of the Universe, Radiation Laws and Big Bang
beginning, there was nothing at all. Earth was not found, nor Heaven above,
a Yawning-Gap there was, but grass nowhere."
Edda -- collection of Norse Myths dating to 1200
In this lecture period
What we know about
the origin of the Universe?
How we determine the
age of the Universe
What is the evidence for the Big Bang?
From the University of Colorado
planet with a rocky composition and a temperate climate at an intermediate
distance from an average star. The Earth is unique in many ways. For example,
it is the only body we know of where water can exist in all three
phases: vapor, liquid and solid forms. Click on the image for a general tour
that is offered through
Windows to the Universe.
A medium-sized, moderately bright, middle
aged star, born ~5 billion years ago from a gaseous nebula, with perhaps
another 4-5 billion years to live before expanding to a "red-giant", engulfing
the Earth and finally cooling to become a fading "white dwarf" star. This
image of today's
Sun, as seen in hydrogen emissions that show the turbulent solar
atmosphere, is the most recent available from NASA.
birthplace of stars. Our own sun formed in just such a nebula. The example
shown here is the Great
Orion Nebula, one of the youngest objects in the sky - thought to be
less than 20,000 years old. Orion is very hot (about 20,000 K). Other spectacular
examples are the Cat's
Eye Nebula, and the Trifid
clusters are recently-born families of stars that form in such nebula
and then gradually drift apart. Click on Orion and see the Horsehead Nebula,
which is 1500 light years away. A "light year" is a unit of distance which
represents the distance traveled by light in one year.
collection of billions of stars, held together by gravity. Our own galaxy
is known as the "Milky-Way"
Galaxy and is 100,000 light years across. Galaxies are in many ways
"Island Universes". Each galaxy contains billions of stars, with some having
more than 1000 billion stars. This Hubble
Space Telescope true-color image is the Cartwheel Galaxy, located 500
million light-years away in the constellation Sculptor. Click on
it to see our nearest neighbor galaxy, Andromeda.
visible universe contains at least 100 billion galaxies - these are incredible
numbers. This image is of very young galaxies observed by the Hubble Space
Telescope at the very limit of its range. The sky is full of such strange
looking galaxies in all directions (except where masked by intervening
dust clouds). The universe is home to a variety of exotic objects. For
which were first discovered in 1960, are still baffling
objects. Incredibly energetic, they are found at great distances near
what is thought to be the edge of the known universe (the most distant
one has been estimated to be 10 billion light years away). Some quasars
produce more energy than 100 large galaxies. Some scientists think that
quasars may represent holes to other universes.
Radiation includes visible light, radio waves, microwaves, x-rays,
gamma rays, and infra-red (heat) rays. All these forms of radiation
are characterized by traveling oscillations of a combined electric
and magnetic field. These electromagnetic waves differ by the
wavelength of the oscillation, with shorter wavelength radiation
carrying more energy than longer wavelength radiation.
All possible wavelengths make up the
spectrum can be expressed in terms of energy, wavelength, or frequency.
Planck's Law is sometimes
called the "black-body" formula works very well for
Where E (lambda) is the
amount of radiant energy emitted at a given wavelength, lambda. T is
the temperature of the object, and a and b are constants.
The spectrum of wavelengths
emitted by a body at a temperature, T, has a characteristic shape that
is strongly dependent on the wavelength (to the inverse fifth power).
Black-body radiation curves, showing the wavelength
distribution of emitted photons at different temperatures, on a
logarithmic scale. Note the different regions of the
This law describes the
spectral distribution of radiation emitted by a black body. Very hot bodies (3000 -
20,000 K) like our Sun emit a lot of light at visible wavelengths. The Sun acts like a black
body near 6000K, whereas the Earth acts like a black body near 300 K (can you
guess where its curve would lie?).
("E equals sigma T to the fourth")
where E is the total energy emitted (calculated by adding up the areas
under the curves of Figure 1), sigma is a constant, and T is temperature.
of the peak radiance [lambda (max)] decreases linearly as the temperature
increases, where c is a constant:
Summary of radiation laws
gives us the shape of the curves.
Law tells us that much more energy comes from the Sun than
from the Earth.
tells us that the hot Sun is much bluer than the cooler Earth.
The Doppler Effect
||The Doppler effect shifts the
light to longer wavelengths (red shift) for a receding object, and to
shorter wavelengths (blue shift) for an approaching object.
to see how the speed of an object affects wavelength.
The Doppler Effect for Light
is calculated by:
Change of wavelength
Speed of source
Speed of light
Big Bang Theory
Big Bang theory states that the Universe began when primordial mass exploded. This fireball gradually cooled as it expanded outward,
and giant clouds of swirling gas formed the celestial bodies. The Big Bang theory does
not explain why the bang occurred, but predicts (with surprising
accuracy) what the consequences of the event.
Evidence #1: atomic
If the big bang occurred, the initial
temperatures must have been so unimaginably high that matter could only
have existed in exotic and unstable forms. As the temperature cooled in
the first second, free hydrogen nuclei (atomic mass 1) were formed that
could undergo fusion reactions to give heavier forms of hydrogen (atomic
mass 2, 3) and helium (atomic mass 3 and 4). Thus, the theory predicts a early universe with
only a mixture of ~75% hydrogen and 25% helium (by weight) and no heavier
species. This ratio is exactly what is observed in stars.
Evidence #2: red shift
Measurements of the red
shifts of virtually all galaxies (except a few in our immediate vicinity)
show that the visible universe is expanding in all directions. The constant of proportionality
between the distance and velocity of recession is known as Hubble's constant.
Evidence #3: microwave
Measurements of low
energy microwave radiation show that the visible universe is permeated by "cosmic background"
microwave radiation, coming from all directions
and similar to what is expected from a black body at 3K. The Big Bang
theory predicts that such radiation is the red-shifted remnant of the
radiation released when matter and light became decoupled about 1 million
years after the Big Bang. The American scientists who first made this
measurement in 1965 (Penzias and Wilson) obtained the Nobel Prize.
Measurements from the
Cosmic Ray Background Explorer (COBE) mission provide additional
results show that the cosmic ray background is not
completely uniform in direction (see Figure), but that there is some clumping in
preferred directions with differences in effective temperature of only
one hundred millionth of a degree. This clumpiness would have been necessary
for the big bang to produce galaxies, since a perfectly uniform explosion
would not produce localized high densities.
A timeline for the
Big Bang model of the Universe. At ~1 million years after the Big
Bang, temperatures cool sufficiently to allow hydrogen- and
helium-neutral atoms to form from the plasma (charged particles).
This event freed up radiation that was previously contained in
thermal equilibrium with matter. Since then, radiation and matter
have gone their own ways. When astronomers observe the cosmic ray
background, they are looking at photons released from the big bang
when radiation and matter became uncoupled.
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