BIG BANG: THE BEGINNING OF THE UNIVERSE
Introduction
Since humans appeared, the world around is always a mystery.
Thanks to explore the world around that people know things and phenomena and
their laws. The human understanding about the world around is called knowledge.
From knowledge of many are aggregated into knowledge of humanity.
Because human knowledge is limited by the perception of the
world should not know what the very remote, vast and abstrate sothat human
beings have supernatural like the God, Budda and the divines…created the world
and the universe. Thus, humans have always believed that supernatural powers
available plan for all things in the nature and people only to comply with and
not allowed to know.
Therefore, the theocracy exited millions of years and humanity
under the lull may not know about the world. Today many people still believe in
the supernatural power as in the religions. People think that there is the the supernatural
creator that created all natural things including human races, the world and
the universe as they knew.
The scientific understanding and to create tools for
scientific research that humanities have made discoveries about the world
around, including of space and the universe based on the scientific basis.
Thanks to scientific knowledge that humanities has know
about the world around them, about the structure of land warrants, the space
around Earth, the Solar system and to galaxy…
To learn about space, the universe… reqires a vast amount of
knowledge of humanities. First is the theory posed by exploring the results of
the scientists, and then proven true test of that theory. The conclusions from
proven theories would add to knowledge base of humanities. Understanding the
origin of the universe is also in this rule.
To understand the origin of the universe is a great discover
of humanities. With scientific basis we need to understand the world around us
and the universe that we inhabit. First we learn about the Big Bang theory, an
event began to shape the universe based on science, not based on blind trust
from the spirit and religious rights, including the explanation prevailing
religions of the world.
The history of the Big Bang theory began with the Big Bang's
development from observations and theoretical considerations. Much of the
theoretical work in cosmology now involves extensions and refinements to the
basic Big Bang model.
History of discovering Big Bang theory
In 1910s
Observationally, in the 1910s, Vesto Slipher and
later, Carl Wilhelm Wirtz, determined
that most spiral nebulae were receding from Earth. Slipher used spectroscopy to investigate the rotation periods of
planets, the composition of planetary atmospheres, and was the first to observe
the radial velocities of galaxies. Wirtz observed a systematic Redshift of
nebulae, which was difficult to interpret in terms of a cosmology in which the
Universe is filled more or less uniformly with stars and nebulae. They weren't
aware of the cosmological implications, nor that the supposed nebulae were
actually galaxies outside our own Milky Way.
Also in that decade, Albert Einstein's
theory of general relativity was
found to admit no static cosmological solutions,
given the basic assumptions of cosmology described in the Big
Bang's theoretical underpinnings. The universe (i.e., the space-time
metric) was described by a metric tensor that was either expanding or shrinking
(i.e., was not constant or invariant). This result, coming from an evaluation
of the field equations of the general theory, at first led Einstein himself to
consider that his formulation of the field equations of the general theory may
be in error, and he tried to correct it by adding a cosmological constant. This constant would restore to the
general theory's description of space-time an invariant metric tensor for the
fabric of space/existence. The first person to seriously apply general
relativity to cosmology without the stabilizing cosmological constant was Alexander Friedmann.
In 1920s
In 1922 Friedmann
derived the expanding-universe solution to general relativity field equations.
In 1924 Friedmann's
papers included "Über die Möglichkeit einer Welt mit konstanter
negativer Krümmung des Raumes" (About the possibility of a world with
constant negative curvature) which was published by the Berlin Academy of
Sciences on 7/1/ 1924. Friedmann's equations
describe the Friedmann-Lemaître-Robertson-Walker universe.
In 1927 Georges Lemaître (1894-1966) was a Belgian priest, astronomer and professor of physics at
the Catholic University of
Leuven. He was the first person to propose the theory of the expansion of the Universe.
The
famous work of Lemaître, G. (1927) was "Un univers homogène de masse
constante et de rayon croissant rendant compte de la vitesse radiale des
nébuleuses extragalactiques". Annals of the Scientific
Society of Brussels 47A:
41. (French).
(Translated in: "A
Homogeneous Universe of Constant Mass and Growing Radius Accounting for the
Radial Velocity of Extragalactic Nebulae").
