Tycho Brahe Johannas Kepler Galileo Galilei
(1546-1601) (1571-1630) (1564-1642)
Timeline of Cosmological Theories
This timeline of cosmological theories and discoveries is a
chronological record of the development of humanity's
understanding of the cosmos over the last two-plus
millennia. Modern cosmological ideas follow the development
of the scientific discipline of physical cosmology.
Review of atomic spectral lines and Doppler shift
From Wikipedia - Doppler Effect
From Wikipedia - Redshift
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.
The Big Bang model was a natural outcome of Einstein's
General Relativity as applied to a homogeneous universe.
However, in 1917, the idea that the universe was expanding
was thought to be absurd. So Einstein invented the
cosmological constant as a term in his General Relativity
theory that allowed for a static universe.
In 1929, Edwin Hubble announced that his observations of
galaxies outside our own Milky Way showed that they were
systematically moving away from us with a speed that was
proportional to their distance from us. The more distant
the galaxy, the faster it was receding from us. The
universe was expanding after all, just as General
Relativity originally predicted!
Hubble observed that the light from a given galaxy was
shifted further toward the red end of the light spectrum
the further that galaxy was from our galaxy.
The specific form of Hubble's expansion law is important:
the speed of recession is proportional to distance. Hubble
expressed this idea in an equation - distance/time per
megaparsec. A megaparsec is a really big distance (3.26
The expanding raisin bread model illustrates why this
proportion law is important. If every portion of the bread
expands by the same amount in a given interval of time,
then the raisins would recede from each other with exactly
a Hubble type expansion law.
In a given time interval, a nearby raisin would move
relatively little, but a distant raisin would move
relatively farther - and the same behavior would be seen
from any raisin in the loaf. In other words, the Hubble law
is just what one would expect for a homogeneous expanding
universe, as predicted by the Big Bang theory.
Moreover no raisin, or galaxy, occupies a special place in
this universe - unless you get too close to the edge of the
loaf where the analogy breaks down.
From Wikipedia - Hubble's Law
Where was the center of the Big Bang?
How can the Universe be infinite if it was all concentrated into
a point at the Big Bang?
Why the universe probably is "flat"
http://www.youtube.com/watch?v=LQL2qiPsHSQ (1hr 17)
Lawrence Krauss makes the case for a flat universe, where
the total amount of mass-energy in the universe is and
always has been zero.
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.