Gravitational wave was directly detected for the first time in 2015 after a century of its prediction by Einstein’s General Theory of Relativity in 1916. But, the continuous, low frequency Gravitational-wave Background (GWB) that is thought to be present throughout the universe has not been detected directly so far. The researchers at North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have recently reported detection of a low-frequency signal that could be ‘Gravitational-wave Background (GWB)’.
General theory of relativity propounded by Einstein in 1916 predicts that major cosmic events such as supernova or merger of black holes should produce gravitational waves that propagate through the Universe. Earth should be awash with gravitational waves from all directions all the time but these are undetected because they become extremely weak by the time they reach earth. It took about a century to make a direct detection of gravitational ripples when in 2015 LIGO-Virgo team was successful in detecting gravitational waves produced due to merger of two black holes situated at a distance of 1.3 billion light-years from the Earth (1). This also meant the detected ripples were bearer of information about the cosmic event that took place about 1.3 billion years ago.
Since the first detection in 2015, a good number of gravitation ripples have been recorded till date. Most of them were due to merger of two black holes, few were due to collision of two neutron stars (2). All detected gravitational waves so far were episodic, caused due to binary pair of black holes or neutron stars spiralling and merging or colliding with each other (3) and were of high frequency, short wavelength (in milliseconds range).
However, since there is possibility of large number of sources of gravitational waves in the universe hence many gravitational waves together from all over the universe may be continuously passing through the earth all the time forming a background or noise. This should be continuous, random and of low frequency small wave. It is estimated that some part of it may even have originated from the Big Bang. Called Gravitational-wave Background (GWB), this has not been detected so far (3).
But we may be on the verge of a breakthrough – the researchers at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have reported detection of a low-frequency signal that could be ‘Gravitational-wave Background (GWB) (4,5,6).
Unlike LIGO-virgo team who detected gravitational wave from individual pairs of black holes, NANOGrav team have looked for persistent, noise like, ‘combined’ gravitational wave created over very long period of time by countless blackholes in the universe. The focus was on ‘very long wavelength’ gravitational wave at the other end of ‘gravitational wave spectrum’.
Unlike light and other electromagnetic radiations, the gravitational waves cannot be observed directly with a telescope.
The NANOGrav team chose millisecond pulsars (MSPs) that rotate very rapidly with long term stability. There is steady pattern of light coming from these pulsers which should be altered by the gravitational wave. The idea was to observe and monitor an ensemble of ultra-stable millisecond pulsars (MSP) for correlated changes in the timing of the arrival of the signals at the Earth thus creating a “Galaxy-sized” gravitational-wave detector within our own galaxy. The team created a pulsar timing array by studying 47 of such pulsars. The Arecibo Observatory and the Green Bank Telescope were the radio telescopes used for the measurements.
The data set obtained so far includes 47 MSPs and over 12.5 years of observations. Based on this, it is not possible to conclusively prove direct detection of GWB though the detected low frequency signals very much indicate that. Perhaps, the next step would be to include more pulsars in the array and study them for longer period of time to enhance sensitivity.
To study the universe, scientists were exclusively dependant on electromagnetic radiations like light, X-ray, radio wave etc. Being completely unrelated to electromagnetic radiation, detection of gravitational in 2015 opened a new window of opportunity to scientists to study celestial bodies and understanding the universe especially those celestial events which are invisible to electromagnetic astronomers. Further, unlike electromagnetic radiation, gravitational waves do not interact with matter hence travel virtually unimpeded carrying information about their origin and source free of any distortion.(3)
Detection of Gravitational-wave Background (GWB) would broaden the opportunity further. It may even become possible to detect the waves generated from Big Bang which may help us understand origin of universe in a better way.
- Castelvecchi D. and Witze A.,2016. Einstein’s gravitational waves found at last. Nature News 11 February 2016. DOI: https://doi.org/10.1038/nature.2016.19361
- Castelvecchi D., 2020. What 50 gravitational-wave events reveal about the Universe. Nature News Published 30 October 2020. DOI: https://doi.org/10.1038/d41586-020-03047-0
- LIGO 2021. Sources and Types of Gravitational Waves. Available online at https://www.ligo.caltech.edu/page/gw-sources Accessed on 12 January 2021.
- NANOGrav Collaboration, 2021. NANOGrav Finds Possible ‘First Hints’ of Low-Frequency Gravitational Wave Background. Available online at http://nanograv.org/press/2021/01/11/12-Year-GW-Background.html Accessed on 12 January 2021
- NANOGrav Collaboration 2021. Press briefing – Searching for the Gravitational-Wave Background in 12.5 years of NANOGrav Data. 11 January 2021. Available online at http://nanograv.org/assets/files/slides/AAS_PressBriefing_Jan’21.pdf
- Arzoumanian Z., et al 2020. The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background. The Astrophysical Journal Letters, Volume 905, Number 2. DOI: https://doi.org/10.3847/2041-8213/abd401