According to the modern modified version of the Big Bang theory, called the inflationary theory, the universe underwent a rapid period of expansion shortly after the Big Bang, which accounts for its current large size and uniform nature. The inflationary theory is supported by the most recent observations of the cosmic background radiation.

Present quantum theory and relativity theory break down when pushed back to earlier than 10⁻⁴³ seconds, which is known as the Planck time. (10⁻⁴³ is equal to 1÷10⁴³; and 10⁴³ is equal to 1 followed by 43 zeroes.) The present-day observable universe was then smaller than a proton and the temperature was 10³² K (kelvins - equal in size to degrees Celsius). Inflation began now or an instant later, and ended when 10⁻³³ seconds had passed. The universe observable today was then a metre across.

One ten-thousandth of a second after the Big Bang, the temperature had decreased enormously, but was still 10 billion K (10¹⁰ K). Subatomic particles had formed by the collision of quarks. After 10 seconds, neutrons had combined with protons to form nuclei of deuterium (an isotope of hydrogen). The nuclei of deuterium then joined together to form helium nuclei. As the universe continued to expand for the next 300,000 years, the temperature cooled to 10,000 K. Under these conditions helium nuclei were able to join with electrons to form helium atoms. Some hydrogen nuclei joined to form lithium nuclei and thence lithium atoms. After millions of years, at lower temperature and pressure, the force of gravity was able to draw particles together. After millions more years, matter clumped together to form galaxies, stars, planets, and moons.

The first detailed images of the universe as it existed 300,000 years after the Big Bang were released by the US National Aeronautics and Space Administration (NASA) in April 2000. The images were created by mapping cosmic background radiation.

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