NANOGrav Astronomers Proved the Existence of the Gravitational-Waves Background

Scientists have been observing gravitational waves for years, describing them as ‘ripples’ in space-time. What gives life to these waves are highly energetic and violent phenomena that occur in space: collision between black holes, supernovae, and colliding neutron stars. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, whose mathematics showed that massive accelerating objects (for example, black holes orbiting each other) would disrupt space-time resulting in the propagation of undulating waves in all directions away from the source.

Animation that shows the merger of two black holes and the creation of gravitational waves. Credits: LIGO Scientific Collaboration

They are invisible and incredibly fast, traveling at the speed of light (300 thousand kilometers per second) and squeezing and stretching anything in their path as they pass by. Therefore, they are formed in the universe in immeasurable quantities, from all directions and from all times. The set of all these signals constitutes the Cosmic Background of Gravitational Waves.

North American Nanohertz Observatory for Gravitational Waves

NANOGrav is an international collaboration of astronomers dedicated to studying low-frequency gravitational waves through radio pulsar timing. Founded in October 2007, it has more than 190 members. On the basis of their studies, there are some of the world’s largest radio telescopes, such as the Green Bank Telescope, Arecibo, and the Very Large Array, to monitor millisecond pulsars at multiple radio frequencies.

Image showing the biggest telescopes in the world — *On the left is the Green Bank Telescope, central the Arecibo Telescope, and on the right is the Very Large Array Telescope. Credits: PeakPx*

The focal point of the research is in the pulsars. They are cores of massive stars that have reached the end of their life and rotate very quickly on themselves, and are so called because their light, seen from the Earth, pulsates: they project radio light rays in specific directions, and by rotating very quickly on themselves, sometimes these beams point in our direction and sometimes not. Their pulse rate is extremely precise.

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NANOGrav‘s idea is that radio pulsar timing can be broken by the passage of gravitational waves. By warping space-time, they would slightly delay the arrival of the pulsating signal. This would entail a tiny deviation but sufficient to determine the passage of gravitational waves.

NANOGrav indeed collected data from 68 pulsars over 15 years (between July 2004 and August 2020), a time needed to obtain a statistically significant number of observations. The result obtained seems to be the confirmation of the existence of the Cosmic Fund of Gravitational Waves. NANOGrav results are currently in the public domain on “The Astrophysical Journal Letters” website.

Data set elaborated by the astronomers who evidenced the fundamental role of pulsar in the studies of gravitational waves. Credits: NANOGrav — Upper right: data set showing timing observations from 68 pulsars and their locations in equatorial coordinates, using the Arecibo Observatory, the Green Bank Telescope, and the Very Large Array. Left: Correlations between timing measurements. Credits: NANOGrav

“The extensive array of pulsars analyzed by NANOGrav has empowered us to witness the initial signs of the correlation pattern foreseen by general relativity.”

— Dr. Xavier Siemens, co-Director of the NANOGrav Physics Frontiers Center