Here is how we think it all happened (From R. U. Buehler,  http://www.hgb-leipzig.de/weltbilder/) Key points: Grand Unified Theories; Horizon Problem; Flatness Problem; Inflation; assembly of hydrogen and helium atoms; matter vs. antimatter

We need to take things one "era" at a time.
Big Bang (Cosmic timeline from M. Norman, http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/CosmicMysteryTour.html)

Planck Era

Conditions were so extreme in the Planck Era that our current understanding of physics is inadequate to tell us much about them.

"The Universe is not only queerer then we suppose, it is queerer than we can suppose." -Mark Twain

GUT Era
Under conditions of extreme temperature and density, the four fundamental forces of physics look more and more like different manifestations of a single force law.

Physicists have successfully  developed a theory that unifies the strong, weak, and electromagnetic forces, called (with a little puffery) the Grand Unified Theory (or GUT for short), which includes the standard model of particle physics that we discussed earlier. The three forces would have been unified and described by this theory under the conditions in the first 10-38 seconds or so. Physicists would like to believe that gravity can be unified under the extreme conditions in the first 10-40 seconds, but so far this has not been demonstrated necessarily to be true from our current understanding of the laws of physics. (From http://zebu.uoregon.edu/~imamura/123/lecture-9/lecture-9.html)

The GUT Era ended when the strong force separated from the others, resulting in release of a huge amount of energy that caused the Universe to expand very quickly. In the resulting brief interval of "inflation", the Universe expanded by

about 10³⁵ times in 10^-32 seconds, from less than the size of a single electron to the size of a golf ball.

A simple expanding universe has two difficult problems to solve, both of which are neatly accounted for by this rapid inflation.
The Horizon Problem

How can the Universe be so uniform? Now, the time for light to cross a significant part of the Universe is billions of years. We call this time the light communication time, and it is the shortest time required for any changes to be felt between two parts of the Universe. (From J. Schombert, http://zebu.uoregon.edu/~js/ast123/lectures/lec19.html)

If the Universe just expanded in a uniform way, it would have developed large uniformities over distances where the light communication time would be too long to even them out. The observed high degree of uniformity (to about 1 part in 100,000 for the 3K radiation!) must have been locked in at an early stage and maintained since then. We illustrate the problem with a band that synchronizes on the beat of a drum on the left side in this animation. By the time the sound reaches the right side, the band is half a second behind over there compared with where it started on the left side - we have a nonuniformity of the band playing for half a second on the left side before it knows it should be playing on the right side. Similarly, indications of an event at one place in the early Universe - say an explosion that heated up its surroundings tremendously - would travel outwards from it at the speed of light. Therefore, there would be a delay in the rest of the Universe responding - for example, heating up due to the energy it released. Since the early Universe was a very dynamic place, it would be expected to be heated very non-uniformly - not the very smooth, uniform place we know it was!! (animation by G. Rieke)

The high degree of uniformity was established in the first 10^-35 seconds and then locked in by the expansion of inflation that was so rapid that it was impossible for large scale interactions to upset it!

To see how this would work, go back to the marching band we discussed just above. Suppose the band started out very small and got the "beat" from the drummer, and then expanded much faster than the speed of sound, before anyone could get out of synchronization. Then when it began to play, both the left and right sides would be playing together. This is exactly the idea behind inflation in the early Universe, strange as it seems. Everything gets evened out when the Universe is very tiny and then the Universe expands extremely rapidly and becomes 1035 times bigger but retains the uniformity from when it was tiny. (animation by G. Rieke)

The Flatness Problem
Having the Universe come out just at the critical density is a lot like balancing a pencil on its tip -- it can be done only with the greatest delicacy.

This figure shows that the Universe has to be exactly at the critical density (the curve labeled 1.0) at the first second of its evolution, or it rapidly departs from that condition toward being strongly open or closed. The other curves are labeled with the density compared with the critical value at the first second, and they evolve quickly to values differing by huge amounts from critical. (From J. Schombert, http://zebu.uoregon.edu/~js/ast123/lectures/lec19.html)

No matter what you start with, if something is expanded by such a big (1035) factor, it will look very flat anyplace you look. Thus, inflation also explains how the Universe came out exactly at . An ant crawling on a balloon experiences a curved surface. See how the ant's Universe is flattened by blowing up the balloon! (From J. Brau, http://blueox.uoregon.edu/~jimbrau/astr123/Notes/Exam3rev.html.)

