Scattered across the universe

landmark find

The ubiquitous presence of several more super massive black holes in different parts of the universe is being uncovered. A recent landmark discovery was made by the Laser Interferometer Gravitational-Wave Observatory’s (LIGO) gravitational wave detector was a rare merger of two massive black holes a crore light years away. With their masses several tens that of a solar mass, their merger at light speeds unleashed a stupendous burst of gravitational wave energy.

Just over 15 years ago, evidence was accumulated for the presence of a supermassive black hole that was weighing several 10 lakh solar masses in the centre of the Milky Way galaxy. Stars in the vicinity of the galactic centre were moving at several thousand kilometres per second when approaching this large invisible central mass concentration, whose mass was then deduced. 

It turns out that many large galaxies have such supermassive black holes lurking in their core. The velocities of stars, molecular clouds and other objects in their vicinity reach speeds of 20,000 to 30,000 km/s. For instance, the supergiant elliptical galaxy, M87 in the Virgo cluster, has a black hole of at least 10 crore solar masses.

Binary systems
A few crore years ago, we had large numbers of spectacularly luminous objects like quasars, which emit energy several thousand times that of a typical galaxy, from a region hardly the size of our solar system. A typical quasar like 3C 273 has a luminosity that is about 4 lakh crore times that of the Sun. As a result, it emits luminosity in a second the Sun would emit in 10 crore years. These energetic objects are believed to be powered by matter (such as gas and stars) falling on to the supermassive black holes that have a mass that is 100 crore times or more than the solar mass. 

The huge gravitational potential energy (associated with these compact objects) that is converted to kinetic energy heats up the gas several 10 lakh degrees making them grow into X-rays and other energetic radiation. Once they have consumed all or most of the surrounding matter, they can become dormant. The supermassive black hole in our galaxy’s centre is right now in this dormant state. However, occasionally, a passing star or planet could be tidally split by the black hole giving rise to energetic flares.

Several binary systems of supermassive black holes are also known to exist. One such is the OJ 287, a binary system of two supermassive black holes, where one weighs 18 crore solar masses and the other,  10 crore solar masses. They orbit each other with just a 12-year period, implying that they could merge in less than  1,00,000 years. Such a merger would unleash energy that is several lakh times more intense than the present LIGO event, in gravitational waves. 

However, the much lower frequencies would make this easily detectable only in future space borne gravitational wave detectors, like the Laser Interferometer Space Antenna (LISA), which consists of three spacecrafts in an equilateral triangle, separated by more than 50 lakh km. While this may be the case, we can also witness the merger of many galaxies through large telescopes. And when they merge, their respective supermassive black hole could also merge. The Milky Way and our near large neighbouring galaxy, the Andromeda is expected to merge in 3 to 4 crore years. 

The recent discovery of a supermassive black hole with an estimated mass of 1,700   crore solar masses in a smaller galaxy like NGC 1600 was somewhat of a surprise as such black holes were expected to mainly lurk in huge star rich galaxies or gigantic galaxy clusters.

Tracing the origins
Sometime back, a supermassive black hole weighing 1,200 crore solar masses was discovered in the much earlier epoch of the universe, hardly 100 crore years since the Big Bang expansion. This implies that this gigantic black hole is almost 1,300 crore years old, almost the age of the universe. Many such black holes have been found at such large distances.

How could such supermassive black holes form so soon after the universe originated? Possibly, a generation of several thousand massive stars formed in a cluster evolved to become individual black holes in a few lakh years, all of which then merged together. Two or more body interactions or encounters in a dense cluster of objects such as stars can make them all collapse gravitationally together to form a supermassive black hole. However, there is no universally accepted model for the origin of these supermassive black holes.

It would be true to say that much less is known about their origins. Interestingly, in 1916, Karl Schwarzschild gave the first exact solution to the Einstein field equations for general relativity. Known as the Schwarzschild solution, this solution first indicated the possibility of black holes. When the radius of the collapsing massive star reaches the Schwarzschild radius (for a solar mass this is just 3 km), a trapped surface forms and all matter and radiation is confined for ever inside this surface (also called one-way membrane). The term ‘black hole’ to objects with gravitational collapse was coined by John Wheeler in 1968.

Detailed study 
Unlike white dwarfs and neutron stars, which have upper mass limits of 1.5 and a little over 2 solar masses respectively, there is no upper limit for the mass of a black hole. Several hundred crore solar mass heavyweights are being discovered all over the universe. The Sun would only end up as a white dwarf. Only stars that begin the life with 30 or more solar masses could end up as black holes. In our own galaxy, several dozens of such mass black holes are known, and which are strong periodic x-ray sources like Cygnus X-1. Several interesting properties of black holes are being studied in detail. 

They are indeed the biggest powerhouses on the universe. The supermassive ones powering the quasars generate several lakh crore more power than the Sun and in many cases, matter is ejected in jets of 10 lakh light years long. The origins of these, some of which have recently been shown to be present 1,300 crore years ago, continue to be a mystery, and challenge our understanding. And with the use of large telescopes, we would know more about black holes and uncover the mysteries behind them as well.