The Four Pillars of the Standard Cosmology
`The evolution of the world can be compared to a display of fireworks that has just ended; some few red wisps, ashes
and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanishing brilliance
of the origin of the worlds.' Lemaitre.
The four key observational successes of the standard Hot Big Bang model are the following:
Expansion of the Universe
Origin of the cosmic background radiation
Nucleosynthesis of the light elements
Formation of galaxies and large-scale structure
The Big Bang model makes accurate and scientifically testable hypotheses in each of these areas and the
remarkable agreement with the observational data gives us considerable confidence in the model.
Expansion of the Universe
The Universe began about ten billion years ago in a violent explosion; every particle started rushing apart from
every other particle in an early super-dense phase. The fact that galaxies are receding from us in all directions is
a consequence of this initial explosion and was first discovered observationally by Hubble. There is now excellent
evidence for Hubble's law which states that the recessional velocity v of a galaxy is proportional to its
distance d from us, that is, v=Hd where H is Hubble's constant. Projecting galaxy trajectories backwards in time
means that they converge to a high density state - the initial fireball.
The Copernican or cosmological principle states that the Universe appears the same in every direction from every
point in space. It amounts to asserting that our position in the Universe - with respect to the very largest scales - is
in no sense preferred. There is considerable observational evidence for this assertion, including the measured
distributions of galaxies and faint radio sources, though the best evidence comes from the near-perfect uniformity
of the relic cosmic microwave background radiation. This means that any observer anywhere in the Universe will
enjoy much the same view as we do, including the observation that galaxies are moving away from them.
The fact that the Universe is expanding - about every point in space - can be a difficult concept to grasp. The
analogy of an expanding balloon may be helpful: Imagine residing in a curved flatland on the surface of a
balloon. As the balloon is blown up, the distance between all neighbouring points grows; the two-dimensional
universe grows but there is no preferred centre.
Origin of the cosmic background radiation
About 100,000 years after the Big Bang, the temperature of the Universe had dropped sufficiently for electrons
and protoons to cobine into hydrogen atoms, p + e --> H. From this time onwards, radiation was effectively unable
to interact with the background gas; it has propagated freely ever since, while constantly losing energy because
its wavelength is stretched by the expansion of the Universe. Originally, the radiation temperature was about
3000 degrees Kelvin, whereas today it has fallen to only 3K.
Observers detecting this radiation today are able to see the Universe at a very early stage on what is known as
the `surface of last scattering'. Photons in the cosmic microwave background have been travelling towards us for
over ten billion years, and have covered a distance of about a million billion billion miles.
Nucleosynthesis of the light elements
Prior to about one second after the Big Bang, matter - in the form of free neutrons and protons - was very hot and
dense. As the Universe expanded, the temperature fell and some of these nucleons were synthesised into the light
elements: deuterium (D), helium-3, and helium-4. Theoretical calculations for these nuclear processes predict, for
example, that about a quarter of the Universe consists of helium-4, a result which is in good agreement with current
stellar observations.
The heavier elements, of which we are partly made, were created later in the interiors of stars and spread widely
in supernova explosions.
Formation of galaxies and large-scale structure
The standard Hot Big Bang model also provides a framework in which to understand the collapse of matter to form
galaxies and other large-scale structures observed in the Universe today. At about 10,000 years after the Big
Bang, the temperature had fallen to such an extent that the energy density of the Universe began to be dominated
by massive particles, rather than the light and other radiation which had predominated earlier. This change in the
form of the main matter density meant that the gravitational forces between the massive particles could begin to
take effects, so that any small perturbations in their density would grow. Ten billion years later we see the results
of this collapse.
The standard cosmology, then, provides a framework for understanding galaxy formation, but it does not tell us
about the origin of the primordial fluctuations required at 10,000 years. We must seek answers to questions like
these from earlier epochs in the history of the Universe.
[Back][Hot big bang][Galaxies][Relic radiation][Cosmic strings][Inflation][Cosmology][Next]
Steady State Theory in Cosmology
Victor Habbick Visions / Getty Images
byAndrew Zimmerman Jones
Updated April 15, 2018
Steady State Theory was a theory proposed in twentieth-century cosmology to explain evidence that the universe
was expanding, but still retain the core idea that the universe always looks the same, and is therefore unchanging
in practice (and has no beginning and no end). This idea has largely been discredited due to astronomical evidence
that suggests the universe is, in fact, changing over time.
