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Stable Violation of Null Energy Condition in Alternative Cosmologies

Explore the stable violation of the Null Energy Condition (NEC) and non-standard cosmologies. Discuss motivations behind today's acceleration, necessity to violate NEC, and examples of alternative cosmologies such as the Bouncing Cosmology and the Cyclic Universe. Our approach addresses the issue through low-energy Effective Field Theory (EFT) and stable NEC violation is showcased. Discover potential applications and observational signatures of these alternative cosmologies, bridging the gap between past inflation and today's acceleration. Conclude with open questions and future research directions.

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Stable Violation of Null Energy Condition in Alternative Cosmologies

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  1. Starting the Universe:Stable violation of the Null Energy Conditionand Non-Standard Cosmologies hep-th/0606090 Leonardo Senatore (Harvard) With P.Creminelli, M. A. Luty, A. Nicolis

  2. Outline • Motivation: Today’s Acceleration and Alternative Cosmologies • Necessity to violate the Null Energy Condition (NEC) • Our Approach to the Problem and its Solution • Examples of Alternative Cosmologies • Conclusions

  3. Motivation • Today’s acceleration: ? ? • Alternative Cosmologies: • Inflation has singularity in the past • Avoid it: • Start the Universe: in the past • Bouncing Cosmology: from contraction to expansion • Cyclic Universe • All these require NEC violation to have

  4. Why NEC ? • NEC: for every null . • With : • Singularity Theorems (Borde and Vilenkin)

  5. Energy Conditions • Assumptions, not Deductions • Most are violated: • Strong: for timelike • Weak: for timelike Are violated by the C.C. • But NEC is hard: instabilities appear: • S.Dubovsky, T.Gregoise, A.Nicolis, R.Rattazzi, • S.D.H.Hsu, A.Jenkins, M.B. Wise, • Phantom fields. • Question: does NEC really holds in Cosmology? • We will see the answer is NO.

  6. Our Approach • Question: can NEC be violated? • Build general low energy EFT for FRW background: how? • Action for the Goldstone of spontaneously broken time diffeomorphisms. • This is the relevant fluctuation inside the horizon

  7. How to build it? • Go to a “unitary” gauge where • Fluctuation “inside” the metric. • Write action withall terms compatible with residual gauge inv. Ex.: • Reintroduce Goldstone: • Gauge invariant combination: • Concentrate on the action

  8. FRW goldstone effective action • Write all the relevant terms: • Dispersion relation: • Instability for: , • Cured for:

  9. Mixing with Gravity • Usually negligible inside the horizon: • The higher derivative terms mix more strongly: • Jeans-like instability • Final parametric range for stable NEC violation

  10. Applications • Possibility that today. From WMAP: • New possibilities for the Early Universe: • Starting the Universe: in the past. • Eternally expanding cyclic universe • The Big Bounce

  11. Tilted Ghost Condensation • Ghost Condensation (N. Arkani-Hamed, H.C. Cheng, M.A.Luty,S. Mukohyama) (N. Arkani-Hamed, P. Creminelli, S.Mukohyama, M.Zaldarriaga) (L. Senatore) • Add a tilt • Today’s acceleration with :

  12. Starting the Universe • No Big Bang: in the past. • Ghost condensate with • In the past the Cosmology is: • Need to ensure stability in the past:

  13. Ever Expanding Cyclic Universe • Connect today’s acceleration with past inflation. • Model always stable • Observational signatures • Tilt of Power Spectrum • Gravity wave with Blue tilt

  14. The Big Bounce

  15. Conclusions • No Obstacle to violating NEC at EFT level Models with are now conceivable • Two Instabilities: • Gradient Instability Higher derivative terms • Jeans Instability • Safe for (parametrically for ) • Examples of Alternative Cosmologies • today • Starting the Universe: in the past. • Cyclic Ever Expanding Universe • Bouncing Universe • Open questions and future work: • More degrees of freedom stronger NEC violation? • Realistic and predictive Cosmological Models • UV completion

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