The discovery of an expanding Universe implies the obvious, that the Universe must have an initial starting point, a Creation. A point in the past when the radius of the Universe was zero. Since all the matter in the Universe must have been condensed in a small region, along with all its energy, this moment of Creation is referred to as the Big Bang.
A common question that is asked when considering a Creation point in time is ``What is before the Big Bang?''. This type is question is meaningless or without context. For example, it is similar to asking ``What is north of the North Pole?''. The question itself can not be phrased in a meaningful way.
Four main pieces of scientific evidence support the Big Bang
The Big Bang theory has been supported by numerous observations and, regardless of our final theories of the Universe, remains the core element to our understanding of the past. Note that a Creation point automatically implies two things: 1) the Universe has a finite age (about 15 billion years) and 2) the Universe has a finite size.
Geometry of the Universe :
There are basically three possible shapes to the Universe; a flat Universe (Euclidean or zero curvature), a spherical or closed Universe (positive curvature) or a hyperbolic or open Universe (negative curvature). Note that this curvature is similar to spacetime curvature due to masses, like stars, in that the entire mass of the Universe determines the curvature.
All three geometries are classes of what is called Riemannian geometry, based on three possible states for parallel lines
or one can think of triangles where for a flat Universe the angles of a triangle sum to 180 degrees, in a closed Universe the sum must be greater than 180, in an open Universe the sum must be less than 180.
Its important to remember that the above images are 2D shadows of 3D space, it is impossible to draw the geometry of the Universe, it can only be described by mathematics. All possible Universes are finite since there is only a finite age and, therefore, a limiting horizon. The geometry may be flat or open, and therefore infinite, but the amount of mass and time in our Universe is finite.
Density of the Universe:
The key factor that determines which history is correct is the amount of mass/gravity for the Universe as a whole. If there is sufficient mass, then the expansion of the Universe will be slowed to the point of stopping, then retraction to collapse. If there is not a sufficient amount of mass, then the Universe will expand forever without stopping. The flat Universe is one where there is exactly the balance of mass to slow the expansion to zero, but not for collapse.
The parameter that is used to measure the mass of the Universe is the critical density, Omega. Omega is usually expressed as the ratio of the mean density observed to that of the density in a flat Universe.
Given all the range of values for the mean density of the Universe, it is strangely close to the density of a flat Universe. And our theories of the early Universe (see inflation) strongly suggest the value of Omega should be exactly equal to one. If so our measurements of the density by galaxy counts or dynamics are grossly in error and remains one of the key problems for modern astrophysics.
Birth of the Universe :
Our physics can explain most of the evolution of the Universe after the Planck time (approximately 10-43 seconds after the Big Bang). However, the time before this undefined in our current science and, in particular, we have no solid understanding of the origin of the Universe (i.e. what started or `caused' the Big Bang). At best, we can describe our efforts to date as probing around the `edges' of our understand in order to define what we don't understand, much like a blind person would explore the edge of a deep hole.
Cosmic Singularity :
Extrapolation from the present to the moment of Creation implies an origin of infinite density and infinite temperature (all the Universe's mass and energy pushed to a point of zero volume). Such a point is called the cosmic singularity.
Infinites are unacceptable as physical descriptions but our observers are protected by the principle of cosmic censorship. This means that singularities exists only mathematically and not as a physical reality that we can observe or measure. A typical solution to this problem is an event horizon as with black holes.
Planck Era :
Although we have little knowledge of the Universe before the Planck time, we can calculate when this era ends and when our physics begins. This occurs when the Universe is at the Planck scale in its expansion.
Remember, there is no `outside' to the Universe. One could, in theory, measure the size of the Universe much like you measure the radius of the Earth. You don't dig a hole in the Earth and lower a tape measure, you measure the circumference (take an airplane ride) of the Earth and divide by 2 pi (i.e. C = 2 x pi x radius).
The Universe expands from the moment of the Big Bang, but until the Universe reaches the size of the Planck scale, there is no time or space. Time remains undefined, space is compactified. Superstring theory maintains that the Universe had 10 dimensions during the Planck era, which collapses into 4 at the end of the Planck era (think of those extra 6 dimensions as being very, very small hyperspheres inbetween the space between elementary particles, 4 big dimensions and 6 little tiny ones).
During the Planck era, the Universe can be best described as a quantum foam of 10 dimensions containing Planck length sized black holes continuously being created and annihilated with no cause or effect. In other words, try not to think about this era.
An example of unification is the consider the interaction of the weak and electromagnetic forces. At low energy, photons and W,Z particles are the force carriers for the electromagnetic and weak forces. The W and Z particles are very massive and, thus, require alot of energy (E=mc**2). At high energies, photons take on similar energies to W and Z particles, and the forces become unified into the electroweak force.
There is the expectation that all the nuclear forces of matter (strong, weak and electromagnetic) unify at extremely high temperatures under a principle known as Grand Unified Theory, an extension of quantum physics of as yet undiscovered relationships between the strong and electroweak forces.
The final unification resolves the relationship between quantum forces and gravity (supergravity).
In the early Universe, the physics to predict the behavior of matter is determined by which forces are unified and the form that they take. The interactions just at the edge of the Planck era are ruled by supergravity, the quantum effects of mini-black holes. After the separation of gravity and nuclear forces, the spacetime of the Universe is distinct from matter and radiation.
Spacetime Foam :
Most of these black holes and wormholes are leftover from the Planck era, remnants of the event horizon that protected the cosmic singularity. These conditions are hostile to any organization or structure not protected by an event horizon. Thus, black holes are the only units that can survive intact under these conditions, and serve as the first building blocks of structure in the Universe, the first `things' that have individuality.
