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10.5: Big Bang Theory

  • Page ID
    52215
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    Skills to Develop

    • Describe the statement "Big Bang Theory"
    • Give evidence for the Big Bang Theory

    Introduction

    The Big Bang is the currently accepted theory of the early development of the universe. Cosmologists study the origin of the universe and use the term Big Bang to illustrate the idea that the universe was originally an extremely hot and dense point in space at some finite time in the past and has since cooled by expanding to the present state. The universe continues to expand today. The theory is supported by the most comprehensive and accurate explanations from current scientific evidence and observation. According to the best available measurements the big bang occurred about 13.75 billion years ago.

    According to the theory the Universe would have cooled sufficiently to allow energy to be converted into subatomic particles (protons, neutrons, and electrons and many other particles). While protons and neutrons would have formed the first atomic nuclei only a few minutes after the Big Bang, it would then have taken thousands of years for electrons to lose enough energy to form neutral atoms. The first element produced would be hydrogen. Giant clouds of these primordial elements would then form stars and galaxies. Other elements were formed by fusion within the stars.

    Evidence for the Big Bang Theory

    Many scientists have contributed to gathering evidence and developing theories to contribute to our understanding of the origin of the universe and the Big Bang Theory. Georges Lemaître, a Belgian priest, physicist, and astronomer was the first person to propose the theory of the expansion of the Universe and proposed his hypothesis of the primeval atom which later became known as the Big Bang Theory. Lemaître's hypothesis used the work of earlier astronomers and proposed that the inferred recession of the nebulae (later shown to be galaxies) was due to the expansion of the Universe.

    More evidence of the expanding universe was provided by Alexander Friedmann, Russian cosmologist and mathematician. He derived the "Friedmann" equations from Albert Einstein's equations of general relativity, showing that the Universe might be expanding in contrast to the static Universe model advocated by Einstein at that time. Albert Einstein had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a "fudge factor" to the equations to avoid this "problem". When Einstein heard of Hubble's discovery, he said that changing his equations was "the biggest blunder of his life".

    Edwin Hubble is regarded as the leading observational cosmologist of the 1900's. He is credited with the discovery of galaxies other than the Milky Way. In 1929 Hubble presented evidence that galaxies were moving away from each other and that galaxies that are further away are moving faster, as first suggested by Lemaître in 1927. Hubble's evidence is now known as red shift. This discovery was the CK12 Screenshot 10-5-1.pngfirst observational support for the Big Bang Theory. If the distance between galaxies is increasing today, then galaxies and everything else in the universe must have been closer together in the past. In the very distant past, the universe must have indeed been extremely small and had extreme densities and temperatures.

    The opponents to the Big Bang Theory argued that if the universe had existed as a point in space, large amounts of radiation would have been produced as the subatomic particles formed from the cooling and expanding energy. After cosmic microwave background radiation was discovered in 1964 and the analysis matched the amount of missing radiation from the Big Bang, most scientists were fairly convinced by the evidence that some Big Bang scenario must have occurred.

    In the last quarter century, large particle accelerators have been built to provide significant confirmation of the Big Bang Theory. Several particles have been discovered which support the idea that energy can be converted to particles which combine to form protons. Although these accelerators have limited capabilities when probing into such high energy regimes, significant evidence continues to support the Big Bang Theory.

    Elements and the Big Bang Theory

    If the Big Bang Theory was correct, scientists predicted that they should still find most of the universe to be still composed of the hydrogen that was formed in the first few minutes after the Big Bang as the universe cooled and expanded. The observed abundances of hydrogen and other very light elements throughout the universe closely match the calculated predictions for the formation of these elements from the rapid expansion and cooling in the first minutes of the Universe. Over \(90\%\) of the entire universe remains in the lightest of the elements, hydrogen and helium. The heavier elements, from helium to iron were formed from fusion within stars. Fred Hoyle, who originally criticized the Big Bang Theory, provided an explanation of nuclear fusion in stars that later helped considerably in the effort to describe how heavier elements were formed from the initial hydrogen.

    The earth consists of much heavier elements. The most abundant elements in the earth's crust include oxygen, silicon, and aluminum. These elements were formed by fusion of the earliest (and heaviest stars) formed. The core of the earth is primarily iron. This iron was also formed is these very early, heavy stars. The radioactive elements found on the earth were most probably formed as these heavy stars died the violent death known as supernovae. The iron (and other elements near it on the periodic table) were thrown into the void of space with very high speeds allowing them to form still heavier elements by a similar process to which transuranium (artificial or man-made) elements have been formed during the 20\(^\text{th}\) century.

    Lesson Summary

    • The Big Bang Theory proposes that all matter in the universe was once contained in a small point, but has since expanded and cooled.
    • The theory is supported by scientists as it provides a satisfactory explanation for the observations that the universe is expanding today, that the universe is composed mostly of hydrogen and oxygen, cosmic background radiation, etc.
    • The theory also provides an explanation for where elements heavier than hydrogen were formed, through fusion, into heavier elements.

    Vocabulary

    • Big Bang Theory: The idea that the universe was originally extremely hot and dense at some finite time in the past and has since cooled by expanding to the present state and continues to expand today.
    • Cosmic background radiation: Energy in the form of radiation left over from the early Big Bang.

    Further Reading/Supplemental Links

    Contributors


    10.5: Big Bang Theory is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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