The Big Bang

How did the universe begin?  Were we created by some higher being, or is our entire universe in all it’s beauty a fluke of evolution?  These are questions humans have been attempting to answer for all humanity.   Many people disagree on what happened in the beginning moments of our universe. Some believe that God created the universe with a divine hand, other say that the universe evolved after a massive explosion.  We may never be able to prove beyond a doubt how the universe came into existence, but science can at least point us in the right direction.  The most widely accepted theory in science today dealing with how the universe came into existence is the big bang theory, specifically the "hot big bang" model.   The big bang theory tells us that millions of years ago there was a point when the universe had zero size and was infinitely hot.  The big bang was a giant explosion that caused the matter in the universe to expand and cool down, eventually forming the planets and stars we see today.  It was at the big bang that time itself began.  What proof do we have that suggests that the big bang is a probable theory?

All light gives off electromagnetic radiation.   This electromagnetic radiation travels through space as a wave, with a wavelength and frequency.   On the spectrum waves with the lowest frequency are red, waves with the highest frequency are blue.    The Doppler effect tells us that waves propagate out from a stationary source of light at a constant frequency.  When the light source (a star or galaxy, in this case) is moving away from us, the frequency of the light is lower.   In 1929, using these principles, Edwin Hubble demonstrated that the universe is expanding.  He observed that light from the distant parts of the galaxies was not at the frequency he knew it should be. Instead, it was shifted towards the red end of the spectrum when compared to the wavelengths of the nearer galaxies.  This shift in wavelength is called the redshift.  This means that the frequency of the light is lower than expected, therefore these galaxies are moving away from us.  It has also been shown that the further a galaxy is away from us, the faster it is moving.  The expanding universe is just one of the reasons science points to the big bang.  Another strong proof is the comic microwave background radiation.
Cosmic Microwave Background Radiation
           The universe is filled with microwaves, or short wavelength radio waves.  In 1965 Penzias and Wilson were making a series of measurements with a radio telescope when they discovered background radio noise that appeared to be independent of direction.  Initially, there was much uncertainty as to the origin of this microwave radiation.  But Robert Dicke, another physicist, realized that the background radiation provided clues to the origin of the universe and was able to begin to put the pieces together.

Years earlier George Gamow had predicted that radiation should be left over from the big bang, currently at a temperature of about 10 degrees Kelvin, just above absolute zero.  Until Penzias and Wilson’s discovery, this idea had all but been forgotten about.  Experiments showed that the cosmic microwave background radiation found by Penzias and Wilson was highly uniform, more than one part in a thousand.  If this radiation was from a nearby star or galaxy it would not be evenly distributed throughout the universe at it was.  Since it was so uniform we can assert that it came from the most distant parts of the universe and is left over from the big bang.  The microwaves corresponds to a characteristic temperature of about three degrees Kelvin, close to what Gamow predicted.  The cosmic microwave background radiation is one of the best arguments for the big bang. 

 The existence and amount of helium in the universe today also points scientists to the possibility of a big bang at some point in the past.  Today the universe consists of about 88.6% hydrogen and 11.3% helium.  The rest of the elements are all heavier than helium and are formed when trace impurities are added to a mixture of helium and hydrogen.  In 1957 it was shown that all elements heavier than helium could be built up inside of stars and other explosions.  So where did helium come from?  Scientists argue that helium could have only come from the big bang for two basic reasons.  The first is that older stars contain no less helium than younger stars.  The second reason is that a large amount of energy is required to form helium, if this much energy were present things would currently be different in the universe.  For example, the stars would shine brighter than they do.  The redshift, cosmic background radiation, and helium in the universe give us reason to believe that the big bang is a possible answer to the problem of the beginning of the universe.  Before we can take a deeper look at the big bang itself and what happened shortly afterwards, we need to understand the particles that were involved in the early stages of the big bang.

For many years, the atom was considered to be the fundamental building block of all matter.  By combining with other atoms to form molecules, the atom makes up everything in the universe.  But in 1911, Rutherford discovered that the atom had a nucleus.  This caused scientists to realize that the atom itself could be divided into smaller parts and is not the smallest building block of life.   Later, the electron with its negative charge was discovered to be part of the atom, as well as the positively charged proton and the zero charge neutron.  These particles were considered the most basic particles until the 1960's when once again even smaller particles, called quarks were discovered to make up the proton and neutron.  Scientists now know that all particles in the universe are composed of either quarks or leptons, called elementary particles.  

