In 1922, a Russian mathematician, Alexander A. Friedman, discovered an error in Einstein's proof for a static universe.  In carrying out his proof, Einstein had divided both sides of an equation by a quantity, which Friedman found could become zero under certain circumstances.  Since division by zero is not permitted in algebraic computations, the possibility of a non static universe could not be excluded under the circumstances in question.  Friedman showed that two non static models were possible.  One pictured the universe as expanding with time;  the other, contracting.*

*Scientific American, Sept. 1956, p. 140.

Both Friedman and Hubble laid the groundwork for the theory of the expanding universe, which was then developed further by a Belgian theoretical astronomer, Georges Lemaitre, who proposed that our universe started from a highly compressed and extremely hot state, which he called the "primeval atom".  By extrapolating the expansion of galaxies backwards in time to a common point of origination, this state of our universe in the form of a "primeval atom", is estimated to have occurred about 19 billion years ago, with the universe undergoing expansion ever since.

Today our galaxy and earth are somewhere in the midst of all this, where all around us, looking back in time, we can detect the residual traces of a once great cosmic fireball, in the form of what is called background radiation (BG).

In 1946, George Gamow predicted the existence of such a thermal background entirely from the theoretical framework of the Big-Bang model.  He estimated its present temperature as being about five degrees Kelvin.  The general agreement between Gamow's prediction and the observations of Penzias and Wilson is the most compelling evidence for the Big-Bang.*

*Scientific American, Mar. 1976, p. 65.

Besides the usual starlight flooding us from space are cosmic rays from deep space;  high energy alpha particles.  All of these fall within a class called the electromagnetic spectrum;  ranging from radio waves at a wavelength of 300 meters to gamma rays at a wavelength of 10^-14 meters.  Within this range are a number of wavelengths which have been detected as coming from all directions in space and not favoring any particular direction (thus being isotropic), and when plotted somewhat describe a portion of a smooth curve belong to a family of curves, some claim to be nearly identical to a black body radiation curve, of a specific radiation temperature, a bit above absolute zero.

Reacquainting ourselves with quantum properties, consider then an ordinary tungsten lamp burning brightly with a yellow white glow.  In actuality, the color we see is a composite of many different wavelengths striking our eyes with different intensities, which, if  carefully measured and plotted, would form a smooth curve, called a black body radiation curve, with its peak intensity somewhere near the wavelength we see (yellow).  In contrast, the glowing coals of a fireplace also emit a variety of wavelengths, which, if measured and plotted, would produce a smooth curve peaking somewhere in the vicinity of reddish light.  Both the light bulb and hot coals are considered radiators, the temperatures of which can be determined on the basis of where each of theses curves peak:  the difference in their temperatures corresponding to the difference in wavelength between peaks (Wien's Displacement Law for black body radiators.)  Now in fact, all objects (including gases) are radiators;  even ice.  The colder the radiator, the more theoretically perfect it becomes, by more closely obeying Plank's Radiation Law for black body radiators, obviously implying that a perfect black body radiator is very cold (ideally zero degrees Kelvin).

These wavelengths, coming from all directions in space, somewhat describe a perfect radiation curve, which, by using Wien's Displacement Law, project a radiation source at a temperature of 2.76 degrees Kelvin:  the now greatly cooled gas remnants of a cosmic fireball.

The first background radiation was discovered in 1965 by Arno A. Penzias and Robert W. Wilson of Bell Laboratories, confirming an earlier suggestion by Robert H. Dicke of Princeton University that one ought to be able to detect a new kind of cosmic radiation:  a primeval fireball of radiation surviving from the earliest days of the universe, when the universe was enormously hot and contracted.*

*Scientific American, June 1967, p. 28.

This view was shared by a host of others besides Dicke, including George Gamow, Ralph Adler and Robert Herman, all who predicted background temperatures below three degrees Kelvin, and by R.C. Tolman (1930) and C.F. von Weizsacker (1938) who pioneered theoretical development on thermodynamics and thermal radiation in an expanding universe, and by T.F. Howell and J.R. Shakeshaft at the University of Cambridge, and by P.J.E. Peebles, David T. Wilkinson, P.G. Roll, and R.B. Partridge at Princeton (circa 1965 through 1967), all of who contributed greatly in the discovery and measurement of background radiation.

