Big Bang Cosmology
The Big Bang Model is a broadly accepted theory for the origin and evolution of our universe
. It postulates that 12 to 14 billion years ago, the portion of the universe we can see toda
y was only a few millimeters across. It has since expanded from this hot dense state into th
e vast and much cooler cosmos we currently inhabit. We can see remnants of this hot dense ma
tter as the now very cold cosmic microwave background radiation which still pervades the uni
verse and is visible to microwave detectors as a uniform glow across the entire sky.
Foundations of the Big Bang Model
The Big Bang Model rests on two theoretical pillars:
General Relativity
Albert Einstein at the Chalkboard- source unknownThe first key idea dates to 1916 when Einst
ein developed his General Theory of Relativity which he proposed as a new theory of gravity.
His theory generalizes Isaac Newton's original theory of gravity, c. 1680, in that it is su
pposed to be valid for bodies in motion as well as bodies at rest. Newton's gravity is only
valid for bodies at rest or moving very slowly compared to the speed of light (usually not t
oo restrictive an assumption!). A key concept of General Relativity is that gravity is no lo
nger described by a gravitational "field" but rather it is supposed to be a distortion of sp
ace and time itself. Physicist John Wheeler put it well when he said "Matter tells space how
to curve, and space tells matter how to move." Originally, the theory was able to account f
or peculiarities in the orbit of Mercury and the bending of light by the Sun, both unexplain
ed in Isaac Newton's theory of gravity. In recent years, the theory has passed a series of r
igorous tests.
The Cosmological Principle
APM Galaxy SurveyAfter the introduction of General Relativity a number of scientists, includ
ing Einstein, tried to apply the new gravitational dynamics to the universe as a whole. At t
he time this required an assumption about how the matter in the universe was distributed. Th
e simplest assumption to make is that if you viewed the contents of the universe with suffic
iently poor vision, it would appear roughly the same everywhere and in every direction. That
is, the matter in the universe is homogeneous and isotropic when averaged over very large s
cales. This is called the Cosmological Principle. This assumption is being tested continuous
ly as we actually observe the distribution of galaxies on ever larger scales. The accompanyi
ng picture shows how uniform the distribution of measured galaxies is over a 70° swath of t
he sky. In addition the cosmic microwave background radiation, the remnant heat from the Big
Bang, has a temperature which is highly uniform over the entire sky. This fact strongly sup
ports the notion that the gas which emitted this radiation long ago was very uniformly distr
ibuted.
These two ideas form the entire theoretical basis for Big Bang cosmology and lead to very sp
ecific predictions for observable properties of the universe.
From NASA
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