1,721,070 research outputs found
Exploring and constraining new physics in the dark sector
No full textThe standard cosmological model that has emerged in the last decades describes an acceleratingly expanding universe where the familiar baryonic matter accounts for a very small fraction of the overall energy budget. The vast majority of the energy content of the Universe appears to belong to an elusive dark sector made up of dark matter and dark energy. In this thesis, we explore the cosmological consequences of new physics that could govern this unknown dark sector.
We first consider a model where dark matter can annihilate to Standard-Model particles through a Breit-Wigner resonance. We show in this case that the energy released by dark matter annihilating in the first proto-halos is likely substantial. We determine that the bounds on the allowed energy injection into the primordial gas and the energy density of the diffuse gamma-ray background strongly constrain the magnitude of the resonantly-enhanced annihilation cross-section.
We then perform a thorough analysis of a dark sector made of atom-like bound states. This so-called Atomic Dark-Matter model predicts novel dark-matter properties on small scales but retains the success of cold dark matter on cosmological scales. We revisit the atomic physics necessary to capture the thermal history of the dark atoms and discuss the required improvements over the hydrogen calculation. To solve the perturbation equations, we develop a second-order tight-coupling approximation and further discuss its implications for the baryon-photon case. We compute the matter power spectrum in this model and show that it displays strong dark-matter acoustic oscillations and a cutoff on small scales. Interestingly, we also identify key cosmic microwave background signatures that distinguish the atomic dark-matter scenario from other dark-matter theories. We determine that astrophysical constraints on this model generally favour dark atoms that are both more massive and have higher binding energies than standard atomic hydrogen.
We finally consider how oscillations in the bispectrum of primordial fluctuations affects the clustering of dark-matter halos. We discover that features in the inflaton potential such as oscillations and bumps become imprinted in the mass dependence of the non-Gaussian halo bias. This finding opens the possibility of characterizing the inflationary potential with large-scale-structure surveys.Science, Faculty ofPhysics and Astronomy, Department ofGraduat
Study of entropy perturbations in MSSM flat direction decay
No full textWe study how the resonant decay of moduli fields from the scalar potential of the MSSM could affect large scale curvature perturbations. After introducing the theory of cosmological perturbations and of broad and stochastic resonance, we present the supergravity inputs necessary to study the MSSM moduli in a cosmological context. We find that the resonant amplification of large scale field fluctuations is allowed for a very small range of parameters and is harmless for large scale curvature modes.Nous étudions l'effet de la désintégration des champs de module du MSSM sur les perturbations gravitationelles adiabatiques à grandes échelles. Après avoir introduit la théorie des perturbations cosmologiques et la théorie de la resonance stochastique, nous présentons les éléments de la supergravité nécessaire à l'étude des champs de module dans un contexte cosmologique. Nous trouvons que l'amplification non-perturbative des fluctuations d'ordre cosmologiques du champ scalaire n'est permise pour que pour un mince interval de paramètres. L'effet de la résonance sur les perturbations gravitationelles à grandes échelles est négligeable
Statistical challenges in substructure lensing
No full textNon UBCUnreviewedAuthor affiliation: Harvard UniversityPostdoctora
Thomson scattering: One rate to rule them all
Full text linkThe enduring tension between local and distant measurements of remains
unresolved. It was recently pointed out that cosmic microwave background (CMB)
and large-scale structure (LSS) observables are invariant under a uniform
rescaling of the gravitational free-fall rates of all species present and the
Thomson scattering rate between photons and electrons. We show that a unique
variation of the fine-structure constant and the electron mass can leverage this scaling transformation to reconcile the CMB and LSS data
with a broad spectrum of Hubble constant values, encompassing those inferred
from local measurements. Importantly, this study demonstrates that the
constraints on the variation of fundamental constants imposed by the specific
recombination history are not as stringent as previously assumed. Our work
highlights the critical role of the Thomson scattering rate in the existing
Hubble tension and offers a distinct avenue of exploration for particle model
builders.Comment: 20 pages + references, 6 figures, comments welcome
Absence of concordance in a simple self-interacting neutrino cosmology
No full textSome cosmic microwave background (CMB) data allow a cosmological scenario in
which the free streaming of neutrinos is delayed until close to
matter-radiation equality. Interestingly, recent analyses have revealed that
large-scale structure (LSS) data also align with this scenario, discarding the
possibility of an accidental feature in the CMB sky and calling for further
investigation into the free-streaming nature of neutrinos. By assuming a simple
representation of self-interacting neutrinos, we investigate whether this
nonstandard scenario can accommodate a consistent cosmology for both the CMB
power spectra and the large-scale distribution of galaxies simultaneously.
