The Sciences

The Arrow of Time in Scientific American

Cosmic VarianceBy Sean CarrollMay 21, 2008 8:47 PM

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Greetings from Paris! Just checking in to do a bit of self-promotion, from which no blog-vacation could possibly keep me. I've written an article in this month's Scientific American about the arrow of time and cosmology. It's available for free online; the given title is "Does Time Run Backward in Other Universes?", which wasn't my choice, but these happenings are team events. As a teaser, here is a timeline of the history of the universe according to the standard 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.

Despite appearances, this really is just the standard cosmology, not some fairy tale. I just chose to tell it from the point of view of a time coordinate that is oriented in the opposite direction from the one we usually use. Given that the laws of physics are reversible, this choice is just as legitimate as the usual one; nevertheless, one must admit that the story told this way seems rather unlikely. So why does the universe evolve this way? That's the big mystery, of course.

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