Although the general outline of the classical Big Bang cosmology has served well to provide an understanding of both the present nature of the universe and a large part of its past history (after a time of about 30 seconds), there are several matters that this theory currently cannot explain. One of these issues is the communication problem. The large‐scale uniformity of the properties of the universe requires that every region of the observable universe must once have been able to share information with every other region, a possibility ruled out by the finite speed of light and the nature of expansion in a Big Bang universe.
The existence of galaxies is actually also a problem. In the Big Bang theory, density fluctuations in the early universe that left their mark on the temperature fluctuations (1 part in 10 5) of the cosmic background radiation grew into the galaxies of today. But why did these density fluctuations actually exist at the time of decoupling? For the average density at that time, the statistical laws of variability, that is, random chance, require an exceedingly uniform universe, much smoother than observed! Some physical effect stemming from the even earlier universe must be responsible for beginning the rearrangement of matter from an earlier homogeneous density state to the weakly nonuniform state at the time of decoupling.
The very existence of normal matter represents a third problem. In the physics of the present day universe, there is a symmetry in the relationship between matter and energy (in the form of electromagnetic radiation). Nature, on the one hand, can create matter (and antimatter) in the reaction
and destroy both forms of matter through the reaction
The two sides of each equation represent different aspects of what is essentially identical, and both reactions can be summarized in a single expression where the double‐ended arrow indicates that the reaction is permitted to go in both directions:
The reaction can go back and forth any number of times, and after an even number of reactions (no matter how large), the physical situation is exactly where it started: Nothing has been changed, lost, or gained. Thus there should be no excess of one type of matter over the other, unless during an early epoch in the history of the universe the physics of the electromagnetic radiation‐matter interaction was different. If the physical rules were different, then
leaving behind in the present universe about one nuclear particle for every 10 9 photons.
Related to this is the question of the dark matter, or the invisible matter whose existence is postulated by astrophysicists to account for the large amount of observed gravitation that cannot be accounted for by visible matter. The dynamics of normal galaxies suggest that perhaps only 10 percent or less of the gravitating matter in the universe is observable with visible light or some other form of electromagnetic radiation that can be detected on Earth and from which the state of the material that emitted the radiation can be deduced. As every form of known matter, regardless of its temperature of other physical conditions, emits some form of this radiation, this matter must exist in some form not described by the physics of today's universe.
To all the other aspects of the universe scientists wish to understand would be the question of why there exist four distinct forces of nature. Gravity is the weakest of the four forces. Electromagnetism is some 10 40 times stronger. The other two forces act at the nuclear level. The weak nuclear force is involved in electron reactions (such as 1H + 1H → 2H + e + + ν), and the strong nuclear force holds protons and neutrons together in the atomic nuclei.
A final problem is that the Big Bang cosmology alone is not able to address why the geometry of the universe is so close to being flat. The Big Bang cosmology allows for a variety of geometries, but makes no specification as to what the geometry should be. Observation suggests the geometry is very close to being flat, but this is a difficult result to understand. If the initial universe began ever so slightly different from being flat, then over its evolution until today the curvature should have become enhanced. In other words, some unknown cause very early in the history of the universe appears to have forced a flat geometry.
Grand Unified Theories (GUTs)
The apparent resolution to understanding the origin of these six additional aspects of the universe has come not from refinement of cosmological theory, but from theory aimed at understanding the interrelation between the four forces of nature and their further relation to the existence of the many types of particles that physicists have produced in high‐energy particle accelerators (over 300 so‐called elementary particles are now known). Each force appears to have an association with a particle that transmits that force: The electromagnetic force is carried by the photon, the weak force by the Z particle, the strong force through gluons. No one knows if gravity has an associated particle or not, but quantum theory predicts that the graviton does indeed exist.
Einstein tried (and failed) to unify gravity and electromagnetism. Modern theorists have succeeded in a theoretical unification of the electromagnetic force and weak force (theory of the electroweak force). In turn, various theoretical schemes ( Grand Unified Theories or GUTs) to unite the electroweak force and strong force (into a superforce) are being investigated at the present time. Ultimately, the theoretical goal is to unite gravity and a Grand Unified Theory into a single theoretical formulism, a theory of everything, in which there would be a single unified force (for example, Quantum Gravity or Supergravity). Each stage of unification, however, occurs at successively higher energies and therein lies the cosmological connection — the early universe was a high temperature, high energy density situation at which time existed vast quantities of the exotic particles associated with each of these unifications.
From these theoretical developments, an outline of the very earliest history of the universe may be deduced. The universe began with a single (unified) force in existence, but the physics of this era before a time of 10 −43 seconds will be known only when the final unification of gravity into the theory has been achieved. Before 10 −43 seconds, the so‐called Planck time, is an unknown era for which existing gravitational theory (general relativity) and Grand Unified Theories are in conflict. After this time, however, the expanding universe evolved monotonically to lower temperatures. As temperatures and energies dropped, the several forces became distinguishable in their behavior:
This is a symmetry breaking in the sense that in the present universe, the opposite reactions, a recombination of these forces into, a single force, won't occur.
