Earth Energy

Carl Sagans Cosmic Calendar

Carl Sagan popularized the Cosmic Calendar as a method to visualize the chronology of the universe, scaling its current age of 13.8 billion years to a single year in order to help intuit it for pedagogical purposes.

If you would like to download a copy of this for yourself you can go here to wiki commons where they have multiple sizes for you.

Please watch Carl Sagans 1994 lecture on The Age of Exploration on Youtube.  Here is the link.


The chronology of the universe describes the history and future of the universe according to Big Bang cosmology.

The earliest stages of the universe’s existence are estimated as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.

Chronology in five stages

Diagram of evolution of the (observable part) of the universe from the Big Bang (left), the CMB-reference afterglow, to the present.

For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts. It is generally considered meaningless or unclear whether time existed before this chronology:

  1. The very early universe -The first picosecond (10−12) of cosmic time. Tiny ripples in the universe at this stage are believed to be the basis of large-scale structures that formed much later. 
  2. The early universe – Lasting around 370,000 years.  The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form He+
    . At around 47,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. (Much later, hydrogen and helium hydride react to form molecular hydrogen (H2) the fuel needed for the first stars.) At about 370,000 years, neutral hydrogen atoms finish forming (“recombination”), and as a result the universe also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium—quickly reach their lowest energy state (ground state) by releasing photons (“photon decoupling”), and these photons can still be detected today as the cosmic microwave background (CMB). This is the oldest observation we currently have of the universe. 
  3. The Dark Ages and large-scale structure emergence–  From 370,000 years until about 1 billion years. After recombination and decoupling, the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies, so there were no new sources of light. 
  4. The universe as it appears today– From 1 billion years, and for about 12.8 billion years, with the earliest traces of life on Earth emerging by about 10.3 billion years (3.5 Gya). The present-day universe is understood quite well, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), uncertainties in current knowledge mean that we are less sure which path our universe will take.  This is the period mostly covered in Carl Sagan’s Cosmic Calendar.
  5. The far future and ultimate fate– At some time the Stelliferous Era will end as stars are no longer being born, and the expansion of the universe will mean that the observable universe becomes limited to local galaxies. There are various scenarios for the far future and ultimate fate of the universe. More exact knowledge of our current universe will allow these to be better understood. 

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