Steady State Theory Background and Development
When Einstein created his theory of general relativity, early analysis showed that it created a universe that was
unstable—expanding or contracting—rather than the static universe that had always been assumed. Einstein also
held this assumption about a static universe, so he introduced a term into his general relativity field equations called
the cosmological constant, which served the purpose of holding the universe in a static state. However, when Edwin
Hubble discovered evidence that distant galaxies were, in fact, expanding away from the Earth in all directions,
scientists (including Einstein) realized that the universe didn't seem to be static and the term was removed.
Steady state theory was first proposed by Sir James Jeans in the 1920s, but it really got a boost in 1948, when
it was reformulated by Fred Hoyle, Thomas Gold, and Hermann Bondi. (There is an apocryphal story that they
came up with the theory after watching the film Dead of Night, which ends exactly as it began.) Hoyle particularly
became a major proponent of the theory, especially in opposition to the big bang theory. In fact, in a British radio
broadcast, Hoyle coined the term "big bang" somewhat derisively to explain the opposing theory.
In his book, physicist Michio Kaku provides one reasonable justification for Hoyle's dedication to the steady state
model and opposition to the big bang model:
One defect in the [big bang] theory was that Hubble, because of errors in measuring light from distant galaxies, had
miscalculated the age of the universe to be 1.8 billion years. Geologists claimed that Earth and the solar system were
probably many billions of years old. How could the universe be younger than its planets?
In their book Endless Universe: Beyond the Big Bang, cosmologists Paul J. Steinhardt and Neil Turok are a bit less
sympathetic to Hoyle's stance and motivations:
Hoyle, in particular, found the big bang abhorrent because he was vehemently antireligious and he thought the
cosmological picture was distrubingly close to the biblical account. To avoid the bang, he and his collaborators were
willing to contemplate the idea that matter and radiation were continually created throughout the universe in just such
a way as to keep the density and temperature constant as the universe expands. This steady-state picture was the last
stand for advocates of the unchanging universe concept, setting off a three-decade battle with proponents of the big
bang model.
As these quotes indicate, the major goal of the steady state theory was to explain the expansion of the universe
without having to say that the universe as a whole looks different at different points in time. If the universe at any
given point in time looks basically the same, there is no need to assume a beginning or an end. This is generally
known as the perfect cosmological principle. The major way that Hoyle (and others) were able to retain this
principle was by proposing a situation where as the universe expanded, new particles were created. Again, as
presented by Kaku:
In this model, portions of the universe were in fact expanding, but new matter was constantly being created out of
nothing, so that the density of the universe remained the same.[...] To Hoyle, it seemed illogical that a fiery cataclysm
could appear out of nowhere to send galaxies hurtling in all directions; he preferred the smooth creation of mass out
of nothing. In other words, the universe was timeless. It had no end, nor a beginning. It just was.
Disproving the Steady State Theory
The evidence against the steady state theory grew as new astronomical evidence was detected. For example,
certain features of distant galaxies—such as quasars and radio galaxies—weren't seen in nearer galaxies. This
makes sense in the big bang theory, where the distant galaxies actually represent "younger" galaxies and nearer
galaxies are older, but the steady state theory has no real way to account for this difference. In fact, it's precisely
the sort of difference that the theory was designed to avoid!
The final "nail in the coffin" of steady state cosmology, however, came from the discovery of the cosmological
microwave background radiation, which had been predicted as part of the big bang theory but had absolutely
no reason to exist within the steady state theory.
In 1972, Steven Weinberg said of the evidence opposing steady state cosmology:
In a sense, the disagreement is a credit to the model; alone among all cosmologies, the steady state model makes such
definite predictions that it can be disproved even with the limited observational evidence at our disposal.
Quasi-Steady State Theory
There continue to be some scientists who explore the steady state theory in the form of quasi-steady state theory. It
is not widely accepted among scientists and many criticisms of it have been put forth that have not been adequately
addressed.
Stellar Evolution and Nucleosynthesis
Teacher Background
1 - Origin of the Stars
A � The Big Bang
The current theory for the origin of the Universe, the Big Bang, is very successful in describing the observed
Universe today. The four key observational successes of the standard Hot Big Bang model are the following:
Expansion of the Universe
Origin of the cosmic background radiation
Nucleosynthesis of the light elements
Formation of galaxies and large-scale structure
Here we will concentrate our attention in the Nucleosynthesis of the light elements.