The use of an analogy with simple geometry to explain something so fundamental as the Universe being at the critical density may seem overly simplistic. However, the relation between the curvature of space and its contents is very fundamental in the Einstein-Lemaitre-Friedmann version of the Universe. Thus, the concept of inflation solves both the Horizon Problem and the Flatness Problem.

Electroweak Era

This picture tries to capture the collision of super-energetic particles to make a Higgs Boson. Artist's concept of Higgs Boson, http://news.msn.com/science-technology/physicists-on-higgs-boson-hunt-nearly-there-but-not-yet

The recent discovery of the Higgs boson, if confirmed, suggests that our picture of  the early Universe with the the GUT era followed by the Electroweak Era is correct. The Higgs boson would be the only particle present during the GUT era. In the Electroweak Era, Higgs bosons could collide to create W and Z bosons that carry the electroweak force and quarks, which are fundamental to matter as we know it. The Electroweak Era ended when the Universe cooled sufficiently that W and Z bosons were no longer being created; they decayed away and without them the electroweak force separated from the electromagnetic one and became the short-range weak nuclear force.

The Universe was from then on governed by the physical laws as we now experience them.
 
Particle Era
At first, it was too hot for protons and neutrons to survive. Instead, there was a dense sea of "quarks" and "anti-quarks", the underlying particles out of which protons and neutrons (and their anti-particles) are made. There was a nearly equal mixture of quarks and anti-quarks -- anti particles in physics behave exactly as their counterpart particles but have opposite charge and annihilate if they collide with particles, so the mass of both the particle and anti-particle is converted to energy and emerges as gamma rays. As the Universe expanded and cooled, annihilation proceeded; either because of a slight asymmetry in the behavior of the particles or a slight excess of particles over antiparticles to start with, all the anti-quarks were annihilated and only quarks were left  -- along with a lot of gamma rays.

As the Universe cooled further, the quarks slowed down until the strong nuclear force could draw them together to make protons and neutrons (at about 1 second after the Big Bang).See: Cosmos in a Computer, U. Illinois http://www.ncsa.uiuc.edu/Cyberia/Cosmos/CosmosCompHome.html

Particle Era

Era of Nucleosynthesis
The temperature remained high enough for the first 10 seconds that energy was still passed back and forth freely between electrons/antielectrons (the latter called positrons) and photons. (figures from J. Schombert, http://zebu.uoregon.edu/~js/ast123/lectures/lec18.html)

At this time, the conditions were at such high temperature and density that fusion reactions of protons into helium nuclei started.  Hydrogen fusion, often called the proton-proton chain is shown to the left.   (From Nick Strobel Go to his site at www.astronomynotes.com for the updated and corrected version.) (Deuterium is an "isotope" of hydrogen)

A summary of how it works is below. Take a close look, since we will come back to it when we talk about the sun.

(From U. Tenn Ast  162, http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html)

There is a whole chain of fusion reactions going from hydrogen to helium, from helium to beryllium and lithium and carbon, from carbon to oxygen, and so forth, building up a variety of more complex elements from pure hydrogen.

Models of "different" Universes show that the reactions proceed farther the greater the amount of protons and neutrons to interact. Thus, with imaginary universes of increasing density, we get increasing amounts of helium 4 and lithium, but decreasing amounts of helium 3 and hydrogen 2 (deuterium). (From M. White, http://astron.berkeley.edu/~mwhite/darkmatter/bbn.html)

The current-day abundances of deuterium and lithium are indicated by the light vertical bar through the center of the image. They show that our Universe must have had a relatively low density of protons and neutrons to start with  -- about 4% of the total. That is, the density of the Universe was not high enough to sustain the reactions building more complex elements than lithium, and only a small amount of lithium was built compared with the possibilities for a denser Universe. These particles are called altogether "baryons". The remaining 96% of the mass is in the dark matter and dark energy, which have different properties than baryons. We know very little about what they are or how they behave. and Particle Eras