The theory which changed what we think about the Universe
I | INTRODUCTION |
One way to understand the concept of an expanding
universe is to draw dots, representing galaxies, on a balloon. As the balloon is
inflated, each dot moves away from all the others. To a person viewing the
universe from a galaxy, all other galaxies would seem to be receding. The
distant galaxies appear to be moving away faster than the near ones, which
demonstrates Hubble’s law. Most astronomers now believe that this expansion will
continue forever.
Big Bang
Theory, currently accepted explanation of the beginning of the universe.
The big bang theory proposes that the universe was once extremely compact,
dense, and hot. Some original event, a cosmic explosion called the big bang,
occurred about 13.7 billion years ago, and the universe has since been expanding
and cooling.
The theory is based on the mathematical
equations, known as the field equations, of the general theory of
relativity set forth in 1915 by Albert Einstein. In 1922 Russian physicist
Alexander Friedmann provided a set of solutions to the field equations. These
solutions have served as the framework for much of the current theoretical work
on the big bang theory. American astronomer Edwin Hubble provided some of the
greatest supporting evidence for the theory with his 1929 discovery that the
light of distant galaxies was universally shifted toward the red end of the
spectrum (see Redshift). Once “tired light” theories—that light slowly
loses energy naturally, becoming more red over time—were dismissed, this shift
proved that the galaxies were moving away from each other. Hubble found that
galaxies farther away were moving away proportionally faster, showing that the
universe is expanding uniformly. However, the universe’s initial state was still
unknown.
In the 1940s Russian-American physicist George
Gamow worked out a theory that fit with Friedmann’s solutions in which the
universe expanded from a hot, dense state. In 1950 British astronomer Fred
Hoyle, in support of his own opposing steady-state theory, referred to Gamow’s
theory as a mere “big bang,” but the name stuck. Indeed, a contest in the 1990s
by Sky & Telescope magazine to find a better (perhaps more dignified)
name did not produce one.
II |
HISTORY |
The overall framework of the big bang theory
came out of solutions to Einstein’s general relativity field equations and
remains unchanged, but various details of the theory are still being modified
today. Einstein himself initially believed that the universe was static. When
his equations seemed to imply that the universe was expanding or contracting,
Einstein added a constant term to cancel out the expansion or contraction of the
universe. When the expansion of the universe was later discovered, Einstein
stated that introducing this “cosmological constant” had been a mistake.
After Einstein’s work of 1917, several
scientists, including the abbé Georges Lemaître in Belgium, Willem de Sitter in
Holland, and Alexander Friedmann in Russia, succeeded in finding solutions to
Einstein’s field equations. The universes described by the different solutions
varied. De Sitter’s model had no matter in it. This model is actually not a bad
approximation since the average density of the universe is extremely low.
Lemaître’s universe expanded from a “primeval atom.” Friedmann’s universe also
expanded from a very dense clump of matter, but did not involve the cosmological
constant. These models explained how the universe behaved shortly after its
creation, but there was still no satisfactory explanation for the beginning of
the universe.
In the 1940s George Gamow was joined by his
students Ralph Alpher and Robert Herman in working out details of Friedmann’s
solutions to Einstein’s theory. They expanded on Gamow’s idea that the universe
expanded from a primordial state of matter called ylem consisting of
protons, neutrons, and electrons in a sea of radiation. They theorized the
universe was very hot at the time of the big bang (the point at which the
universe explosively expanded from its primordial state), since elements heavier
than hydrogen can be formed only at a high temperature. Alpher and Hermann
predicted that radiation from the big bang should still exist. Cosmic background
radiation roughly corresponding to the temperature predicted by Gamow’s team was
detected in the 1960s, further supporting the big bang theory, though the work
of Alpher, Herman, and Gamow had been forgotten.
III |
THE THEORY |
The big bang theory seeks to explain what
happened at or soon after the beginning of the universe. Scientists can now
model the universe back to 10-43 seconds after the big bang. For the
time before that moment, the classical theory of gravity is no longer adequate.
Scientists are searching for a theory that merges gravity (as explained by
Einstein's general theory of relativity) and quantum mechanics but have not
found one yet. Many scientists have hope that string theory, also known
as M-theory, will tie together gravity and quantum mechanics and help scientists
explore further back in time (see Physics: Unified Field Theory).
