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In this second week of the course, I plan to use our lecture time together so that we
can discuss how scientists account for the origins of the universe.
Today, the explanation most physicists and astronomers favour what has become
known as the “Big Bang” Theory. This theory postulates that the universe suddenly
came into existence around 13.7 billion years ago. Scientists estimate that this is the
age of universe on the basis of measuring various cosmic phenomena, such as light
emitted by stars, and temperature fluctuations in cosmic microwave background
radiation (this is the radiation that permeates the universe and which scientists think
was to produced in the first moments of its creation).
As the name “Big Bang” Theory suggests, at the core of this theory is the
assumption that the creation of the universe was an incredible, sudden explosive
event.
What caused this Big Bang? Scientists agree that the universe came into being by
what they term a singularity – a remarkable extraordinary event unlike anything that
has occurred before or since. The singularity of the universe’s creation is something
that our current understanding of physics can’t really explain. It appears to have been
created at a point or zone that had no dimension or volume.
The Expansion of the Universe, public domain image,
http://commons.wikimedia.org/wiki/File:Universe_expansion.png
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It defies our every-day common sense understanding of reality that something without
dimension or volume could be the origin of something. Could something emerge
from nothing?
Mathematically, it is possible to conceive of our universe as having originated at a
point at which the force of gravity acting on matter was so great that it was infinitely
dense. However, scientists generally concur we cannot say anything with any
certainty about the universe before its existence. Further, as to what occurred
happened it came into existence, well, that is something about which we cannot
construct anything like a robust scientific explanation; and as the eminent
mathematician and physicist, Stephen Hawking, has pointed out, whatever events that
may have occurred before the creation of the universe have had no consequences after
its creation, and thus should not form part of any scientific model of the universe.
Scientists have hypothesized that in the first few infinitesimal fractions of a second
after the Big Bang, remarkable things occurred in a number of successive distinct
phases.
The first phase between the moment the Big Bang occurred and around 10-43 of a
second later, is what scientists have called the Planck Epoch, in honour of the great
German physicist Max Planck (1858-1947).
It’s thought appropriate this earliest phase of the universe’s history be named after
Plank because his work on quantum theory suggests that in this initial phase of the
universe the laws of physics as we know them did not operate.
The universe at this moment had zero size, was infinitely hot and the fundamental
forces governing its subsequent regularities were one unified electronuclear force.
Some scientists have suggested that at around 10-34 of a second after the Big Bang
the universe entered a new phase, which lasted until just 10-35 of a second after the
Big Bang. This phase they call the Grand Unification Epoch because the temperature
of the universe was still incredibly hot, if no longer infinite.
In the Grand Unification Epoch, the universe grew in size from being no bigger
than that of type of sub-atomic particle we call a quark to the size of a proton, the
elementary particle we find in the nuclei of atoms.
The electromagnetic and strong and weak forces that now determine how the
simplest particles in the universe interact with each other were still one force.
However, by the end of the Grand Unification Era, the strong force that holds together
quarks and another type of sub-atomic particle called gluons had into existence. Once
this happened it allowed the formation of protons and neutrons, the key parts of
atomic nuclei.
Scientists differ about what have occurred in the universe prior to10-35 of a second
after the Big Bang. Where there is greater consensus is about what happened from
around 10-35 of a second. This is because scientists are able to move beyond the realm
of theoretical speculation to study the effects of what happened after 10-35 of a second
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after the Big Bang. For the effects of what happened from this time can still be
observed today.
From 10-35 of a second after its creation, the universe entered a phase that scientists
since the early 1980s have called the inflationary epoch.
During this inflationary epoch, the universe began expanding faster than the speed
of light. What caused this expansion is unknown. Some scientists have suggested that
it was due to some sort of unknown anti-gravitational effect; but this runs counter to
how gravity is understood by modern physics. However, some astronomers have
reasoned that anti-gravitational forces may exist, as their observations of certain stars
have produced evidence highly suggestive that the universe is expanding at an
accelerating rate. If this is so, then the most plausible explanation of this acceleration,
they have argued, is that it is caused by some kind of anti-gravitational phenomena.
The inflationary epoch is estimated to have ended somewhere between 10-33 and
10-32 of a second after the creation of the universe. By this time, the universe had
expanded from a size possibly less than the volume of an atom to being larger than
the size of galaxy. Moreover, as this expansion occurred at speeds beyond those of
light, what we can now see of the universe is only a microscopic proportion of the
entire universe.
By the end of the inflationary period, the universe was still mostly made up of
particles of light with no mass called photons, which scientists classify by the name of
quarks and anti-quarks – two kinds of particle that are identical apart from possessing
opposite electrical charges. What happened at this point I the early history of the
universe was that every time a quark met an anti-quark, the two particles were
transformed into energy – the energy known as cosmic microwave background
radiation that pervades the universe, and which can be observed today.
