3.5 Theories of the Universe

3.5.1 History

- Earth Centred Universe
Until Copernicus and Galileo Galilei (16th, 17th century AD) most scientists and philosophers believed that the earth was the centre of the universe, that it was not moving, but that the sun, the planets and the other known celestial bodies were revolving around it. This theory is known as "Geocentrism".

The Christian Church, in particular, made this part of its doctrine. They went so far to pretend that the Bible confirmed this view and anybody that said the opposite was accused of heresy and, more often that not, burned at the stake. Galileo escaped this punishment because he was well connected and aged. But he spent the last years of his life in house arrest. Formally the Roman Catholic Church, through the pope John Paul II, changed its view in the 20th century.

Until the 17th century the universe was thought to be limited to part of the Milky Way.

- Sun-Centred theory of the universe
In astronomy, heliocentrism is the theory that the sun is at the centre of the Solar System. Although some early cosmologists such as the Greek Aristarchus of Samos (circa 310-230 BC) speculated about the motion of the Earth around a stationary Sun, it was not until the 16th century that Copernicus presented a fully predictive mathematical model of a heliocentric system, which was later elaborated by Kepler and defended by Galileo. Their universe was also limited to the Milky Way although, with the invention of the telescope, astronomers were able to see deeper in it.

-The universe as we see it at the beginning of the 21t century
(The distinction between the Solar System and the Universe was not clear until modern times, but extremely important relative to the controversy over cosmology and religion.)

It is now generally accepted that our universe started through a big bang. The Big Bang Theory was first proposed by Alan Guth of MIT. It was improved by the "Inflationary Universe Theory". According to this latest theory, in the first trillion of a trillion of a second after the big bang, an unknown anti-gravity force caused the universe to expand very, very fast, faster than the speed of light.

This does not contradict the law that say that nothing can travel faster than the speed of light because it is the empty space that was expanding, not any matter. During this short time the universe expanded by a factor of 10E50. We still do not know what caused this inflation and what this anti-gravity force behind it is.

3.5.2 New theories

- Big Bang (BBT) is a cosmological model of the universe which has the primary assertion that the universe has expanded into its current state from an initial state of infinite density and temperature. The term is also used in a narrower sense to describe the rapid expansion of space-time that started at or close to an initial event in the history of our observed space-time. Theoretical support for the Big Bang comes from mathematical models, called Friedmann models. These models are framed in the context of general relativity and are based on the cosmological principle, which states that the properties of the universe is everywhere similar, and that there is no preferred orientation or in other words the universe is homogeneous and isotropic when viewed over sufficiently large spatial scales.

- Big Bounce: It is a scientific model related to the creation of the known Universe. It derives from the cyclic model or oscillatory universe interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe. According to one version of the Big Bang theory, in the beginning the universe had infinite density. Such a description seems to be at odds with everything else in physics, and especially quantum mechanics and its uncertainty principle. According to some oscillatory universe theorists, the Big Bang was merely the beginning of a period of expansion that followed a period of contraction. In this view, one could talk of a Big Crunch followed by a Big Bang, or more simply, a Big Bounce. As density approaches infinity, the behaviour of the quantum foam changes. All the so-called fundamental physical constants, including the speed of light in a vacuum, were not so constant during the Big Crunch, especially in the interval stretching 10?43 seconds before and after the point of inflection. (One unit of Planck time is about 10?43 seconds.)

- Cold dark matter (or CDM): It is a refinement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and hence is dark while at the same time the particles making up this matter are slow and hence are cold. As of 2006, most cosmologists favour the cold dark matter theory as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation), to the lumpy distribution of galaxies and their clusters we see. In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects.

- Cosmic inflation theory is the idea that the nascent universe passed through a phase of exponential expansion that was driven by a negative-pressure vacuum energy density. As a result the observable universe originated in a small causally-connected region. Inflation theory justifies the big bang cosmology and explains why does the universe appear flat, homogeneous and isotropic when one would expect, on the basis of the physics of the big bang, a highly curved, inhomogeneous universe? Inflation also explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the universe. Inflation was proposed by American physicist and cosmologist Alan Guth in 1981.

Alan Guth improved the Big Bang Theory when he observed that the universe went through hyperinflation immediately after the big bang. It explained many unsolved problems, one of them being the fact that the curvature of the universe is close to zero.

- Cosmic expansion of space (metric): It is a key part of science's current understanding of the universe, whereby space-time itself is described by a metric which changes over time in such a way that the spatial dimensions grow or stretch as the universe gets older. It explains how the universe expands in the Big Bang model, a feature of our universe supported by all cosmological experiments, astrophysics calculations, and measurements to date.

