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The universe's explosion into being is known as the Big Bang. In this article 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.

For centuries, humans have gazed at the stars and wondered how the universe developed into what it is today. It's been the subject of religious, philosophical, and scientific discussion and debate. 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 development is the big bang theory.

The earliest stages of the big bang focus on a moment in which all the separate forces of the universe were part of a unified force. The laws of science begin to 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.

Timeline of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. (Not to scale)

Timeline of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. (Not to scale)

Today, when we look at the night sky, we see galaxies separated by what appears to be huge expanses of empty space. At the earliest moments of the big bang, all of the matter, energy and space we could observe was compressed to an area of zero volume and infinite density. Cosmologists call this a singularity.

There was so much energy in the universe during those first few moments that matter as we know it couldn't form. But the universe expanded rapidly, which means it became less dense and cooled down. As it expanded, matter began to form and radiation began to lose energy. In only a few seconds, the universe formed out of a singularity that stretched across space.

One result of the big bang was the formation of the four basic forces in the universe. These forces are:

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

At the beginning of the big bang, these forces were all part of a unified force. It was only shortly after the big bang began that the forces separated into what they are today.

Many physicists and cosmologists are still working on forming the Grand Unified Theory, which would explain how the four forces were once united and how they relate to one another.

Evidence for the Big Bang

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

  1. The first is that the universe is expanding - galaxies are flying apart so in the past everything was closer together and must once have been in one spot.
  2. The second is the discovery of the cosmic microwave background radiation (CMB), the afterglow of the hot young universe.
  3. The third is that the abundances of the abundance of light elements - deuterium, helium and lithium - in stars and gas clouds are as expected if they formed by the nuclear process in the Big Bang fireball.

The evidence points to the big bang occurring 13.77 billion years ago. The Universe then swelled rapidly for a fraction of a second - a period known as cosmic inflation - and expanded 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 380,000 years the temperature of the Universe was about 2,727°C. This was cool enough for protons and electrons to combine into hydrogen atoms. The universe changed from being a plasma - a soup of charged particles - to being neutral, allowing photons to travel freely across space. Today we see these photos due to the expansion of the universe at a temperature far cooler: -270°C. This warm glow appears as a weak microwave signal coming from all over the sky, which 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 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

Singularity

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone can not extrapolate toward the singularity beyond the end of the Planck epoch.

Inflation

The earliest phases of the Big Bang are subject to much speculation. In the most common models the universe was filled homogeneously and isotropically 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 for the production of 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 being continuously created and destroyed in collisions.

Baryogenesis

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

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 relic radiation 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

Over a long period of time, 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%.

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 apparently 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 braking 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.

Ultimate Fate of the Universe

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.

If the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organised form of energy could be extracted from it, a scenario known as heat death.

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.

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