Cosmology in the nonlinear domain

The introduction of a so-called dark sector in cosmology resolved many inconsistencies between cosmological theory and observation, but it also triggered many new questions. Dark Matter (DM) explained gravitational effects beyond what is accounted for by observed luminous matter and Dark Energy (DE) accounted for the observed accelerated expansion of the universe. The most sought after discoveries in the field would give insight into the nature of these dark components. Dark Matter is considered to be the better established of the two, but the understanding of its nature may still lay far in the future. This thesis is concerned with explaining and eliminating the discrepancies between the current theoretical model, the standard model of cosmology, containing the cosmological constant Λ as the driver of accelerated expansion and Cold Dark Matter (CDM) as main source of gravitational effects, and available observational evidence pertaining to the dark sector. In particular, we focus on the small, galaxy-sized scales and below, where N-body simulations of cosmological structure in the ΛCDM universe predict much more structure and therefore much more power in the matter power spectrum than what is found by a range of different observations. This discrepancy in small scale power manifests itself for example through the well known "dwarf-galaxy problem'" (e.g. Klypin, 1999), the density profiles and concentrations of individual haloes (Donato, 2009) as well as the properties of voids (Tikhonov, 2009). A physical process that would suppress the fluctuations in the dark matter density field might be able to account for these discrepancies. Free-streaming dark matter particles dampen the overdensities on small scales of the initial linear matter density field. This corresponds to a suppression of power in the linear matter power spectrum and can be modeled relatively straightforwardly for an early decoupled thermal relic dark matter particle. Such a particle would be neutrino-like, but heavier; an example being the gravitino in the scenario, where it is the Lightest Supersymmetric Particle and it decouples much before neutrinos, but while still relativistic. Such a particle is not classified as Hot Dark Matter, like neutrinos, because it only affects small scales as opposed to causing a suppression at all scales. However, its free-streaming prevents the smallest structures from gravitationally collapsing and does therefore not correspond to Cold Dark Matter. The effect of this Warm Dark Matter (WDM) may be observable in the statistical properties of cosmological Large Scale Structure. The suppression of the linear matter density field at high redshifts in the WDM scenario can be calculated by solving the Boltzmann equations. A fit to the resulting linear matter power spectrum, which describes the statistical properties of this density field in the simple thermal relic scenario is provided by Viel (2004). This linear matter power spectrum must then be corrected for late-time non-linear collapse. This is rather difficult already in the standard cosmological scenario, because exact solutions the the evolution of the perturbed density field in the nonlinear regime cannot be found. The widely used approaches are to the 'halofit' method of Smith (2002), which is essentially a physically motivated fit to the results of numerical simulations or using the even more physical, but slightly less accurate halo model. However, both of these non-linear methods were developed assuming only CDM and are therefore not necessarily appropriate for the WDM case. In this thesis, we modify the halo model (see also Smith, 2011) in order to better accommodate the effects of the smoothed WDM density field. Firstly, we treat the dark matter density field as made up of two components: a smooth, linear component and a non-linear component, both with power at all scales. Secondly, we introduce a cut-off mass scale, below which no haloes are found. Thirdly, we suppress the mass function also above the cut-off scale and finally, we suppress the centres of halo density profiles by convolving them with a Gaussian function, whose width depends on the WDM relic thermal velocity. The latter effect is shown to not be significant in the WDM scenario for the calculation of the non-linear matter power spectrum at the scales relevant to the present and near future capabilities of astronomical surveys in particular the Euclid weak lensing survey. In order to determine the validity of the different non-linear WDM models, we run cosmological simulations with WDM (see also Viel, 2012) using the cutting edge Lagrangian code Gadget-2 (Springel, 2005). We provide a fitting function that can be easily applied to approximate the non-linear WDM power spectrum at redshifts z = 0.5 - 3.0 at a range of scales relevant to the weak lensing power spectrum. We examine the simple thermal relic scenario for different WDM masses and check our results against resolution issues by varying the size and number of simulation particles. We finally briefly discuss the possibility that the effects of WDM on the matter power spectrum might resemble the analogous, but weaker and larger scale effects of the free-streaming of massive neutrinos. We consider this with the goal of re-examining the Sloan Digital Sky Survey data (as in Thomas, 2010). We find that the effects of the neutrinos might just differ enough from the effects of WDM to prevent the degeneracy of the relevant parameters, namely the sum of neutrino masses and the mass of the WDM particle.Mit der Einfuehrung eines so genannten "dunklen Sektors'" in der Kosmologie konnten zwar Ungereimtheiten zwischen kosmologischer Theorie und Beobachtungen geloest werden - er wirft allerdings auch viele neue Fragen auf. Dunkle Materie (DM), erklaert gravitative Effekte, die nicht durch die beobachtete leuchtende Materie verursacht werden koennen. Dunkle Energie (DE) erklaert die beobachtete beschleunigte Expansion des Universums. Zu den begehrenswertesten Entdeckungen des gesamten Feldes gehoeren jene, die unser Verstaendnis bezueglich der Dunklen Materie und Dunklen Energie erweitern. Obwohl die Dunkle Materie die etabliertere der beiden Theorien ist, steckt unser Verstaendnis auch ihrbezueglich noch in den Kinderschuhen. Diese Doktorarbeit befasst sich mit der Erklaerung und Beseitigung von Unstimmigkeiten zwischen dem gaengigen theoretischem Modell, dem ΛCDM-Modell - welches die kosmologische Konstante (Λ) als Ursache fuer die beschleunigte Ausbreitung des Universums und kalte dunkle Materie (CDM) als die Quelle fuer Gravitationseffekte beinhaltet - und den verfuegbaren Beobachtungsdaten ergeben. Dabei wird der Schwerpunkt auf kleine Massstaebe - Galaxiengroesse und kleiner - gelegt, wo N-Teilchensimulationen der kosmologischen Strukturbildung im ΛCDM-Modell viel mehr Struktur und folglich viel mehr Leistung im Materieleistungsspektrum voraussagten, als viele andere Beobachtungen. Diese Unstimmigkeiten im Leistungspektrum auf kleinen Massstasben asussern sich zum Beispiel im so genannten Zwerggalaxienproblem (z.B. Klypin, 1999), in der Konzentration und den Dichteprofilen individueller Halos (Donato, 2009) und auch in den Eigenschaften so genannter Voids, grosser Leerraeume im Universum (Tikhonov, 2009). Diese Ungereimtheiten koennten durch einen physikalischen Prozess erklaert werden, welcher die Schwankungen des DM-Dichtefeldes zu unterdruecken vermag. Frei stroehmende dunkle Materieteilchen daempfen auf kleinen Abstaenden das urspruengliche lineare Materiedichtefeld. Dies deckt sich mit einer Unterdrueckung der der Leistung im Leistungsspektrum und erlaubt eine relativ einfache Erstellung von Modellen von frueh abgekoppelten thermischen Reliktteilchen. Solche Teilchen waeren neutrinoaehnlich, allerdings schwerer. Ein Beispiel waere das Gravitino in einem Szenarium wo es das leichteste supersymmetrische Teilchen ist und sich viel frueher abkoppelte als Neutrinos, aber noch waehrend es sich in einem relativistischen Zustand befand. Diese Teilchen koennen nicht wie Neutrinos als heisse dunkle Materie klassifiziert werden, da sie nur auf kleinen und nicht auf allen Abstaenden einen Einfluss auf das Leistungsspektrum haben. Allerdings bewahrt das freie Stroemen dieser Teilchen die kleinsten Strukturen vom Gravitationskollaps, womit sie auch nicht in die Kategorie der kalten dunklen Materie fallen koennen. Der Einfluss dieser warmen dunklen Materie kann in den statistischen Eigenschaften von kosmologischen Strukturen beobachtet werden. Die Unterdrueckung des linearen Materiedichtefeldes bei hohen Rotverschiebungen im WDM-Szenarium kann durch Loesen der Boltzmann-Gleichungen berechnet werden. Ein Fit an das resultierende lineare Materieleistungsspektrum, welches die statistischen Eigenschaften dieses Materiedichtefeldes im einfachen thermischen Reliktszenarium beschreibt, wird von Viel (2004) bereitgestellt. Dieses lineare Materieleistungsspektrum muss als naechstes korrigiert werden um die nichtlineare Strukturbildung im heutigen Universum miteinzubeziehen. Dies erweist sich als schwierig, da schon im kosmologischen Standard-Modell exakte Loesungen fuer die Entwicklung von gestoerten Dichtefeldern im nichtlinearen Regime nicht analytisch berechenbar sind. Die weitverbreiteten Ansaetze sind die 'halofit'-Methode von Smith (2002), welche einen physikalisch motivierten Fit an die Ergebnisse von numerischen Simulationen vornimmt, oder das noch physikalischere, jedoch weniger akurate 'Halomodel'. Beide nichtlinearen Methoden wurden jedoch nur unter der Annahme von kalter dunkler Materie entwickelt und sind daher nicht unbedingt fuer den Fall der warmen dunklen Materie anwendbar. In dieser Doktorarbeit wird das Halomodel abgeaendert (siehe auch Smith, 2011) um die Auswirkungen eines geglaetteten WDM-Dichtefeldes miteinzubeziehen. Erstens wird das dunkle Materiedichtefeld in zwei Hauptkomponenten geteilt: einen geglaetteten linearen Bestandteil und einen nichtlinearen Bestandteil, wobei jedoch beide Leistung auf allen Skalen haben. Zweitens wird eine Mindestmasse vorgeschlagen unter welcher keine Halos gefunden werden koennen. Drittens wird die Massenfunktion auch ueberhalb der Mindestmasse unterdrueckt. Viertens werden die Zentren der Halodichteprofile durch eine Faltung mit einer Gaussschen Funktion geglaettet, dessen Breite von der thermische WDM-Geschwindigkeit bestimmt wird. Es wird gezeigt, dass im WDM-Szenarium der letztere Effekt nicht relevant fuer die Berechnung des nichtlinearen Materie-Leistungsspektrums ist, auf allen Skalen relevant fuer aktuelle astronomische Surveys, insbesondere das Euclid Weak-Lensing-Survey. Um die Gueltigkeit der verschiedenen nichtlinearen WDM-Modelle zu ueberpruefen, wurden mit Hilfe des innovativen Lagrange-Code Gadget-2 (Springel, 2005) kosmologische N-Teilchensimulationen durchgefuert (siehe auch Viel, 2012). Diese Arbeit stellt eine leicht zu benutzende Fitfunktion zur Verfuegung, welche das nichtlineare WDM-Leistungsspektrum bei Rotverschiebungen zwischen z = 0.5 - 3.0 und im Bereich der fuer Weak-Lensing relevanten Skalen approximiert. Dabei wird das einfache thermische Reliktszenarium fuer verschiedene WDM-Massen untersucht und mit unseren Ergebnissen auf Aspekte der Aufloesungskraft durch Variation der Groesse und Zahl der simulierten Teilchen ueberprueft. Die Doktorarbeit endet mit einer Diskussion der Moeglichkeit, dass die WDM-Effekte auf das Materie-Leistungsspektrum Aehnlichkeiten aufweisen koennen mit den schwaecheren und grossskaligeren Effekten von freistroemenden massiven Neutrinos. Dies dient dem Ziel, die Daten des Sloan Digital Sky Survey daraufhin zu untersuchen (wie in Thomas, 2011). Es wird ermittelt, dass Neutrinoeffekte sich gerade genug von WDM-Effekten unterscheiden um eine Entartung von relevanten Parametern - die Summe der Neutrinomassen und die WDM-Masse - zu verhindern

