Bulletin of the American Physical Society
APS April Meeting 2018
Volume 63, Number 4
Saturday–Tuesday, April 14–17, 2018; Columbus, Ohio
Session S15: Early Universe |
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Sponsoring Units: DAP Chair: Keith Olive, University of Minnesota Room: B230-231 |
Monday, April 16, 2018 1:30PM - 1:42PM |
S15.00001: How kinetic decoupling of dark matter during an early matter-dominated era affects small-scale structure Isaac Waldstein, Adrienne Erickcek An early matter-dominated era (EMDE) occurs when the energy content of the Universe is dominated by either massive particles or an oscillating scalar field before big bang nucleosynthesis. An EMDE enhances the small-scale matter power spectrum and increases the abundance of microhalos. This enhancement in the abundance of microhalos implies a substantially boosted dark matter (DM) annihilation rate. However, estimates of the DM annihilation rate show that it is highly sensitive to the free-streaming cutoff scale in the matter power spectrum, which is set by the kinetic decoupling temperature of DM. DM kinetic decoupling is radically different in an EMDE than it is in a radiation-dominated era: DM enters a quasidecoupled state in an EMDE, during which the DM temperature cools faster than the plasma temperature but slower than it would cool if the DM were fully decoupled. Moreover, radiation perturbations do not oscillate during an EMDE. We establish that interactions between DM and relativistic particles during an EMDE do not significantly suppress the growth of perturbations on small scales, in contrast to the impact of these interactions in a radiation-dominated era. [Preview Abstract] |
Monday, April 16, 2018 1:42PM - 1:54PM |
S15.00002: BBN and the Lithium Problem Nishanth Sasankan Big Bang Nucleosynthesis has been a computational tool to test the earliest conditions after big bang. It has given very precise results for most of the primordial nuclei abundances. These values have been confirmed by multiple observations. However the computational model of BBN predicts almost two to three times more Lithium-7 abundance than what is confirmed by observations. This is known as the cosmic lithium problem. Many solutions has been proposed for this problem. One such solution is to re-evaluate the nuclear reaction rates with some modified particle energy distribution, namely Tsallis distribution. However there is no physical motivation behind the assumption. We explore physical conditions which can provide distributions that are similar to Tsallis distribution. One such distribution is the Maxwell-Juttner distribution. It is a purely relativistic distribution accounting for the fact that no particle can exceed the speed of light. Which is in contrast to the Maxwell-Boltzmann distribution of velocities. We re-calculate the nuclear reaction rates in the BBN network, to study the effects it will have on the final primordial abundances and the Lithium problem. [Preview Abstract] |
Monday, April 16, 2018 1:54PM - 2:06PM |
S15.00003: Accurately constraining the primordial power spectrum using minihalos Sten Delos, Adrienne Erickcek, Avery Bailey, Marcelo Alvarez Small-scale fluctuations in the primordial density field are difficult to probe today due to the complexity of physics at small scales. However, these fluctuations can still leave observable effects. Primordial density contrasts with $\delta\rho/\rho\sim 10^{-3}$ collapse into dark matter halos at early times, leading to an abundance of so-called ultracompact minihalos (UCMHs). The absence of observational signatures of UCMHs then yields an upper bound on the primordial power spectrum of density fluctuations at small scales. UCMHs have attracted considerable interest, but most previous treatments assumed that halos collapsing at early times possess an extremely steep $\rho\propto r^{-9/4}$ radial density profile. We recently found that UCMHs forming due to a spike in the power spectrum develop $\rho\propto r^{-3/2}$ inner profiles instead, but these halos are still highly concentrated due to their early formation. We discuss these results and show how new power spectrum constraints are obtained in this more accurate picture by relating the distribution of UCMHs today to that of primordial density peaks. Since we are now able to include halos that collapse at any time, our preliminary constraints are stronger than prior constraints from UCMHs. [Preview Abstract] |
Monday, April 16, 2018 2:06PM - 2:18PM |
S15.00004: Growth of Dark Matter Perturbations during Kination Kayla Redmond, Adrienne Erickcek Our ignorance of the period between the end of inflation and the beginning of Big Bang Nucleosynthesis limits our understanding of the origins and evolution of dark matter. We cannot calculate the dark matter relic abundance without knowing when the Universe became radiation dominated. If the Universe's energy density was dominated by a fast-rolling scalar field while the radiation bath was hot enough to thermally produce dark matter, then dark matter with larger-than-canonical annihilation cross sections can generate the observed dark matter relic abundance. To further constrain these scenarios, we investigate the evolution of small-scale density perturbations during such a period of kination. We determine that once a perturbation mode enters the horizon during kination, the gravitational potential drops sharply and begins to oscillate and decay. Nevertheless, dark matter density perturbations that enter the horizon during kination grow linearly with the scale factor prior to radiation domination. This linear growth generates enhanced substructure and effectively increases the dark matter annihilation rate, which could make thermal dark matter production during kination incompatible with observations. [Preview Abstract] |
Monday, April 16, 2018 2:18PM - 2:30PM |
