In the twentieth century, physics was divided into two major parts; classical physics and modern physics. Modern physics includes relativity and quantum mechanics. In recent decades, physics has encountered numerous problems and unanswered questions. The problems and unanswered questions are related to the particle physics and astrophysics. Since particle physics is beyond the classical mechanics and Newton's universal gravitational laws has replaced by general relativity, physicists are trying to solve the physics problems in the context of modern physics or to think the beyond of the modern physics while they have not cared classical physics. Some physicists believe that by combining general relativity and quantum mechanics, these problems may be resolved and the unanswered questions will be answered.

However, in all of these efforts, the classical physic has been ignored, while nature is unique and all physical phenomena, from the microscopic or the macroscopic ones are obeying the same law. Therefore to solve the contemporary physics problems, the basic concepts and relations of physics should be the foundation of classical mechanics which have to be reviewed and analyzed. Then, we have to combine these three theories of classical mechanics, quantum mechanics and relativity in order to reach to a unique physics.

https://www.researchgate.net/publication/310628928_Rooting_the_biggest_problems_in_physics?ev=prf_pub

Article Rooting the biggest problems in physics

Dear Hossin

In the beginning, when the big bang (BB) supposedly went bang, the universe was very very hot. According to BB theory, essentially all the energy of the universe was a more or less uniform soup of heat energy. But as the universe cooled, beginning immediately after the BB and continuing even today, the more or less uniform heat energy began to sort of congeal. Quarks, a basic mass particle that makes up protons and neutrons, for example, began to pop up among the primordial soup of heat. [See source.] Then photons, the basic electro-magnetic force particle, started to show up and...let there be light. (NB: The reason we can only see back 14 to 15 billion light years across our universe is because light has been around only 14 to 15 billion years.) There are but four fundamental forces in our known universe: electro-magnetic (EM), gravity, and strong and weak atomic. The EM forces stem from photons, like the light that popped up 14 or so billion years ago. We have pretty good idea how all this light works as a particle with enegy E = hf and a group of particles forming waves that travel at the speed of light L/t = c. [h is Plank's constant, f is the particle frequency, L is the wavelength, t is the period, and c is light velocity.] We have less insight on the strong and weak atomic forces, which keep all that positive charge from protons (hadrons made up of quarks) from blowing atomic nuclei apart. Like charges repel, even at the subatomic level. But we do have some experimental indicatiions of their existance as messenger particles called gluons and bosons. But, and this is a big BUT, we have no experimental indication that gravitons, the quantum particle of gravity, exist. There are quantum gravity theories, like the string/M theory, predicting gravitons, but, thus far, none has been observed. To date, prevailing theories have "unified" the EM, and strong and weak atomic forces. That is, a single set of equations can be used to "predict" the three quantum particles that create these three forces.

But there are four forces and if the BB theory explaining the origin of our known universe is correct, all the forces of the universe were once unified during the initial very short length of time after the bang. THEREFORE, until such time that a single set of equations unifies all four forces (and their messenger particles), including gravity, the BB theory, which says all four should be unifiable, remains suspect.

Thus, to answer your question directly, being able to observe gravitons, the gravity quantum, would go a long way in validating the big bang theory that explains the origin of our universe. That results because, at the time of origin and if the BB theory is valid, all four forces were in fact unified; so there has to be a unifiable gravity quantum particle if the BB theory is correct.

Regards 

There are three principal quantum gravity theories. String theory is the most popular, but is a perturbation dependent theory that is not background independent (e.g. the space-time in which string theory operates must be assumed).. Further, because each of the six competing string theories involve mutually exclusive geometries (called Calabi–Yau manifolds), there may be some reasons to believe that string theory is not fundamental (e.g. it is an effect of quantum mecha rather than “the cause”). The second leading candidate is loop quantum loop gravity (LQG). According to the Wikipedia article on Loop Quantum Gravity, “The equations of LQG are not embedded in, or presuppose, space and time, except for its invariant topology. Instead, they are expected to give rise to space and time at distances which are large compared to the Planck length.” In other words, space-time is an emergent property of LQG, but in most of the LQG models, this emergence occurs at dimensions in excess of the planck length. It is this prediction that may have failed in a 2009 experiment.