- Decoupling In physical cosmology, it is often used for the moment during recombination when the rate of Compton scattering became slower than the expansion of the universe, producing Cosmic Microwave Background as we know it.

- Deferent: In the Ptolemaic system, the planets are assumed to move in a small circle, called an epicycle, which in turn moves along a larger circle called a deferent.

- Epicycle: In the Ptolemaic system of astronomy, the epicycle was a geometric model to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. In particular it explained the retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from Earth. In the Ptolemaic system, the planets are assumed to move in a small circle, called an epicycle, which in turn moves along a larger circle called a deferent. Both circles rotate counter-clockwise and are roughly parallel to the Earth's plane of orbit (ecliptic). The orbits of planets in this system are epitrochoids.

- Epitrochoids: In the Ptolemaic system, the planets are assumed to move in a small circle, called an epicycle, which in turn moves along a larger circle called a deferent. Both circles rotate counter-clockwise and are roughly parallel to the Earth's plane of orbit. The orbits of planets in this system are epitrochoids.

- Flatness and horizon problems
The initial Big bang Theory could not explain why the universe was so flat. Until the 1920s it w assume hat the matter density in the universe (omega was about 0.1, the critical density being 1.0, This value of 0.1 billions of years after the big bang could not be explained. it should have been closer o zero by now. Guth Theory claimed that the universe was so flat today due to the hyperinflation following the big bang.

If we look at the sky at night in all directions, the sky seems to be uniform. Satellites have also shown that the cosmic microwave radiation is also uniform. If we look at the sky as it was 380,000 years after the big bang, this was the time when the background radiation was created and it had to be uniform. But the universe was 90 billion light years across then, and there is no way that the light -and information- could have travelled this distance in 380,000 years. To have uniform radiation, information would have had to travel faster than the speed of light, and this is impossible. As a result, the universe should have been appearing as being lumpy but this was not the case. This is called the "Horizon problem".

Guth suggested that our visible universe was probably a small part of the original fireball. This part was uniform in temperature and density and inflation expanded it by a factor o 10E50, faster that the speed of light so that our present universe is uniform.

Physicists accepted Guth Theory but the astronomers had doubts because it gave what they saw as the wrong prediction of omega, the density of the universe. They expected Omega to be equal to 1.0, corresponding to a flat universe. Taking into consideration the Dark matter, Omega rose to 0.3, but this was still not good enough. Guth believed that, in the future, new knowledge would show that Omega was equal to one. The discovery of Dark Energy showed that he was right. Omega is equal to 1.0 and the universe is flat.

- Giant impact hypothesis or big whack or big splash: It is the now-dominant scientific theory for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body.

- Grand Unification, grand unified theory, or GUT: It is one of several very similar unified field theories or models that predict that at extremely high energies (above 1014 GeV), the electromagnetic, weak nuclear, and strong nuclear forces are fused into a single unified field. Thus far, physicists have been able to merge electromagnetism and the weak nuclear force into the electroweak force, and work is being done to merge electroweak and quantum chromodynamics into a QCD-electroweak interaction sometimes called the electrostrong force. Beyond grand unification, there is also speculation that it may be possible to merge gravity with the other three gauge symmetries into a theory of everything.

- Horizon problem: It is a problem with the standard cosmological model of the Big. It points out that different regions of the universe have not "contacted" each other due to the great distances between them, but nevertheless they have the same temperature and other physical properties. This should not be possible, given that the exchange of information (or energy, heat, etc.) can only take place at the speed of light. The horizon problem may have been answered by inflationary theory, and is one of the reasons for that theory's formation.

- Hot dark matter: In this theory, popular in the early eighties, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way. The predictions of hot dark matter strongly disagree with observations of large-scale structure, whereas the cold dark matter paradigm is in general agreement with the observations.

- Many-worlds interpretation or MWI: It is an interpretation of quantum mechanics. Many-worlds deny the objective reality of wavefunction collapse. Many-worlds then explain the subjective appearance of wavefunction collapse with the mechanism of quantum decoherence. Consequently, many-worlds claims this resolves all the "paradoxes" of quantum theory since every possible outcome to every event defines or exists in its own "history" or "world". This means that there are an infinite number of universes and that everything that could possibly happen in our universe (but doesn't) does happen in another. MWI reconciles how we can perceive non-deterministic events (such as the random decay of a radioactive atom) with the deterministic equations of quantum physics.