Those quarks that did not meet an anti-quark formed the matter that exists in our
universe. Scientists estimate that probably one in every billion quarks did not meet
with an anti-quark and survived to provide the basis of protons and neutrons – the
elementary building blocks of atomic nuclei.
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“The quark structure of the neutron. There are two down quark in it and one up quark. The
strong force is mediated by gluons (wavey). The strong force has three types of charges, the so
called red, green and the blue. Note that the choice of blue for the up quark is arbitrary; the
"color charge" is thought of a circulating between the three quarks.” Image: Arpad Horvath,
Creative Commons licensed,
http://commons.wikimedia.org/wiki/File:Quark_structure_neutron.svg
David Christian amusingly describes this process by which the matter in our universe
was formed in his Maps of Time as a “perverse subatomic game of musical chairs, in
which quarks were the players, anti-quarks were the chairs, and the winner was the
one quark in a billion that could find a particle chair.” 1
About a second after the Big Bang the universe had become so big that its
temperature dropped considerably. However, the universe was still very hot, with a
temperature of around 10 billion degrees Celsius – which is about a thousand times
hotter than the temperature of the sun.
At this early point in its history, the universe was comprised of particles such as
electrons, protons and photons that interacted to produce energy. The Cosmologist
Eric Chaisson has called this time the era of radiation, describing universe as a
“macroscopic glowing ‘fog’ of dense, brilliant radiation”, suspended in which was a
“relatively thin microscopic precipitate” of matter.2 A hundred or so seconds later, the
temperature of the universe had dropped to a billion degrees, which is about how hot
1
David Christian, Maps of Time: An Introduction to Big History (Berkeley: California University Press,
2005)., pp. 24-5.
2
See especially Eric Chaisson, Epic of Evolution : Seven Ages of the Cosmos (New York, N.Y. ; Chichester:
Columbia University Press, 2006).
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it is in the core of the hottest stars. What physicists call the strong force now caused
protons and neutrons to bind to each other to form simple atomic nuclei – the simplest
being deuterium (a heavy form of hydrogen).
Diagram of hydrogen atom Author: Webber Source: "own work" Date: 2005, public domain,
http://commons.wikimedia.org/wiki/File:Hydrogen_Atom.jpg
When deuterium nuclei collided with other protons and neutrons, the strong force
ensured they were turned mostly into helium nuclei, but also much frequently into
heavier nuclei such as those of lithium and beryllium.
Scientists think that about a quarter of all protons and neutrons were turned into
helium nuclei. However, helium was only produced in the first hours of the universe.
Thereafter protons and neutrons became hydrogen nuclei, as the neutrons in
deuterium (heavy hydrogen) lost energy and became protons.
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Model of the Helium atom, showing the nucleus with two protons (blue) and two neutrons (red),
orbited by two electrons (waves), public domain,
http://commons.wikimedia.org/wiki/File:Helium_atom_with_charge-smaller.jpg
For the next million or so years the universe was mostly empty space permeated by
radiation, but in which there was this thin precipitate made up mostly of hydrogen and
helium atoms.
As the universe expanded, the energy and matter it contained became subject to the
force of gravity. As Isaac Newton, the great late seventeenth century scientist was to
show, the force of gravity causes particles of matter to be attracted to each other.
Moreover, as Newton further showed, the closer together particles move, the greater
the gravitational attraction between them becomes.
Now, had the hydrogen and helium atoms that made up most of the matter in the
early universe been completely evenly distributed in empty space, the universe, as we
know it today would not exist. Had these atoms been evenly distributed, all that
would have happened would have been a deceleration in the rate of the universe’s
expansion. There would have been no formation of stars, planets or living beings.
Stars, planets and living beings were formed because the atoms within the early
universe were not uniformly distributed. There were slight variations in the heat and
density of matter in the early universe that resulted in the denser regions of matter
becoming denser as they became subject to the force of gravity.
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“A Volume plot of the logarithm of gas/dust density in an Enzo star and galaxy simulation.
Regions of high density are white while less dense regions are more blue and also more
transparent. The data used to make this image were provided by Tom Abel Ph.D. and Matthew
Turk of the Kavli Institute for Particle Astrophysics and Cosmology.” Public Domain:
http://commons.wikimedia.org/wiki/File:Star_formation.jpg
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“Located some 13 million light-years from Earth, NGC 4214 is currently forming clusters of new
stars from its interstellar gas and dust. In this Hubble image, we can see a sequence of steps in
the formation and evolution of stars and star clusters. The picture was created from exposures
taken in several color filters with Hubble's Wide Field Planetary Camera 2.” Public Domain:
http://commons.wikimedia.org/wiki/File:Fireworks_of_Star_Formation_Light_Up_a_Galaxy__GPN-2000-000877.jpg
The force of gravity first acted on those atoms of hydrogen and helium that were no
evenly distributed. As these atoms gravitated to each other, they formed denser
clouds of gas that then became compact. As they compacted, the pressure at the
centres of these masses caused temperature increases as high as 10 million degrees
centigrade.
