The first 10⁻⁴³ seconds
General relativity describes a universe that began as a gravitational singularity—a point of zero volume and infinite density. Our current understanding of physics breaks down at this initial moment. The earliest period of time that science can describe is the Planck Epoch, which lasted from the moment of the Big Bang to 10⁻⁴³ seconds after. During this sliver of time, the universe was contained in a region just 10⁻³⁵ meters across and had a temperature of over 10³² Kelvin. The four fundamental forces we know today—gravity, electromagnetism, and the strong and weak nuclear forces—were believed to be unified into a single force.
At the end of the Planck Epoch, gravity separated from the other unified forces. This event triggered a period of exponential expansion known as cosmic inflation, which began around 10⁻³⁶ seconds after the Big Bang. In an astonishingly brief period, lasting until about 10⁻³² seconds, the universe increased in size by a factor of at least 10²⁶. An object the size of a proton would have swelled to be about 100 million light years across. The portion of the universe that is observable to us today expanded from a subatomic scale to roughly the size of a grapefruit.