A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions.
For reasons we do not fully understand, a consequence of the odd half-integer spin is that fermions obey the Pauli Exclusion Principle and therefore cannot co-exist in the same state at same location at the same time.
Quarks are one type of matter particle. Most of the matter we see around us is made from protons and neutrons, which are composed of quarks.
There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.) Be glad that quarks have such silly names -- it makes them easier to remember!
Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another type of charge called color charge, which we will discuss later.
The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years.
Want to see a particle physicist's idea of a good pun?The naming of quarks...
...began when, in 1964, Murray Gell-Mann and George Zweig suggested that hundreds of the particles known at the time could be explained as combinations of just three fundamental particles. Gell-Mann chose the name "quarks," pronounced "kworks," for these three particles, a nonsense word used by James Joyce in the novel Finnegan's Wake:
"Three quarks for Muster Mark!"
In order to make their calculations work, the quarks had to be assigned fractional electrical charges of 2/3 and -1/3. Such charges had never been observed before. Quarks are never observed by themselves, and so initially these quarks were regarded as mathematical fiction. Experiments have since convinced physicists that not only do quarks exist, but there are six of them, not three.
How did quarks get their silly names? |   |
| There are six flavors of quarks. "Flavors" just means different kinds. The two lightest are called up and down. |
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| The third quark is called strange. It was named after the "strangely" long lifetime of the K particle, the first composite particle found to contain this quark. |
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| The fourth quark type, the charm quark, was named on a whim. It was discovered in 1974 almost simultaneously at both the Stanford Linear Accelerator Center (SLAC) and at Brookhaven National Laboratory. |
The fifth and sixth quarks were sometimes called truth and beauty in the past, but even physicists thought that was too cute.
| The bottom quark was first discovered at Fermi National Lab (Fermilab) in 1977, in a composite particle called Upsilon ( |
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| The top quark was discovered last, also at Fermilab, in 1995. It is the most massive quark. It had been predicted for a long time but had never been observed successfully until then. |
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).
Like social elephants, quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called
Although individual quarks have fractional electrical charges, they combine such that hadrons have a net integer electric charge. Another property of hadrons is that they have no net color charge even though the quarks themselves carry color charge (we will talk more about this later).
There are two classes of hadrons (try putting your mouse on the elephants):
...are any hadron which is made of three quarks (qqq).
Because they are made of two up quarks and one down quark (uud), protons are baryons. So are neutrons (udd).
...contain one quark (q) and one antiquark (
).
One example of a meson is a pion (
+), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (-) is made up a down quark and an up antiquark.
Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K-) meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components.
A weird thing about hadrons is that only a very very very small part of the mass of a hadron is due to the quarks in it.
The other type of matter particles are the leptons.
There are six leptons, three of which have electrical charge and three of which do not. They appear to be point-like particles without internal structure. The best known lepton is the electron (e-). The other two charged leptons are the muon(
) and the tau(
), which are charged like electrons but have a lot more mass. The other leptons are the three types of neutrinos (
). They have no electrical charge, very little mass, and they are very hard to find.
Quarks are sociable and only exist in composite particles with other quarks, whereas leptons are solitary particles. Think of the charged leptons as independent cats with associated neutrino fleas, which are very hard to see.
For each lepton there is a corresponding antimatter antilepton. Note that the anti-electron has a special name, the "positron."
The heavier leptons, the muon and the tau, are not found in ordinary matter at all. This is because when they are produced they very quickly decay, or transform, into lighter leptons. Sometimes the tau lepton will decay into a quark, an antiquark, and a tau neutrino. Electrons and the three kinds of neutrinos are stable and thus the types we commonly see around us.
When a heavy lepton decays, one of the particles it decays into is always its corresponding neutrino. The other particles could be a quark and its antiquark, or another lepton and its antineutrino.
Physicists have observed that some types of lepton decays are possible and some are not. In order to explain this, they divided the leptons into three lepton families: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino. The number of members in each family must remain constant in a decay. (A particle and an antiparticle in the same family "cancel out" to make the total of them equal zero.)
Although leptons are solitary, they are always loyal to their families!Lepton Type Conservation
Leptons are divided into three lepton families: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino.