The extremely high temperatures of the early moments of the universe did not allow nuclei to exist. At these
temperatures, quarks had too much energy to be confined in protons and neutrons. (The Nuclear Wall Chart has
details on the phases of nuclear matter.). At about 1 second after the big bang the temperature fell to 100
billion degrees Kelvin and the synthesis of light elements begun. Since the cool down of the universe was very
rapid, there was not enough time for the synthesis of heavier elements.
From observation of spectra of the interstellar medium, astronomers have determined the abundance of most
common elements in the universe. The 2 most abundant elements in the universe are hydrogen and helium.
Hydrogen is about 73% of all the visible mass in the universe. Helium accounts for about 25%. This account
leaves only 2% of the visible mass of the universe to all other elements, including C, O, Ca and other elements
that are found in living organisms. This "imbalance" is evidence that heavier elements were not formed in the Big
Bang.
The web page maintained by the Isotopes Project group at the Lawrence Berkeley Laboratory,
has information on the elemental composition of the Solar System and the Earth. The graph below shows the
relative abundance of the elements in the solar system.
Learn more about the Big Bang on the Cosmic Mystery Tour
So where were they formed? and how?
B - Proto-stars and Star formation
Stars form in nebulae and clouds. About 90% of the material in the Milky Way is contained in the stars, whereas
the remaining 10% is distributed among the stars in the form of gas (interstellar gas) and, in lesser degree, in the
form of small interstellar dust particles. The major part of interstellar gas exists in the form of neutral material,
basically atomic and molecular hydrogen. Spectroscopic observations at centimeter and millimeter wavelengths
have allowed astronomers to determine that the major part of interstellar material is concentrated in giant
molecular clouds with a mass ranging between ten thousand and one million times the mass of the Sun. This fact
makes them the most massive objects in the galaxy.
Observations of interstellar molecules have revealed that the major part of the gas in these clouds is extremely
cold, with temperatures ranging between
-268 and -253 degrees Celsius and with average densities of a few hundreds of molecules every cubic
centimeter. However, there exist small areas in these
clouds where the densities are thousands of times higher than the average, up to 10^7 molecules every cubic
centimeter. One of the most important aspects
in the study of molecular clouds is the fact that it is in the most dense condensations of these clouds where new
stars are born. In the case of the Milky
Way this takes place at a rate of 4-5 suns per year, approximately.
Due to the high visual extinction associated with these condensations, the physical processes involved in the star
formation take place in extremely dark
regions and, therefore, are only accessible by means of observations in the far infrared and at radio
wavelengths -- from submillimeter to centimeter
wavelengths --. In the last decade, the observations with large telescopes and millimeter-wave interferometers
have contributed in an important way to the
understanding of the first stages of star formation and evolution, one of the most important and yet unresolved
questions in today's Astrophysics.
This image, taken by the Hubble Space Telescope, shows gaseous pillars in a star-formation region of the Eagle
Nebula (also known as M16, or Messier 16). Within the pillars are denser regions dubbed "Evaporating
Gaseous Globules" (EGGs) containing embryonic stars. For further details, read the complete caption. Visit
the Hubble server for more images of this phenomenon.
This is a schematic view of the process of star formation, obtained from Prof. Alyssa Goodman. Check out
her talk on Star formation.
2 - Origin of stellar energy and the elements
A - Energy production and Stellar classes
The initially uniform distribution of matter from the Big Bang somehow was broken to form the clumps that were
the proto galaxies. Inside these galaxies, other clumps began to collapse under gravity into smaller bodies. The
compression of this collapse heated the gas until it began to radiate light into the universe. These new stars
continued to collapse and heat up. After perhaps 10 or 100 million years of this steady collapse, the internal
temperature of the new star reached a value of about 10 million degrees (proton energies of about 1 keV), and
thermonuclear reactions between the protons in the gas began.
Nuclear fusion reactions are now accepted to be the source of energy in the starts. Until recently this fact was
based on circumstantial evidence only. This is because the light we observe from stars is emitted in their surface,
and we cannot look inside them to determine what is going on. One of the early evidences that nuclear reactions
occur in stars was the observation of spectral lines of an element called technetium on the surfaces of certain old
stars. Technetium has no stable isotopes, i.e., all technetium decays into other elements. The observed isotope (99Tc) has a half life of 0.2 million years. This is very short compared to the life of a star. The conclusion is that 99Tc
is being "produced" in the star somehow...
