Question

Does the the universal law of gravitation works everywhere in universe? If no, then where?

Answer 1

The Universal Law of Gravitation was given by Sir Isaac Newton.

He stated that the law is applicable to every two pairs of masses in the universe, no matter how far they are.

This law has some drawbacks, which were addressed by Albert Einstein in his Theory of Relativity.

Now, there are quite some cases where Newton's Laws don't perfectly agree with observations:

1) Precession Of Perihelion of Mercury's Orbit:
Perihelion is the point where a planet is the closest to Sun. Now, it is observed that the actual orbit of Mercury deviates from predictions by Newton's Laws. What happens is that the Perihelion of Mercury doesn't occur always at the same place. Instead, the point of Perihelion moves around the Sun. I have attached the figure.

This anomaly was resolved in Einstein's General Theory of Relativity, by using the concept of four-dimensional space-time.

2) Bending of Light:
Newton's theory is applicable to only objects which have masses. But in Einstein's Theory, energy and mass are equivalent. So, light is affected by gravity, as it tries to follow the shortest path (geodesic) around curved space-time . (Space-Time gets curved due to presence of mass in it).

So, it means that every object bends light around it. But the effect is considerable only for very massive objects like stars.

3) Black Holes:
Newton's Theory didn't predict black holes. But today we know that black holes exist and we have detected them. (The first being in a binary system in Cygnus X-1).
If we apply Newton's laws to black holes, we find that on supplying a given amount of energy, we can escape from a black hole. But in reality it is not possible to do so.

4) Mechanism Of Gravity
Newton's theory gives no explanation of where the gravitational force originates. In fact there is no successful theory to date which completely explains gravity.

There is, though, a big flaw in Newton's Law. In two bodies which are exerting gravitational force on each other, if the distance between the bodies is, say, increased, then the gravitational force immediately decreases.
This means that gravitational effects travel at infinite speed.

Einstein's theory says that gravitational effects travel at speed of light. This means that if the sun suddenly vanished, Earth would continue to orbit an apparently invisible object for next ~8 minutes, and only then it will be set free.