S15.00005: Simulations of Microhalo Formation After an Early Matter-Dominated Era Sheridan B. Green, Adrienne L. Erickcek, Marcelo A. Alvarez The evolution of the Universe between inflation and the onset of Big Bang nucleosynthesis is largely unknown. Several theories include an early matter-dominated era (EMDE) during the Universe's first second, driven by either a massive particle or an oscillating scalar field. During the EMDE, sub-horizon perturbations in the dark matter (DM) density grow linearly with the scale factor, as opposed to logarithmically during the radiation-dominated era, dramatically enhancing the microhalo abundance below the horizon scale at the end of the EMDE and above the free-streaming cut-off scale. We analyze two suites of small-box, high-redshift cosmological simulations: one suite based on a power spectrum that includes an EMDE enhancement and a small-scale cut-off and one suite based on a power spectrum with the same cut-off scale but no enhancement. The EMDE halo mass functions are in agreement with Press-Schechter theory, and $70\%$ of DM is bound into microhalos by $z=20$. The EMDE microhalos have steeper inner density profiles and higher concentrations than their standard counterparts due to their earlier formation times. They also contain significantly more substructure. We use these simulations to estimate how an EMDE boosts the DM annihilation rate in dwarf spheroidal galaxies. [Preview Abstract] |
Monday, April 16, 2018 2:30PM - 2:42PM |
S15.00006: If dark matter dominates before BBN: implications, from gravitational wave detection to the growth of structure Tanja Rindler-Daller, Bohua Li, Paul Shapiro There is recently great interest in ultralight scalar-field dark matter (SFDM) models, in which structure formation is supposed to be similar to standard CDM on large scales, while suppressed on small scales by quantum effects. Complex field SFDM with a global U(1)-symmetry, having a conserved charge, can emerge after reheating, along with the standard model particles. By setting this conserved charge to the present observed DM density, complex SFDM obeys a stiff equation of state in the early Universe. As a result, DM dominates all the other cosmic components during this phase. We have studied in great detail and accuracy the evolution of such LSFDM models, whose cosmic inventory is the same than for LCDM, except that CDM is replaced by SFDM. We have previously constrained the epoch of stiff-SFDM-domination by studying the impact onto the SFDM model parameter space, using cosmological observables from Big Bang nucleosynthesis (BBN) and the CMB to the inflationary stochastic gravitational wave background which gets amplified during such a stiff phase. We will present these findings, along with new ones concerning the implications of stiff-SFDM-domination for the growth of perturbations prior to and after BBN, with interesting consequences for structure formation in the Universe. [Preview Abstract] |
Monday, April 16, 2018 2:42PM - 2:54PM |
S15.00007: Gravitational Leptogenesis, Reheating, and Models of Neutrino Mass Peter Adshead, Andrew Long, Evangelos Sfakianakis Gravitational leptogenesis refers to a class of baryogenesis models in which the matter-antimatter asymmetry of the universe arises through the standard model lepton-number gravitational anomaly. In these models chiral gravitational waves source a lepton asymmetry in standard model neutrinos during the inflationary epoch. We point out that gravitational leptogenesis can be successful in either the Dirac or Majorana neutrino mass scenario. In the Dirac mass scenario, gravitational leptogenesis predicts a relic abundance of sterile neutrinos that remain out of equilibrium, and the lepton asymmetry carried by the standard model sector is unchanged. In the Majorana mass scenario, the neutrinos participate in lepton-number-violating interactions that threaten to washout the lepton asymmetry during post-inflationary reheating. However, we show that a complete (exponential) washout of the lepton asymmetry is prevented if the lepton-number-violating interactions go out of equilibrium before all of the standard model Yukawa interactions come into equilibrium. The baryon and lepton asymmetries carried by right-chiral quarks and leptons are sequestered from the lepton-number violation, and the washout processes only suppress the predicted baryon asymmetry by an order unity factor. The sign of the resulting baryon asymmetry depends on the model parameters in such a way that a future measurement of the primordial gravitational wave chirality would constrain the scale of lepton-number violation (heavy Majorana neutrino mass). [Preview Abstract] |
Monday, April 16, 2018 2:54PM - 3:06PM |
S15.00008: Partially Massless Gravity in de Sitter Space Laura Johnson, Claudia de Rham, Kurt Hinterbichler Adding a mass to the graviton is a promising way to get an accelerating universe without dark energy. When a mass term is added to the Einstein-Hilbert action, it gives the graviton 3 extra degrees of freedom, in addition to the standard 2 of a massless graviton. The new degrees of freedom become strongly coupled at a low scale, limiting the theory's naive range of applicability. However, in the space-time of an accelerating universe (which approximates our early universe during inflation and the phase our universe is currently entering into), a linear massive graviton can obtain a new symmetry known as the partially massless symmetry. If this partially massless symmetry were exact, the value of the cosmological constant would be set by the graviton mass, potentially explaining of the smallness of our cosmological constant. We examine the partially massless symmetry in the full non-linear theory of massive gravity. The full non-linear theory does not have this symmetry, but we find that nevertheless the strong coupling scale is raised, giving a wider range of applicability of the theory. [Preview Abstract] |
Monday, April 16, 2018 3:06PM - 3:18PM |
S15.00009: Non-Abelian gauge preheating Zachary Weiner, Peter Adshead, Tom Giblin We study preheating in models where a scalar inflaton is directly coupled to a non-Abelian $SU(2)$ gauge field. In particular, we examine $m^2 \phi^2$ inflation with a conformal, dilatonlike coupling to the non-Abelian sector. We describe a numerical scheme that combines lattice gauge theory with standard finite difference methods applied to the scalar field. We show that a significant tachyonic instability allows for efficient preheating, which is parametrically suppressed by increasing the non-Abelian self-coupling. Additionally, we comment on the technical implementation of the evolution scheme and setting initial conditions. [Preview Abstract] |
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