We use the terms "electron number," "muon number," and "tau number" to refer to the lepton family of a particle. Electrons and their neutrinos have electron number +1, positrons and their antineutrinos have electron number -1, and all other particles have electron number 0. Muon number and tau number operate analogously with the other two lepton families.
One important thing about leptons, then, is that electron number, muon number, and tau number are always conserved when a massive lepton decays into smaller ones.
Let's take an example decay.
A muon decays into a muon neutrino, an electron, and an electron antineutrino:
As you can see, electron, muon, and tau numbers are conserved. These and other conservation laws are what we believe define whether or not a given hypothetical lepton decay is possible.
Neutrinos
Neutrinos are, as we've said, a type of lepton. Since they have no electrical or strong charge they almost never interact with any other particles. Most neutrinos pass right through the earth without ever interacting with a single atom of it.
Neutrinos are produced in a variety of interactions, especially in particle decays. In fact, it was through a careful study of radioactive decays that physicists hypothesized the neutrino's existence.
For example: (1) In a radioactive nucleus, a neutron at rest (zero momentum) decays, releasing a proton and an electron. (2) Because of the law of conservation of momentum, the resulting products of the decay must have a total momentum of zero, which the observed proton and electron clearly do not. (3) Therefore, we need to infer the presence of another particle with appropriate momentum to balance the event. (4) We hypothesize that an antineutrino was released; experiments have confirmed that this is indeed what happens.
Because neutrinos were produced in great abundance in the early universe and rarely interact with matter, there are a lot of them in the Universe. Their tiny mass but huge numbers may contribute to total mass of the universe and affect its expansion.
Note that both quarks and leptons exist in three distinct sets. Each set of quark and lepton charge types is called a generation of matter (charges +2/3, -1/3, 0, and -1 as you go down each generation). The generations are organized by increasing mass.
All visible matter in the universe is made from the first generation of matter particles -- up quarks, down quarks, and electrons. This is because all second and third generation particles are unstable and quickly decay into stable first generation particles.
Wait a minute. If the higher generations of matter decay quickly, are rarely observed, and do not make up any of the stable matter around us, why do they exist at all?
Good question. In fact, when the muon was discovered physicist I.I. Rabi asked,
So why do we have generations of matter at all? Why three of them? We don't know. And without understanding why the second and third generation particles exist, we cannot rule out the possibility that there are yet more quarks and leptons that we have not discovered yet. Or perhaps the answer is that quarks and leptons aren't fundamental, but are made up of even more elementary particles whose composite particles we observe as quarks.
Matter Summary
Now we know of what the world is made. The most fundamental matter particles are the six quarks and the six leptons.
Well, it is a bit more complicated than that. Remember that we never find isolated quarks, we only observe them in composite particles called hadrons. And there is an antimatter particle for every matter particle.
The Four Interactions
Now we think we have a good idea of what the world is made of: quarks and leptons. So...
What holds it together?
The universe, which we know and love, exists because the fundamental particles interact. These interactions include attractive and repulsive forces, decay, and annihilation. 
There are four fundamental interactions between particles, and all forces in the world can be attributed to these four interactions!
That's right: Any force you can think of -- friction, magnetism, gravity, nuclear decay, and so on -- is caused by one of these four fundamental interactions.
What's the difference between a force and an interaction?
This is a hard distinction to make. Strictly speaking, a force is the effect on a particle due to the presence of other particles. The interactions of a particle include all the forces that affect it, but also include decays and annihilations that the particle might go through. (We will spend the next chapter discussing these decays and annihilations in more depth.)
The reason this gets confusing is that most people, even most physicists, usually use "force" and "interaction" interchangeably, although "interaction" is more correct. For instance, we call the particles which carry the interactions force carrier particles. You will usually be okay using the terms interchangeably, but you should know that they are different.How Does Matter Interact?
One tricky question that plagued physicists for many years was...
How do matter particles interact?
The problem is that things interact without touching! How do two magnets "feel" each other's presence and attract or repel accordingly? How does the sun attract the earth?
We know the answers to these questions are "magnetism" and "gravity," but what are these forces?