See: (http://www.sciencenews.org/view/generic/id/48891/title.../Gamma-ray_observations_shrink_known_grain_size_of_spacetime_). Specifically, the experiment provided some evidence that space-time is less “grainy” than most LQG theories had predicted.

The third approach, which I favor, is called "Causal Dynamical Triangulation" (CDT). Every theory must begin somewhere and CDT begins with tiny building blocks of space, a 4-dimensional tetrahedron (called a 4-simplice) and, by applying certain rules of causality, to construct the space-time geometry of our universe. This system was proposed by Renate Loll of Utrecht University in the Netherlands, and her colleagues Jan Ambjørn and Jerzy

Dear Saeed

You have described important notes of modern physics relative to this issue, thank you very much.

I agree with you that "... predicting gravitons, but, thus far, none has been observed."

If graviton be able to describe physical phenomena that are regarding with gravity, so we should accept it exists.

Same question arises for virtual photons.

As you know better than me, QED rests on the idea that charged particles (e.g., electrons and positrons) interact by emitting and absorbing photons, the particles that transmit electromagnetic forces. These photons are “virtual”; that is, they cannot be seen or detected in any way because their existence violates the conservation of energy and momentum.

Thank you Dear Hossein

may be you will see this link its helpful for you question

http://www.physicsoftheuniverse.com/glossary.html

regards 

Quantum gravity seems unavoidable since the known sources of gravity are discrete. Observations of distant beginnings can help to decide which version of quantum gravity is best. Also lab experiments with falling neutrons are helpful to define lower limits.

Dear Saeed

Thank you for link. I saw it.

There is two remarkable stages in cosmological theory, before Big Bang and after Big Bang.

Big Bang Theory and inflation are related to the after of Big Bang. But I think knowing these two are not separable from each other.

So, I have reviwed the Friedman equation.

https://www.researchgate.net/publication/279446746_Graviton_and_cosmology_equations_before_the_Big_Bang

Article Graviton and cosmology equations, before the Big Bang

Dear Jerry

I agree with you that quantum gravity is unavoidable.

In spite of publishing many articles about graviton, but it has not been done any considerable work about mechanism of graviton exchange between bodies/particles. The reason is that the old graviton definition (in modern physics) is unable to describe this mechanism and also it is impossible to get the theory of the quantum gravity.

In this article with re-considering physical phenomena, a new definition of graviton is given which by its using; the mechanism of graviton exchange between bodies/particle is described and surveyed.

https://www.researchgate.net/publication/310457967_The_Mechanism_of_Graviton_Exchange_between_Bodies_Part_II?ev=prf_pub

Article The Mechanism of Graviton Exchange between Bodies, Part II

If graviton be able to describe physical phenomena that are regarding with gravity, so we should accept it exists.

Same question arises for virtual photons or electromagnetic interaction.

QED rests on the idea that charged particles (e.g., electrons and positrons) interact by emitting and absorbing photons, the particles that transmit electromagnetic forces. These photons are “virtual”; that is, they cannot be seen or detected in any way because their existence violates the conservation of energy and momentum.

However, two items need to be answered

A - force carrier exchange mechanism between objects

B - mathematical justification by using Newton's Second Law

These are what I have done about gravitons and electromagnetic force (virtual photons).

We need a new definition of graviton.

Photon-Graviton Interaction and CPH Theory

https://www.researchgate.net/publication/308890409_Photon-Graviton_Interaction_and_CPH_Theory?ev=prf_pub

The Mechanism of Graviton Exchange between Bodies

https://www.researchgate.net/publication/310457967_The_Mechanism_of_Graviton_Exchange_between_Bodies_Part_II

Article Photon-Graviton Interaction and CPH Theory

Article The Mechanism of Graviton Exchange between Bodies, Part II