The Inflationary Universe. A major aspect of applying Grand Unified Theories to the early history is the recognition that the universe did not always expand at a rate that can be determined from observations of the present day universe. At an epoch of 10 −35 seconds after the initial infinite density, it is theorized that there occurred a surge in the expansion, an inflation by perhaps 10 30 times. In an instant, everything within the present‐day observable universe (a diameter of about 9 billion parsecs or 30 billion light‐years) went from approximately the size of a proton to the size of a grapefruit. Why? Because in the GUTs, the description of what we think of as space requires additional factors than things like familiar length, density, and so forth; more importantly as the universe evolved, these factors changed with the accompanying release of immense energy. In the jargon of physicists, one talks about there being a “structure” to the vacuum (this use of the word is very different from the normal usage of meaning “completely empty space”). As the universe expanded and the temperature dropped, the vacuum underwent a phase change from one state of existence to another. This change is analogous to the phase transition of water from gaseous steam to liquid. Liquid water is a lower‐energy phase, and the energy released by water condensing from steam to liquid can produce work in a steam engine. In a similar manner, as the vacuum went from a high‐energy to a low‐energy phase, the energy released drove a momentary inflation in the size of the universe, followed by the much slower rate of expansion that continues today. This phase transition was responsible for the separation of the strong force from the electroweak force; in the higher‐energy, preinflation state, these two forces were linked into a single force. In the lower‐energy, postinflation state, the two forces are no longer identical and could be distinguished from each other.
There is a further significant consequence of the inflation that is important in understanding the present universe. Nearby regions that were in communication with each other before the inflationary expansion (the communication distance is the speed of light times the age of the universe), and that therefore had the same physical properties of energy density, temperature, and so on, ended up at a later time, after the rapid expansion, much further apart than estimated on the basis of using only the present expansion rate. Because these regions evolved over time, the laws of physics starting with their original similar conditions produced the present day similar conditions. This explains why regions now widely separated in opposite directions in our sky have the same properties even though these regions are no longer in communication (distance apart now being greater than the speed of light times the present age of the universe).
A second and more consequential result is present: The GUTs do allow a symmetry breaking in the interaction between matter and photons, allowing an excess of normal matter (proton, neutrons, and electrons—the material that makes up matter as we know it) to be present after the universe cooled to its present state. However, this is only part of the existence of gravitating material in the universe. GUTs force a major inflation in the universe. No matter how curved the early universe was, this inflation in size forces the universe to have a flat geometry. (By analogy, a basketball has a surface that is obviously curved, but if suddenly increased in size by 10 30 times, making it about 1,000 times larger than the present visible universe, then any local area of the surface would appear very flat). A flat geometry means that the true density of the universe must be equal to the critical density that divides universes between those that will expand forever and those that will collapse back into themselves. Dynamical studies of galaxies and clusters of galaxies have been suggesting that 90 percent of the gravitating material of the universe is not visible, but all their matter, visible plus dark, if spread uniformly over the volume of the universe, yields only ∼10 percent of the critical density. GUTs demand a density equal to the critical density, thus it is not 90 percent of the mass of the universe that is invisible, but 99 percent! (See Figure .)
Evolution of the universe including the inflationary era.
Dark Matter. GUTs predict on the one hand far more dark matter in the universe than implied by studies of galaxies. But on the other hand, GUTs also predict the existence of many particles other than the material (protons, neutrons, electrons, photons) that make up the visible universe. An abundance of possibilities exist for the dark matter, depending upon which version of Grand Unified Theory you consider. Sophisticated physical experiments are being designed and put into operation to attempt to test for the existence of these possibilities, both to eliminate incorrect versions of GUTs as well as to identify the true nature of the dark matter. Some dark matter possibilities are WIMPs ( Weakly Interacting Massive Particles), axions (lightweight particle types that again interact poorly with everything else), strings (features in the structure of space that are analogous to the boundaries between different crystals in a solid material), magnetic monopoles (in essence, incredibly tiny pieces of the early universe, with the conditions of temperature, energy, and the physical laws of the preinflation universe preserved behind a shell of exotic particles), and shadow matter (a second form of matter that has evolved independently of normal matter, whose presence is detectable only through its gravity). Which, if any, of these ideas are correct will be determined only through significant research effort.
One additional factor may influence cosmological evolution. The mathematical equations describing the evolution of the universe allow for a cosmological constant, a factor originally introduced by Einstein. This factor would act as a repulsive force working against gravity. The evolution of the universe at any era would thus depend on which factor is stronger. It also is interpreted as an energy density of the vacuum, which would exist even if there were no matter and no electromagnetic radiation in the universe, hence another contributor to the dark matter. Most theory considers the cosmological constant to be zero, but its true value is yet to be determined. Ironically, Einstein introduced the cosmological constant erroneously; because he thought the universe was static and constant in size, he used the cosmological constant as a force to oppose gravity. Without it, he predicted the universe would collapse. However, a few years later it was discovered that the universe was expanding, and he realized the constant wasn't needed. He called it the biggest blunder of his life! The findings using Type I supernovae that the universe may be accelerating its expansion has reawakened interest in the cosmological constant. Future research and further observations will help shed light on this old problem.