At a fundamental level, a force isn't just something that happens to particles. It is a thing which is passed between two particles.
The Unseen Effect
You can think about forces as being analogous to the following situation:
Two people are standing on an ice pond. One person moves their arm and is pushed backwards; a moment later the other person grabs at an invisible object and is driven backwards. Even though you cannot see a basketball, you can assume that one person threw a basketball to the other person because you see its effect on the people. (Click on the checkmark or cross below the animation in order to make the basketball appear or disappear.)
It turns out that all interactions which affect matter particles are due to an exchange of force carrier particles, a different type of particle altogether. These particles are like basketballs tossed between matter particles (which are like the basketball players). What we normally think of as "forces" are actually the effects of force carrier particles on matter particles.
The basketball animation is, of course, a very crude analogy since it can only explain repulsive forces and gives no hint of how exchanging particles can result in attractive forces.
We see examples of attractive forces in everyday life (such as magnets and gravity), and so we generally take it for granted that an object's presence can just affect another object. It is when we approach the deeper question, "How can two objects affect one another without touching?" that we propose that the invisible force could be an exchange of force carrier particles. Particle physicists have found that we can explain the force of one particle acting on another to INCREDIBLE precision by the exchange of these force carrier particles.
One important thing to know about force carriers is that a particular force carrier particle can only be absorbed or produced by a matter particle which is affected by that particular force. For instance, electrons and protons have electric charge, so they can produce and absorb the electromagnetic force carrier, the photon. Neutrinos, on the other hand, have no electric charge, so they cannot absorb or produce photons.
Electromagnetism
The electromagnetic force causes like-charged things to repel and oppositely-charged things to attract. Many everyday forces, such as friction, and even magnetism, are caused by the electromagnetic, or E-M force. For instance, the force that keeps you from falling through the floor is the electromagnetic force which causes the atoms making up the matter in your feet and the floor to resist being displaced.
The carrier particle of the electromagnetic force is the photon (
). Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth.
Photons have zero mass, as far as we know, and always travel at the "speed of light", c, which is about 300,000,000 meters per second, or 186,000 miles per second, in a vacuum.
Residual E-M ForceAtoms usually have the same numbers of protons and electrons. They are electrically neutral, therefore, because the positive protons cancel out the negative electrons. Since they are neutral, what causes them to stick together to form stable molecules?
The answer is a bit strange: we've discovered that the charged parts of one atom can interact with the charged parts of another atom. This allows different atoms to bind together, an effect called the residual electromagnetic force. 
So the electromagnetic force is what allows atoms to bond and form molecules, allowing the world to stay together and create the matter you interact with all of the time. Amazing, isn't it? All the structures of the world exist simply because protons and electrons have opposite charges!
See? Now you know the meaning of life!
What about the Nucleus?
We have another problem with atoms, though. What binds the nucleus together? 
The nucleus of an atom consists of a bunch of protons and neutrons crammed together. Since neutrons have no charge and the positively-charged protons repel one another, why doesn't the nucleus blow apart? 
We cannot account for the nucleus staying together with just electromagnetic force. What else could there be? Gravity? Nope! The gravitational force is far too weak to overpower the electromagnetic force.
So how can we account for this dilemma?
To understand what is happening inside the nucleus, we need to understand more about the quarks that make up the protons and neutrons in the nucleus. 
Quarks have electromagnetic charge, and they also have an altogether different kind of charge called color charge. The force between color-charged particles is very strong, so this force is "creatively" called
The strong force holds quarks together to form hadrons, so its carrier particles are whimsically called 
gluons because they so tightly "glue" quarks together. (Other name candidates included the "hold-on," the "duct-tape-it-on," and the "tie-it-on!")
Color charge behaves differently than electromagnetic charge. Gluons, themselves, have color charge, which is weird and not at all like photons which do not have electromagnetic charge. And while quarks have color charge, composite particles made out of quarks have no net color charge (they are color neutral). For this reason, the strong force only takes place on the really small level of quark interactions, which is why you are not aware of the strong force in your everyday life.
