Research and observation suggest that there is a supermassive black hole (SMBH) at the centre of most galaxies with the SMBH being proportional to the size of the galaxy. Approximately ten percent of galaxies merge with another galaxy of similar mass within their lifetime and so one would expect that when two galaxies merge this would include the merger of two SMBHs. However, this assumption faces some challenges.

Gravitational drag also known as dynamical friction occurs when stars, gas and black holes lose energy by interacting with other entities in space. As the more massive SMBH interacts with stars and gas in the galaxy, it  loses energy while the star or gas will gain energy and subsequently get flung out into space.

After many of these interactions the SMBH  slows down and begins to sink towards the centre of mass of the system, which is typically the middle of the galaxy due to the loss in energy; this happens over a huge timescale as many interactions are needed for SMBH to lose enough energy. 

During a galaxy merger the blackholes from both galaxies undergo this process  and eventually both SMBH will end up orbiting the centre of the galaxy. Once in orbit around the centre of mass they will stay there unless they lose more energy. However, as the SMBH are so massive they clear the space around them from all of the previous interactions and so there is nothing left for them to interact with to make them lose energy, slow down and merge together. This happens when the two blackholes are around 1 parsec from each other and this is what is known as ‘The Final Parsec Problem’.

So why is it deemed a problem? Observations that suggest that SMBH actually do merge, one such observation of particular relevance is the newly observed low frequency gravitational waves, which scientists suggest occur due to the merging of Super massive black holes. 

To resolve this problem one can acknowledge that there is a problem with our mathematical model that suggests that SMBH cannot merge, or one can consider that our observations of low frequency gravitational waves are misinterpreted or incomplete.  The LISA mission, expected to launch in 2035, will be crucial to further observing these low frequency gravitational waves which will undoubtedly provide clarity on whether it is our mathematical model or observations that are in need of refinement and resolution.


Image credit: By NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA – APOD 2004-06-12, Public Domain,

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