Quarks come in pairs: up and down, strange and charm, bottom and top.  Protons and neutrons are composed of three quarks each.  A proton has two ups and a down (u,u,d).  A neutron made up of the opposite: two downs and an up (d,d,u).  Since protons have a positive charge and neutrons zero charge, the quarks themselves must have individual charges to make the larger particles charged.  By solving for “u” and “d” in these equations:
Proton = u + u + d = +1        
Neutron = d + d + u = 0

we find that the charge on an up quark is +2/3, while the charge on a down quark is  -1/3.  The other quarks also have the same charges - up, charm, and top quarks all have a charge of +2/3.  Down, strange, and bottom quarks all have a charge of -1/3.  Quarks are grouped into generations based on how they respond to the fundamental forces.  The quarks in the first generation (up and down) are the most common in the universe because these are the quarks that make up the nucleus in the atom.  The mass of the quarks increase as you move across the generations.  The second and third generation quarks are much more rare because of their higher masses.  For these quarks to be created, particles must collide at high velocities.  These second and third generation quarks were much more common in the earlier universe, because matter was more energetic it traveled at higher velocities, yielding more of these heavier quarks to be produced in collisions.  
            As well as the elementary particles called quarks, there are six kinds of leptons.  The electron is a well-known lepton and is very similar to two other leptons, the muon and the tau.  Essentially the only things that distinguish an electron from a muon or a tau is it's mass.  Both particles have the same charge as an electron and respond to the fundamental forces in the same way.   But unlike the electron, the muon and the tau are allowed to decay into other particles whereas the electron is a stable particle.  The other three leptons are called neutrinos because they are electrically neutral.  This is different from the zero charge on a neutron - the neutron is made up of fundamental particles with charges adding up to zero, but the neutrinos themselves have no charge and are called “genuinely neutral”.
An Elementary Particle Example: The electron
            The electron is one example of a lepton, it is the earliest known elementary particle.   In 1891, George Stoney named the theoretical unit that would link electricity and matter the electron.  In 1897, through a series of cathode ray tube experiments, J.J. Thomson actually discovered the electron.  The electron has a mass of 9x10-31 kilograms, the same mass as one million millionth of a spec of dust.  The electron has a charge of 1.6x10-19 Coulombs.  The radius of the electron is so small that it cannot be measured, although it is known to be less than 10-18 meters in radius.  In fact, the electron's properties are consistent with a radius equal to zero!  It is known that electrons can only exist in certain discrete energy levels in an atom.  DeBroglie concluded that electrons act not only like a particle but also as a wave.  This is called wave-particle duality.  The implications of wave-particle duality are astonishing.  Because a wave spreads through space, we cannot know the exact velocity and location of a particle at the same time.
            Electrons, as well as all other elementary particles display rotational motion, or spin.  The spin of a particle shows us what the particle looks like when viewed from different directions.  If a particle has a spin 0 it looks the same from all directions; it is like a dot.  A particle with a spin 1 is like an arrow, it looks the same only if rotated 360 degrees.  A particle with a spin 2 is like a double-headed arrow, it looks the same after every 180-degree rotation.  Some particles (electrons for example) look the same only after rotating them through two complete revolutions or 720 degrees.  These particles are said to have a spin 1/2.