Georges
Lemaître proposed an
expanding model for the universe to explain the observed redshifts of spiral
nebulae, and forecast the Hubble law. He
based his theory on the work of Einstein and De Sitter, and
independently derived Friedmann's equations for an expanding universe. Also,
the red shifts themselves were not constant, but varied in such manner as to
lead to the conclusion that there was a definite relationship between amount of
red-shift of nebulae, and their distance from observers.
This discovery was the first
observational support for the Big Bang theory.
In 1929, Edwin Hubble (1889-1953) provided a comprehensive
observational foundation for Lemaître's theory. Hubble's experimental
observations discovered that, relative to the Earth and all other observed
bodies, galaxies are receding in every direction at velocities (calculated from
their observed red-shifts) directly proportional to their distance from the
Earth and each other.
Hubble and Milton Humason formulated the empirical Redshift
Distance Law of galaxies, nowadays known as Hubble's law,
which, once the Redshifts is interpreted as a measure of recession speed, is
consistent with the solutions of Einstein’s General Relativity Equations for a
homogeneous, isotropic expanding space. The isotropic nature of the expansion
was direct proof that it was the space (the fabric of existence) itself that
was expanding, not the bodies in space that were simply moving further outward
and apart into an infinitely larger preexisting empty void. It was this
interpretation that led to the concept of the expanding universe. The law
states that the greater the distance between any two galaxies, the greater
their relative speed of separation. This discovery later resulted in the
formulation of the Big Bang model.
Hubble discovered that the distances to far
away galaxies were
generally proportional to
their Redshifts - an idea
originally suggested by Lemaître in 1927. Hubble's observation was taken to
indicate that all very distant galaxies and clusters have an apparent velocity
directly away from our vantage point: the farther away, the higher the apparent
velocity.
Hubble formulated the empirical
Redshift Distance Law of galaxies, nowadays termed simply Hubble's law,
which, if the redshift is interpreted as a measure of recession speed, is
consistent with the solutions of Einstein’s equations of general relativity for a homogeneous, isotropic
expanding space.
So the discovery by Edwin Hubble that the Universe is in
fact expanding at enormous speed was revolutionary. Hubble noted that galaxies
outside our own Milky Way were all moving away from us, each at a speed
proportional to its distance from us. He quickly realized what this meant that
there must have been an instant in time (now known to be about 14 billion years
ago) when the entire Universe was contained in a single point in space. The
Universe must have been born in this single violent event which came to be
known as the "Big Bang."
In 1930s
In 1931
Lemaître went further and suggested that the evident expansion of the universe,
if projected back in time, meant that the further in the past the smaller the
universe was, until at some finite time in the past all the mass of the
Universe was concentrated into a single point, a "primeval atom"
where and when the fabric of time and space came into existence.
Lemaître proposed in his "hypothèse
de l'atome primitif" (hypothesis of the primeval atom) that the
universe began with the "explosion" of the "primeval atom" - what was later called the Big
Bang. Lemaître first took cosmic rays to be the remnants of the event,
although it is now known that they originate within the local galaxy. Lemaître had to wait until shortly
before his death to learn of the discovery of cosmic microwave background radiation, the
remnant radiation of a dense and hot phase in the early Universe.
In the 1930s Hubble was involved in
determining the distribution of galaxies and spatial curvature. These data seemed to indicate that the
universe was flat and
homogeneous, but there was a deviation from flatness at large redshifts.
According to Allan Sandage.
During the 1930s other ideas were
proposed as non-standard cosmologies to
explain Hubble's observations, including the Milne model, the oscillatory Universe (originally
suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman) and
Fritz Zwicky's tired light hypothesis.
In 1940s
After World War II, two
distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the
Universe seemed to expand. In this model the Universe is roughly the same at
any point in time. The other was Lemaître's Big Bang theory, advocated and
developed by George Gamow, who
introduced big bang nucleosynthesis (BBN)
and whose associates, Ralph Alpher and Robert Herman,
predicted the cosmic microwave background radiation (CMB). Ironically, it was Hoyle who
coined the phrase that came to be applied to Lemaître's theory, referring to it
as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between
these two theories. Eventually, the observational evidence, most notably from
radio source counts,
began to favor Big Bang over Steady State.
In 1949 Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast (on 28/3/1949,
on the BBC Third Programme).
