Einstein was not alone in his endeavors, nor was Planck, nor was Darwin, nor was Galileo, etc. Many of Einstein’s contemporaries contributed so much to the development of the theory of relativity that some came close to preempting Einstein. If ever a scientific idea was crying out to be discovered at a certain time, that idea was the special theory of relativity.
In 1917, Einstein and all other cosmologists thought that the universe was static, neither expanding nor collapsing. At that time, Einstein considered gravitation and he put the lambda constant in his equations to prevent the universe from collapsing on itself from the gravitational pull of matter. He founded a theory of gravity in which the flow of time from place to place, and the creation and destruction of space, depends on matter. The priest Lemaitre4 pointed out that this theory implied that the universe had a beginning and he proposed the theory of the Big Bang in 1927, with the notion that the earth is only a grain of dust in the cosmos. Einstein found Lemaitre’s conclusion so repugnant that he tried to change his theory. In 1929, Hubble confirmed that the Universe is expanding. Space really does grow and time has a beginning. The telescope used by Hubble heralded a change of cosmology from a field dominated by speculation to an observational science. Now cosmological theories die because of disagreement with observation and not because of the death of their proponents.
With the formidable numbers handled, cosmologists and astronomers turned to shortcuts. One of these is the light-year. It is the distance traveled by light during one year. This unit of measure is not entirely satisfactory because it relies on a local condition: the planet Earth that revolves around the sun in 365 days and rotates on its axis in 24 hours. A more acceptable unit is the Parsec that stands for Parallax Second. It is the distance from which one would see the astronomical unit, i.e. the distance between earth and sun, under an angle of one second. It amounts to 3.26 light-years or 3 x 1013 km: one light-year = 9.46 x 1012 km = 0.307 parsecs.
Considering the universe on the scale of the gigaparsec (3 x 1022 km), it is composed of molecules of a gas. These molecules are galaxies. The density of the gas is about 3 x 10-31 gm/cm3 and this is exceedingly small. About seventy years ago, it was recognized that this gas is in expansion: the current paradigm is that the galaxies are moving away from each other with a speed that is proportional to their distance. The speed (v) at which galaxies move away from each other is proportional (H) to their distance (d): V =Hd, where H is the Hubble constant, taken as 60 Km/sec/mega parsec. I will come back to the Hubble constant later. Since all galaxies speed away, there is supposed to have been a time when they were all together. The universe must have been at one time in a very condensed form, a ball of energy no bigger perhaps than the sun. The heat of the compressed gas must have been such that matter must have disintegrated to the state of elementary particles bathing in a gas of photons. If elementary particles have no size, then the size of the primeval ball of energy may have been that of a ping-pong ball or less, which is absurd.
If such were the initial conditions, then we must still bathe in a gas of photons, but this gas has cooled down tremendously due to its expansion; everywhere in space we should find a diffuse electromagnetic radiation at low temperature. Such a radiation was found in 1965. This cosmic background lies at 2.7° K (Kelvin temperature). The absolute cold state is reached with a temperature of -273.3° Celsius. The Kelvin temperatures are measurements starting from this ultimate cold state, using the same scale as the one used to define the Celsius: 0° is melting ice and 100° is boiling water at a pressure of 760 mm of mercury, that is, at sea level. The observation of such a background practically proves that the universe at one time existed in a very condensed form.
At time 0, the condensed Universe was of infinite thermal energy. An absurdity, of course, but what is the alternative, as long as electrons, photons, quarks and other elementary particles are claimed to have no size? Elementary reactions took place, of the type e+ + e- = photon+photon. This equation means that pairs of electrons of opposite sign (the e+ is a positron) interact and are annihilated producing two photons of high energy, and the equation is reversible. Other elementary particles such as protons (hydrogen nucleus), neutrons and mesons are also present as well as their opposites, the antiprotons, antineutrons, etc. We suppose that matter and antimatter were continuously created and annihilated, with the production of thermal energy, photons. The assumption of the existence of antimatter may however be disputed, since no antimatter has ever been detected in space. The antimatter has the same properties as matter and follows the same physical laws. The luminous spectrum of antimatter is identical to that of matter. No radio or optical observation is able to tell us if we are dealing with a galaxy of matter or of antimatter. If, however, a collision occurs between two oppositely signed galaxies, then annihilation takes place with emission of energy under the form of, mainly, gamma rays.
