The Understanding of the History of Our Universe by Cosmologists Evolves
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Introduction
For most of the 20th century, cosmologists held a fairly simple picture of the evolution of the Universe: It began with the Big Bang in which the Universe was very hot and extremely dense. From that inception, the Universe expanded, meaning that space stretched, thereby causing the temperature to drop and matter/energy to spread out. Eventually, the structures we observe today (everything from atoms to galaxy clusters) emerged. The theoretical foundation for the development of our Universe, known as the Friedmann-Robertson-Walker cosmological model or the Standard Model of cosmology, underwent an important modification in the late 20th century and, due to recent precise astronomical measurements, has experienced dramatic changes. Jupiter Scientific is pleased to report these developments to its readers.
Background The Theoretical Foundation. Modern cosmology is based on Einstein's fundamental theory of gravity: the theory of general relativity. It supersedes Newton's mechanical picture of gravitation with a fundamental, esthetically pleasing idea: The curvature of space-time causes the force of gravity.
In general relativity, space is dynamic, meaning that it can bend, twist and stretch. A massive body such as the Earth causes a deformation of space-time. When a second object, such as an apple, is introduced, it moves in a natural way along geodesics in the curved space-time. Because the motion of the object in a curved space-time is not necessarily along a line nor necessarily uniform, the object (the apple) appears to accelerate. It changes its direction or its speed or both. The response of space-time to a massive body (such as the Earth) and the natural motion of an object (such as an apple) in the curved space-time of Einstein almost exactly reproduce Newton's version of gravity. Differences between the two theories are quite small but increase if the curvature of space is great, as is the case near a pulsar or black hole, or if one or both of the gravitationally interacting bodies is moving at speeds approaching that of light.
The earlier 20th-century simple picture
of our world as a de-accelerating, expanding Universe
driven by the mass of ordinary matter
has been supplanted by a new cosmology.
When general relativity is applied to the Universe as a whole, it turns out that space should be either stretching or contracting. In 1929, the astronomer Edwin Hubble observed that distant galaxies are all moving away from the Earth. He concluded that the Universe is expanding. As space stretches, galaxies are pulled away from one another and the farther two galaxies are apart the faster they move away from each other. The standard analogy is that of a balloon with dots on it:
Figure: An Expanding Balloon
As one blows up the balloon, the dots move apart.
The dots are the galaxies; the balloon is the Universe.
Since Hubble's initial findings, many observations have confirmed that the Universe is expanding. The fractional speed of stretching is known as the Hubble expansion rate.
The analogy with a balloon is defective in one respect. An outside influence (a human being with lungs) must blow air into it to make it inflate. For the Standard Model of cosmology, there are no external influences. So, what causes the expansion of the Universe? The answer is the matter and radiation within it. In the Friedmann-Robertson-Walker model, this matter/radiation is taken to be uniformly distributed. Observations of our Universe indicate that this is a good approximation at sufficiently large distances. Locally, the Universe is lumpy, stars and galaxies being the lumps, but such lumps form a rather even distribution if viewed from sufficiently far away. Furthermore, radiation left over from the early Universe, known as the cosmic microwave background radiation, is smooth
to one part in 100,000!
The Geometry of Space. In the Friedmann-Robertson-Walker model, the
geometry of space depends of whether the mass/energy density is greater than, equal to, or less than a critical value.
Let the capital Greek letter Omega W [If you see W instead of Omega then your computer does not have the Symbol Font and W will appear below instead of Omega.] be the ratio of the mass/energy density of the Universe to this critical value. Then one has
W > 1 <=> closed Universe (a 3-d sphere)
W = 1 <=> flat Universe (regular flat space)
W < 1 <=> open Universe (a 3-d hyperbolic space)
Independent of the geometry of space, it turns out that, if the mass/energy density in the Universe is due to matter and radiation, which is what one expects, then the rate of expansion of the Universe must be slowing. In other words, the Universe should have been expanding faster in the past and should be expanding more slowly in the future. This is called de-acceleration.
Although astronomical observations in the 20th century were unable to determine whether the Universe was open, flat or closed, they were able to show that space was fairly flat. In other words, W was not extremely small or very large. Despite the lack of a good measurement of W, this crude observation had important theoretical implications. The goal of observational astronomy became that of determining the fundamental parameters of the Friedmann-Robertson-Walker model such as the Hubble expansion rate, the mass/energy density of the Universe and the de-acceleration parameter. When these were known, one would know the geometry of space and the general history of the Universe.
Predictions and Confirmations. The Standard Model of cosmology, when combined with atomic and nuclear physics, makes two interesting predictions about the early history of the cosmos that were confirmed by observations and led to the strong belief that the above picture of our expanding Universe was valid. The first of these was the existence of the cosmic microwave background radiation. This
radiation was created when atoms formed at around 300,000 years after the Big Bang, an event known as recombination. This radiation started out as visible light but has evolved to
microwaves due to the expansion of the Universe.