In 1950s
Hoyle repeated the term in further
broadcasts in early 1950, as part of a series of five lectures entitled The Nature of The Universe. The
text of each lecture was published in The Listener a week after the broadcast, the first
time that the term "big bang" appeared in print.
As evidence in favour of the Big Bang
model mounted, and the consensus became widespread, Hoyle himself, albeit
somewhat reluctantly, admitted to it by formulating a new cosmological model
that other scientists later referred to as the "Steady Bang".
For a number of years, the support for
these theories was evenly divided, with a slight imbalance arising from the
fact that the Big Bang theory could explain both the formation and the observed
abundances of hydrogen and helium, whereas the Steady State could
explain how they were formed, but not why they should have the observed
abundances. However, the observational evidence began to support the idea that
the universe evolved from a hot dense state. Young objects such as quasars were only observed at the very edges
of the universe, indicating that such objects only existed in times long past,
whereas the Steady State predicted that young galaxies should be scattered all
over the universe, both near and far.
Before the late 1960s, many
cosmologists thought the infinitely dense and physically paradoxicalsingularity at
the starting time of Friedmann's cosmological model could be avoided by
allowing for a universe which was contracting before entering the hot dense
state, and starting to expand again. This was formalized as Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea
was unworkable, and the singularity is an essential feature of the physics
described by Einstein's gravity. This led the majority of cosmologists to
accept the notion that the universe as currently described by the physics of
general relativity has a finite age. However, due to a lack of a theory of quantum gravity,
there is no way to say whether the singularity is an actual origin point for
the universe, or whether the physical processes that govern the regime cause
the universe to be effectively eternal in character.
In 1960s
In 1964 After
the discovery of the cosmic microwave background radiation, and
especially when its spectrum (i.e., the amount of radiation measured at each
wavelength) was found to match that of thermal radiation from a black body, most
scientists had become fairly convinced that some version of the Big Bang
scenario must have occurred.
The discovery and confirmation of the
cosmic microwave background radiation in 196 4 secured the Big Bang as the best
theory of the origin and evolution of the cosmos. Much of the current work in
cosmology includes understanding how galaxies form in the context of the Big
Bang, understanding the physics of the Universe at earlier and earlier times,
and reconciling observations with the basic theory.
In 1965 In
addition, the discovery of the cosmic microwave background radiation in 1965
was considered the death knell of the Steady State, although this prediction
was only qualitative, and failed to predict the actual temperature of the CMB.
After some reformulation, the Big Bang has been regarded as the best theory of
the origin and evolution of the cosmos.
In 1990s
Significant progress in Big Bang
cosmology have been made since the late 1990s as a result of advances in telescope technology as well as the analysis of
data from satellites such as COBE, the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise
and accurate measurements of many of the parameters of the Big Bang model, and
have made the unexpected discovery that the expansion of the Universe appears
to be accelerating.
Huge advances in Big Bang cosmology
were made as a result of major advances in telescope technology in combination with large
amounts of satellite data, such as that from COBE and
the Hubble Space Telescope.
In 1989 NASA launched
the Cosmic Background Explorer satellite (COBE). Its findings were consistent
with predictions regarding the CMB, finding a residual temperature of
2.726 K (more recent measurements have revised this figure down slightly
to 2.725 K) and providing the first evidence for fluctuations
(anisotropies) in the CMB, at a level of about one part in 105. John C. Mather and George Smoot were awarded the Nobel Prize for their
leadership in this work. During the following decade, CMB anisotropies were
further investigated by a large number of ground-based and balloon experiments.
In 2000s
In 2000–2001 several experiments, most notably BOOMERanG, found the shape of the Universe to
be spatially almost flat by measuring the typical angular size (the size on the
sky) of the anisotropies.
In 2003, NASA's WMAP took
more detailed pictures of the universe by means of the cosmic microwave
background radiation. The images can be interpreted to indicate that the
universe is 13.7 billion years old (within 1% error) and that the Lambda-CDM model and the inflationary theory are
correct. No other cosmological theory can yet explain such a wide range of
observed parameters, from the ratio of the elemental abundances in the early
Universe to the structure of the cosmic microwave background, the observed
higher abundance of active galactic nuclei in the early Universe and the observed
masses of clusters of galaxies.