The sensitivity of the instruments used in these studies, particularly the Dicke radiometer, can be appreciated by imagining the difficulty in measuring the heat coming from, say an ice cube at 273 degrees Kelvin;  a temperature many times hotter than the background radiation actually detected by the radiometer!

At the initial onset of the Big Bang, the state of matter was non-existent, the primeval atom, so to speak, being pure radiation for the first few minutes.  Within the first five minutes, elementary particles such as protons, neutrons and electrons began to appear in a dissociative state.  After five minutes and up to the first thirty minutes, thermal agitation is thought to have declined sufficiently for these elementary particles to have combined together into more complex nucleons such as a deuteron (one proton and one neutron) and helium-4 (two protons and two neutrons).  Further expansion and cooling would have allowed the combining of electrons into orbit, forming hydrogen atoms (one proton and one electron).  Eventually in time, estimated to be about one hour, the fireball temperature would have dropped to about 250 million degrees, too low for further thermonuclear reactions to take place.  Thus during this brief moment at the very onset of the Big Bang, was all the matter in the universe created.

Subsequent forms of matter, such as carbon, oxygen, uranium and so  forth, find their creation within the stars as byproducts of stellar combustion (fusion).

What was once the primeval atom is most certainly the center of the universe;  at the onset, the center of energy, and now the center of mass as well.  All matter moving away from this center is of course being continually attracted back to this center by the gravitational forces working across the full range of the universe.  The question to be asked, will this matter as a whole eventually fall back to its center, or does it have sufficient momentum (escape velocity) to remain perpetually in a state of expansion?  In the case that the explosion energy does not override the total mass, and the universe does fall back to its center (at least another 19 billion years), the energy of implosion will allow for another explosion to follow, and the process repeated over and over again.  Such a universe is said to be pulsating, and is wholly conceivable within the principles and framework of modern science.  Each time it collapses, temperatures will again rise to levels for dissociation of matter to occur, returning to the state of pure radiation.  If in the event the explosion energy is sufficient to override the total mass, in essence producing an escape velocity for virtually all matter, the universe will continue to expand;  never stopping.  Such a universe is called a hyperbolic universe, and occurs only once.   In between these two possibilities, is a flat universe.

Indeed, if the evolutionary universe is open, the big question is, what caused it to explode only once in eternity?  And if it is closed, why has it not already run down?  Really important question, since there is no known laboratory example of any explosion not initiated by some means, nor any known contradiction to entropy.  Then there are questions of space-gauge and similitude, two notions which the Big-Bang cannot contend, yet which are necessary in understanding the consistency of time, space and matter.

What is really most peculiar about the Big-Bang is how in the world, in a universe, presumably of immense time and space, are we as observers caught again in the middle;  this being one of the criticisms of the geocentric model.

You would think that the first red flag should have been recognized when Hubble law placed us somewhere near the center of a cosmic expansion!  But astronomers ignored that, just as they ignored Ptolemy's strange presentation placing the earth at the center.

The general distribution of galaxies shows and excess of radio galaxies nearest the fringes of our optically resolvable universe as well as a number of celestial oddities, such as QSS (quasi-stellar sources, also known as quasars) and QSG (quasi-stellar galaxies).  As recently as 1970, there has been no explanation for them, or at least none that has met with the consensus of scientific acceptance.  It has been surmised, that from the very beginning of the explosion, large groups of matter were ejected with exceptionally higher velocities than normal, some of which are still moving outward (with excessive red shifts) while others are returning from their outward excursions and moving towards us with their light shifted away from the red.  Another view is that we are looking back so far in time, we are seeing radiation from the formative period of our universe, when immense clouds were collapsing into nebulae and star clusters, something not occurring to the same extent today.  In addition to these possibilities, if in fact more than one universe exist just like ours, we may in fact be seeing the encroachment of a neighboring universe into our own.  This latter view might neatly explain these various quasi-stellar and powerful radio sources at the distant fringes of our universe.

Also associated with the Big-Bang, is the postulation of dark matter;  which is unseen mass (hence its name) essential to the proponents of the Big-Bang model, as of yet to be detected.