Color Charge
Quarks and gluons are color-charged particles. Just as electrically-charged particles interact by exchanging photons in electromagnetic interactions, color-charged particles exchange gluons in strong interactions. When two quarks are close to one another, they exchange gluons and create a very strong color force field that binds the quarks together. The force field gets stronger as the quarks get further apart. Quarks constantly change their color charges as they exchange gluons with other quarks.
How does color charge work?
There are three color charges and three corresponding anticolor (complementary color) charges. Each quark has one of the three color charges and each antiquark has one of the three anticolor charges. Just as a mix of red, green, and blue light yields white light, in a baryon a combination of "red," "green," and "blue" color charges is color neutral, and in an antibaryon "antired," "antigreen," and "antiblue" is also color neutral. Mesons are color neutral because they carry combinations such as "red" and "antired."
Because gluon-emission and -absorption always changes color, and -in addition - color is a conserved quantity - gluons can be thought of as carrying a color and an anticolor charge. Since there are nine possible color-anticolor combinations we might expect nine different gluon charges, but the mathematics works out such that there are only eight combinations. Unfortunately, there is no intuitive explanation for this result.
Important Disclaimer:
"Color charge" has nothing to do with the visible colors, it is just a convenient naming convention for a mathematical system physicists developed to explain their observations about quarks in hadrons.
Quark Confinement
Color-charged particles cannot be found individually. For this reason, the color-charged quarks are confined in groups (hadrons) with other quarks. These composites are color neutral.
The development of the Standard Model's theory of the strong interactions reflected evidence that quarks combine only into baryons (three quark objects), and mesons (quark-antiquark objects), but not, for example, four-quark objects. Now we understand that only baryons (three different colors) and mesons (color and anticolor) are color-neutral. Particles such as ud or uddd that cannot be combined into color-neutral states are never observed.
Color-Force Field
The quarks in a given hadron madly exchange gluons. For this reason, physicists talk about the color-force field which consists of the gluons holding the bunch of quarks together.
If one of the quarks in a given hadron is pulled away from its neighbors, the color-force field "stretches" between that quark and its neighbors. In so doing, more and more energy is added to the color-force field as the quarks are pulled apart. At some point, it is energetically cheaper for the color-force field to "snap" into a new quark-antiquark pair. In so doing, energy is conserved because the energy of the color-force field is converted into the mass of the new quarks, and the color-force field can "relax" back to an unstretched state.
Quarks cannot exist individually because the color force increases as they are pulled apart.
Quarks Emit Gluons
Color charge is always conserved.
When a quark emits or absorbs a gluon, that quark's color must change in order to conserve color charge. For example, suppose a red quark changes into a blue quark and emits a red/antiblue gluon (the image below illustrates antiblue as yellow). The net color is still red. This is because - after the emission of the gluon - the blue color of the quark cancels with the antiblue color of the gluon. The remaining color then is the red color of the gluon.
Quarks emit and absorb gluons very frequently within a hadron, so there is no way to observe the color of an individual quark. Within a hadron, though, the color of the two quarks exchanging a gluon will change in a way that keeps the bound system in a color-neutral state.
Residual Strong Force
So now we know that the strong force binds quarks together because quarks have color charge. But that still does not explain what holds the nucleus together, since positive protons repel each other with electromagnetic force, and protons and neutrons are color-neutral.
So what holds the nucleus together? Huh?
The answer is that, in short, they don't call it the strong force for nothing. The strong force between the quarks in one proton and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force.
Weak
There are six kinds of quarks and six kinds of leptons. But all the stable matter of the universe appears to be made of just the two least-massive quarks (up quark and down quark), the least-massive charged lepton (the electron), and the neutrinos.
Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. When fundamental particles decay, it is very strange: we observe the particle vanishing and being replaced by two or more different particles. Although the total of mass and energy is conserved, some of the original particle's mass is converted into kinetic energy, and the resulting particles always have less mass than the original particle that decayed.
The only matter around us that is stable is made up of the smallest quarks and leptons, which cannot decay any further.
When a quark or lepton changes type (a muon changing to an electron, for instance) it is said to change flavor. All flavor changes are due to the weak interaction.
The carrier particles of the weak interactions are the W+, W-, and the Z particles. The W's are electrically charged and the Z is neutral.