The Universe's
"baby picture".
Big Bang
Theory
The Premise
The Big
Bang theory is the prevailing cosmological model that explains the early development of
the Universe.
Singularities are zones which defy our current understanding
of physics. They are thought to exist at the core of "black holes."
Black holes are areas of intense gravitational pressure. The pressure is
thought to be so intense that finite matter is actually squished into infinite
density (a mathematical concept which truly boggles the mind). These zones of
infinite density are called "singularities." Our universe is thought
to have begun as an infinitesimally small, infinitely hot, infinitely dense,
something - a singularity.
After its initial appearance, it apparently inflated (the
"Big Bang"), expanded and cooled, going from very, very small and
very, very hot, to the size and temperature of our current universe. It
continues to expand and cool to this day and we are inside of it: incredible
creatures living on a unique planet, circling a beautiful star clustered
together with several hundred billion other stars in a galaxy soaring through
the cosmos, all of which is inside of an expanding universe that began as an
infinitesimal singularity which appeared out of nowhere for reasons unknown.
This is the Big Bang theory.
According to the Big Bang theory, the
Universe was once in an extremely hot and dense state which expanded rapidly.
This rapid expansion caused the Universe to cool and resulted in its present
continuously expanding state. According to the most recent measurements and
observations, the Big Bang occurred approximately 13.75 billion years ago,
which is thus considered the age of the Universe.
After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy
to be converted into
various subatomic particles, including protons, neutrons, and electrons. While
protons and neutrons combined to form
the first atomic nuclei only a few minutes after the Big Bang, it would take
thousands of years for electrons to combine with them and
create electrically neutral atoms. The first element produced was hydrogen, along
with traces of helium and
lithium. Giant
clouds of these primordial elements would coalesce through gravity to form stars and galaxies, and the heavier elements would be synthesized either within stars or during supernovae.
The Big Bang is a well-tested scientific theory which is widely accepted within the
scientific community because it is the most accurate and comprehensive
explanation for the full range of phenomena astronomers observe. Since its
conception, abundant evidence has arisen to further validate the model.
The framework for the Big Bang model relies on Albert Einstein's general relativity and
on simplifying assumptions such as homogeneity and isotropy of space. The governing equations had
been formulated by Alexander Friedmann.
According to the standard theory, our universe sprang into
existence as "singularity" around 13.7 billion years ago.
According to the Big Bang model, the Universe expanded from an extremely dense and
hot state and continues to expand today. A common analogy explains that space itself
is expanding, carrying galaxies with
it, like spots on an inflating balloon. The graphic scheme above is an artist's
concept illustrating the expansion of a portion of a flat universe.
Extrapolation of the expansion of the Universe
backwards in time using general relativity yields
an infinite density and temperature at a finite time in the past. This singularity signals
the breakdown of general relativity. How closely we can extrapolate towards the
singularity is debated-certainly no closer than the end of the Planck epoch. This
singularity is sometimes called "the Big Bang",but the term can also
refer to the early hot, dense phase itself, which can be considered the
"birth" of our Universe. Based on measurements of the expansion using Type Ia supernovae,
measurements of temperature fluctuations in the cosmic microwave background, and
measurements of the correlation function of
galaxies, the Universe has a calculated age of 13.75 ± 0.11 billion years,
The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents
of the Universe.
The earliest phases of the Big Bang are
subject to much speculation. In the most common models the Universe was filled homogeneously and isotropically with an incredibly high energy density and huge temperatures and pressures and was very rapidly expanding and
cooling. Approximately 10−37 seconds
into the expansion, a phase transition caused a cosmic inflation,
during which the Universe grew exponentially. After inflation stopped, the Universe consisted
of a quark-gluon plasma, as well as all other elementary particles. Temperatures were so high that the
random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously
created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number,
leading to a very small excess of quarks and leptons over
antiquarks and antileptons - of the order of one part in 30 million. This
resulted in the predominance of matter over antimatter in the present Universe.