Dark matter is a great mystery.
The Standard Model of cosmology, when combined with nuclear physics, makes a second important prediction. The very lightest nuclei, those of hydrogen, helium, lithium and beryllium and their isotopes, which compose the first four elements of the periodic table, should have been generated when the Universe was one to three minutes old. This
process is known as Big Bang nucleosynthesis. Big Bang nucleosynthesis predicts
that the contribution to W from protons and neutrons is about 5%, a result
that has been confirmed by astronomical observations.
Some Theoretical Difficulties
Given the successful agreement of predictions with measurements, few astrophysicists doubted the correctness of the Big Bang and of the Friedmann-Robertson-Walker cosmology. However, several problems existed of a highly theoretical nature. For the most part, these issues went unnoticed, were ignored or were assumed to be solved by some yet-to-be-understood means.
The flatness problem arises because W must have been highly fine-tuned in the past to give an approximately flat Universe today,
the smoothness problem deals with trying to understand how the Universe can be so smooth if different parts of it were never in contact or in communication,
the inhomogeneity problem is how the appropriate density fluctuations are generated at various scales to produce the astronomical structures that are observed,
and
the magnetic monopole problem, which only arises in grand unified gauge theories, is why magnetic monopoles have never
been detected.
Inflation
Alan Guth proposed a mechanism known as inflation to resolve the above four problems of the Standard Model of cosmology. He proposed that, when the Universe was less than a billionth of a trillionth of a trillionth of a second old, that it underwent a tremendous expansion in which space was stretched by an enormous factor greater than 1050.
It is not too hard to understand how inflation resolves the four problems of the standard model of cosmology.
Because no other mechanism resolves the problems of the Friedmann-Robertson-Walker cosmology so well, cosmologists have been willing to believe in inflation even before any experimental evidence of it existed. The late 20th-century view of cosmology modifies the Standard Model to include a period of rapidly expansion at very early times. Inflation makes a number of predictions that can be tested including that W must be quite close to one today; that is, the visible Universe
must be almost flat.
Dark Matter
At first, it appears that there is a problem with the W=1 prediction of inflation. Neutrons and protons, which make up virtually all the weight in known matter (since electrons are relatively light), generate a value of W of about 0.05 (see above). If W equals one then most of the matter in the Universe must be of some exotic form. Indeed, even before the inflation theory was invented, astronomers had detected some mysterious material that accumulates in and around galaxies. Because this material does not interact with light, it cannot be seen in telescopes and for this reason, astronomers name it dark matter. Astronomers deduce its existence through its gravitational effects: The speeds of the stars in the outer parts of a galaxy are greater than expected. In other words, the force of gravity on these stars is much more than the force generated from the stars and gas in the interior regions of a galaxy. Alternatively, one can say that, without dark matter and its gravitational pull, the stars in a galaxy would fly off and the galaxy would disintegrate. About 80% of the mass in galaxy is in dark matter. In short, dark matter holds a galaxy together.
The existence of dark matter also effects the large-scale structure of the Universe. Cosmologists are able to perform computer simulations to see its influence on the development of the Universe and its contents.
Dark matter is a great mystery. Astronomers and theorists do not know what it is. Neutrinos contribute to dark matter, but recent measurements indicate that neutrinos can only be a small component.
Originally, it was thought that the presence of dark matter would provide a value of W of one. Although the dark matter in a galaxy was not sufficiently dense to generate such a flat Universe, dark matter was deduced, again through its gravitational effects, in galaxy clusters at greater densities. Although the measurements were crude, most cosmologists assumed that the presence of dark matter would yield W=1, at least up until the mid 1990's.
During the late 1990's, more precise observations suggested that the dark matter contribution to W could be no bigger than 60%. It seemed that the case for inflation was in jeopardy. However, at around the same time, a measurement of supernovae produced a result that would radically change the way cosmologists view the evolution of our Universe.
New Observational Developments
In 1995 and 1996, measurements of type Ia supernovae began that would suggest an astonishing result. Supernovae are stars that explode. They can do so in various ways and the label "type Ia" indicates a certain kind of supernova. Astronomers
can see these very bright objects are faraway distances and use them to measure how fast space is stretching.
By 1998, a sufficient number of type Ia supernovae had been observed to conclude that the expansion of the Universe was accelerating. This came as a complete surprise since cosmologists thought that the expansion rate should be slowing. See above. Science Magazine called the result "The Breakthrough of the Year".
It's a strange, strange world indeed!
