__THEORTICAL PHYSICS-__

*This first pillar of this physics is einstein’s 10 tensor field partial differential equations based upon higher mathematics known as Tensor Calculas . These equations are a set of 10 equatons in einstein’s theory of general relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy.*

*Einstein’s field equations are solutions of spacetime . the study of exact solutions of EFE is one of the activities of cosmology . it leads to prediction of black holes and the expanding universe . dutch physicist willem de sitter solved the equations of general relativity for a simplified universe incorporating einstein’s cosmological constant.*

*Einstein’s equations predict that universe has to expand or contract so from this realization the Big Bang Theory was born .*

*The next pillar came in 1920s , when an American astronomer vesto sipher detected the expansion of the universe and who used the spectra of stars to measure the velocities of nearby galaxies . waves of light from a star moving toward earth are compressed , shortening the wavelength and making the light bluer . light waves from a object moving away from us are stretched , making the wavelength longer and the light redder . by measuring the lengthening or compression of the light waves from galaxies , sipher was able to determine whether they moving toward us or away from us and at what speed sipher found that almost all these galaxies were moving away from us .*

*Later the person who is generally credited for discovering the expansion of the universe is not sipher but American astronomer Edwin hubble . hubble determined not just the velocities of nearby galaxies but also their distances. His measurements led to two conclusions that justify his fame. First , hubble showed that galaxies were so far away that they were really independent collections of stars, just our own galaxy. Second he discovered a simple relation between the distances to galaxies and their velocities. The velocity was directly proportional to its distance from us . a galaxy twice as far away as another was moving twice as sadt.*

*The Third Pillar Is the fiant glow of the cosmic microwave background , discovered in 1965 by bell labs physicists arno Penzias and and roberet willson as they tracked down sources of radio interference . this radiation was quickly recognized from the early stages of the expansion of the universe . it indicates that the universe began hot and dense and hence has since cooled and thinned out .*

*The final observationol pillar of the Big Bang is that the hot , dense early universe was a perfect location for nuclear fusion when the temperature of the universe was one billon to 10 billion kelvins , lighter nuclei could fuse into heavier nuclei , a process known as Big Bang nucleosynthesis . this process can occur for only a few minutes as the universe expands and cools , so fusion was limited to the lightest elements .*

*Today Theortical Physics want a theory of quantum gravity or theory of every thing .*

*Einstein’s general theory of relativity says that the universe began with theBig Bang singularity a moment when all the matter we see was concentrated at a single point of infinite density .but the theory does not capture the fine quantum structure of spacetime, which limits how tightly matter can be concentrated and how strong gravity can become . to figure out what really happened , physicists need a quantum theory of gravity.*

*According to one candidate for such a theory , loop quantum gravity , space is divided into “atoms” of volume and has a finite capacity to store matter and energy , thereby preventing true singularities from exiting.*

__Theories of Quantum Gravity__

*1-*

*Loop Quantum Gravity- gravity is typically an attractive force . but loop gravity suggests that the atomic structure of spacetime changes the nature of gravity at very high energy densities ,mking it repulsive . imagine when energy densities become too large , repulsive force come into play. The continous space of general relativity ,in contrast , can store a limitless amount of energy. In fact , the repulsive gravity caused space to expand at an accelerate rate.*

*It Invokes a new technique for applying quantum rules to einstein’s general theory of relativity.*

*2-*

*String Theory – The Approach favored by most therotical physicists , it is a theory not just of quantum gravity but of all matter and forces . It is based on the idea that particles (including the hypothetical ones that transmit gravity ) are vibrating strings.*
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