The Universe continued to grow in size
and fall in temperature, hence the typical energy of each particle was
decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into
their present form. After about 10−11 seconds, the picture becomes less
speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to
form baryons such
as protons and neutrons. The small excess of quarks over antiquarks led to a
small excess of baryons over antibaryons. The temperature was now no longer
high enough to create new proton–antiproton pairs (similarly for
neutrons–antineutrons), so a mass annihilation immediately followed, leaving
just one in 1010 of
the original protons and neutrons, and none of their antiparticles. A similar
process happened at about 1 second for electrons and positrons. After these
annihilations, the remaining protons, neutrons and electrons were no longer
moving relativistically and the energy density of the Universe was dominated by photons (with
a minor contribution from neutrinos).
A few minutes into the expansion, when
the temperature was about a billion (one thousand million; 109; SI
prefix giga-) kelvin and
the density was about that of air, neutrons combined with protons to form the
Universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei
combined into atoms (mostly hydrogen); hence
the radiation decoupled from matter and continued through space largely
unimpeded. This relic radiation is known as the cosmic microwave background radiation.
Over a long period of time, the
slightly denser regions of the nearly uniformly distributed matter
gravitationally attracted nearby matter and thus grew even denser, forming gas
clouds, stars, galaxies, and the other
astronomical structures observable today. The details of this process depend on
the amount and type of matter in the Universe. The four possible types of
matter are known as cold dark matter, warm dark matter, hot dark matterand baryonic matter.
The best measurements available (from WMAP) show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be
cold (warm
dark matter is ruled
out by early reionization), and is
estimated to make up about 23% of the matter/energy of the universe, while
baryonic matter makes up about 4.6%. In an "extended model" which
includes hot dark matter in the form of neutrinos, then if
the "physical baryon density" Ωbh2 is estimated at about 0.023 (this is
different from the 'baryon density' Ωb expressed as a fraction of the total
matter/energy density, which as noted above is about 0.046), and the
corresponding cold dark matter density Ωch2 is about 0.11, the corresponding
neutrino density Ωvh2 is
estimated to be less than 0.0062.
Independent lines of evidence from Type Ia supernovae and the CMB imply
that the Universe today is dominated by a mysterious form of energy known as dark energy, which
apparently permeates all of space. The observations suggest 73% of the total
energy density of today's Universe is in this form. When the Universe was very
young, it was likely infused with dark energy, but with less space and
everything closer together, gravity had the upper hand, and it was slowly
braking the expansion. But eventually, after numerous billion years of
expansion, the growing abundance of dark energy caused the expansion of the Universe to slowly begin to accelerate. Dark
energy in its simplest formulation takes the form of the cosmological constant term
in Einstein's field equations of
general relativity, but its composition and mechanism are unknown and, more
generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be
investigated both observationally and theoretically.
All of this cosmic evolution after the inflationary epoch can
be rigorously described and modeled by the ΛCDM model of cosmology, which uses the
independent frameworks of quantum mechanics and Einstein's General Relativity.
As noted above, there is no well-supported model describing the action prior to
10−15 seconds or so.
Apparently a new unified theory of quantum gravitation is needed to break this barrier.
Understanding this earliest of eras in the history of the Universe is currently
one of the greatest unsolved problems in physics.
Evidence for the Theory
The major evidences which support the Big Bang theory are:
1-First of all, we are reasonably certain that the universe
had a beginning.
2-Second, galaxies appear to be moving away from us at
speeds proportional to their distance. This is called "Hubble's Law,"
named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929.
This observation supports the expansion of the universe and suggests that the
universe was once compacted.
3-Third, if the universe was initially very, very hot as the
Big Bang suggests, we should be able to find some remnant of this heat. In
1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree
Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave
Background radiation (CMB) which pervades the observable universe. This is
thought to be the remnant which scientists were looking for. Penzias and Wilson
shared in the 1978 Nobel Prize for Physics for their discovery.
4-Finally, the abundance of the "light elements"
Hydrogen and Helium found in the observable universe are thought to support the
Big Bang model of origins.
Big Bang under NASA’s Vision
WMAP's map of the
temperature of the microwave background radiation
shows tiny variations
(of few microdegrees) in The 3K background.
Hot spots show as red,
cold spots as dark blue.
The night sky presents the viewer with a picture of a calm
and unchanging Universe.