The expansion of the Universe can be made to accelerate by introducing a cosmological constant, denoted by the Greek letter Lambda L by physicists, into Einstein's general theory of relativity. This
term modifies the Standard Model of cosmology by introducing a uniform background mass/energy density that has the unusual property of
providing a negative pressure that compels the Universe to expand increasingly fast. The
type Ia supernova results supported a non-zero value for L.
The cosmological constant contributes to W. The type Ia supernova data was consistent with a flat Universe. In others words, the contributions from ordinary matter (protons and neutrons), from dark matter, and from the cosmological constant to W were adding up to something close to one. Inflation was no longer in jeopardy at least as far as its prediction for W was concerned.
However, it turns out
that the value of L needed is theoretically unnaturally small. Because of this, many scientists thought that there was some systematic error in the supernova data; for example, perhaps unseen intergalactic dust was making the supernova appear dimmer. However, during the past few years (1999-2002), exciting, new, detailed measurements of the cosmic microwave background radiation also reveal that the Universe is accelerating. See below. It appears that L is non-zero and that the cosmological
problem exists. But hold on!
Quintessence
Theorists recently developed a new way of driving the acceleration of the Universe called quintessence that may solve the problem. The idea uses a scalar particle field. A particle field is the quantum source of an elementary particle; basically, the vibrational modes of the field produce one or more (possibly moving) elementary particles of a particular type. For example, the quantum electromagnetic field is the source of all photons. The quintessence scalar field produces both a cosmological constant and a mass/energy density, something that is sometimes called "dark energy." The dynamics is adjusted so that the contribution to the cosmological constant changes with time. It is then possible to arrange things so that L is close to its large, natural value in the early Universe but diminishes as the Universe evolves to a value that is small and acceptable today. The idea of quintessence is speculative but is the only reasonable way theorists have found to explain the conclusions drawn from the Type Ia supernova data.
Detailed Cosmic Microwave Background Radiation Measurements
In 1999 and 2000, instruments in balloons were sent to the upper reaches of Earth's atmosphere to measure more precisely the cosmic microwave background radiation. In particular, astronomers observed a pronounced bump in temperature fluctuations at what corresponds to an angular separation of about one degree. The fundamental mode of sound waves in the original atom/photon gas at recombination is believed to be the source of this bump. (Overtones, or higher modes, should produce additional bumps at smaller angular separations and they do. See below.) Since sound waves are propagating density variations (moving rarified and concentrated regions), astronomers were seeing in these measurements the density fluctuations that eventual led to galaxy clusters and giant voids. The location, size and shape of bumps change according to various cosmological parameters such as the contributions to W from protons/neutrons, from dark matter and from the cosmological constant. By 2001, two additional peaks were measured, and the data was sufficiently accurate to conclude that L was non-zero. This independent confirmation of the type Ia supernova results left little doubt that besides dark matter there is another mysterious dark energy pervading throughout the Universe whose effect is to create a negative pressure that causes the stretching of space to accelerate.
For most of the 20th century, cosmologists held
a fairly simple picture of the evolution of the Universe.
Last month (May 2002), radio telescopes on top of high mountains measured the temperature fluctuations at smaller angular separations using interference among multiple elements. The first three bumps were confirmed, a fourth bump was seen and the fluctuations dropped off in agreement with theory for angles down to about one-tenth of a degree. The data from many experiments are summarized in the following figure
by Max Tegmark that originally was at the website http://www.hep.upenn.edu/~max/cmb/experiments.html:
Figure: Temperature Fluctuations Versus Multipole l
The temperature fluctuations, which are related to density perturbations,
are in units of micro-kelvins (10-6 degrees K). The multipole l can be
thought of as the inverse angular separation with angle equal to (180 degrees)/l.
Summary
When all the recent astronomical observations are taken into account, the parameters that govern the cosmology of our Universe are now determined with unprecedented precision. The age of the Universe is about 13.5 billion years with an uncertainty of less than 500 million years! The contributions to W from the cosmological constant, from dark matter and from proton/neutron are respectively about 0.7, 0.25 and 0.045. The Hubble expansion rate is 0.69 kilometers per second per megaparsec (to an accuracy of 0.02). Results are consistent with inflation: W=1 to within 10%, and the spectrum of density fluctuations is the predicted one within experimental errors.
The earlier 20th-century simple picture of our world as a de-accelerating, expanding Universe driven by the mass of ordinary matter has been supplanted by a new cosmology: It is now believed, and there is some evidence to support this, that inflation took place at less than a tiny fraction of a second during which space underwent an enormous expansion. During the past 13 billion years, space has been expanding but at increasing rates, that is, the Universe is accelerating. Most of this expansion is driven not by the mass in protons and neutron but by a cosmological constant and by mysterious, unknown dark matter. It's a strange, strange world indeed!
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