In the most anticipated announcement in physics since the discovery of the Higgs Boson, the first detection of a gravitational wave has been reported. If verified, the find will dispel any lingering doubts about Relativity theory, transform our understanding of the universe’s beginning and provide astrophysicists with a new tool to probe the universe. The importance of the detection is hard to overstate.
As part of his General Theory of Relativity, Einstein predicted that acceleration of large masses would cause waves to ripple through space in a manner analogous to ripples on the surface of a pond. Indirect evidence abounds for gravitational waves, but almost a century after Einstein predicted it direct evidence remained elusive – until today’s announcement by the Harvard-Smithsonian Center for Astrophysics. The paper is now available on arXiv.
The Cosmic Microwave Background (CMB) is the left over radiation from a four hundred thousand years after the Big Bang stretched by the expansion of the universe to peak in the microwave part of the spectrum. In the mid 1990s astrophysicists proposed that the polarization of the CMB could provide evidence for gravitational waves from the birth of the universe.
Photons can oscillate in different directions as they travel; up or down, side to side or even in a circular manner clockwise or anticlockwise. Hot sources produce photons with random orientations, but certain forces can create a bias where there is a preponderance of photons oscillating in a particular direction as they travel, making the radiation as a whole polarized.
The CMB was found to have a very slight polarization in 2002 as a result of density perturbations in the universe. Gravitational waves however, would be expected to induce a slightly different form of polarization. However, this pattern is so slight, and so vulnerable to false positives caused by other things, that there has been considerable skepticism that we would be able to detect the gravitational wave-induced polarization, at least with existing instruments.
The Plank space observatory has been studying the CMB since 2009, and some astronomers hoped it would be able to provide the evidence, but in the end the results came from an even more remote location, the Background Imaging of Cosmic Extragalactic Polarization (BICEP) detector located at the South Pole, where the cold dry air makes microwave astronomy possible.
Technical papers are available and are being poured over by researchers from teams worldwide.
The discovery of the CMB polarization by gravitational wave, should it stand the test of time, settles one question on its own, the debate over whether the early universe was inflationary. According to the most popular, but not universally accepted, theory of the early universe, 10-34 seconds after it began the universe experienced a period of rapid growth – expanding 100 trillion trillion times to something the size of a marble.
An inflationary period would produce larger gravitational waves than would have been generated without. Nevertheless, even most inflationary models do not predict a gravitational wave large and polarizing enough to be detected by BICEP.
The signal BICEP has found is so strong it makes many of the inflationary models of the early universe untenable, and leaves non-inflationary versions completely on the outer, suggesting the energy in the universe at that moment was well very much at the upper end of what was previously thought possible.
One of the reasons gravitational waves are so keenly sought is the hope that they will provide information about the crucial first moments of the universe in ways other instruments cannot. “People talk about the Square Kilometre Array as enabling us to detect the radiation from the Big Bang, but that is not strictly correct, Professor Jesper Munch of Adelaide University told Australasian Science. For the first 300 million years the universe was opaque to all electromagnetic radiation. However, gravitational waves could propagate through this early universe, and we can thus in principle detect signatures from the time of the Big Bang. It is probably the only way we can get signals from the origin of the universe.
Merely detecting a way is exciting, but we want more information than that it exists. The strength of the wave is expected to vary at different wavelengths. Finding out where it is strongest and weakest will tell us a lot about how the inflation occurred. The most important information of all is how energy dense the universe was during this era, and this could potentially be found by comparing wavelengths.
Gravitational wave perturbations from those first moments are directly dependent on the inflation, unlike density perturbations which are modulated by an unknown potential energy function. Consequently they would give us direct evidence of the details of energy of inflation in those first moments.