- Multiverse
Some scientists believe that inflation happened again and again creating other new parallel universes. According to this theory, a small part of a universe can inflate producing a "Baby universe" which, in its turn, can also produce another universe. This process can go on for ever, each universe giving rise to one or more new universes. This, of course, means that big bangs happen all the time and that all the universes are like bubbles floating in a sea of bubbles.
Andrei Linde, a physicist, proposed a new version of the Inflation Theory. According to him, spontaneous breakings occur at random points in space and time. When breaking happens, a new universe is created. Very often the following inflation is minor but, from time to time, inflation lasts long enough so that a new universe is created. This means that big bangs happen all the time with new universes coming out of the old ones. From this one can say that our universe emerged from a previous one. In this chaotic inflation model, the multiverse is eternal but the individual universes are not. Some universes have an Omega larger than 1.0 and they soon vanish in big crunches after their big bangs. Those with an Omega smaller than 1.0 expand for ever.
The Multiverse Theory seems at first to violate the laws of conservation of matter and energy. However, the total amount of matter/energy required to make a new universe can be small. The matter in the universe (stars, planets, galaxies,...) may be large and positive but the energy stored in gravity can be negative. The result is that the total matter plus energy can be small and these universes can spring out of vacuum.
Even a universe like ours can be created from a very small amount of matter/energy, practically out of nothing.
Unfortunately we cannot devise experiments to test the Multiverse Theory in laboratories. To do it, to reach other universes, we would have to travel faster than the speed of light and this is not possible.
The Inflationary Theory is a Quantum Theory and as such it obeys the Heisenberg uncertainty principle. Applied to the fireball that produced the big bang, this means that the original cosmic explosion was not infinitely "Smooth". There were some ripples, or fluctuations, in the fireball. Taking into consideration that these small quantum fluctuations inflated, scientists can calculate the minimum number of ripples one should see in the microwave background radiation left 380,000 years after the big bang. Expanding these ripples to today, one should find the current distribution of the galaxy cluster.
The data collected by the CORE satellite show a blurry set of ripples, variations in temperature of 1 part in 100,000, and these results are consistent with the Inflationary Theory.
Calculations made in 1998 of the expansion rate of the universe using type 1A Super Nova instead of Cepheid variables, have shown that the rate of expansion of the universe is increasing all the time.
It was impossible to fit the data with the known value of Omega, the density of the universe, To make the data fit the theory one had too reintroduce Lambda, the energy of the vacuum first introduced by Einstein. When the value of Omega (0.3) was added to the value of Lambda (0.7) the sum (1.) equalled the value foreseen by the Inflationary Theory. The WMAP satellite showed that Lambda, or dark energy, makes 73% of all matter and energy in the universe.
General Relativity introduces the notion of black holes, white holes, wormholes, time machines, ... by the back door. Even Einstein had doubts about these anomalities, but today they are considered to be integral part of the General Relativity Theory. However, the strangest anomalies are the parallel universes and the links between them.

- Plasma cosmology is a non-standard cosmology that attempts to explain the development of the visible universe through the interaction of electromagnetic forces on astrophysical plasma. According to this theory the universe was an equal mixture of ionized matter and anti-matter that would naturally separate as annihilation reactions occurred accompanied by a tremendous release of energy. Plasma cosmology contradicts the current consensus of astrophysicists that Einstein's Theory of general relativity explains the origin and evolution of the universe on its largest scales, relying instead on the further development of classical mechanics and electrodynamics as applied to astrophysical plasmas.

- Steady state versus Big Bang Theory
Fred Hoyle did not believe in the Big Bang Theory. His Steady State Theory states that, even if it is true that the universe is expanding, new matter are created out of nowhere all the time and, as a result, the density of the universe remains the same.

However, later evidences showed that the Steady State Theory was wrong:
1- Since the universe did not evolve but was creating new matter all the time, the early universe should have looked the same as it does now. In other words, today's galaxies should look the same as they did billions of years ago. Large energy sources known as "Quasars" have been found to be billions of light years away in space. That means that they were created at that time. As one does not find any today, the universe must have changed with time.
2- There is about 25% of helium in the universe. According to the Steady State Theory it is mainly created in stars but then it should be quite rare. As it is not the case, the Steady State Theory is wrong, and the Big bang Theory is right.
3- The microwave background radiation discovery of Penzias and Wilson brought the Steady State Theory to an end.