Astronomers combine mathematical models with observations to
develop workable theories of how the Universe came to be. The mathematical
underpinnings of the Big Bang theory include Albert Einstein's general theory
of relativity along with standard theories of fundamental particles. Today NASA
spacecraft such as the Hubble Space Telescope and the Spitzer Space Telescope
continue Edwin Hubble's work of measuring the expansion of the Universe. One of
the goals has long been to decide whether the Universe will expand forever, or
whether it will someday stop, turn around, and collapse in a "Big
Crunch?"
Background Radiation
According to the theories of physics, if we were to look at
the Universe one second after the Big Bang, what we would see is a 10-billion
degree sea of neutrons, protons, electrons, anti-electrons (positrons),
photons, and neutrinos. Then, as time went on, we would see the Universe cool,
the neutrons either decaying into protons and electrons or combining with
protons to make deuterium (an isotope of hydrogen). As it continued to cool, it
would eventually reach the temperature where electrons combined with nuclei to
form neutral atoms. Before this "recombination" occurred, the
Universe would have been opaque because the free electrons would have caused
light (photons) to scatter the way sunlight scatters from the water droplets in
clouds. But when the free electrons were absorbed to form neutral atoms, the
Universe suddenly became transparent. Those same photons - the afterglow of the
Big Bang known as cosmic background radiation - can be observed today.
Missions Study Cosmic
Background Radiation
NASA has launched two missions to study the cosmic
background radiation, taking "baby pictures" of the Universe only
400,000 years after it was born.
In 1992, the COBE team announced that they had mapped the
primordial hot and cold spots in cosmic background radiation. These spots are
related to the gravitational field in the early Universe and form the seeds of
the giant clusters of galaxies that stretch hundreds of millions of light years
across the Universe. This work earned NASA's Dr. John C. Mather and George F.
Smoot of the University
of California
The second mission to examine the cosmic background
radiation was the Wilkinson Microware Anisotropy
Probe (WMAP). With
greatly improved resolution compared to COBE, WMAP surveyed the entire sky,
measuring temperature differences of the microwave radiation that is nearly
uniformly distributed across the Universe. The picture shows a map of the sky,
with hot regions in red and cooler regions in blue. By combining this evidence
with theoretical models of the Universe, scientists have concluded that the Universe
is "flat," meaning
that, on cosmological scales, the geometry of space satisfies the rules of
Euclidean geometry (e.g., parallel lines never meet, the ratio of circle
circumference to diameter is pi, etc).
A third mission, Planck, is led by
the European Space Agency with significant participation from NASA. Launched in
2009, Planck is making the most accurate maps of the microwave background
radiation yet. With instruments sensitive to temperature variations of a few
millionths of a degree, and mapping the full sky over 9 wavelength bands, it
measures the fluctuations of the temperature of the CMB with an accuracy set by
fundamental astrophysical limits.
The Universe's "baby picture". WMAP's map of the
temperature of the microwave background radiation shows tiny variations (of few
microdegrees) in The 3K background. Hot spots show as red, cold spots as dark
blue.
Inflation
One problem that arose from the original COBE results, and
that persists with the higher-resolution WMAP data, was that the Universe was too homogeneous.
How could pieces of the Universe that had never been in contact with each other
have come to equilibrium at the very same temperature? This and other cosmological
problems could be solved, however, if there had been a very short period
immediately after the Big Bang where the Universe experienced an incredible
burst of expansion called "inflation." For this inflation to have
taken place, the Universe at the time of the Big Bang must have been filled
with an unstable form of energy whose nature is not yet known. Whatever its
nature, the inflationary model predicts that this primordial energy would have
been unevenly distributed in space due to a kind of quantum noise that arose
when the Universe was extremely small. This pattern would have been transferred
to the matter of the Universe and would show up in the photons that began
streaming away freely at the moment of recombination. As a result, we would expect
to see, and do see, this kind of pattern in the COBE and WMAP pictures of the
Universe.
But all this leaves unanswered the question of what powered
inflation. One difficulty in answering this question is that inflation was over
well before recombination, and so the opacity of the Universe before
recombination is, in effect, a curtain drawn over those interesting very early
events. Fortunately, there is a way to observe the Universe that does not
involve photons at all. Gravitational waves, the only known form of information
that can reach us undistorted from the instant of the Big Bang, can carry
information that we can get no other way. Two missions that are being
considered by NASA, LISA and
the Big Bang Observer, will look for the gravitational waves from the epoch of
inflation.
