 meters to almost 1 meter in under a second!  During cosmic inflation, massive amounts of energy were released into space-time which would have created gravitational waves. So the detection of these primordial gravitational waves lends support to the idea that the Universe was created via the Big Bang and then expanded through cosmic inflation.

How do astronomers detect primordial gravitational waves created at the dawn of time, some 13.8 billion years ago? Because the Universe continues to expand, albeit very slowly compared to the period of cosmic inflation, the signature of gravitational waves would be too weak to detect in the nearby Universe. However, the discovery of the cosmic microwave background (CMB) has given us a time stamp of the distant Universe. Some 372,000 years after the Big Bang, the Universe was cool enough that matter became decoupled from radiation.  Before this time, the Universe was opaque to radiation and light could not escape. The cosmic microwave background can be thought of as the most distant part of the Universe that we can observe, when the Universe first because transparent and light could travel freely through space.

If gravitational waves were present in the early Universe, they would have left a distinct pattern on the cosmic microwave background  because they literally alter the space-time in which the photons moved. As gravitational waves propagate through space-time, they will condense space-time in one direction, making it look a little hotter, and stretch space-time in another direction, making it look a little cooler.  These temperature variations are very, very small, but detectable. As photons move through  rippled space-time, they will scatter in a preferred direction, resulting in polarization. The type of polarization that would have been created by primordial gravitational waves is called B-mode polarization, producing a curly, vortex-like pattern.

B-mode polarization pattern seen in the cosmic microwave background as measured by the BICEP2 instrument. The colours show the spin intensity and orientation (red clockwise, blue anti-clockwise) and the lines show the polarization strength and direction. (Credit: BICEP2 collaboration)

To detect the very faint signal of B-mode polarization in the cosmic microwave background requires a precision of one ten-millionth of a kelvin to measure the tiny temperature fluctuations.  Astrophysicists from Harvard-Smithsonian Center for Astrophysics, led by John Kovac, used the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) instrument at the South Pole to study the “southern hole”, a patch of sky free of other forms of emission, to measure temperature variations in the cosmic microwave background to extremely high precision. And what they found was amazing – a signature in the cosmic microwave background that is consistent with the pattern left by primordial gravitational waves.

Of course additional measurements have been made to confirm this ground-breaking discovery. The Keck Array, also located at the South Pole has provided data with the same implications as BICEP2, and will continue to run for another two  years. The Planck telescope, which has provided the most precise measurements of the Cosmic Microwave Background to date, will be used to provide a more extensive all-sky map of the B-mode polarization.

So what are the consequences of detecting primordial gravitational waves and confirming cosmic inflation? Inflation solves two major issues that arise from the Big Bang theory. First, we know that in any direction you observe, space-time is flat. Secondly, if you measure the temperature of space where there is no matter, it is isotropic. But the Big Bang theory has no explanation for why the Universe would evolve to be this way. Alan Guth, from Massachusetts Institute of Technology, developed the theory of cosmic inflation to understand why particles that should have been created during the Big Bang are not present today, but his theory also solves these other two problems. Essentially, if the Universe began very small but then grow gigantic very quickly, several things could happen. First, if the Universe were small enough the temperature would balance out and be in equilibrium, and so once it expanded it would be the same temperature everywhere. And if the Universe expands enough during the preriod of cosmic inflation, it would wash out any  curvature in space-time.

The confirmation of primordial gravitational waves would also be the first experimental evidence that quantum mechanics and gravity are in fact linked. This is because part of what started cosmic inflation was the existence of quantum fluctuations. These fluctuations are very small waves that propagate through empty space and ignited cosmic inflation. If quantum mechanics and gravity can be linked, than a theory of quantum gravity cannot be dismissed and could be used to explain other extreme phenomena in our Universe, such as the physics that govern the centers of black holes.

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[Rebecca Allen & Sarah Maddison]

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