At this temperature, hydrogen atoms were no longer subject to repulsion between
their positively charged nuclei. They broke up to form helium atoms, each with two
protons in its nucleus.
This process of nuclear fusion – the same reaction that occurs when a hydrogen
bomb explodes - released incredible amounts of energy. What is more, the energy
released by these fusion reactions was powerful enough to resist the force of gravity
and stop the atoms from collapsing into ever-denser masses. In short, the force of the
nuclear reaction and that of gravity balanced each other out, thus allowing stars to
form.
As David Christian observes, stars can be understood as “…the result of a
negotiated compromise between gravity, which crushes matter together, and the
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explosive force of fusion reactions, which forces matter apart.”3
The oldest stars in the universe those in the centre of galaxies, or in globular
clusters orbiting their centre. We can tell that they are of great age because they
generally have no elements heavier than hydrogen or helium.
NASA public domain image,
http://commons.wikimedia.org/wiki/File:Hubble_Space_Telescope,_better_than_new.jpg
In April 2002, NASA's Hubble Space Telescope (pictured above) discovered what are
probably the oldest stars in our galaxy, the Milky Way, in a globular cluster called
M4, some 5,600 light years away from Earth. These stars have been estimated as
having formed less than 1 billion years after the Big Bang.
3
Christian, Maps of Time: An Introduction to Big History, 44.
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NASA
image
of
artist's
impression
of
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the
Milky
Way,
in
public
domain:
http://commons.wikimedia.org/wiki/File:Milky_Way_galaxy.jpg
The shapes of galaxies reflect the influence of gravity. Our own galaxy, the Milky
Way, is very typical in having become by gravitational force a large spinning disk.
We would do well to keep in mind that the observable universe is only a very minute
portion of the entire universe. The theoretical physicist and cosmologist Alan Guth, a
key figure in the development of the Big Bang Theory, has suggested that the entire
universe could be as much as 1023 to 1026 bigger than the universe we can observe.
Further, it is clear that the universe contains things other than stars. There are entities
known as “black holes”, which are parts of space so dense that neither matter nor
energy can resist their gravitational force. Cosmologist and science writer Timothy
Ferris has vividly likened the density of a black hole to the earth compressed into a
ball about 0.7 inches in diameter.”4
In all likelihood a massive black hole lies at the centre of our galaxy, for there is a
powerful source of radio waves astronomers have detected in the area of the
constellation known as Sagittarius. So powerful are these waves that they suggest this
black hole has a density around 2.5 million times that of our sun.
4
Timothy Ferris, The Whole Shebang: A State-of-the-Universe(s) Report (New York Simon and
Shuster, 1997), 79-80. Cited Christian, Maps of Time: An Introduction to Big History, 46.
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NASA sponsored artistic representation of a black hole. Public domain,
http://commons.wikimedia.org/wiki/File:BlackHole.jpg
As to the significance of black holes in the creation of the universe, there has been
much speculation, including the suggestion that they might be separate universes that
we are seeing from the outside. This may seem a wild idea, but it plausibly explains
why the nature of the physical forces and relative size of nuclear particles in our
universe have been such as to allow the formation of stars, the range of elements that
exist, and ultimately the development of complex life-forms.
Stars
Our focus here, however, is on stars: their creation, life and inevitable death.
The life cycles of stars are largely determined by the size of the cloud of matter
from which they formed. If the cloud resulting in their formation is less than around 8
percent of the size of our sun, the centre of the star will not reach the temperature
required for hydrogen to be transformed by nuclear fusion into helium and
progressively heavier elements. They will become what astronomers call a brown
dwarf, or sometimes refer to as a “failed star” because these entities failed to reach the
point of hydrogen fusion and hence are objects somewhere between being a star and a
planet. The existence of “brown dwarfs” was first hypothesized in the mid-1970s, but
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was not confirmed by observational evidence until 1995 using advanced imagining
techniques and spectroscopic analysis.
The more matter there is in a stellar cloud the greater will be the gravitational force
to which it is subject. The greater the gravitational force, the faster it will compact.
The faster the compaction, the denser and hotter this ball of matter will be. This in
turn will determine the speed at which the star that forms will consume the material it
contains. Hence the largest stars in the universe may have more matter but nuclear
fusion takes place at a faster rate, and decay is much faster than in smaller stars. The
life expectancy of our sun, for example, is estimated to be 5 billion years. A star ten
times the size of our sun may last no more than 30 million years. The largest stars in
the universe may have a life cycle of no more that several hundred thousand years.
