* Our galaxy is a huge, rotating disk-shaped group of stars that we see in the sky as the Milky Way from our location about two-thirds of the way out from the center. Most of the stars of the galaxy are found in two spiral arms that extend outward from the center. Spiral galaxies are other collections of stars that resemble our galaxy. The universe apparently consists of widely separated galaxies of stars.
a) Illustrations(1) The Milky Way Galaxy (2) The Milky Way (3) Galaxies (4) Exploring our Galaxy (5) The Hot Big Bang (6) A brief introduction to Universe (7) The nature of the Universe (8) The Big Bang model of the Universe. (9) The hot Big Bang model (10) The Big Bang (11) God, Genesis and the Big Bang (12) Closed and Open Model of the Universe (13) The Big Bang
* Tracing the observed
motions of galaxies back in time implies that, about 15 billion
years ago, the universe consisted of a single point that expanded
rapidly in the Big Bang. Space itself was compressed to a point
at that instant-the Big Bang happened everywhere at once. The
cosmological redshift occurs as a photon's wavelength is "stretched"
by cosmic expansion. The extent of the observed redshift is a
direct measure of the expansion of the universe since the photon
was emitted. The spectral lines of distant galaxies show a doppler
shift to the red arising from motion away from the earth. Since
the speed of recession is observed to be proportional to distance,
the red shift means that all the galaxies in the universe are
moving away from one another. This expansion of the universe began
about 15 billion years ago.
* The big bang theory holds that the universe originated in a great explosion about 15 billion years ago. Radiation left over from the big bang and doppler-shifted to radio frequencies has been detected. If the explosion was violent enough, the expansion of the universe will continue forever; if not, the universe will eventually begin to contract and will end up in a big crunch after which another cycle of expansion and contraction may occur. It is likely that the solar system originated as part of an evolutionary process, and that planetary systems are quite common elsewhere in the universe.
b) Illustrations(1) Expansion of the Universe (2) Doppler shift (3) The Expansion of the Universe (4) Hubble Expansion of the Universe (5) The Expanding Universe (6) Doppler shift (7) Hubble's Law and the Expansion of the Universe (8) Expansion of the Universe (9) Hubble's law and the Expanding Universe (10) Expansion of Universe simulation (11) Cosmology Tutorial (12) Expansion of Universe (13) Hubble's Law
* The cosmic microwave
background is isotropic blackbody radiation that fills the entire
universe. Its present temperature is about 3 K. Its existence
is evidence that the universe expanded from a hot, dense state.
As the universe has expanded, the initially high-energy radiation
has been redshifted to lower and lower temperatures.
c) Illustrations(1) The Cosmic Microwave Background Radiation (2) Frequently Asked Questions (3) Cosmic expansion
* Radio waves from
space are produced by extremely hot gases, by fast electrons that
move in magnetic fields and by atoms and molecules excited to
radiate. Especially notable sources are quasars, distant objects
that emit both light and radio waves strongly and that may be
powered by supermassive black holes at their centers.
d) Illustrations(1) Radio Waves (2) What are radio waves? (3) Frequently Asked Questions About Quasars (4) Quasars (5) Quasars (6) Black Hole (7) Quasi Stellar Objects
* Cosmic rays are
atomic nuclei, mostly protons, that travel through the galaxy
at speed close to that of light. They probably were ejected during
supernova explosions and are trapped in the galaxy by magnetic
fields.
e) Illustrations(1) Cosmic Rays (2) Cosmic Rays (3) Cosmic Rays: What Are They? (4) Supernova Explosions (5) The Supernova Explosions
*
Cosmological principle: In cosmology-the study of the universe
as a whole assume that the universe is homogeneous and isotropic
on large scales.
f) Illustrations(1) Cosmological Principles (2) Cosmological Principle
* Olbers's paradox:
If the universe were homogeneous, isotropic, infinite, and
unchanging, then the night sky would be bright because any line
of sight would eventually intercept a star. The fact that the
night sky is instead dark is called Olbers's paradox. Its resolution
is that we see only a finite part of the universe-the region within
about 15 billion light-years, beyond which light has not yet had
time to reach us.
g) Illustrations(1) Olbers' Paradox and the Dark Night Sky (2) The darkness of the night sky (3) Olbers' Paradox
* The large-scale
structure observed in the universe today formed when density inhomogeneities
in the dark matter clumped and grew to create the "skeleton"
of the structure now observed. Normal matter then flowed into
the densest regions of space, eventually forming the galaxies
we now see. "Ripples" in the microwave background are
the imprint of these early inhomogeneities on the radiation field.
h) Illustrations(1) Dark Matter (2) Dark Matter