The Planckian Paradigm for Dark Matter

Andrea Palessandro

Research output: ThesisPh.D. thesis

Abstract

Numerous cosmological and astrophysical observations show that a significant fractionof matter in the Universe is non-baryonic, cold and dark. The existence of dark matter provides strong evidence for physics beyond the Standard Model of particle physics.Extensions of the Standard Model involving the Peccei-Quinn symmetry or supersymmetrygive rise to compelling dark matter candidates. Nonetheless, the Standard Modelcould be self-consistent up to the Planck scale according to the present measurements ofthe Higgs boson mass and top quark Yukawa coupling. It is therefore possible that newphysics is only coupled to the Standard Model through Planck suppressed higher dimensionaloperators. In this case the weakly interacting massive particle miracle is a mirage,and instead minimality as dictated by Occam’s razor would indicate that dark matter isrelated to the Planck scale, where quantum gravity is anyway expected to manifest itself.Planckian Interacting Dark Matter (PIDM) is a minimal scenario of dark matter assumingonly gravitational interactions with the standard model and with only one freeparameter, the PIDM mass. PIDM can be successfully produced by gravitational scatteringin the thermal plasma of the Standard Model sector after inflation in the PIDM massrange from TeV up to the GUT scale, if the reheating temperature is sufficiently high.The minimal assumption of a GUT scale PIDM mass can be tested in the future by measurementsof the primordial tensor-to-scalar ratio. While large primordial tensor modeswould be in tension with the QCD axion as dark matter in a large mass range, it wouldfavour the PIDM as a minimal alternative to WIMPs.In a simple extension of the PIDM framework, the PIDM is charged under an unbrokenU(1) gauge symmetry, but remains only gravitationally coupled to the StandardModel (SM). Contrary to "hidden charged dark matter", the charged PIDM never reachesthermal equilibrium with the SM. The dark sector is populated by freeze-in via gravitationalinteractions at reheating. If the dark fine-structure constant aD is larger than about10􀀀3, the dark sector thermalizes within itself, and the PIDM abundance is further modifiedby freeze-out in the dark sector. Interestingly, this largely reduces the dependence ofthe final abundance on the reheating temperature, as compared to an uncharged PIDM.The observed CDM abundance can be obtained over a wide mass range from the weak tothe GUT scale, and for phenomenologically interesting couplings aD _ 10􀀀2. Due to thedifferent thermal history, the charged PIDM can be discriminated from "hidden chargeddark matter" by more precise measurements of the effective number of neutrino speciesNeff.Purely gravitational atoms are created together with dark matter particles in the minimalPIDM scenario. In general, particles in a yet unexplored dark sector with sufficientlylarge mass and small gauge coupling may form purely gravitational atoms (quantum gravitational bound states) with a rich phenomenology. Decays of gravitational atomscan source gravitational waves or ultra high energy cosmic rays. If ordinary Einsteingravity holds up to the Planck scale, then, within the LCDM model, the frequency ofthe gravitational wave signal produced by the decays is always higher than 1013 Hz. Anobservable signal of gravitational waves with smaller frequency from such decays, inaddition to probing near Planckian dark physics, would also imply a departure fromEinstein gravity near the Planck scale or an early epoch of non-standard cosmology. Forexample, an early universe cosmology with a matter-dominated phase can give a signalin an interesting frequency range for near Planckian bound states.If Einstein gravity with minimally coupled matter is valid up to the Planck scale,gravitational as well as non-gravitational atoms absorb gravitons of a specific frequencywith Planckian cross section, sabs _ l2p. Consequently, one can show that gravitationalabsorption by bound states is inefficient in ordinary gravity. If observed, gravitationalabsorption lines would therefore constitute a powerful smoking gun of new exotic astrophysicalbound states (near extremal bound states) or new gravitational physics, aswell as give direct evidence of the quantized nature of the gravitational field. A nonminimalcoupling of the matter fields which breaks the equivalence principle on-shellprovides a clear example of new gravitational physics near the Planck scale. In certainmodels, absorption lines in the primordial gravitational wave spectrum can be producedas a consequence of this coupling.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Sloth, Martin Snoager, Principal supervisor
Publisher
Publication statusPublished - 14. Jan 2020

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