Video

Here is a video overview of the Big Bang Theory.

Big Bang Theory

• In 1929, Edwin Hubble discovered the Universe is expanding.
• The Hubble Law is the observation that the recessional velocity of distant galaxies (measured by their redshift) is proportional to their distance from the observer. This simple model/activity can help us understand this phenomenon.
• The Hubble Law implies that:
  • The Universe is expanding (like a giant balloon), as shown in this video.
  • The Universe has no center and no edge.
  • The Universe is about 14 billion years old, depending on the exact value of the Hubble constant (slope). This video explains the simplified calculation.
• The Big Bang theory actually consists of two types of expansion:
  • A tiny fraction of a second after the Big Bang, the Universe expanded dramatically in an event known as inflation.
  • Inflation was followed by a more gentle Hubble expansion that we still observe today.
• On the basis of internal consistency and supporting observations, the Big Bang Theory has been one of the most successful theories in astronomy. The Big Bang Theory makes several predictions, which have been found to be true:
  1. Currently observed expansion of the Universe.
  2. The oldest stars in the universe are all a little younger than the Big Bang.
  3. The relative amounts of hydrogen and helium, the lightest elements in the periodic table, are consistent with the amounts that would have been produced soon after the Big Bang.
  4. Cosmic Background Radiation emanating almost equally from all directions -- a faint remnant of the radiation of the Big Bang.

Cosmic Background Radiation

• Cosmic background radiation (CBR) emanates from all parts of space which are not lit up by bright objects (hence "background").
• CBR resembles the radiation that would emanate from a dense object with a temperature of about 2.7 K. The dense object is presumably the original dense Universe.
• CBR was discovered by accident by Penzias and Wilson in the early 1960s.
• CBR is consistent with an expanding and cooling Universe.
  • Right after the Big Bang, the temperature of the Universe was in the millions of degrees and the CBR peaked in the gamma-ray part of the electromagnetic spectrum.
  • As the Universe expanded and cooled, the peak wavelength also increased by the same factor as the temperature decreased, consistent with the inverse relationship given by Wien's law.
  • Today, CBR peaks in the microwave part of the EM spectrum and the Universe is around 2.7 K (away from the bright areas).
  • In 1992, NASA's Cosmic Background Explorer (COBE) satellite detected tiny fluctuations (a few parts in 100,000), or anisotropy, in CBR. The slight fluctuations in temperature are related to the primordial structure of the Universe which has given rise to the distribution of matter today (warmer areas correspond to galaxies, etc.).
• The CBR is slightly warmer (0.0033 K) in the direction of Leo and slightly cooler toward Aquarius. This effect is directly related to the movement of the Milky Way toward Leo, which causes the radiation from Leo to be Doppler blue-shifted and radiation from Aquarius to be red-shifted.

Cosmic abundance of elements

• In the very early Universe, the temperature was so great that all matter was fully ionized and dissociated (i.e., in subatomic form). Roughly 3 minutes after the Big Bang itself, the temperature of the Universe rapidly cooled from its phenomenal 10³² Kelvin to approximately 10⁹ Kelvin. At this temperature, nucleosynthesis, or the production of light elements, could take place. In a short time interval, protons and neutrons collided to produce deuterium (one proton bound to one neutron). Most of the deuterium then collided with other protons and neutrons to produce helium and a small amount of tritium (one proton and two neutrons). Lithium-7 could also arise form the coalescence of one tritium and two deuterium nuclei.
• Aside from a tiny amount of lithium, nucleosynthesis beyond helium could not take because the Universe expanded and cooled too quickly. (The heavier the elements to be fused, the greater the required temperature.) Once all nuclear reactions ceased, 75% of the mass of the Universe was in the form of hydrogen and 25% in the form of helium. Since helium is 4 times more massive than hydrogen, the original hydrogen-helium ratio was 12:1 by number of atoms (or 92% to 8%). Today, after many cycles of stellar evolution, the Universe is still mostly hydrogen and helium, in roughly the same proportions, although 1% now fills out the rest of the periodic table. Your body is made mostly of these `trace' elements. Your body is made mostly of these `trace' elements.
• The light-element abundance in the universe is an important piece of evidence supporting the Big Bang theory. It is now known that the elements observed in the Universe were created in either of two ways. Light elements (namely deuterium, helium, and lithium) were produced in the first few minutes of the Big Bang, while elements heavier than helium are thought to have their origins in the interiors of stars which formed much later in the history of the Universe. Both theory and observation lead astronomers to believe this to be the case.

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