According to law of conservation of energy;
"Energy can neither be created nor be destroyed but may change it's form"
Where did the energy come from which led to the creation of universe by big bang?
[Physics] What was the initial source of energy
conservation-lawsenergyenergy-conservation
Related Solutions
The point of defining energy is to obtain a conserved scalar quantity -- as an example of how this works out, have a look at the derivation of the Newtonian formula for kinetic energy, where energy conservation is disguised as "kinetic energy is equal to the heating produced when it's destroyed".
There are plenty of conserved scalar quantities of course -- charge, each component of momentum, whatever -- so the real definition of energy is as the conserved quantity mandated by time-translational invariance through Noether's theorem.
We usually say that the ultimate source of energy for earth is the Sun. So this means that sun creates energy but according to the law of the conservation of energy, energy can neither be created nor destroyed, so how does the energy come into the sun?
The ultimate source of energy in our present universe is the Big Bang. Here is a history of the universe from the interpretation of astrophysical data.
All the energy was created at the beginning of time as we know it, and as the universe expanded various stages of particulate matter appeared, created from that energy. By the time of the Modern Universe at the right of the figure, the energy was apportioned either into mass (nuclear energy is there), or kinetic energy or chemical energy in lumps of matter, or radiant energy. The masses are organized into galaxies and stars within them. The sun is one of those stars and got the energy from the primordial Big Bang.
That is just an example. We know that energy is transformed from one form to another but is not created and destroyed. But that is confusing because there must be an ultimate source of energy.
Astrophysicists have observed and documented the appearance of this energy that is presently locked into nuclear energy and chemical energy, at the beginning of the universe as we know it.
How is energy created in this source?
How all that energy appeared around time=zero is a matter of theoretical speculations.
The fact is that we have a model where once the energy appeared at the beginning of the universe we can explain the whole progress to our present day energy sources.
Best Answer
Well, if you want an answer at the 9-grader level, it's probably this:
We don't know, and it's mostly irrelevant to how the universe behaves now. In particular, the Big Bang Theory doesn't care about what happened before the Big Bang. According to many interpretations of different branches of physics, the question doesn't even make sense, e.g. what happened before time existed? Well... nothing. How could it? Time didn't exist. Things don't happen without time.
Asking the Big Bang Theory to explain how the universe came to being is like asking evolution theory to explain particle physics. It's just outside of the scope of the theory.
Now, there's a lot of hypotheses that try to explain it anyway. However, the problem is that we can only use behaviours we observe now - e.g. we can observe nowadays that there are some asymmetries in the universe. For example, anti-matter and matter are mostly symmetric, and when they come in contact, they annihilate each other. So... why is almost all of the visible universe formed out of matter? People trying to gain insight into issues like this will gain more evidence to theories about how the universe formed in the first place (within the general assumption that the fundamental laws of the universe do not change with time).
Finally, it should be noted that we don't actually know if we know any fundamental law. It's not even something that's decidable from within the universe, we just get closer and closer to reality with our models. For example, thermodynamics is usually considered as fundamental, and thermodynamics says (extremely simplified) that the entropy of a closed system will equalize over time: for example, if you put hot water and cold water in the same container, they will eventually mix to form a body of warm water. However, this is actually only a statistical effect, so when you reach maximum entropy, you will get localised pockets of lower entropy (e.g. some parts of the container have water warmer than average, and some colder). In fact, there is a hypothesis that tries to explain why our universe doesn't have maximum entropy yet, based on this behaviour - the idea is that our universe is a tiny pocket of locally low entropy inside the "real" universe, which is close to maximum entropy.