The Standard Model has united electromagnetic interactions and weak interactions into one unified interaction called electroweak.
Electroweak
In the Standard Model the weak and the electromagnetic interactions have been combined into a unified electroweak theory.
Physicists had long believed that weak forces were closely related to electromagnetic forces.
Eventually they discovered that at very short distances (about 10-18 meters) the strength of the weak interaction is comparable to that of the electromagnetic. On the other hand, at thirty times that distance (3x10-17 m) the strength of the weak interaction is 1/10,000th than that of the electromagnetic interaction. At distances typical for quarks in a proton or neutron (10-15 m) the force is even tinier.
Physicists concluded that, in fact, the weak and electromagnetic forces have essentially equal strengths. This is because the strength of the interaction depends strongly on both the mass of the force carrier and the distance of the interaction. The difference between their observed strengths is due to the huge difference in mass between the W and Z particles, which are very massive, and the photon, which has no mass as far as we know.
Gravity
What about gravity?
Gravity is weird. It is clearly one of the fundamental interactions, but the Standard Model cannot satisfactorily explain it. This is one of those major unanswered problems in physics today.
In addition, the gravity force carrier particle has not been found. Such a particle, however, is predicted to exist and may someday be found: the graviton.
Fortunately, the effects of gravity are extremely tiny in most particle physics situations compared to the other three interactions, so theory and experiment can be compared without including gravity in the calculations. Thus, the Standard Model works without explaining gravity.
Interaction Summary
This is a summary of the different interactions, their force carrier particles, and what particles they act on:
The nucleus of an atom is a fermion or boson depending on whether the total number of its protons and neutrons is odd or even, respectively. Recently, physicists have discovered that this has caused some very strange behavior in certain atoms under unusual conditions, such as very cold helium.
Bosons are those particles which have an integer spin (0, 1, 2...).
All the force carrier particles are bosons, as are those composite particles with an even number of fermion particles (like mesons).
Big Bang
Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.[20] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang",[21] and is considered the "birth" of our universe. Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.73 ± 0.12 billion years.[22] The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the universe.
The earliest phases of the Big Bang are subject to much speculation. In the most common models, the universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. Approximately 10−35 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially.[23] After inflation stopped, the universe consisted of a quark-gluon plasma, as well as all other elementary particles.[24] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and anti-leptons—of the order of 1 part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.[25]
The universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form.[26] After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton-antiproton pairs (similarly for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI prefix giga) Kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis.[27] Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.[28]
Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types of matter are known as cold dark matter, hot dark matter and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the universe.[22]
Independent lines of evidence from Type Ia supernovae and the CMB imply the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 72% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.[8]
All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10−15 seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics.
Big bang theory assumptions
The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10−5.[29] Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.[30]
If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the CMB.[31] The universe has been measured to be homogeneous on the largest scales at the 10% level.[32]
Big Bang Theory
The Big Bang is the cosmological model of the universe that is best supported by all lines of scientific evidence and observation. As used by scientists, the term Big Bang generally refers to the idea that the universe has expanded from a primordial hot and dense initial condition at some finite time in the past, and continues to expand to this day. Georges Lemaître proposed what became known as the Big Bang theory of the origin of the Universe, although he called it his 'hypothesis of the primeval atom'. The framework for the model relies on Albert Einstein's General Relativity as formulated by Alexander Friedmann. After Edwin Hubble discovered in 1929 that the distances to far away galaxies were generally proportional to their redshifts, this observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.[1] If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of the theory, but these accelerators have limited capabilities to probe into such high energy regimes. Without any evidence associated with the earliest instant of the expansion, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the universe since that instant. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.
Fred Hoyle is credited with coining the phrase 'Big Bang' during a 1949 radio broadcast, as a derisive reference to a theory he did not subscribe to.[2] Hoyle later helped considerably in the effort to figure out the nuclear pathway for building certain heavier elements from lighter ones. After the discovery of the cosmic microwave background radiation in 1964, and especially when its collective frequencies sketched out a blackbody curve, most scientists were fairly convinced by the evidence that some Big Bang scenario must have occurred.
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