How Earth rocks on the sea of space-time
A constant radiation in the microwave range, the background radiation, has long told cosmologists that something important happened 380,000 years after the Big Bang. At that time, electrons and protons recombined to form molecular hydrogen, so that space finally became transparent, allowing light to propagate. We can still measure the remnants of this light, shifted into the infrared.
But a lot happened shortly after the Big Bang. The very early universe was determined by time-varying scalar fields, after the inflation phase there should have been an energy transfer from inflaton particles to regular matter, there were various phase transitions, where the universe changed hard from one to the following state (as in the electroweak phase transition).
These phenomena are still hypothetical today, but they could be proved: Namely, a gravitational wave background would have to have been created in the process. The detection of a background from these sources would thus be a significant discovery of new physics and would have a profound impact on the cosmology of the early universe and on high-energy physics.
How can we imagine a gravitational wave background? Researchers today use LIGO to repeatedly measure gravitational echoes from black hole collisions and similarly violent events. These waves are strong and fast, racing through the Earth in microseconds. They are tsunamis in space-time. The gravitational wave background, on the other hand, is the normal swell of the sea we call space-time. The Earth and all other objects in the universe float on it without us noticing. They are long waves, their passage through the Earth takes a year or more.
Accordingly, it is difficult to detect the gravitational wave background. For more than 13 years, researchers have been sifting through the light of dozens of pulsars in the Milky Way as part of the NANOGRAV project. The team hasn’t reached that goal yet, but it’s getting closer than ever, says Joseph Simon, an astrophysicist at the University of Colorado Boulder and lead author of a new paper.
«We found a strong signal in our data set,» Simon says. «But we can’t yet say this is the gravitational wave background. These tantalizing first hints of a gravitational wave background suggest that supermassive black holes probably are indeed merging and that we are swimming in a sea of gravitational waves emanating from the merger of supermassive black holes in galaxies all over the universe,» says Julie Comerford, associate professor of astrophysics and planetary sciences at CU Boulder and a member of the NANOGrav team.
With their work on NANOGrav, Simon and Comerford are part of an international race. The NANOGrav team uses telescopes on the ground to observe pulsars. These collapsed stars are the lighthouses of the galaxy. They spin at incredible speeds, sending streams of radiation toward Earth in a blinking pattern that remains largely unchanged over the eons. Gravitational waves, however, alter the steady pattern by tugging or squeezing the distances these beams travel through space. Scientists can thus detect the gravitational wave background simply by monitoring pulsars for correlated changes in timing.
«These pulsars are spinning about as fast as your kitchen blender,» Simon says. «And we detect variations in their timing of just a few hundred nanoseconds.» To do that, the NANOGrav team needs to observe as many pulsars as possible, for as long as possible. To date, the group has observed 45 pulsars for at least three years, and in some cases well over a decade.
The hard work appears to be paying off. In their latest study, Simon and his colleagues report that they have detected a clear signal in their data: A common process appears to be affecting the light from many of the pulsars. «We went through each of the pulsars individually. I think we all expected to find a few that would mess up our data,» Simon says. «But then we went through them all and said, ‘Oh my God, there’s actually something here.'»
The researchers still can’t say for sure what’s causing that signal, however. They need to add more pulsars to their data set and observe them over longer periods of time to determine if it is indeed the gravitational wave background.