The Gaia spaceship is an impressive technical achievement. Its main task is to map the position and movement of more than a billion stars in our galaxy and to create the most comprehensive map of the Milky Way to date. Gaia collects so much precise data that it can make discoveries that go well beyond its primary purpose. For example, by looking at the spectra of stars, astronomers can measure the mass of individual stars with an accuracy of 25%. Astronomers can use the movement of stars to measure the distribution of dark matter in the Milky Way. Gaia can also spot exoplanets when they pass in front of a star. However, one of the more surprising uses is that Gaia could help us capture cosmic gravitational waves.
The area of the Milky Way as observed by Gaia. Photo credit: X. Luri & DPAC-CU2
A new study shows how this can be done. The work is based on an earlier study carried out with Very Long Baseline Interferometry (VLBI), in which radio telescopes measured the position and apparent movement of quasars. Quasars are bright radio sources that are billions of light years away. Because quasars are so far away, they look like fixed points in the sky. By accurately measuring quasars, we can pinpoint positions on Earth so accurately that we can see how continents are drifting due to plate tectonics and how the Earth’s rotation slows down over time.
While the quasars are essentially fixed points, their light can be easily deflected by gravitational lenses. When a star comes within the line of sight of a quasar, the quasar appears to shift slightly. Since gravitational waves can also deflect light, we can detect the presence of gravitational waves through the apparent wobbling of quasars. The VLBI observations of quasars have found no evidence of gravitational waves, which is an upper limit for these in our space region.
The grazing of the starlight through the sun. Photo credit: Wikipedia
While Gaia’s position measurements are not as accurate as VLBI’s, they are accurate enough to detect gravitational lenses. In fact, when analyzing Gaia data, astronomers must consider the sun’s lens flare. The team therefore examined Gaia’s position data for 400,000 quasars. Although quasars are not stars, many of them are optically bright, and Gaia measures their position as if they were stars. The team looked for statistical evidence of wobble in the Gaia quasar data and found none. However, given the large number of quasars observed, they could place a stronger upper limit on local gravitational waves. From this study it emerged that there are no binary supermassive black holes in our local group, which includes both the Milky Way and the Andromeda galaxy.
The great thing about this study is that it shows the power of big data. When we observe the sky at both a large scale and with great precision, astronomers can use the data in innovative ways. Gaia was never intended to study gravitational waves, and still can. Who knows what else we will discover if we move further into the realm of big data astronomy?
Reference: Shohei Aoyama et al. “Gaia 400,894 QSO Restriction on Energy Density of Low Frequency Gravitational Waves.” arXiv preprint arXiv: 2105.04039 (2021).