Early Models of the Universe
Space exploration missions contribute greatly to our understanding of the Universe.
With our new understanding of the fundamental workings of light, matter, and gravity, we can begin to examine the theories and discoveries that led to the development of the Big Bang model. Currently the most widely accepted scientific model of the Universe, the Big Bang model describes a dynamic evolving Universe that began in an extremely hot dense initial state nearly 14 billion years ago, and has since expanded into the complex structure that we see today.
The Newtonian Model
Scientists following Newton used his theory of gravity to successfully explain the arrangement and motion of the planets in our solar system. However when they considered the structure of the Universe on larger scales an apparent paradox arose: if everything in the universe is gravitationally attracted to everything else, the stars would accelerate towards each other causing the universe to collapse.
To avoid this problem, it was hypothesized that the universe is infinitely large and that the stars are positioned perfectly so as to hold each other in gravitational equilibrium. However even the slightest disruption of a single star could create a domino effect, resulting in a collapse. Something obviously wasn't right.
In 1915, Albert Einstein expanded Newtonian gravity into the theory of General Relativity. The theory describes space and time as a 4-dimensional entity called space-time. The presence of a massive object warps or bends the fabric of space-time such that the path of other objects (even light itself) is curved inwards towards the mass.
The more massive an object is, the greater the degree of warping it causes, and hence, the greater the tendency of other objects to "fall" toward it.
When Einstein formulated General Relativity he believed, as did most of his contemporaries, that the Universe was static and unchanging. To his surprise, however the solutions to the relativistic equations indicated that the universe must either be expanding or contracting. To fix this troubling anomaly Einstein postulated an additional term in the equations called the cosmological constant, Λ, which counteracted the attraction caused by gravity at large distance scales and preserved the hypothesis of a static universe. He would later come to regret this move calling it his "greatest blunder", when observational data showed that the universe is expanding.