- Time machines

- Van Stockum time machine
In 1933 Van Stockum found a solution to Einstein's equations that allowed time travel. He showed that an infinite, fast spinning -near the speed of light- cylinder would drag space-time along with it. A person travelling along the cylinder would be swept along at speed exceeding the speed of light. The person making a complete trip around the cylinder would go back in time by an amount depending of the speed of the cylinder.
Every time the persona walks around the cylinder, the farther back in time he, or she, goes. Of course, one cannot build an infinite cylinder and, if one could, it would fall apart under the centrifuge force.

- Godel universe
Godel assumed that the whole universe was rotating. As before, the persons on the earth are swept along. If one person takes a rocket around the universe he, or she, would arrive back at the starting point back in time. In this universe one could travel to any point in space and time. Any event in the past, as far back as one decide, can be visited. However, the universe does not rotate, it expands, so this is only a theoretical solution to the problem of time travel.

- Thorne time machine
In 1988 Thorne found another solution to Einstein's equations that allows time travel without most of the previous problems. However, to do this his machine needs "Negative matter! and "Negative Energy". Negative matter has never been found yet and negative energy can only be produced in very small quantity. These negative matter and energy make a wormhole usable for a return journey.
Negative matter falls up because it has negative gravity. It is also repelled by ordinary and negative matter. So t is difficult, if not impossible, to find some in our universe, but it could be floating in outer space.
Thornes machine could possibly looks like two sets of concentric spheres separated by a very small gap. If one sends one set of concentric spheres in a rocket speeding near the speed of light, time slows down in it. If a person enters the set of spheres left on earth, he or she could be sucked through a wormhole to the set of spheres in space and find him, or her, back in the past. However, getting large amount of negative matter and energy is practically impossible at this time and so, this is again only a theoretical solution.

- Gott time machine
In 1991 Gott found still another solution to Einstein equations that allowed time travel. Gott became interested in "Cosmic Strings", a relic of the big bang. They are thinner than an atomic nucleus but their mass is enormous, and they extent for millions of light years in space. Gott found that if two cosmic strings are sent towards each other, just before they collide they can be used as a time-machine.
If one makes a trip around colliding cosmic strings, space is contacted because the round trip is less than 360o due to the space contraction. As a consequence the rocket passengers make a trip to the past, and this is done using positive matter and energy travelling fast, but below the speed of light. However, the required energy is huge and cosmic strings are rare, if they exist at all, and colliding one are even rarer.

- Unification of the four forces of the universe

These four forces are:
- Gravity, the force holding the sun together and the one that determines the orbits of its planets. Without gravity the stars, and the earth too, would explode throwing us in outer space.
- Electromagnetism, without it there would be no electricity, television, telephone, laser beams, ...
- Weak nuclear force, it permits radioactive decay. As it is weak it allows the nuclei to break down, not being strong enough to keep them together. Radioactive decay keeps the earth very hot. (I THINK THIS IS WRONG.CHECK)
- Strong nuclear force, it keeps the nuclei of the atoms together. Without it they would split apart and life would stop. It is responsible for the creation of the approximatively one hundred elements found in the universe.

These four forces are different from each other. Gravity is the weakest (10E36 times weaker than the electromagnetic force) but it acts on long distances. Gravity is always attractive, while the electromagnetic force acts both ways depending on the electrical charge (positive or negative).

Einstein tried to unify these forces but he failed. In 1967 Steven Weinberg and Abdus Salam unified the weak nuclear force and the electromagnetic one. In the 1970s the strong nuclear force was unified with the previous two but gravity was still left out. This was called the "Standard Model Theory". However, it is not a very nice theory in the sense that 19 parameters have to be introduced by hand whereas they should come directly out of it.

Physicists proposed a better theory - Grand Unified Theory or GUT- but it is still far from perfect.

It is now believed that at the time of the big bang all four forces were united into a single "Super force" with all forces having the same strength. But, as the universe expanded and cooled, the super force split into the four forces known today. In this view the universe started in a state of perfect unity, but it went thought phase transitions that led to the present situation.

- Vacuum and false vacuum
If a water dam breaks, the water flows downhill in the direction of the lowest energy, which is at sea level. This lowest energy state is called a "vacuum". The water in the dam has a higher potential energy and this is called a "False vacuum".

According to the Guth Theory, at the beginning the universe was in a state of false vacuum when all the four forces were unified. This state was unstable and, after the big bang, the universe transited to the state of true vacuum where the four forces were separated.

In the state of false vacuum, the universe expands exponentially. The cosmological constant, the energy o the false vacuum, makes the universe to expand so fast.