This article is a supplement to the feature "The Cosmic Origin of Time Arrow: Does Time Run Backward in Other Universes?" from the June 2008 issue of Scientific American.

Here is a timeline of important events in the history of our observable universe, according to conventional cosmology:

• Space is empty, featuring nothing but a tiny amount of vacuum energy and an occasional long-wavelength particle formed via fluctuations of the quantum fields that suffuse space.
• High-intensity radiation suddenly sweeps in from across the universe, in a spherical pattern focused on a point in space. When the radiation collects at that point, a “white hole” is formed.
• The white hole gradually grows to billions of times the mass of the sun, through accretion of additional radiation of ever decreasing temperature.
• Other white holes begin to approach from billions of light-years away. They form a homogeneous distribution, all slowly moving toward one another.
• The white holes begin to lose mass by ejecting gas, dust and radiation into the surrounding environment.
• The gas and dust occasionally implode to form stars, which spread themselves into galaxies surrounding the white holes.
• Like the white holes before them, these stars receive inwardly directed radiation. They use the energy from this radiation to convert heavy elements into lighter ones.
• Stars disperse into gas, which gradually smooths itself out through space; matter as a whole continues to move together and grow more dense.
• The universe becomes ever hotter and denser, eventually contracting all the way to a big crunch.

Needless to say, this is not the usual way in which we describe the history of the universe—it is the conventional sequence of events told backward in time. But the laws of physics work equally well run forward or backward in time. Thus, this sequence is as legitimate as the usual one. It serves the purpose of driving home just how unlikely the entire history of our observable universe really is.

—S.M.C.