Ðåôåðàò: Big Bang theory
Ðåôåðàò: Big Bang theory
Big
Bang Theory
(essay)
A cosmological model to explain the origins of matter,
energy, space, and time, the Big Bang theory asserts that the universe began at
a certain point in the distant past—current estimates put it at roughly 13.7 billion
years ago—expanding from a primordial state of tremendous heat and density. The
term is also used more generally to describe the vast explosion that erupted at
the beginning of space and time, bringing the universe into being. First conceived
by astronomers and physicists in the early twentieth century, the Big Bang was effectively
confirmed in the middle and latter years of the century, once new telescopes and
computers made it possible to peer further into the universe and process the enormous
amounts of data those observations generated. The term “big bang” comes from its
underlying hypothesis, that the universe has not been eternal but emerged out of
a sudden, almost incomprehensibly vast explosion.
Scientists’ understanding of the Big Bang theory emerges
out of two separate fields of inquiry: theoretical physics and observational astronomy.
According to what are called the Friedmann models, a set of complex metrics named
for Alexander Friedmann, an early twentieth century Soviet physicist who first
developed them, the Big Bang theory fits in with two of the most important theories
of twentieth century physics: the cosmological principle (which says that basic
physical properties are the same throughout the universe) and Albert Einstein’s
General Theory of Relativity of 1915-1916, which conceives of gravity as a curvature
in space and time. That convergence of ideas, say physicists, provides the theoretical
underpinning of the Big Bang theory.
Astronomers have made their own confirmations of the
Big Bang theory. Analyzing the light coming from other galaxies, they have noted
shorter and longer wavelengths proportional to the distances of the galaxies from
Earth, indicating that they are moving away from the Earth and thus that space itself
is expanding. The existence of cosmic microwave radiation, a remnant of hot ionized
plasma of the early universe offers more proof of the Big Bang, as does the distribution
of heavier and lighter elements through the universe.
Timeline of the Big Bang
The Big Bang theory hypothesizes that there were time-based
stages in the origins of the universe. The first stage—or, at least, the first
stage that cosmologists can theorize about given current understanding of physics—is
known as the Planck era, after the German scientist of the late nineteenth and early
twentieth centuries who studied the physics that explain it. The Planck era was
extremely brief—just 10-43 seconds (also known as one Planck time). During
this period, all four forces of the universe—gravity, electromagnetic energy, and
the weak and strong nuclear forces—were theoretically equal to one another, implying
that there may have been just one unified force. The Planck era was extremely
unstable, with the four forces quickly evolving into their current forms, starting
with gravity and then the strong nuclear force (what binds protons and neutrons
together in the nucleus of an atom), the weak nuclear force (associated with radioactive
decay, it is some 100 times weaker than the strong force), and finally electromagnetic
energy. This process is known as symmetry breaking and led to a longer period in
the universe’s history--though, at one millionth of a second, still extremely
brief in ordinary time--known as the “inflation era.” Physicists, however, are
not certain of the energy force that led to this inflation. At one second in age,
the universe now consisted of fundamental energy and sub-atomic particles such as
quarks, electrons, photons, and other less familiar particles.
The next stage in the Big Bang—lasting for roughly
100,000 years and beginning about three seconds after the Planck era—consisted of
the process of nucleosynthesis, as protons and neutrons came into being and began
to the form the nuclei of various elements, predominantly hydrogen and helium,
the two lightest elements in the periodic table and the two most common elements
in the universe. Yet matter as we know it still did not exist and for those
hundred thousand or so years, the universe essentially consisted of radiation in
the form of light, radio waves, and X-rays. This period, known as the “radiation
era,” came to a gradual end as free floating atomic nuclei bonded with free-floating
electrons to produce the matter with which the universe would subsequently consist.
While time was critical to the process so was temperature and density, with the
various changes corresponding to a gradual cooling of the universe and the gradual
dispersing of matter.
It took some 200 million years for gravity to begin
coalescing these free-floating atoms into the primordial gas out of which the first
stars and galaxies would emerge. Over billions of years, such early stars and galaxies
phased through their lifecycle, using up their nuclear fuel and collapsing in on
themselves, spewing out vast new clouds of matter and energy that would eventually
form new generations of stars and galaxies. The sun around which the earth and
the solar system rotate is one of these later generation stars, formed roughly
five billion years ago.
Fate
of the Universe
The Big Bang theory concerns not just the origins of
the universe but its ultimate fate. The critical question, of course, is whether
the universe will continue expanding forever or eventually fall back into itself,
creating, perhaps, the conditions for the next Big Bang. Gravity is the critical
factor here, with three outcomes possible. The first, and most widely accepted
by physicists, is that there is not the critical density, known as omega and estimated
at roughly six hydrogen atoms per cubic meter, necessary to pull the universe back
in on itself. In this model, referred to as the “open” model, the universe will
continue to expand and cool indefinitely. If however, the density of he universe
is greater than omega then the universe will eventually, after billions of years,
collapse in what physicists call the “big crunch.” A third and highly unlikely
possibility is that omega equals precisely one; in this model, the universe gradually
slows and cools to a static state.