NASA / ESA sponsored artist's impression of Sirius star system. Public domain image:
http://commons.wikimedia.org/wiki/File:Sirius_A_and_B_artwork.jpg
Often two or more stars are created when gas clouds between more than 60 to 100
times bigger than the size of our sun have gravitationally compacted. The best-known
example of a binary star system is the Sirius system (represent above), which is
situated about 8.6 light years from Earth. One star, Sirius A, is the brightest start
visible in Earth’s Northern Hemisphere. It is a hot white star twenty-five times
brighter than our sun and around twice its mass. Its companion star, Sirius B, is a
very dense white star that has ceased nuclear fusion and is gradually cooling. Sirius
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A, incidentally, was of great significance in ancient Egyptian and ancient Greek
cultures. Its movement from just above the eastern horizon and steady westward
movement at about one degree a day was used by the ancient Egyptians and Greeks to
calculate when to plant crops.
The best-known example of a triple star system is the Alpha Centuri system, which
is made up of a pair of yellow dwarf stars some 4.4 light years from earth and a dwarf
red star that is estimated to be about 4.2 light years from the sun. There are also
systems containing between four to seven stars.
A comparison of the sizes and colors of the stars in the Alpha Centauri system with the Sun made
by
David
Benbennick,
and
licensed
under
Creative
Commons,
http://commons.wikimedia.org/wiki/File:Alpha_Centauri_relative_sizes.png
Nuclear fusion within a star will gradually turn all its hydrogen into helium. As
helium is a heavier element it will sink to the star’s core. As a star exhausts its
available hydrogen, it will cool and gravity will cause it to collapse into a denser
mass. This collapse will increase the temperature at the core of the star. If the star’s
temperature becomes greater then 100 million degrees, nuclear fusion will begin
again, though this time with helium being converted into heavier elements such as
carbon, oxygen and nitrogen. This secondary fusion process produces much less
energy and ends fairly rapidly with the heavier elements causing the core of the star to
collapse into an even denser mass. When this happens, the energy produced will push
the outer layers of the star out into space. If the star is large then the pressure at its
core will push up the temperature again so that even heavier elements are created via
nuclear fusion.
Consider, for example, the star known as Betelgeuse in the constellation of Orion.
This star is estimated to be about 640 light years from earth. It is a large star that has
a diameter about 700 times that of our sun that has probably existed for around 8
million years.
Betelgeuse is now burning helium, having exhausted its available hydrogen, and is
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shrinking to become denser in mass. Scientists call large stars that have reached this
point in their life cycle a red supergiant, because the burning of helium causes the
emission of large amounts of red light. Many scientists think that when Betelgeuse
finally runs out of available fuel a supernova will occur – that is to say the star will
contract at so great a speed that the resulting pressure and heat will cause a massive
explosion, liberating so much energy that for several weeks the light from the
explosion of the star will be as bright as a galaxy.
In the year 1054 CE, many peoples throughout the Northern Hemisphere saw light
from a supernova occurring within a region of our galaxy some 6,300 million light
years from Earth. According to Arab astronomers, the light from the explosion could
be clearly seen during the day for just over three weeks and was visible at night for
around 650 days. The stellar region known as the Crab Nebular is the cloudy remains
of that explosion.
NSA / ESA image. Public domain, http://commons.wikimedia.org/wiki/File:Crab_Nebula.jpg
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The death of the largest stars can be even more spectacular. They can collapse to
create pressures great enough at their core to create a black hole, causing protons and
electrons to form neutrons that are explosively released into space. This phenomenon
generates temperatures so high that elements as heavy as uranium are formed, which
are likewise explosively released into space.
Most stars in our galaxy are of a size and mass that renders their explosive end
unlikely. They will eventually exhaust their hydrogen and become helium burning
red giants. Most will very likely then contract causing the outer layers of the star to
be pushed into space away from its core, now largely consisting of carbon and
oxygen. These outer layers will become ionized by ultra-violet radiation from the
core, which will become what scientists term a “white dwarf.” While white dwarfs
are initially very hot and appear as bright white stellar objects light they do not
undergo further violent compactions and expansions, but gradually cool down
becoming redder in colour. Scientists hypothesize that they eventually become cold
black dwarfs. However, this remains a hypothesis because so far no signs of the
existence of black dwarfs have been observed, and scientists have estimated that the
time required for a white dwarf to become a black dwarf is longer than the generally
accepted age of the universe (approximately 13.7 billion years).
Biblio graphy
Chaisson, Eric. Epic of Evolution : Seven Ages of the Cosmos. New York, N.Y. ;
Chichester:
Columbia University Press, 2006.
Christian, David. Maps of Time: An Introduction to Big History. Berkeley:
California University Press, 2005.
Ferris, Timothy. The Whole Shebang: A State-of-the-Universe(S) Report. New
York Simon
and Shuster, 1997.
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