Now that we have a basic understanding of the elementary particles that played a role in the big bang, we need to understand the fundamental forces that allow these particles to interact.
Fundamental Forces
            There are many different forces in our world today, but all of them can be summed up as a combination of the four fundamental forces.  These four are gravity, the electromagnetic force, the strong force, and the weak force.  Gravity is a familiar force.  This is the force that works only between massive objects.  It has no role at the microscopic level.  But it effects essentially everything that has mass.  Gravity is also the weakest of the four forces.  The electromagnetic force is also a fairly well known force.  The electromagnetic force is the force that holds electrons to the nucleus.  This force exists only between charges.    The strong force is what causes quarks to stick together.  The strong force allows protons and neutrons to form.  The range of the strong force is only 10 -15 meters, or approximately the diameter of an atom.  This force allows the nucleus to resist being blown apart by electrostatic repulsion.  The final of the four fundamental forces is the weak force.  The weak force is the most difficult to understand.  It facilitates the decay of heavy particles into smaller particles.  The range of the weak force is 10 -17 m.  The weak force changes particles from one type to another.  Leptons can only be changed to other leptons of the same generation.  For example, an electron could be changed to an electron neutrino, or a muon neutrino to a muon.  But in quarks, the weak force not only changes the quarks within generations, but also across generations.    For example a bottom quark could be changed to an up quark or a top quark.  
Characteristics of the Universe
            Gases are made up of particles moving at a variety of speeds.  The temperature of a gas can be defined by the way the particles are colliding with one another.  The particles interact with one another in certain ways because of the different fundamental forces that control them.  But the particles interact in such a way that the average energy of the gas is always constant.  This is called thermal equilibrium.  Our universe is in thermal equilibrium.  Also, each particle in the universe has a threshold temperature.  When the temperature of the universe drops below this threshold temperature for the particle, these particles disappear from the universe.  They will be annihilated due to particle/antiparticle collisions faster than they will be produced.  But the energy of the universe will remain constant.
The Big Bang Itself
            The point in time just before the big bang is called singularity; this is the point where the laws of physics breaks down.   At this point the universe was zero in size and infinitely hot. Because all of our laws of physics break down at singularity, scientists cannot possibly understand what caused the big bang to happen.  The moment after the big bang happened, the universe began to expand and the matter and radiation in the universe began to cool.  In fact, when the size of the universe doubles, the temperature falls by one half.  

At 10 –43 seconds after the big bang, the universe was at 10 33 degrees Kelvin.  At this point all four fundamental forces were equal in strength.   The universe was filled with quarks and leptons in approximately equal numbers.  The universe continued to expand and cool.  At 10 –35 seconds after the big bang, the electromagnetic and weak forces were equal in strength, gravity and the strong force were weaker.    As time continued, the universe is expanded enough for quarks and anitquarks to bond, which means that protons and neutrons are forming.   At 7x10 –7 seconds after the big band the universe is at a temperature of about 1013 Kelvin.  This is below the threshold for protons and neutrons, so these particles stop being created.  But the remaining protons and neutrons are kept in thermal equilibrium with the rest of the particles by neutrino interactions.  One hundred seconds after the big bang, the universe was at one thousand million degrees.  At this point, protons and neutrons could no longer escape the strong nuclear force so they would start to come together to form deuterium (heavy hydrogen).  The earliest stars condensed from this deuterium.  Finally, about 3.2 minutes after the big bang occurred, the deuterium begins to interact with other particles to produce helium.

D + D → ³He + n

³He + n → ³H + p

³H + d → 4He + n

Reactions similar to these continue to occur as hydrogen and helium combine to produce the heavier elements.  As time continued, the deuterium combined with more protons to produce other elements, especially helium.  A few hours later, the production of helium stopped.  Over the next million years the universe continued to expand without much other major activity.

            The big bang theory gives us a probable explanation for how the universe came into existence.  The amount of information we can learn about things that happened so long ago is incredible.  Of course, we still don’t know for sure whether the big bang is even the correct theory of how the universe came to be.   We many never know.  And even if we can prove that the big bang is correct, scientists still have no idea how or why the big bang actually happened.  Stephen Hawking, in his book A Brief History of Time writes, “It would be very difficult to explain why the universe should have begun in just this way, except as the act of a God who intended to create beings like us.” 





Works Cited

Allday, Jonathan. Quarks, Leptons and the Big Bang.  Philadelphia, PA: Institute of Physics Publishing. 1998.

Fermi National Accelerator Laboratory.  What is the world made of?  Internet.  February 28, 2001.

Greenwood, N.N. and A. Earnshaw.  Chemistry of the Elements.  Pergamon Press: Tarringtown, New York.  1984.

Hawking, Stephen W.  A Brief History of Time.  New York, New York: Bantam Books. 1988.

NSSDC Photo Gallery.  Galaxies and Globular Clusters.  Internet.  May 31, 2001

Parker, Barry.  Creation:  The Story of the Origin and Evolution of the Universe.  New York.  Plenum Press.  1988.

The Particle Data Group.  The Particle Adventure. Internet. 2000.

Silk, Joseph.  The Big Bang:  The Creation and Evolution of the Universe.  San Francisco.  W. H. Freeman and Company.  1980.

Science Museum and Institute of Physics.  Life, the Universe, and the Electron. Internet. 1997.


(1) Science Museum and Institute of Physics
(2) The Particle Data Group
(3) The Particle Data Group
(4) NSSDC Photo Gallery