While it would seem at first glance that the fate of
the universe—that is, whether matter exceeded omega or not--could be determined
by the admittedly complex but not impossible task of calculating the amount of
matter and dividing it by the dimensions of the universe, in fact, there is a complicating
factor. The galaxies and nebulae, or primordial dust clouds out of which stars and
galaxies, do not pull on themselves or on each another as they should. That is
to say, they behave as if there was more mass and, hence, gravitational pull than
can be observed. For example, the Andromeda galaxy, the nearest neighbor to our
own Milky Way galaxy, is rushing toward us at 200,000 miles per hour, a speed
that cannot be explained by the gravitational force of the matter in the two galaxies.
In fact, the two galaxies are coming together at a pace requiring some 10 times
that amount of matter. Physicists offer the possibility that there is dark matter
in the universe, that is, an unknown type of matter that does not emit or reflect
enough electromagnetic energy to be observable by current means. Such dark matter,
according to this hypothesis, exists in haloes around galaxies and may be what
composes black holes and massive clouds of neutrinos, particles formed from radioactive
decay with little mass and no electric charge. Such dark matter would imply a
universe that eventually collapses in on itself, except for an additional complicating
factor.
Scientists hypothesize that there is also a dark energy
in the universe counteracting both matter and dark matter, a kind of anti-gravitational
force that is also undetectable with existing technology. While dark matter is
believed to constitute 22 percent of the universe, dark energy is believe to compose
74 percent. These numbers, along with the difficulties of detecting dark matter
and energy make it impossible for physicists as of the early twenty-first century
to come to a definitive conclusion about the ultimate fate of the universe.
Pre-Twentieth
Century Ideas of Universe’s Origins
The origins of creation have, of course, preoccupied
humanity since at least the beginning of civilization itself. Virtually every
culture around the world has created myths to explain how the universe came into
being, even if they did not necessarily comprehend the universe’s magnitude and
complexity. These cosmologies, or explanations for the existence of creation, generally
share four basic ideas. First, there is an intelligence or creator behind creation.
Second, the universe came into being at a specific point in time and that what existed
before the universe came into being is irrelevant as there was no existence or
time before it. A major exception to this model of a universe created at a single
moment in time comes from Hindu cosmology which states that the universe exists
in cycles, of roughly 4.5 billion years, or one day in the life of the Brahma,
the creator, endlessly being born, dying, and being reborn. The third component
of most ancient cosmologies was that the Earth stood at the center of creation.
And the final element was that, once the universe was
created, it remained essentially static--nothing added, nothing taken away, all
matter and energy in perpetual balance. That, too, was the model advanced by English
scientist Isaac Newton in the late seventeenth and early eighteenth centuries,
whose understanding of the laws of the universe dominated physics for more than
200 years. But even in Newton’s own time, the idea of a perpetually balanced creation
was questioned by some thinkers, who pointed out that the universe would come apart
if just one object should slip out of balance. And while Newton’s laws attempted
to explain how the universe operated, they did not offer much insight into its origins.
Immanuel Kant, a German philosopher of the late eighteenth
century, was the first major Western thinker to tackle the question that the Big
Bang theory would eventually answer—had the universe always existed or did it come
into existence at a specific point in time? Kant concluded that since both arguments
were equally valid on the face of things and that it was impossible to determine
which was fundamentally true, the question of the universe’s origins, or lack
thereof, was beyond human comprehension. Even as nineteenth century astronomers
began to push back the envelope of what was known about the universe’s scale,
they did not have the means or, given their religious faith, the inclination to
grapple with Kant’s question.
Early Hypotheses
Early twentieth century physicists and astronomers, of
course, would prove Kant wrong. In 1912, an American astronomer named Vesto Slipher
noted a Doppler shift in the wavelengths of light coming from spiral nebulae, an
antiquated term for galaxies, dating from before the existence of other galaxies
was confirmed. (It was American astronomer Edwin Hubble who first concluded in
the mid-1920s that the nebulae were, in fact, galaxies similar to our own Milky
Way.) The Doppler shift, named after Christian Doppler, the early nineteenth century
Austrian mathematician who discovered it, says that waves alter in relation to
the movement of the observer or the object causing the wave. While Slipher noted
that almost all such spiral nebulae were moving away from the Earth, he failed
to reach the conclusion that this meant the universe was expanding.
Around the same time, Slipher was making his observations,
Friedmann, the Soviet physicist, explained how Einstein’s General Relativity Theory
might prove that the universe was expanding. Einstein’s theory updated and revised
Newton’s gravitational laws, for conditions where enormous mass and energy existed.
Newton concluded that gravity was a force between two masses; Einstein argued,
correctly as it was proved by later experiments, that gravity was the warping of
space and time caused by mass. While Newton’s model of gravity was not consistent
with the Big Bang theory—since there was no mass in the primordial state of heat
and density at the beginning of time—Einstein’s allowed for the possibility of
gravity itself coming into being, though, ironically, Einstein himself held to a
static view of the universe when he came up with his General Relativity Theory.