Dark Energy
During the years following Hubble and COBE, the picture of
the Big Bang gradually became clearer. But in 1996, observations of very
distant supernovae required a dramatic change in the picture. It had always
been assumed that the matter of the Universe would slow its rate of expansion.
Mass creates gravity, gravity creates pull, the pulling must slow the
expansion. But supernovae observations showed that the expansion of the
Universe, rather than slowing, is accelerating. Something, not like matter and
not like ordinary energy, is pushing the galaxies apart. This "stuff"
has been dubbed dark energy, but to give it a name
is not to understand it. Whether dark energy is a type of dynamical fluid,
heretofore unknown to physics, or whether it is a property of the vacuum of
empty space, or whether it is some modification to general relativity is not
yet known.
The future of theory
In the past and current time
In the past, there was much discussion
as to whether the Big Bang would need to be completely abandoned as a
description of the universe, but such proponents of non-standard cosmology have become fewer in number over the
last few decades.
Much of the current work in cosmology
includes understanding how galaxies form in the context of the Big Bang,
understanding what happened at the Big Bang, and reconciling observations with
the basic theory. Cosmologists continue to calculate many of the parameters of
the Big Bang to a new level of precision. Observations suggest that the
expansion of the universe appears to be accelerating, a fact that is not yet
explained, and may call for modifications of the underlying theory.
The future according to the Big Bang theory
Before observations of dark energy,
cosmologists considered two scenarios for the future of the Universe. If the
mass density of the Universe were greater than the critical density,
then the Universe would reach a maximum size and then begin to collapse. It
would become denser and hotter again, ending with a state similar to that in
which it started -a Big Crunch. Alternatively,
if the density in the Universe were equal to or below the critical density, the
expansion would slow down but never stop. Star formation would cease with the
consumption of interstellar gas in each galaxy; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very
gradually, collisions between these would result in mass accumulating into
larger and larger black holes. The average temperature of the Universe would
asymptotically approach absolute zero - a Big Freeze.
Moreover, if the proton were unstable, then
baryonic matter would disappear, leaving only radiation and black holes.
Eventually, black holes would evaporate by emitting Hawking radiation.
The entropy of the Universe would increase to the
point where no organized form of energy could be extracted from it, a scenario
known as heat death.
Modern observations of accelerating expansion imply
that more and more of the currently visible Universe will pass beyond our event horizon and out of contact with us. The
eventual result is not known. The ΛCDM model of the Universe contains dark energy in the form of a cosmological constant. This theory suggests that only
gravitationally bound systems, such as galaxies, will remain together, and they
too will be subject to heat death as the Universe expands and cools. Other
explanations of dark energy, calledphantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei,
and matter itself will be torn apart by the ever-increasing expansion in a
so-called Big Rip.
The one of galaxies in the Universe
Underlying
assumptions
The Big Bang theory depends on two
major assumptions: the universality of physical laws, and
the cosmological principle. The cosmological principle states that
on large scales the Universe is homogeneous and isotropic.
These ideas were initially taken as
postulates, but today there are efforts to test each of them. For example, the
first assumption has been tested by observations showing that largest possible
deviation of the fine structure constant over
much of the age of the universe is of order 10−5. Also, general relativity has
passed stringent tests on
the scale of the Solar System and binary stars while extrapolation to
cosmological scales has been validated by the empirical successes of various
aspects of the Big Bang theory.
If the large-scale Universe appears
isotropic as viewed from Earth, the cosmological principle can be derived from
the simpler Copernican principle, which states that there is no preferred
(or special) observer or vantage point. To this end, the cosmological principle
has been confirmed to a level of 10−5 via observations of the CMB. The
Universe has been measured to be homogeneous on the largest scales at the 10%
level.
The Big Bang is not an explosion of
matter moving outward to fill an empty universe. Instead, space itself expands with
time everywhere and increases the physical distance between two comoving
points. Because the FLRW metric assumes a uniform distribution of mass and
energy, it applies to our Universe only on large scales—local concentrations of
matter such as our galaxy are gravitationally bound and as such do not
experience the large-scale expansion of space.
References
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