The laws of physics are, in principle, time-symmetric (apart from the waves-packet reduction problem). It is obvious to link the arrow of time (defined by thermodynamics) to the cosmological expansion of the universe. Lawrence Schulman (Clarkson University in New York) is questioning if could there be regions in the universe where the time-arrows point in opposite directions? He claims that it is possible and that some contact between these regions will not destroy the arrows of time. How could regions of opposite-running time have formed? In the same way that electrons and positrons run in opposite time directions. Schulman suggests a cosmological picture with a time-line running between the Big Bang and the Big Crunch. Then what would appear to us as time-reversed regions are just matter following the time-line in reverse. Although current cosmological thinking disfavours the Big Crunch scenario, if it does happen, it is still certainly a very long time away. Therefore, if there are galaxies with backward-running time, following the timeline from Big Crunch to Big Bang, at this instant they are very old and hence not very luminous, although they are still exerting a gravitational pull. Dark matter problem solved?
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