At the initial concentration of matter that is assumed, and this represents much more than all the matter now present in the whole of the Universe, the particles present would interact many times within a short period of 10-23 seconds. This period started at time 0 when there was neither time nor even space. And it may have lasted for an eternity. Then, suddenly, the process changed. At 10-5 seconds after time 0 (that is a fraction of a 10,000th of a second), the "Big Bang" entered in a new phase. The temperature, after these 0.00001 seconds, dropped to 3.5 x 1012 degrees K. This apparently extremely hot temperature is, however, sufficiently low to favor the spontaneous separation of nucleons from antinucleons. As said before, no antimatter has been detected and perhaps there is none left. If matter and antimatter were exactly the same, then the energy from the big bang would have created an equal amount of matter and antimatter, which would have annihilated each other. But the early universe had a preference for matter, which became the stuff of stars, planets and life. This preference implies that at least some elements making up the matter differ from antimatter.
There are four fundamental forces recognized in the universe: the force of gravity that keeps stars and galaxies from flying apart, the electro-magnetic force, the strong force that binds protons to neutrons, and the nuclear weak force (W). Physicists long assumed that any experiment performed with matter would give the same result with antimatter. This symmetry is known as "charge", or C(harge) symmetry. Similarly, the physicists thought that experiments should be identical, regardless if you swap right and left, up and down, front and back. This property is known as parity, or P(arity) symmetry. The strong force, the electromagnetic force and the gravitational force obey C and P symmetries. The weak force W exerts a subtle pull on quarks and on leptons, i.e. the neutrinos and the electrons, and ignores the common-sense rules that the three other forces obey.
The origin of this concept started in the early 1930s, when Wolfgang Pauli attempted an explanation for the decay of cobalt into nickel (the beta decay): a neutron in an atom of cobalt releases an electron, turning therewith into a proton and changing the cobalt atom into a nickel atom. There remains, however, a tiny gap between the electron and the proton, which Pauli reduced by assuming that the neutron also emitted a tiny neutral particle. Enrico Fermi gave it the name neutrino in 1934, and thought that the neutrino was released by the action of a weak force (W). Today, we know that the beta decay is caused by the interaction of a quark within the cobalt neutron with a carrier force known as the W particle, changing the neutron into a proton and emitting an electron and an antineutrino.
In the early 1950s, it became clear that the weak force at work on cobalt neutrons forced the released electrons in a preferential direction. The Weak force was left-handed. It violated Parity symmetry. Soon thereafter, it was found that it also violated Charge symmetry. Immediately followed the observation that it violated CP symmetry, when the weak force acted on these two symmetries together. However, the tiny asymmetry between quarks and anti quarks cannot account for the preponderance of matter in our universe because the difference is too small. The extra asymmetry discovered recently between leptons and antileptons would make up the difference.
In the very beginning of time, about 10-35 seconds after the Big Bang, when elementary particles began to be formed, nature showed a slight preference for quarks and neutrinos over antiquarks and antineutrinos. In the late 1990s, it was discovered in Japan that neutrinos change as the particles pass through earth (i.e. a muon neutrino changes into a tau neutrino). This indicates that neutrinos have a mass. It is possible that neutrinos also violate CP symmetry. These two asymmetries together would explain the preponderance of matter.
At the time 10-4 seconds, the temperature had dropped to 1 x 1012 degrees K. The separation of matter and antimatter present as an emulsion like oil on water proceeded further, with annihilation of each other in an anarchic way when both types came into contact, until the presence of nucleons within a sphere of 10-4 cm3 exceeded the antinucleons in the same sphere by a factor at least 1028. In the meantime, however, this anarchic annihilation of matter and antimatter produced photons. These photons separate the small units of matter formed. They establish a buffer zone. The pressure of photons is not a negligible factor. J. Keppler advanced in 1619 AD the hypothesis that the tail of comets is pointing away from the sun’s rays because of the pressure of light. It is currently assumed that large quantities of gas are ejected from very hot stars at velocities as great as 2 x 106 meters/second because the star produces intense UV light whose pressure on the ionized gas exceeds the gravity pull of the star itself.
Twenty minutes after the original explosion, the units of matter and antimatter coalesced and united into larger, rounded units. These were less vulnerable to annihilation because the total surface of contact is reduced with this process.
The temperature continued to drop and considerable annihilation proceeded further, with only one nucleon still being ultimately present for the 108 or 109 (hundred million to thousand million) initially separated. About one million years after the Big Bang, the surviving amount of matter had dropped to the level presently known and the great journey into space started, with the general electromagnetic radiation being down to a level of about 3,000° K. Today, the level is down to 2.7°K.