Because scientists cannot look back in time
beyond that early epoch, the actual big bang is hidden from them. There is no
way at present to detect the origin of the universe. Further, the big bang
theory does not explain what existed before the big bang. It may be that time
itself began at the big bang, so that it makes no sense to discuss what happened
“before” the big bang.
According to the big bang theory, the
universe expanded rapidly in its first microseconds. A single force existed at
the beginning of the universe, and as the universe expanded and cooled, this
force separated into those we know today: gravity, electromagnetism, the strong
nuclear force, and the weak nuclear force. A theory called the electroweak
theory now provides a unified explanation of electromagnetism and the weak
nuclear force theory (see Unified Field Theory). Physicists are now
searching for a grand unification theory to also incorporate the strong nuclear
force. String theory seeks to incorporate the force of gravity with the other
three forces, providing a theory of everything (TOE).
One widely accepted version of big bang
theory includes the idea of inflation. In this model, the universe
expanded much more rapidly at first, to about 1050 times its original
size in the first 10-32 second, then slowed its expansion. The theory
was advanced in the 1980s by American cosmologist Alan Guth and elaborated upon
by American astronomer Paul Steinhardt, Russian American scientist Andrei Linde,
and British astronomer Andreas Albrecht. The inflationary universe theory
(see Inflationary Theory) solves a number of problems of cosmology. For
example, it shows that the universe now appears close to the type of flat space
described by the laws of Euclid’s geometry: We see only a tiny region of the
original universe, similar to the way we do not notice the curvature of the
earth because we see only a small part of it. The inflationary universe also
shows why the universe appears so homogeneous. If the universe we observe was
inflated from some small, original region, it is not surprising that it appears
uniform.
Once the expansion of the initial
inflationary era ended, the universe continued to expand more slowly. The
inflationary model predicts that the universe is on the boundary between being
open and closed. If the universe is open, it will keep expanding forever. If the
universe is closed, the expansion of the universe will eventually stop and the
universe will begin contracting until it collapses. Whether the universe is open
or closed depends on the density, or concentration of mass, in the universe. If
the universe is dense enough, it is closed.
IV |
SUPPORTING EVIDENCE |
Most scientists believe that the microwave background
radiation is left over from the big bang at the beginning of the universe. The
Wilkinson Microwave Anisotropy Probe (WMAP) produced this image, which shows
variations in cosmic microwave radiation. The colored spots correspond to
fluctuations in the density of matter and energy in the early universe, about
380,000 years after the big bang. Cosmologists believe that as the universe
expanded and cooled, these fluctuations structured the formation of
galaxies.
NASA/WMAP Science
Team
The universe cooled as it expanded. After
about one second, protons formed. In the following few minutes—often referred to
as the “first three minutes”—combinations of protons and neutrons formed the
isotope of hydrogen known as deuterium as well as some of the other light
elements, principally helium, as well as some lithium, beryllium, and boron. The
study of the distribution of deuterium, helium, and the other light elements is
now a major field of research. The uniformity of the helium abundance around the
universe supports the big bang theory and the abundance of deuterium can be used
to estimate the density of matter in the universe.
From about 380,000 to about 1 million years
after the big bang, the universe cooled to about 3000°C (about 5000°F) and
protons and electrons combined to make hydrogen atoms. Hydrogen atoms can only
absorb and emit specific colors, or wavelengths, of light. The formation of
atoms allowed many other wavelengths of light, wavelengths that had been
interfering with the free electrons prior to the cooling of the universe, to
travel much farther than before. This change set free radiation that we can
detect today. After billions of years of cooling, this cosmic background
radiation is at about 3 K (-270°C/-454°F).The cosmic background radiation
was first detected and identified in 1965 by American astrophysicists Arno
Penzias and Robert Wilson.
The Cosmic Background Explorer (COBE)
spacecraft, a project of the National Aeronautics and Space Administration
(NASA), mapped the cosmic background radiation between 1989 and 1993. It
verified that the distribution of intensity of the background radiation
precisely matched that of matter that emits radiation because of its
temperature, as predicted for the big bang theory. It also showed that cosmic
background radiation is not uniform, that it varies slightly. These variations
are thought to be the seeds from which galaxies and other structures in the
universe grew.
