The remarkable detection of gravitational waves by the LIGO collaboration recently has drawn much attention to the fundamental and intriguing workings of gravity in our universe. Finding these gravitational waves, inferred to be produced by merger of two stellar mass black holes, has been like listening to the very distant sound of the universe. The natural question that arises is: What do such phenomena tell us about the cosmos, and what new information can they bring on the amazing nature and structure of the universe?
The key point to note here is, gravity is the supreme force ruling such happenings which are on cosmic scales and also at hugely faraway distances. The dynamical process that governs such phenomena, especially the formation of stellar mass black holes, is gravitational collapse of massive stars, that is, shrinking and contracting of the big stars in the universe under the force of their own gravity. Also, galaxy formation and in general, structure formation in the universe, are largely governed by such collapse processes.
We know, from the time of Newton, that any two bodies will be attracted to each other by their own gravity with a force that is proportional to their masses, and the farther they are the less force. But we now know that while such a simple law would hold in approximation, this Newtonian gravity picture assumes the force of gravitation to move at an infinite speed, inconsistent with the special relativity theory which is experimentally well tested.
This is why Einstein developed the general theory of relativity, which is the best available theory of gravity today. For weak gravity fields such as those on earth or in planetary systems, we do not see much observable differences from Newtonian theory. However, when gravity fields are strong, as would be the case when massive stars contract and collapse under their own gravity at the end point of their stellar evolution, we then need to use the Einstein theory of gravity to probe their dynamics. It is Einstein’s gravity that rules the universe of stars and galaxies, their evolution and dynamics, and tells us about the amazing phenomena in the universe such as the black holes and space-time singularities, including the Big Bang singularity as the origin of universe.
The sun, and other massive stars many times the sun, give out light and heat by burning their internal nuclear fuel such as hydrogen. When this fuel is exhausted finally, the star reaches the end point of its evolution and the nuclear reactions within subside. There are no pressures present within now which can resist the ever present force of gravity of the star. The massive body then starts contracting and shrinking onto itself, which is the gravitational collapse of the star. For stars which are about seven to eight times the mass of the sun, this collapse can still be halted due to a newly generated quantum pressure within which builds up as the star contracts, due to rapid motion of neutrons inside. Then the contracting stellar core gives birth to a neutron star, of the order of ten kilometers in radius, whereas the outer layers of the star are blasted off in a supernova explosion.
If the star is, however, much more massive, of tens of solar masses, then there is no halting of its collapse under gravity, resulting in a continual gravitational collapse. Then, the general theory of relativity predicts that a space-time singularity must be the final fate of such a collapsing star. The singularity is an epoch where all physical quantities such as the mass-energy density, the curvatures of space-time, and such others blow up and diverge arbitrarily high.
In the past few decades, researchers have now analysed extensively many gravitational collapse models within the framework of Einstein’s gravity. The key point here is, as the collapse of a massive star evolves to form the singularity, at times an event horizon of gravity develops before the singularity happens. Then we have a “black hole” forming as the collapse end-state. This is because the event horizon is a “one-way membrane” which allows entry for material particles and light, but they can never escape the black hole region. On the other hand, if the event horizon fails to form or is delayed during collapse, then we have a singularity not hidden inside the horizon, visible to faraway observers in the universe, sometimes called a “naked singularity.” Whether the collapse produces a black hole or a naked singularity depends on the internal structure of the star and how its dynamical evolution proceeds.
Einstein’s theory treats the universe as a “space-time continuum,” or “fabric,” wherein matter and its motion curves the space-time. It is like putting a marble on a rubber sheet curving the same. The formation of a black hole or singularity from the collapse of a massive star creates strong ripples in the space-time geometry. Merger events of black holes or singularities create such ripples or vibrations, propagating at the speed of light. These perturbations in the space-time geometry are what we call gravitational waves.
The two black holes that collided to produce the gravitational waves that LIGO detected recently were stellar mass black holes of 36 and 29 solar masses, which were produced by such gravitational collapse. This involved very intricate measurements of contraction in space through laser beams.
This is of course one way to begin scratching the surface of the ultimate reality that the universe is. While gravitational waves provide a strong confirmation of general relativity, there are major advances to be made theoretically and experimentally and challenges to be resolved. What is important and crucial here is we have now started probing into ultra-strong gravity regimes in the universe as never done before, and where the gravitation and quantum forces may unite to produce hitherto unknown and unseen unique physical effects. The future may therefore herald many surprises in our understanding of the universe and is certainly exciting!
Featured image credit: Artist’s illustration of galaxy with jets from a supermassive black hole by ESA/Hubble. CC BY 3.0 via Wikimedia Commons.