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Cosmology - The Big Bang



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The Universe is expanding. This fact alone suggests a starting point. But unlike circular expansion as we know it, Astronomers abide by the cosmological principle:

"Viewed on sufficiently large distance scales, there are no preferred directions or preferred places in the Universe."

In other words, there is no known point of origin of the Big Bang and the expansion of the Universe is relative to the observers position in the Universe.

The more we learn about our Universe, the more we know the Universe began from a single point of energy some 13.6 billion years ago.

The most compelling evidence: The Cosmic Background Radiation.

The temperature of the Cosmic Background Radiation is about 2 Kelvin. With such low temperature, this effect is not the result of any emission from stars or galaxies. Think of compressed gas - at its highest compression, the heat can increase very rapidly. But release the gas and the outer envelope cools rapidly - just like the Cosmic Background Radiation (or CMB - Cosmic Microwave Background Radiation).

At a redshift of 1000 (z=1000), decoupling occurred (around 300,000 years after the Big Bang).

Decoupling is just another term for recombination when electrons were bound up by protons.

The overall temperature of the CMB is 2.735 Kelvin.

(Image credit: Brooks/Cole Thomson Learning)

As an aside, the CMB is the only object in the Universe that represents a near perfect black body (a black body is a theoretical object that absorbs all light that fall onto it - no no transmission or reflection - any emission from the object will be 100% of all light that was absorbed). Black body radiation is a term used to study the effects of radiation from stars (and other things that emit radiation).

To help understand the Big Bang, we first identify the material involved:

Note that this includes matter and antimatter - i.e. the proton is positive and the neutron is not; the electron spins in one direction while the positron spins in the other. For more information on this, please take a look at the introduction to Quantum Physics.

The timeline (note: we have no idea what happened at exactly T=0 seconds)

  • T = 1/1,000,000 seconds - temperature is 10 trillion Kelvin - light elements form: photons, quarks, neutrinos, electrons (no protons or neutrons)
  • T = 1/100 seconds - temperature  is 100 billion Kelvin - protons and neutrons form
  • T = 1/10 seconds - temperature  is 50 billion Kelvin - Neutrons less stable and convert back to photons - 60% protons, 40% neutrons - protons join to form deuterium, but unstable at high temperature - deuterium bottleneck
  • T = 1 second - temperature is 10 billion Kelvin - deuterium bottleneck still, now 75% protons and 25% neutrons
  • T = 14 seconds - temperature is 4 billion Kelvin - deuterium bottleneck still, now 82% protons and 18% neutrons
  • T = 3 minutes - temperature now under 1 billion Kelvin - deuterium can form (2 protons form nucleus), helium (4 protons) also forms

The first 3 minutes of the Big Bang resulted in the nucleosynthesis of both hydrogen and helium - without electrons. Also during this time the temperature rapidly decreased while the expansion rapidly increased.

Now for a more "slow" evolution:

  • T = 35 minutes - temperature is now 300 million Kelvin - temperature still to hot for hydrogen and helium to bind electrons, increased neutrinos and antineutrinos by positron annihilation
  • T = 1000 years - temperature is now 100,000 Kelvin - bridge between radiation dominated and matter dominated Universe - Dark Energy
  • T = 300,000 years - temperature is now only a few thousand Kelvin - recombination transition - electrons can now bind with deuterium and helium nucleus' - Universe becomes transparent - CMB

Prior to T = 1000 years, radiation in the form of photons and neutrinos dominated the Universe. Deuterium and helium nuclei were still forming but electrons could not be bound. Prior to T = 300,000 years, the Universe was opaque mainly because of the dominate free electrons. Once the recombination transition was reached, the Universe became transparent.


(Image credit: Brooks/Cole Thomson Learning)

Astronomers continue to study the big bang by using tools like computer simulations of dark matter distribution and looking deep into the early Universe using the Hubble Space Telescope.

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