The perishability of matter is nowadays a commonplace idea. Creation and destruction of matter are intertwined twin concepts that are fully accepted in Western consciousness, yet they are utterly counterintuitive and their identification are a turning point in the history of science. Early mystical approaches concretized the concept of creation into religious creeds but these were by no means universally accepted. The Greek philosopher Democritus, 2400 years ago, regarded the lowest parts of matter as timeless. Atoms were considered by the Greeks as indivisible, as are quarks today. The Roman philosopher Lucretius expressed the idea in the following two principles: "nothing can be created by divine power out of nothing" and "nature resolves everything into its component atoms and never reduces anything to nothing".
The permanence of matter has persisted in scientific thinking until about 1920. In the eighteenth century, Lavoisier enunciated the law of conservation of matter and the concept survives in the laws of conservation of energy. In 1905, Einstein described the photons, which are light quanta that do not exist before they are emitted and vanish at the end of their journey. However, light is not matter but a form of electromagnetic field, and these waves could be conceived as evanescent, as ripples appear on the sea with a puff of wind, to subsequently disperse. In 1932, Chadwick discovered the neutron. The neutron had been postulated by Rutherford as a necessary element constitutive of atoms. The building blocks of matter were then the proton, electron, photon and neutron, which compose the atoms. Quarks were not known in Rutherford’s time. Faithful to the paradigm of his days, Rutherford considered the neutron, made of a proton and an electron, to be a stable entity. This however proved an erroneous concept: a neutron confined to a stable nucleus becomes energetically impotent and can live there forever but the free neutron decays within fifteen minutes into a proton, an electron and an anti-neutrino. The free neutron is not an imperishable particle as the electron and the proton but is intrinsically unstable and vanishes. There is no way to interpret this decay as a rearrangement of preexisting components.
The discovery of the spontaneous decay of the neutron was a turning point in physics: no principle in Newton’s mechanics could deal with this event, nor could quantum mechanics. There existed no explanation for the vanishing of a particle of matter. Around 1930, a number of physicists worked on the enigma and solved it by integrating electromagnetism into the quantum principles. The new scheme, called quantum field theory, provides the framework for understanding mathematically the disparate pieces of elementary particle physics, at the cost of an absurdity: particles with no mass.
All the particles of nature fall into one of two categories. The bosons, which include photons, interact easily between themselves. The fermions, such as electrons, quarks, neutrons and protons, do not so easily interact. They cannot remain at the same time in the same place, and they stack up into successive shells. The different arrays of electrons on the energy ladders of different atoms give rise to the variety of chemical elements we see in nature.
The particles mostly involved in the initial formation of matter were the neutrons, the protons and the electrons. Upon cooling down to below 3,000° K, electrons and protons united to form hydrogen atoms. A more stable structure is reached when two hydrogen atoms combine and share a pair of electrons between the two nuclei, thus forming a hydrogen molecule. A molecule is the smallest unit quantity of a substance, which can exist by itself and retain all the properties of the original substance. In the case of hydrogen, a molecule of hydrogen requires two atoms of hydrogen. Helium is a very stable molecule that is composed of only one atom. It is made of two neutrons, two protons and, of course, two neutralizing electrons. Nothing else but hydrogen, helium and a little lithium was initially present. Also photons, which readily interact with electrons at high temperatures, are abandoned to themselves below the 3,000°K mark. Below that temperature, they are no longer able to react with matter. Together, hydrogen, helium and photons form 99% of the visible matter of the Universe.
We may thus envision the universe as an expanding flow of matter. The matter is condensed into galaxies and the expansion provokes a cooling down of the Universe. Matter itself consists mainly of hydrogen, helium and photons. However, about 80% of the mass in the universe is composed of “dark matter”. The nature of this material, which can be detected only by its effects on gravity, is entirely unknown. The kind of matter we are familiar with, the visible matter, makes 20% of the matter and is known as baryons. This matter forms the stars and galaxies but they contain less than a tenth of the baryons that existed when the universe was young.
Where went the missing baryons?
A few billion years after the big bang, when the universe was about one-quarter of its current age, the baryons were still all there. In the present-day universe, 90% of them have vanished. The gas of baryons now found between the galaxies is not enough to account for the missing baryons. It is suspected that the missing baryons form filaments of matter much larger than galaxy clusters, which connect the galaxy clusters and groups in a cosmic web. Since the filaments are of very low densities, they are particularly difficult to detect, yet, the determination of the mass of baryons in the cosmic web is fundamental to understanding the formation and evolution of the structures in our universe.