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The Opposite of Dark Energy: Limits on Ultralight Energy in the Early Universe

The Opposite of Dark Energy: Limits on Ultralight Energy in the Early Universe. Robert J. Nemiroff Michigan Technological University. Abstract.

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The Opposite of Dark Energy: Limits on Ultralight Energy in the Early Universe

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  1. The Opposite of Dark Energy: Limits on Ultralight Energy in the Early Universe Robert J. Nemiroff Michigan Technological University

  2. Abstract Might energy species "lighter" than radiation, with w > 1/3, exist? A dimensional expansion of the cosmological Friedmann Equation of energy has a clear place for them. Such energies would affect the universe much differently than dark energies, and so are here dubbed "ultralight." As the universe expands, ultralight dilutes even faster than light. Ultralight is not considered a candidate to make a significant contribution to the energy budget of the universe today, although ultralight might have affected the universe in the distant past. In particular, the w=2/3 ultralight energy specie appears to have relatively mundane physical attributes. A discussion of properties and falsifiable attributes of ultralight is given. Limits on the possible cosmological density of ultralight in the early universe are determined from existing microwave background and primordial nucleosynthesis data.

  3. What energy species are there? • w<-1: Phantom Energy • w=-1: Cosmological Constant • w=-2/3: Domain Walls • w=-1/3: Cosmic Strings • w=0: Atoms, molecules, dark matter • w=1/3: Radiation, relativistic matter • w>1/3: Ultralight (this work)

  4. A Brief History of the Universe in Terms of Energy Species First w=1/3 energy dominated. Then w=-1 energy dominated. Then w=1/3 energy dominated again. Then w=0 energy dominated for a while. Now w=-1 energy is beginning to dominate again.

  5. Expanded Friedmann Equation Includes more fun energy species Classic Friedmann EquationAs printed in many textbooks

  6. Sound Speed of Ultralight • Assume energy species are perfect fluids • w=P/ρ • Sound speed: cs=w1/2 • For ultralight with 1/3 < w < 1 then cs<1 • Not unphysical • Sound speed is not imaginary (not true for dark energy) • Sound speed can be less than speed of light (not true for phantom energy)

  7. Ultralight is not a scalar field • Evolving scalar fields can have any effective w=P/ρ • (Possible) examples of scalar fields • Field that caused inflation • Quintessence fields that create dark energy • Higgs fields that give matter mass • Ultralight is stable • not a scalar field • w does not evolve

  8. Ultralight is not significant today • Ultralight is not today’s dark energy • Ultralight dilutes more quickly than any other energy specie including radiation • Ultralight might have been important, even dominant, in the early universe • We might not know since it is effectively gone now

  9. Nucleosynthesis Limits on w=2/3 Ultralight • Radiation dominated nucleosynthesis: a ~ t1/2 • Ultralight dominated nucleosynthesis: a ~ t2/5 • Ultralight dominated nucleosynthesis lasts too long! Therefore, universe not dominated by w=2/3 ultralight at z~3E8. • Extrapolating: Ωultralight < 10-11 today

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