The life time of a star and its fate depends on its mass. The reactions that provide new energy to keep the star
shining and to keep it from collapsing further depend on the amount of fuel available. For a comparatively small
sun, like ours, the burning of hydrogen can last for 10 billion years. We are about 5 billion years into that
period at this time. Large stars can go through their entire life cycle and explode in only 10 million years!
A very useful tool to understand the theoretical evolutionary track of stars is the H-R Diagram, named after 2
famous astronomers, Hertzsprung and Russel. The basics of an HR diagram can be found in this page of the
University of Oregon.
These are two representations of the HR diagram, where the luminosity and temperature of a star are plotted.
Also you can load the Java simulator of the evolution of stars in the HR diagram and use these questions about
stars brightness and temperature relations.
More on stellar evolution can be found at http://mrcohen1.keel.physics.ship.edu/108/evol.htm
B - What makes the Sun shine???
Before we can answer this question, we need some information on the characteristics of the Sun and other
important constants:
M = 2x10^33 g (Mass of the Sun in grams)
L = 4x10^33 ergs/s (Luminosity of the Sun)
A = 4.5 x10^9 years (Age of the Sun)
Since life on Earth has existed for some 2x10^9 years, we can assume that the Sun's luminosity has not changed
dramatically over its lifetime. thus we can say that the Sun has radiated LxA = 6 x 10^50 ergs or about 3
x10^17ergs/g.
In the beginning of this century we learned that nuclear reactions can produce large amounts of energy. Consider
combining 4 nuclei of hydrogen to form on nucleus of helium. He = 4.0026 amu, H=1.0087 amu. Use E=mc2 for
the calculation of how much energy is liberated in this process:
4 x 1.0087 amu=4.0348 amu
4.0348 - 4.0026=0.0322 amu.
amu=atomic mass unit. This and other values of physical constants can be found at the NIST fundamental
constants page. The result is around 30 MeV (MeV stands for mega-electron-volts), or around 5 pJ (5 pico
Joules). Thus the fusion of H into He is an exothermic reaction due to the conversion of mass into energy. Many
interesting facts about the Sun can be found at The NASA's observatorium
of Our Sun .
This process does not occur in one step in the Sun. The burning of hydrogen into helium is a 3 step process. A star
which is burning hydrogen is in the "Main Sequence" (MS) of the HR diagram. One very important step is the
fusion of helium into carbon in stars. The energy generation process in a star is related to the stellar evolution
process.
D - Production of elements beyond iron
Again, the production of elements depends on the mass of stars. Low mass stars will become "white dwarfs". They
do not have enough mass i.e., gravitational pressure, to compress and heat up the carbon-oxygen core. High
mass stars keep on contracting and burning heavier elements until iron is formed in their core. The so-called iron
group elements, have the highest known energy per nucleon. What this means is that no further energy
generating nuclear reactions are possible. Still the core of the star will keep contracting and heating. The details
of what happens next are not clear. What we know is that a supernova explosion occurs. As the core of the star
collapses, the density grows until it becomes possible for protons to capture electrons and become neutrons.
This "neutronization" process is very fast and a burst of neutrinos occurs. Eventually the core of the star reaches
the density of nuclear matter, and a complex hydrodynamic bounce occurs releasing matter into the interstellar
medium, leaving behind a neutron star. Again if the mass of the initial star is big enough, it will continue to
collapse and become a black hole. The Mr. Galaxy introduction to supernovae is a good reference on the
supernovae phenomenon.
This is a picture from the Hubble Space Telescope of a supernova remnant, a supernova which
exploded in our galaxy in 1987. Read the press release for more information.
Elements beyond iron are though to be formed by neutron capture in the so-called S (for slow) and R (for rapid)
processes. Although nobody knows yet for sure, the most likely environment for the production of the heavy
elements (just about everything with a Z higher than 25) is in these explosions.
References:
� "Big Bang Nucleosynthesis and the Baryon Density of the Universe", Craig J. Copi, David N. Schramm and
Michael S. Turner; Science vol 267, 13/january 1995 (page 192)
� "Stellar Alchemy: The origin of the chemical elements", Eric B. Norman, Journal of chemical education, Vol 71,
pages 813-820, October 1994.
�The periodic table of isotopes at Los Alamos National Laboratory http://pearl1.lanl.gov/periodic/
� The University of Oregon "Electronic Universe Project" : http://zebu.uoregon.edu/
� Interactive HR diagram http://www.clockwerks.com/trei/proj/H-R/
� The Cosmic Mystery tour http://www.ncsa.uiuc.edu/Cyberia/Cosmos/CosmicMysteryTour.html