Roughly a decade after Friedmann developed his models
out of Einstein’s General Relativity Theory—models that, while published, generally
got overlooked by other physicists--a Belgian physicist and astronomer Georges
Lemaître, independently coming up with the same theories as Friedmann, used
them to reach the conclusion that had eluded Slipher—that receding nebulae meant
the universe was expanding. In 1931, Lemaître also hypothesized that the
universe must have begun with a single atom, an idea that came to be called the
“cosmic egg” theory. American astronomer Edwin Hubble, the first to realize that
nebulae were in fact other galaxies, also confirmed that the galaxies all seemed
to be moving away from us simultaneously. Extrapolating backward, Hubble believed
that they all had emerged from the same high-density place, exploding outward in
a kind of initial fireball. Hubble made his findings by noting shifts in the light
spectrum of distant galaxies that fit in with the Doppler effect.
Despite such findings, a competing theory emerged in
the years after World War II,. The “steady state” model, advocated by British astronomer
Frederick Hoyle, held that new matter was created as the universe expanded. A confirmed
atheist, Hoyle rejected the “cosmic egg” theory as it seemed to imply the existence
of a creator. Ironically, it was Hoyle who, in the 1950s, coined the term “Big
Bang,” using it in a radio interview to ridicule Lemaître’s
ideas. To reconcile his constant universe idea and the observed fact that galaxies
were moving away from each other, Hoyle hypothesized that new galaxies came into
being as older ones grew apart. While later discounted, Hoyle’s work was useful
in explaining how matter and energy came into existence, a key component of the
Big Bang theory.
Confirmation
of the Big Bang Theory
For two decades the two theories vied with each other,
though Lemaître’s steadily gained more advocates. The critical confirmation
of the Big Bang theory came in 1964. That year, Arno Penzias and Robert Wilson,
two scientists working for Bell Laboratories, noticed that background microwave
radiation, a residual form of energy from the Big Bang, permeated the universe,
confirming an idea first propounded by Soviet physicist George Gamow and American
physicist Ralph Alpher in the late 1940s.
With the development of ever more powerful computers
to crunch the numbers in the 1980s, and the deployment of the Hubble Space telescope
in the 1990s, which allowed for observations above the distortions of the Earth’s
atmosphere and radio waves, astronomers were able to make ever more detailed pictures
of the universe and ever more precise timelines for the Big Bang. Key to this was
a worldwide study in the 1980s and 1990s of supernovas, immense outpourings of
radiation caused by the collapse of massive stars, which pointed to yet another
anomaly about the universe. Rather than expanding at a constant rate, it seemed
to be accelerating. This led to the conclusion that there must be a dark energy
in the universe working to counteract gravity. One recent hypothesis states that
space actually consists of negative pressure, which grows as the universe expands
thereby causing that expansion to accelerate since there is not enough matter—even
with dark matter factored into the equation--to put a brake on the expansion. According
to British scientist Robert Caldwell, this accelerating expansion may lead to
what he calls the “big rip,” in which galaxies, stars, and even atoms are eventually
torn apart by the force of dark energy, leading to the destruction of matter in
the final seconds of time at the end of the universe. Much of this work on dark
matter and energy remains hypothetical, of course, as it has been impossible to
detect either of these two phenomena.
As the twenty-first century dawns, scientists—like
the ancients long before them--are still grappling with the very moment of creation,
before the radiation, inflation, and Planck eras. Many believe that unveiling
that moment is connected to the development of a Grand Unified Theory, a single
explanation that fits all of the known laws of the universe—including Einstein’s
General Relativity Theory and quantum mechanics, the study of energy and matter
at the sub-atomic level—into a single equation. As British physicist Stephen Hawking
notes, "At the Big Bang, the universe and time itself
came into existence, so that this is the first cause. If we could understand the
Big Bang, we would know why the universe is the way it is. It used to be thought
that it was impossible to apply the laws of science to the beginning of the universe,
and indeed that it was sacrilegious to try. But recent developments in unifying
the two pillars of twentieth-century science, Einstein's General Theory of Relativity
and the Quantum Theory, have encouraged us to believe that it may be possible to
find laws that hold even at the creation of the universe."
References
1.
Farrell, John. The Day without Yesterday:
Lemaître, Einstein, and the Birth of Modern Cosmology. New York: Thunder’s
Mouth Press, 2005.
2.
Fox, Karen C. The Big Bank Theory: What It
Is, Where It Came from, and Why It Works. New York: Wiley, 2002.
3.
Hawking, Stephen. A Brief History of Time:
From the Big Bang to Black Holes. New York: Bantam, 1988.
4.
Levin, Frank. Calibrating the Cosmos: How
Cosmology Explains Our Big Bang Universe. New York: Springer, 2007.
5.
Singh, Simon. Big Bang: The Origin of the
Universe. New York: Fourth Estate, 2004.