Employing three different approaches - a profile likelihood exploration, a
nested sampling method, and a heuristic Metropolis-Hasting approximation - we
exhaustively explore the parameter space and demonstrate that galaxy data
exacerbates the challenge already posed by the Planck polarization data for
this nonstandard scenario. We find that the most conservative value of the
Bayes factor disfavors the interactions among neutrinos over a CDM +
+ model with odds of and that the
difficulty of simultaneously fitting the galaxy and CMB data relates to the
so-called discrepancy. Our analysis not only emphasizes the need to
consider a broader range of phenomenologies in the early Universe but also
highlights significant numerical and theoretical challenges ahead in uncovering
the exact nature of the feature observed in the data or, ultimately, confirming
the standard chronological evolution of the Universe.Comment: 18 pages + references, 11 figures, 5 tables. arXiv admin note: text
overlap with arXiv:2309.0394
A Ratio-Preserving Approach to Cosmological Concordance
Full text linkCosmological observables are particularly sensitive to key ratios of energy densities and rates, both today and at earlier epochs of the Universe. Well-known examples include the photon-to-baryon and the matter-to-radiation ratios. Equally important, though less publicized, are the ratios of pressure-supported to pressureless matter and the Thomson scattering rate to the Hubble rate around recombination, both of which observations tightly constrain. Preserving these key ratios in theories beyond the Cold-Dark-Matter (CDM) model ensures broad concordance with a large swath of datasets when addressing cosmological tensions. We demonstrate that a mirror dark sector, reflecting a partial symmetry with the Standard Model, in conjunction with percent level changes to the visible fine-structure constant and electron mass which represent a \textit{phenomenological} change to the Thomson scattering rate, maintains essential cosmological ratios. Incorporating this ratio preserving approach into a cosmological framework significantly improves agreement to observational data () and completely eliminates the Hubble tension with a cosmologically inferred km/s/Mpc when including the SES calibration in our analysis. While our approach is certainly nonminimal, it emphasizes the importance of keeping key ratios constant when exploring models beyond CDM.12 pages, 6 figures, comments welcome! Published in PR.
Limits on Neutrino-Neutrino Scattering in the Early Universe
Full text linkIn the standard model neutrinos are assumed to have streamed across the Universe since they last scattered when the standard-model plasma temperature was ∼MeV. The shear stress of free-streaming neutrinos imprints itself gravitationally on the cosmic microwave background (CMB) and makes the CMB a sensitive probe of neutrino scattering. Yet, the presence of nonstandard physics in the neutrino sector may alter this standard chronology and delay neutrino free streaming until a much later epoch. We use observations of the CMB to constrain the strength of neutrino self interactions G_(eff) and put limits on new physics in the neutrino sector from the early Universe. Within the context of conventional Λ CDM parameters cosmological data are compatible with G_(eff)≲1/(56 MeV)^2 and neutrino free streaming might be delayed until their temperature has cooled to as low as ∼25 eV. Intriguingly, we also find an alternative cosmology compatible with cosmological data in which neutrinos scatter off each other until z∼10^4 with a preferred interaction strength in a narrow region around G_(eff)≃1/(10 MeV)^2≃8.6×10^8G_F, where G_F is the Fermi constant. This distinct self-interacting neutrino cosmology is characterized by somewhat lower values of both the scalar spectral index and the amplitude of primordial fluctuations. While we phrase our discussion here in terms of a specific scenario, our constraints on the neutrino visibility function are very general
Hubble distancing: Focusing on distance measurements in cosmology
Full text linkThe Hubble-Lemaitre tension is currently one of the most important questions
in cosmology. Most of the focus so far has been on reconciling the Hubble
constant value inferred from detailed cosmic microwave background measurement
with that from the local distance ladder. This emphasis on one number -- namely
-- misses the fact that the tension fundamentally arises from
disagreements of distance measurements. To be successful, a proposed
cosmological model must accurately fit these distances rather than simply infer
a given value of . Using the newly developed likelihood package
`distanceladder', which integrates the local distance ladder into MontePython,
we show that focusing on at the expense of distances can lead to the
spurious detection of new physics in models which change late-time cosmology.
As such, we encourage the observational cosmology community to make their
actual distance measurements broadly available to model builders instead of
simply quoting their derived Hubble constant values.Comment: 21 pages, 8 figure
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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