New Pulsar Observations: Still No Gravitational Waves

Bad news in the hunt for the universe's most elusive signal: Once again, astrophysicists hunting for direct observations of gravitational waves have come away empty-handed.

In a paper published this week in Science, a team based at Australia's Curtin University describe pulsar observations collected via the Parkes radio telescope, finding no distortions indicative of gravitational wave influence.

While gravitational waves are expected to be the natural (and detectable) result of the intense space-time warping caused by accelerating black holes in a binary black hole system, we've yet to actually see them. Still.

Astronomers are interested in binary systems of super-massive black holes (SMBHs), in particular. An SMBH is about 106 solar masses (106 times the mass of our own Sun) and would usually be found at the center of a galaxy, a position from which it's able to have effects on the entire galaxy by virtue of having an enormous mass. When two or more galaxies merge, as they often do, an expected result is the formation of binary black hole systems.

"The remnants of mergers can host gravitationally bound binary SMBHs, with orbits decaying through the emission of gravitational waves (GWs)," the paper explains.

A binary system means that one black hole of the pair is always accelerating relative to the other. This acceleration is enough to send out ripples across the fabric of space-time, which are the gravitational waves. Gravity is a very, very, very weak force, however, which makes actually detecting these waves a difficult proposition. But not impossible.

Astronomers use pulsars as natural gravitational wave detectors. These are strange, highly magnetized neutron stars spread throughout the universe spinning at extremely fast rates. They emit electromagnetic radiation in beams, which can only be detected if they're pointing at the Earth. The effect is of a distant cosmic lighthouse making complete rotations in mere milliseconds.

This can be exploited in the hunt for gravitational waves as their combined effect, the gravitational wave background (GWB), should cause slight distortions in the pulsars' rotations.

The Australian team observed 24 pulsars for a period of 11 years, three more than the previous longest observational span (which also didn't find gravitational waves). "We found no evidence for the GWB in our data set," the group reports. This doesn't mean that gravitational waves don't exist, but it puts an upper limit on their energy. A weaker GWB suggests that either binary SMBH activity is stalled or occurring at a much faster rate than expected.

Or we may just be looking for the wrong thing. "It is also possible that there is a more exotic reason for our nondetection," the authors write. "We have not yet tested GWBs expected from alternate theories of gravity."

"In order to get gravitational waves, we need the binaries to evolve from being widely separated to more closely separated," Ryan Shannon, one of the study's co-authors and a Curtin University research fellow, told me. "When they are relatively close—orbiting each other every decade or so—they will be emitting GWs that we can see. To get the orbit to shrink to that size requires that the binary interact with its environment [of] stars and gas, which acts like friction. At this point we expect gravitational radiation to take hold and the system to lose its energy, and the orbits to shrink further, through gravitational waves."

So, if the system's environment is too weak, binary evolution stalls and the black holes won't get close enough to each other to produce observable gravitational waves. If it's too strong, the black holes lose energy as friction instead of converting it to gravitational waves.

The search is far from over.

"The gravitational-wave spectrum is as wide as the electromagnetic spectrum," Shannon said. "There are ground based experiments, like LIGO, looking for much higher frequency gravitational waves. These waves come from different class of sources, so we are doing complementary experiments. However, we are all hoping to use gravitational waves to 'see' portions of the universe that are invisible—too faint—or obscured to other observations."