Evidence indicates that the matter that
scientists detect in the universe is only a small fraction of all the matter
that exists. For example, observations of the speeds at which individual
galaxies move within clusters of galaxies show that a great deal of unseen
matter must exist to exert sufficient gravitational force to keep the clusters
from flying apart. Cosmologists now think that much of the universe is dark
matter—matter that has gravity but does not give off radiation that we can see
or otherwise detect. One kind of dark matter theorized by scientists is cold
dark matter, with slowly moving (cold) massive particles. No such particles
have yet been detected, though astronomers have made up fanciful names for them,
such as Weakly Interacting Massive Particles (WIMPs). Other cold dark matter
could be nonradiating stars or planets, which are known as MACHOs (Massive
Compact Halo Objects).
An alternative theory that explains the
dark-matter model involves hot dark matter, where hot implies that
the particles are moving very fast. Neutrinos, fundamental particles that travel
at nearly the speed of light, are the prime example of hot dark matter. However,
scientists think that the mass of a neutrino is so low that neutrinos can only
account for a small portion of dark matter. If the inflationary version of big
bang theory is correct, then the amount of dark matter and of whatever else
might exist is just enough to bring the universe to the boundary between open
and closed.
Scientists develop theoretical models to show
how the universe’s structures, such as clusters of galaxies, have formed. Their
models invoke hot dark matter, cold dark matter, or a mixture of the two. This
unseen matter would have provided the gravitational force needed to bring large
structures such as clusters of galaxies together. The theories that include dark
matter match the observations, although there is no consensus on the type or
types of dark matter that must be included. Supercomputers are important for
making such models.
V | REFINING THE THEORY |
Models of the Universe
According to the widely accepted theory of the big bang,
the universe originated about 14 billion years ago and has been expanding ever
since. Astronomers recognize four models of possible futures for the universe.
According to the closed model, many billions of years from now expansion will
slow, stop, and the universe will contract back in upon itself. In the flat
model, the universe will not collapse upon itself, but expansion will slow and
the universe will approach a stable size. According to the open model, the
universe will continue expanding forever. In the accelerating expansion model,
the universe will expand faster and faster until even the particles in normal
matter are torn away from each other. Astronomers currently favor the
accelerating expansion model.
Astronomers continue to make new observations
that are also interpreted within the framework of the big bang theory. No major
problems with the big bang theory have been found, but scientists constantly
adjust the theory to match the observed universe. In particular, a “standard
model” of the big bang has been established by results from NASA's Wilkinson
Microwave Anisotropy Probe (WMAP), launched in 2001 (see Cosmology). The
probe studied the anisotropies, or ripples, in the temperature of cosmic
background radiation at a higher resolution than COBE was capable of. These
ripples indicate that regions of the young universe were very slightly hotter or
cooler, by a factor of about 1/1000, than adjacent regions. WMAP’s observations
suggest that the rate of expansion of the universe, called Hubble’s constant, is
about 71 km/s/Mpc (kilometers per second per million parsecs, where a parsec is
about 3.26 light-years). In other words, the distance between any two objects in
space that are separated by a million parsecs increases by about 71 km every
second in addition to any other motion they may have relative to one another. In
combination with previously existing observations, this rate of expansion tells
cosmologists that the universe is “flat,” though flatness here does not refer to
the actual shape of the universe but rather that the geometric laws that apply
to the universe match those of a flat plane.
To be flat, the universe must contain a
certain amount of matter and energy, known as the critical density. The
distribution of sizes of ripples detected by WMAP show that ordinary matter—like
that making up objects and living things on Earth—accounts for only 4.4 percent
of the critical density. Dark matter makes up an additional 23 percent.
Astoundingly, the remaining 73 percent of the universe is composed of something
else—a substance so mysterious that nobody knows much about it. Called “dark
energy,” this substance provides the antigravity-like negative pressure that
causes the universe's expansion to accelerate rather than slow down. This
“accelerating universe” was detected independently by two competing groups of
astronomers in the last years of the 20th century. The ideas of an accelerating
universe and the existence of dark energy have caused astronomers to
substantially modify previous ideas of the big bang universe.
WMAP's results also show that cosmic
background radiation was set free about 389,000 years after the big bang, later
than was previously thought, and that the first stars formed about 200 million
years after the big bang, earlier than anticipated. Further refinements to the
big bang theory are expected from WMAP, which continues to collect data. An even
more precise mission to study the beginnings of the universe, the European Space
Agency’s Planck spacecraft, is scheduled to be launched in 2007.
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