The Birth of a Universe
The Origin of the Four Basic Interactions
Particle physicists believe that, despite their different characteristics, if two particles were to collide at very high energies, these forces would be indistinguishable from each other and have the same strength. In other words, they would all be unified into a single super force and theories which attempt to unify the four interactions in this was are called Grand Unified Theories or GUTs for short.
In the 1970's particle physicists were able to show theoretically that the weak and electromagnetic force, collectively called the electroweak force, could be unified at energies of about 100 GeV (1GeV is a unit equal to 1 billion electron-volts) which corresponds to a temperature of 1015 K. The unification of the electroweak and strong forces does not occur until about 1014 GeV and, according to GUTs, all forces are unified at 1019 GeV. Particle accelerators have been built that can operated at a few 100 GeV, but is simply not possible to construct them capable of producing the full range of unification energies.
The Big Bang model though, offers a way of testing GUTs. According to GUTs during the first 10–43 s of the universe's existence when it was at a temperature exceeding 1032 K, all the forces were unified as a single force. When it cooled, the forces of nature 'froze out' to produce the four interactions that we know today. This would account for the balance of matter over antimatter as before the strong force froze out, equal numbers of particles and antiparticles were being created and destroyed. After the strong force froze out, this symmetry was broken leading to an excess of matter.
GUT's also predict that neutrinos created in the Big Bang should also have a small mass. If this is so then they would be a candidate for dark matter and the value of the mass-energy density might be altered so that
ρ = ρc
where ρc is the critical density. Cosmologists also believe that after decoupling, neutrinos might have aided galaxy formation.
Problems with the Big Bang
Despite its successes the standard big bang theory still leaves some unanswered questions (This is not to say the big bang theory is in danger of being discredited or is a failed scientific theory—it simply means that some facets of a very complex model must be revised as new observations and data are made available. This is the essence of scientific inquiry and integral to the methodology of science.)
The first concern is the geometry of space and time. Observations suggest that Ωo is very close to 1 which would suggest that we are living in a flat universe. What special conditions would have led to the mass-energy density being exactly equal to the critical density ρc? It turns out that if the average density of matter even slightly deviated from ρc, then this variation would have rapidly multiplied as the universe expanded leading to a value very far from Ωo = 1. The fact that Ωo is close to 1 now, means that some process in the big bang resulted in the near flat universe we see today. This is called the flatness problem and it is explained by introducing a concept called inflation.
Inflation explains why the cosmic microwave background is isotropic. Before inflation, the universe was small enough for all parts to reach the same temperature within the light horizon. After inflation, this uniformity of temperature is maintained at larger distances even though it now takes considerably longer for light to travel across the universe. Note that inflation does not violate special relativity's postulate that the speed of light is constant since it is the expansion of spacetime itself and not expansion through space that is at work here.
Recent observations have introduced another "wrinkle" into our understanding of the universe's evolution. In the late 1990s, two teams of astronomers simultaneously reached the conclusion that the expansion of the universe — hitherto assumed to be slowing due to the incessant gravitational pull of the universe's matter — was in fact accelerating! Their conclusion was based on observations of Type Ia supernovae in distance galaxies. This type of supernova (produced when a white dwarf draws material from a companion star and exceeds the Chandrasekhar limit to proceed to fusion of heavier and heavier nuclei) has a very distinctive light curve and peak luminosity. The observed luminosities indicated the host galaxies were farther than expected and obeying a Hubble flow that shows acceleration instead of braking. The cause of this seeming repulsive force that becomes evident at immense cosmological distances has been dubbed Dark Energy. It's nature is still unknown and this area of cosmology is one of the most exciting areas of research today. Stay tuned—recent Hubble Space Telescope observations indicate that Dark Energy began to dominate the universe's evolution when it was only about a 1/4 of its current age.
Before 10–43 s: The Planck
Time
General relativity regards space and time as linked together in a smooth
continuum but, at very small scales of length and time, quantum mechanics
says that this representation of space-time breaks down. This is because
of a law in quantum mechanics called the Heisenberg
Uncertainty Principle named after a German physicist Werner Heisenberg (1901–1976). The
uncertainty principle tells us that there always exists a degree of uncertainty
between the position of a particle and its momentum. The more accurately
you measure the speed of a particle the less certain you are of its position
and the uncertainty principle makes it impossible to measure the exact
position and the exact momentum simultaneously. The principle also extends
to energy and time.
By using the uncertainty principle, cosmologists are able to arrive at an interval after which space and time are measurable called the Planck time tp which is made up of the fundamental constants G, h and c as
tp = G1/2 h1/2 c–5/2 = 1.35 x 10–43 sec
From the beginning of the Big Bang at t = 0 to t = tp, we do not know how the universe behaved and we have no theory that can explain what happened using quantum mechanics or general relativity. Time itself began at the Big Bang and to say what came 'before' is a meaningless statement as time didn't exist then!
Particle physicists believe that the vacuum of space is not empty but is seething with pairs of particles and antiparticles that are constantly being made and destroyed. These particles exist for so brief a time (less than 10–21 s) that they hardly exist at all, and are called virtual particles. These particles may exist for a time that satisfies the uncertainty principle, but as the universe rapidly expanded they became separated and appeared as real particles after the universe was older than the Planck time. The gravitational energy associated with the Big Bang singularity provided enough energy to fill the universe as it expanded. The main point about all these concepts is that the Planck time sets a limit to what we can know about the very early universe according to our current knowledge of physics, however theoretical particle physicists are currently working on a quantum theory of gravity and progress continues.