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NASA\/Adboe Stock\/ Ryan Wills<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Our universe is expanding, so it must have been smaller in the past. Indeed, if we rewind our cosmological movie, we see the universe shrinking back almost to a point \u2013 the big bang \u2013 some 13.8 billion years ago. Is this when time began? Alas, things aren\u2019t so simple. Albert Einstein\u2019s general theory of relativity tells us that the backdrop of the universe is a fluid continuum, space-time, in which neither space nor time has an absolute meaning. What\u2019s more, at the big bang, space-time distorts into a point of infinite density called a singularity. We can\u2019t say this is where time begins, only that it marks a rupture beyond which we cannot extrapolate.<\/p>\n
Even so, some cosmologists believe there was a \u201cbefore\u201d the big bang. Some suggest that another universe preceded ours, and that this one contracted and then \u201cbounced\u201d at the big bang, resulting in the expanding era we now observe. More radically, cosmologist Roger Penrose has proposed that new universes can emerge from ones that don\u2019t contract, through a dramatic \u201crescaling\u201d of all space-time.<\/p>\n
In both these scenarios, time is eternal, but that\u2019s just one possibility. The late cosmologists Stephen Hawking and James Hartle suggested that time was once an ordinary dimension like space, which got derailed at the big bang into space-time. Another outlandish idea is that space-time is made of particle-like pieces. If so, these could be arranged in different phases, akin to steam and liquid water. Maybe the big bang was the point at which they \u201ccondensed\u201d into the fluid, continuous space-time we observe today.\u2026<\/p>\n<\/p><\/div>\n
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