Foundations of the Big Bang Model - Theory
Foundations of the Big Bang Model - Concepts
To this point, the only assumption we have made about the
universe is that its matter is distributed homogeneously and
isotropically on large scales. There are a number of free
parameters in this family of Big Bang models that must be fixed
by observations of our universe. The most important ones are:
the geometry of the universe (open, flat or closed); the
present expansion rate (the Hubble constant); the overall
course of expansion, past and future, which is determined by
the fractional density of the different types of matter in the
universe. Note that the present age of the universe follows
from the expansion history and present expansion rate.
Since both energy density and pressure contribute to the
strength of gravity in General Relativity, cosmologists
classify types of matter by its "equation of state" the
relationship between its pressure and energy density. The basic
classification scheme is:
Radiation: composed of massless or nearly massless particles
that move at the speed of light. Known examples include photons
(light) and neutrinos. This form of matter is characterized by
having a large positive pressure.
Baryonic matter: In this cosmological context, this is
"ordinary matter" composed primarily of protons, neutrons and
electrons. This form of matter has essentially no pressure of
Dark matter: this generally refers to "exotic" non-baryonic
matter that interacts only weakly with ordinary matter. While
no such matter has ever been directly observed in the
laboratory, its existence has long been suspected for reasons
discussed in a subsequent page. This form of matter also has no
cosmologically significant pressure.
Dark energy: this is a truly bizarre form of matter, or perhaps
a property of the vacuum itself, that is characterized by a
large, negative pressure (repelling force). This is the only
form of matter that can cause the expansion of the universe to
accelerate, or speed up.
Tests of Big Bang Cosmology
The Big Bang Model is supported by a number of important
observations, each of which are described in more detail
on separate pages:
1. The expansion of the universe
Edwin Hubble's 1929 observation that galaxies were generally
receding from us provided the first clue that the Big Bang
theory might be right.
2. The abundance of the light elements H, He, Li
The Big Bang theory predicts that these light elements should
have been fused from protons and neutrons in the first few
minutes after the Big Bang.
3. The cosmic microwave background (CMB) radiation
The early universe should have been very hot. The cosmic
microwave background radiation is the remnant heat leftover
from the Big Bang.
These three measurable signatures strongly support the notion
that the universe evolved from a dense, nearly featureless
hot gas, just as the Big Bang model predicts.
Wilkinson Microwave Anisotropy Probe (WMAP) Results
WMAP Technical Papers