| Main current observational data
1) Universe is expanding as seen by the cosmological red shifts of galaxies. 2) Cosmic abundance of hydrogen, helium, and lithium(Note: Caveat here for theory!) agree with Big Bang theory. 3) There is a nearly isotropic emission of Cosmic Microwaves (CMBR) at an equivalent temperature of about 2.725K. 4) About 90% of the matter in the Universe is not "ordinary" matter. This is dark non-baryonic matter. 5) Universe is expanding at an increasing
rate. Dark energy( the quintessence field ?) accounts for about 68% of the
critical energy density of the Universe. Will the accelerated rate of
expansion keep going
leading to a Big Rip? More observations are needed to answer the time
development of
Dark Energy. 6)
Big Bang Nucleosynthesis(BBN) accounts for the abundance of hydrogen,
deuterium, helium and lithium. Abundance of elements beyond these light
elements is
explained by stellar nucleosynthesis. 7) WMAP data fix an age for the Universe at
13.77 billion years. 8) WMAP data claim that the geometry of
the Universe is flat. 9) Planck polarization data claim that the first stars appear at about t = 550 million years. |
Current theoretical explanation
The Big Bang Theory, based on General Relativity, explains observations 1-3. The BB theory states that the universe had an origin in time. The inflationary universe theory clears up
several peculiarities in the BB theory. It gives a plausible
description of the small temperature anisotropy in the CMBR and shows how this
anisotropy leads to galaxy formation. |
Prominent Developments prior to current day cosmology.
Eudoxus, geometrical models of the universe
Aristotle, attempt to explain the motions in the
universe based on physical principles
-
nature of the body ( celestial matter or terrestrial matter )
determines
the type of motion, circular or straight line motion.
Hipparchus, introduces eccentric circle and epicycle and deferent
Ptolemy, major summation of ancient astronomical
and geometrical thought. His Almagest
shows
several different models for planetary, lunar, solar motion
based
on Aristotelian physics. The earth is the center of the universe.
Copernicus, Revives the heliocentric theory of
Aristarchus.
De
Revolutionibus(1543) is not entirely free of epicycles, although
the
epicycles are considerably reduced in size (epicyclets).
Copernicus's
model of the solar system is considerably smaller than
Ptolemy's.
The sphere of the fixed stars is very far away making for
an
"absurd" model of the universe, with the solar system in the center
and
vast space between it and the stars. The Copernican theory
displaces
humanity from the center of creation.
Brahe, precise measurements of planetary motion,
in particular Mars,
without
the aid of telescopes. Attempts to measure stellar parallax to
disprove
Copernicus.
Kepler, Tenaciously works to make sense of
Brahe's data to establish
the
true shape of the planets' orbits. Discovers that Mars' orbit
is an
ellipse with the sun at a focus of the ellipse,
that the area swept out by the radius vector from the sun is
constant
for equal time intervals, discovers the relationship between
the
period of the orbit and the semi-major axis of the orbit.
Proposes that a force from
the sun keeps the planets in orbit. The force becomes
weaker as the radius of the orbit becomes larger.
Galileo, co-discoverer of the telescope, develops
experimental physics, in
particular
mechanics, popularizer of the Copernican system
(
Dialogue Concerning Two Chief World Systems), discoverer of sun
spots,
lunar mountains, phases of Venus, satellites of Jupiter,
abundance
of stars in Milky Way
Newton, co-discoverer of calculus, discovers the
Universal law of gravitation,
mechanics,
optics, discovers the spectrum of light, establishes the
universal
nature of natural law.
Halley, Proposes the transit of Venus technique for determining the astronomical unit, predicts the 76 year return period for Halley's comet
Kant, Wright, postulate the disk shape of the
galaxy from the disposition of
the
Milky Way and suggest that the nebulae are
"Island
Universes"
Herschels , father and son catalog nebulae and
significantly improve
telescope
design. They make stellar astronomy a field in its own
right,
discover the planet Uranus, discover infrared spectrum
Fraunhofer, development of superb optical
equipment, in particular
refracting
telescopes with precision mounts, development of
diffraction
grating, detailed description of the solar spectrum
and
its dark lines
Bessel, discovers stellar parallax using a Fraunhofer refractor
Doppler, predicts that the wave length of light
will change depending on the
relative
motion of the source and observer
Bunsen and Kirchoff, place spectroscopy on a firm
footing, analyse the
chemical
composition
of the sun
Huggins, extends spectroscopy to the stars,
discovers the chemical nature of
the
nebulae, verifies Doppler's effect and determines line of sight
velocity
of stars thereby
Leavitt, establishes the relationship between
luminosity and period
of
cepheid variable stars. These will then be used as "standard
candles"
to determine stellar and galactic distances.
Cannon and Harvard computers, catalogue thousands
of stars by luminosity
and
spectral class
Hertzsprung and Russel, determine the
relationship between the absolute
luminosity
of
a star and its spectral class. This enables the
Leavitt
relationship
to be used for absolute distance
determinations.
Einstein, discovers general relativity, an
indispensable theory for
understanding
cosmic evolution
Slipher, discovers galactic motions and rotations
Hubble, shows that the spiral nebulae are in fact
other galaxies, discovers
with
Humason that the universe is expanding in a regular way. Determines the
distances to m31 and m33 establishing that these
spiral nebula are indeed extra galactic and finally verifying
the Island Universe Theory proposed by Kant.
Zwicky, Discovers that there must be a significant amount of
non-luminous matter (Dark Matter) affecting the motions of galaxies
within clusters.
Discovery of Dark Energy and the accelerating expansion of the
universe. Most of the energy density
of the universe now is in Dark Energy. Will there be a "Big Rip"?
