A long post involving quite a bit of physics and thermodynamics. Starting with Newton's third law of motion, that is so commonly used in daily conversations as a reference for 'tit for tat'.

"Every action has an equal and opposite reaction" NOTE - external forces between two bodies are considered here, not internal forces.

Now let us apply this logic to the universe, which, let us assume has been formed in accordance with the big bang theory.

So there's some small dense superforce in the beginning, and there's a big bang, and then the Higgs Boson particle, proof for the existence of a Boson field that gave mass to all matter.

The universe we know today is still expanding. According to Newton's third law, there must be some force (contracting in action) to counter the force that caused the big bang.

According to Einstein's mass energy equivalence relation, mass is a measure of energy, and they are interconvertible following the relation E=m.c² .

In thermodynamics, according to the first law, "Energy can neither be created nor destroyed. It can only change from one form to another, from a more useful form to a less useful form."

Take any example of conversion/transfer of energy from one form to another - be it the generation of hydro electricity and its utilization (potential energy of water to kinetic energy of water to mechanical energy of the turbine to electromagnetic energy in the coil to electricity which is used to heat up your room during winter) , or the energy transfer from one tropic level to another in a biological biological system (abiding by the ten percent law according to which only 10% of the energy of an organism is delivered in a usable form to the next organism, while the rest is dissipated as unusable heat) ; you'll notice that the end result is heat energy.

Heat energy in these cases cannot be used to get a more useful form of energy (exception of thermal power plant). All forms of energy are bound to end up as unusable heat.

This is due to a concept called entropy, which is a measure of order in a system. Energy flows from a state of higher order to disorder. It cannot be reversed, unless done by the application of external force.

A time will come when the entire energy of the world is converted to heat, which cannot be used in any productive way (of course, we wouldn't be alive then, that's why it's a theory). This is called "HEAT DEATH OF THE UNIVERSE".

According to Newton's law, there must be a force to counter the big bang. But, due to heat death, it is not possible. Also, it is not possible since Newton's law requires the application of external forces. Now, we don't know the exact extent of the presently ever expanding universe, so basically there is no external environment.

By then, if we do know the extent since expansion would've stopped or slowed down, then there can be an external environment to that of the universe, and there is some hope of an external force. But the source of this force cannot be determined (since there can be nothing outside the universe), hence it is only fair we consider it non existant.

So basically, there can be no end to the world, at least not in this manner. Of course, there are situations where Newton's laws don't apply. Even if it were that sort of a situation, the end cannot be explained.

Then there are black holes, in whose case matter shrinks under the influence of it's own gravity so much so that it becomes super duper small, with a gravitational field so enormous and powerful that even the almost massless photons cannot escape it.

Could that be the end of the universe?

PS- I am not too well versed in physics, so those who have a deeper understanding, kindly suggest some points that can be added to the post, or any corrections that need to be made.

Also, kindly suggest if there are any theories that can be added to make this more complicated than it already is :P

Suggestions, views and feedbacks are more than welcome.

Thank you for taking time to read this long post :)

Do let me know your thoughts on this!! My first post on this subreddit.