The Big Bang - The Beginning of the Universe As We Know It

We look at what the Big Bang theory is, the evidence we have for the theory and what the future may hold for the universe.

Throughout the ages, mankind has gazed at the stars and wondered how the universe was created. People who have tried to uncover the mysteries of the universe's development include such famous scientists as Albert Einstein, Edwin Hubble and Stephen Hawking. One of the most famous and widely accepted models for the universe's creation and development is the big bang theory.

The earliest stages of the big bang focus on a tiny fraction of a second in which all the separate forces of the universe were part of a single unified force. The laws of physics and science break down the further back you look. Eventually, you can't make any scientific theories about what is happening because science itself doesn't apply.

When we gaze up at the night sky, we see a multitude of galaxies separated by huge distances and vast space. At the point of the big bang, all the matter and energy were compressed to an area of zero volume and infinite density. This is what cosmologists call a singularity.

During the first few moments, no matter could form as there was far too much energy. The universe expanded rapidly, which means it became less dense and therefore cooled down. As the universe expanded and cooled, matter began to form, and radiation began to lose energy. In just a few short seconds the universe was formed from a singularity, that now stretches across space.

As a result of the big bang, the four basic forces were formed. These four forces are:

1. Electromagnetism
2. Strong nuclear force
3. Weak nuclear force
4. Gravity

At the start of the big bang, those forces had been all part of a single unified force. It was only shortly after the big bang started that the forces separated into what they are nowadays.

Physicists and cosmologists are still working on forming the Grand Unified Theory, which might explain how the four forces were once united and how they relate to each other.

What Evidence Do We Have for the Big Bang?

The big bang theory is backed up by three key lines of evidence. From speeding galaxies to ancient gas clouds, there is a lot of proof that we can detect today - the remnants of the Big Bang, that tell a clear story concerning the origins of our Universe.

1. We can observe that the universe is expanding. We observe that galaxies are shifting apart, so therefore previously everything must have been closer together, and we can further assume that one time everything must have once been in one spot.
2. The discovery of the cosmic microwave background radiation (CMB). This is the faint afterglow of the hot young universe which we can observe.
3. The abundances of light elements - deuterium, helium and lithium - in stars and gas clouds are as predicted by computer models if they formed by the nuclear process in the Big Bang fireball.

The evidence all points to the big bang occurring 13.77 billion years in the past. The Universe then swelled abruptly for a fraction of a second, a period known as cosmic inflation, and the expanded more steadily, cooling as it did so. Protons (hydrogen nuclei) and neutrons appeared within the first second, combining within three minutes to produce the nuclei of light elements. A hundred million years later, the first stars began to grow in hydrogen clouds.

The Cosmic Microwave Background

After about 380,000 years the temperature of the universe fell to around 2,727°C. The universe changed from being a plasma to being neutral, allowing photons to travel freely across space and protons and electrons to combine into hydrogen atoms through fusion. Today we see the echoes of these photons at a temperature far cooler, -270°C. This "warm" glow appears as a weak microwave signal coming from all over the sky. It was first detected in 1965. Its exact temperature was measured in the 1990s by NASA's Cosmic Background Explorer mission, and since then astronomers have mapped the CMB in much more detail. The maps reveal a rash of hot and cold spots indicating high and low-density regions in the young universe from which galaxy structures grew. A new map is currently being made by EAS's Planck satellite.

Stages of the Big Bang

There were several stages to the Big Bang evolution, most of which occurred in the first few million years.

Origin of the Big Bang - A Singularity

When we look at the expansion of the universe in reverse using general relativity, we see find the universe to be infinitely dense and of infinite temperature at a finite time in the past. The Big Bang singularity indicates that general relativity is not an adequate description of the laws of physics for this point in time. Models based on general relativity alone cannot extrapolate toward the singularity beyond the end of the Planck epoch.

Inflation of the Universe

The earliest phases of the Big Bang are subject to much speculation. In the most common models, the universe was filled homogeneously and isotopically with a very high energy density and huge temperatures and pressures and was very rapidly expanding and cooling. Approximately 10^-37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially during which time density fluctuations that occurred because of the uncertainty principle were amplified into the seeds that would later form the large-scale structure of the universe.

After inflation stopped, reheating occurred until the universe obtained the temperatures required to produce a quark-gluon plasma as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were continuously created and destroyed in collisions.

Baryogenesis - Creation of Protons and Neutrons

The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 10^-11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators.

At about 10^-6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. This resulted in the predominance of matter over antimatter in the present universe.

The temperature was now no longer high enough to create new proton-antiproton pairs (similarly for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles.

Cooling of the Universe

A few minutes into the expansion, when the temperature was about a billion (one thousand million) kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei.

As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years, the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This radiation is the left-over radiation from the Big Bang and is known as the cosmic microwave background radiation. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10^-17 million years old.

Structure Formation after the Big Bang

Over a long period, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter, warm dark matter, hot dark matter, and baryonic matter.

The best measurements available, from Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-Cold Dark Matter (Lambda-CDM) model in which dark matter is assumed to be cold (warm dark matter is ruled out by early reionization), and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%.

Dark Matter and Cosmic Acceleration

Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which permeates all of space. The observations suggest 73% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity predominated, and it was slowly breaking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate.

Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theoretically.

How Will the Universe End? A Big Crunch?

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started — a Big Crunch.

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero — a Big Freeze.

Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known.