Based on computer simulations of these early moments, there is the prediction that many small, primordial black holes were created at this time with no large black holes (the Universe was too small for them to exist). However, due to Hawking radiation, the primordial black holes from this epoch have all decayed and disappeared by the present-day.
Matter arises at the end of the spacetime foam epoch as the result of superstrings, or loops in spacetime. The transformation is from ripping spacetime foam into black holes, which then transmute into elementary particles. Thus, there is a difference between something and nothing, but it is purely geometrical and there is nothing behind the geometry. Matter during this era is often called GUT matter to symbolize its difference from quarks and leptons and its existence under GUT forces.
In the early Universe, pressures and temperature prevented the permanent establishment of elementary particles. Even quarks and leptons were unable to form stable objects until the Universe had cooled beyond the supergravity phase. If the fundamental building blocks of Nature (elementary particles) or spacetime itself were not permanent then what remained the same? The answer is symmetry.
Often symmetry is thought of as a relationship, but in fact it has its own identical that is preserved during the chaos and flux of the early Universe. Even though virtual particles are created and destroyed, there is always a symmetry to the process. For example, for every virtual electron that is formed a virtual positron (anti-electron) is also formed. There is a time symmetric, mirror-like quality to every interaction in the early Universe.
Symmetry also leads to conservation laws, and conservation laws limit the possible interactions between particles. Those imaginary processes that violate conservation laws are forbidden. So the existence of symmetry provides a source of order to the early Universe.
Pure symmetry is like a spinning coin. The coin has two states, but while spinning neither state is determined, and yet both states exist. The coin is in a state of both/or. When the coin hits the floor the symmetry is broken (its either heads or tails) and energy is released in the process.
The effect of symmetry breaking in the early Universe was a series of phase changes, much like when ice melts to water or water boils to stream. A phase change is the dramatic change in the internal order of a substance. When ice melts, the increased heat breaks the bonds in the lattice of water molecules, and the ice no longer holds its shape. Phase change in the early Universe occurs at the unification points of fundamental forces. The decoupling of those forces provides the energy input for the phase change.
With respect to the Universe, a phase change during symmetry breaking is a point where the characteristics and the properties of the Universe make a radical change. At the supergravity symmetry breaking, the Universe passed from the Planck era of total chaos to the era of spacetime foam. Spacetime was acquired during the phase transition. During the GUT symmetry breaking, mass and spacetime separated and particles came into existence.
Notice that as symmetry breaks, there is less order, more chaos. The march of entropy in the Universe apples to the laws of Nature as well as matter. The Universe at the time of the cosmic singularity was a time or pure symmetry, all the forces had equal strength, all the matter particles had the same mass (zero), spacetime was the same everywhere (although all twisted and convolved).
There are two major problems for the Big Bang model of the creation of the Universe. They are
The flatness problem relates to the density parameter of the Universe, . Values for can take on any number between 0.01 and 5 (lower than 0.01 and galaxies can't form, more than 5 and the Universe is younger than the oldest rocks). The measured value is near 0.2. This is close to an of 1, which is strange because of 1 is an unstable point for the geometry of the Universe.
Values slightly below or above 1 in the early Universe rapidly grow to much less than 1 or much larger than 1 (like a ball at the top of a hill). So the fact that the measured value of 0.2 is so close to 1 that we expect to find in the future that our measured value is too low and that the Universe has a value of exactly equal to 1 for stability. And therefore, the flatness problem is that some mechanism is needed to get a value for to be exactly one (to balance the pencil).
The horizon problem concerns the fact that the Universe is isotropic. No matter what distant corners of the Universe you look at, the sizes and distribution of objects is exactly the same (the Cosmological Principle). But there is no reason to expect this since opposite sides of the Universe are not causally connected, any information that is be transmitted from one side would not reach the other side in the lifetime of the Universe (limited to travel at the speed of light).
All the Universe has an origin at the Big Bang, but time didn't exist until after the Planck era. And by the end of that epoch, the Universe was already expanding such that opposite sides were not causally connected.
The solution to both the flatness and horizon problems is due to a phase of the Universe called inflation. Currently, inflation is the only theory that explains why the observable Universe is both homogeneous and causally connected. During inflation the Universe expanded a factor of 1054, so that our horizon now only sees a small piece of what was the total Universe from the Big Bang.
The cause of the inflation era was the symmetry breaking at the GUT unification point. At this moment, spacetime and matter separated and a tremendous amount of energy was released. This energy produced an overpressure that was applied not to the particles of matter, but to spacetime itself. Basically, the particles stood still as the space between them expanded at an exponential rate.
Note that this inflation was effectively at more than the speed of light, but since the expansion was on the geometry of the Universe itself, and not the matter, then there is no violation of special relativity. Our visible Universe, the part of the Big Bang within our horizon, is effectively a `bubble' on the larger Universe. However, those other bubbles are not physically real since they are outside our horizon. We can only relate to them in an imaginary, theoretical sense. They are outside our horizon and we will never be able to communicate with those other bubble universes.
Inflation solves the flatness problem because of the exponential growth. Imagine a highly crumbled piece of paper. This paper represents the Big Bang universe before inflation. Inflation is like zooming in of some very, very small section of the paper. If we zoom in to a small enough scale the paper will appear flat. Our Universe must be exactly flat for the same reason, it is a very small piece of the larger Big Bang universe.
The horizon problem is also solved in that our present Universe was simply a small piece of a larger Big Bang universe that was in causal connection before the inflation era. Other bubble universes might have very different constants and evolutionary paths, but our Universe is composed of a small, isotropic slice of the bigger Big Bang universe.