There is no single experiment capable of proving or disproving the Big Bang Theory, however there is a plethora of strong, significant observational evidence which supports the theory as the predominant explanation of the origin of our Universe.
The Universe first existed as a Singularity or an infinitely small, infinitely dense point. The Big Bang occurred about 13.75 Billion years ago and the Universe expanded rapidly. The rapid expansion caused the Universe to cool, forming matter and resulted in its present continuously expanding state. The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe.
The “four pillars” of evidence supporting the Big Bang theory are:
- The observable expansion of the Universe.
- Origin of the cosmic background radiation.
- The abundance of light elements.
- Formation of galaxies and large-scale structure.
Expansion of the Universe
Prior to the early 20th century, the Universe was thought to be unchanging. Observations and analysis in the late 1920s by Edwin Hubble demonstrated that assumption to be false. Hubble found the farther away a galaxy is from our own, the faster it recedes from us. The relationship between a galaxy’s distance and recessional velocity is known as Hubble’s Law.
There is considerable observational evidence for this assertion, including the measured distributions of galaxies and faint radio sources.
Any observer anywhere in the Universe will see galaxies are moving away from them because the Universe is expanding uniformly from every point in space. It can be a difficult concept to grasp, but you could visualize it like this an expanding balloon: Imagine residing in a curved flatland on the surface of a balloon. As the balloon is blown up, the distance between all points grows; the two-dimensional universe grows but there is no preferred center.
Hubble’s observations were later shown to be correct as new technology emerged. The Hubble Space Telescope showed us for the first time, that our view of the Universe was vastly incomplete. By focusing on tiny areas of the night sky previously thought to be empty, we caught a glimpse of the early Universe.
Scientists were able to deduce that as stars, galaxies or clusters move through space and the Universe continues to expand, the light has to travel further to reach Earth and we perceive a redshift. Redshift occurs when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the spectrum.
Cosmological redshift is seen due to the expansion of the Universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase of their distance from Earth.
Cosmic Microwave Background Radiation was discovered in 1965 by Arno Penzias and Robert Wilson from Bell Labs when they were working with a microwave receiver used to communicate with the Telstar satellite. In every direction they pointed their microwave receiver they got “noise” interference. It came from all over the sky at what seemed to be exactly the same frequency. The “noise” came from the first photons that could stream freely through the universe that have become so stretched out over time (due to the universe’s expansion) that they now exist in the microwave range.
These photons are an echo from when the young Universe was around 380,000 years old. Before this time, the Universe was completely opaque to light because electrons at the time were too energetic / too hot to bind with atomic nuclei. The electrons roamed freely about and photons could not move about freely because they were continually being absorbed and re-emitted by the electrons. When the Universe cooled off sufficiently, electrons no longer had enough energy to overcome the attractive force of atomic nuclei, and they became bound, allowing photons to stream forth unimpeded in a process is called recombination.
The Abundance of Light Elements
There was a specific time in the history of our Universe when the temperature and density was in a very specific range to create vast quantities of certain elements. When the Universe was just a few minutes old, the temperature dropped enough so that nucleosynthesis could begin. This is when the light atomic nuclei of deuterium, tritium, and helium formed. After just a few more minutes, the Universe had cooled further to a point where neutrons and protons could no longer combine, and so the primordial ratios of these elements were frozen.
Astronomers have since used telescopes to measure the abundances and distribution of elements in the Observable Universe. The Big Bang predicts about 72% of the material out there is hydrogen, and 28% is helium. Astronomers have found that about 24% is helium and 76% is hydrogen, and with error bars, this is in good agreement with the theory. Because these elements cannot be created (except in stars) and because they exist in such large quantities, our Universe must have existed in a state with temperatures and densities high enough to allow for protons and neutrons to exist as free particles, not bound up in atomic nuclei, thus forming massive quantities of the lighter elements. Later, the heavier elements were created in stars and spread via supernovae and exist in vastly smaller amounts.
About 10,000 years after the Big Bang, the temperature of the Universe dropped low enough to allow matter to take over as the dominant source of energy density, replacing light and other radiation. The attractive gravitational forces exerted by large particles were now stronger than the force exerted by various radiation so that any perturbations in their density would enable them to grow by accreting other matter from their surroundings.
13 Billion years later and matter has accumulated in wildly complex patterns with planets orbiting stars and stars orbiting around objects of much higher density like black holes.
1. TalkOrigins, Björn Feuerbacher and Ryan Scranton, Evidence for the Big Bang
2. University of Cambridge Department of Applied Mathematics and Theoretical Physics, The Four Pillars of the Standard Cosmology
3. Weber State University Department of Physics, Elementary Astronomy, The Four Pillars of Big Bang Cosmology
4. RationalWiki, Big Bang