Stephen Hawking – His Life And Work


Penrose’s discovery that a star of great enough mass undergoing gravitational collapse must form a singularity set fire to Hawking. With Robert Geroch and Penrose, he began to extend ideas about singularities to other physical and mathematical cases. He was certain the discovery had significant implications for the beginning of the universe. This was exhilarating work, with the ‘glorious feeling of having a whole field virtually to ourselves’. Hawking realized that if he reversed the direction of time so that the collapse became an expansion, everything in Penrose’s theory would still hold. If general relativity tells us that any star that collapses beyond a certain point must end in a singularity, then it also tells us that any expanding universe must have begun as a singularity. For this to be true the universe must be like what scientists call a Friedmann model.

Click on the imagem below and catch a glimpse of the Friedmann models of the universe:

Credits: NASA / Chandra X-Ray Observatory

Which model fits our universe?Will the universe collapse some day or go on expanding forever? It depends on how much mass there is in the universe […]. It will take much more mass than we presently observe to close the universe. […]

Penrose’s theory about stars collapsing and becoming singularities only worked with a universe infinite in space that will go on expanding forever, not collapse. Hawking first set out to prove that a universe infinite in space not only would have singularities in black holes but also must have begun as a singularity. He was confident enough by the time he finished his thesis to write: ‘There is a singularity in our past.’

© Kitty Ferguson – Stephen Hawking – His Life and Work (excerto, com comentário) – Bantam Books


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