Most galaxies have a gigantic black hole
From some decades, our observations show that most galaxies include a gigantic black hole in their center and the rate of detection of gravitational waves emitted by black holes, when merging, induces that, likely, a huge number of smaller ones would be present in the galaxies. How explaining that?
One hypothesis is that the black hole results from early instabilities or significant over densities in the cloud of gas in the primordial universe leading to a collapse into a black hole of large quantity of it, which in turn, per its gravitational action, would initiate the formation of the galaxy around it.
Conversely, another hypothesis is that the same over density would fragment the gas cloud, this process initiating a collapse process leading to the galaxy, where in the center, the densest region, a massive, growing, black hole would emerge.
In these hypotheses, there is a chronology, one is before the other and is its cause, but one may also consider the case of a process involving simultaneously the two phenomena.
Let us explain how this could work.
Entropy, information as the key for an explanation
Information measures the quantity of information (usually in bits) needed for describing a system.
More complex is the system, more information is needed.
Entropy is bound to information in a reverse relation. More complex is the system less is the entropy. Such system as black body (where the sole parameter is its temperature), black holes (defined by a maximum of 3 parameters: mass, angular momentum, electric charge) have a huge entropy.
The second principle of the thermodynamic states that the entropy of an isolated system may not decrease, this meaning that whether the entropy of a subsystem decrease, in another part of the system it must increase as well or more for satisfying this principle. This means that “entropy” should be transferred to a surrounding when it decreases in a subsystem. But this may be not easy whether the entropy of the surrounding system is very high! One knows the concept of free energy tanks where energy can be extracted because their entropy may easily increase!
Gravitation by fragmenting the clouds and causing their collapse, contributes to increasing information by producing galaxies, stars, planets, locally lowers the entropy because a cluster of galaxies, with its contents, contains more information than the gas cloud (almost a black body) from which it comes.
If life is a system whose entropy decreases, to the detriment of its environment (see N. Wiener, cybernetics and society), gravitation, always attractive (non-symmetrical, which according to R. Penrose allows us to justify the arrow of time) seems to produce a similar effect.
Therefore, if the second law of thermodynamics remains valid, for the entropy of a subsystem to be able to decrease, that of its environment must be able to increase, which is not so simple if the latter is very high.
Because, just as a star must evacuate its angular momentum to form (hence multiple systems or planets), to collapse, a gas cloud must be able to evacuate part of its entropy into the surrounding environment.
As the formation of black holes locally involves a strong capture of entropy [1], trapped under their horizon, it allows entropy to decrease around and this can explain the presence of gigantic black holes at the heart of galaxies, because by allowing them to evacuate their entropy this contributes to the viability of the process.
Therefore, as without a concomitant central black hole process, a galaxy process struggles to succeed, no wonder we find them in most existing galaxies for the same reason that we the find many planets around single stars systems.
Notes
[1] For showing how a black hole increases the entropy, you will find an example in a synthetic view on entropy of black holes, provided in:
http://scholarpedia.org/article/Bekenstein-Hawking_entropy
(below, an extract of it)
« Note that a one-solar mass Schwarzschild black hole has an horizon area of the same order as the municipal area of Atlanta or Chicago (about 110km²). Its entropy is about 4×1077 , which is about twenty orders of magnitude larger (1020) than the thermodynamic entropy of the sun. This observation underscores the fact that one should not think of black hole entropy as the entropy that fell into the black hole when it was formed.«