Why weak lensing cluster shapes are insensitive to self-interacting dark matter

We investigate whether the shapes of galaxy clusters inferred from weak gravitational lensing can be used as a test of the nature of dark matter. We analyse mock weak lensing data, with gravitational lenses extracted from cosmological simulations run with two different dark matter models (CDM and SIDM). We fit elliptical NFW profiles to the shear fields of the simulated clusters. Despite large differences in the distribution of 3D shapes between CDM and SIDM, we find that the distributions of weak-lensing-inferred cluster shapes are almost indistinguishable. We trace this information loss to two causes. Firstly, weak lensing measures the shape of the projected mass distribution, not the underlying 3D shape, and projection effects wash out some of the difference. Secondly, weak lensing is most sensitive to the projected shape of clusters, on a scale approaching the virial radius (~ 1.5 Mpc), whereas SIDM shapes differ most from CDM in the inner halo. We introduce a model for the mass distribution of galaxy clusters where the ellipticity of the mass distribution can vary with distance to the centre of the cluster. While this mass distribution does not enable weak lensing data to distinguish between CDM and SIDM with cluster shapes (the ellipticity at small radii is poorly constrained by weak lensing), it could be useful when modelling combined strong and weak gravitational lensing of clusters.Comment: 14 pages, 9 figures, submitted to MNRAS, comments welcom

The Non-Linear Matter Power Spectrum in Warm Dark Matter Cosmologies

We investigate the non-linear evolution of the matter power spectrum by using a large set of high-resolution N-body/hydrodynamic simulations. The linear matter power in the initial conditions is consistently modified to accommodate warm dark matter particles which induce a small scale cut-off in the power as compared to standard cold dark matter scenarios. The impact of such thermal relics is addressed at small scales with k > 1 h/Mpc and at z < 5, which are particularly important for the next generation of Lyman-alpha forest, weak lensing and galaxy clustering surveys. We quantify the mass and redshift dependence of the warm dark matter non-linear matter power and we provide a fitting formula which is accurate at the ~2% level below z=3 and for masses m_wdm > 0.5 keV. The role of baryonic physics (cooling, star formation and feedback recipes) on the warm dark matter induced suppression is also quantified. Furthermore, we compare our findings with the halo model and show their impact on the cosmic shear power spectra.Comment: 14 pages, 8 figures, 1 Table. Discussion on AGN feedback and references added. Accepted for publication in MNRA

Produkcija makrozoobentosa u reci Rači uzvodno i nizvodno od pastrmskog ribnjaka

Biomasa (produkcija) makrozoobentosa je odabrana kao osnovni pokazatelj za praćenje promena kvantitativnog sastava naselja dna na lokalitetima uzvodno i nizvodno od pastrmskog ribnjaka u reci Rači. Istraživanje sekundarne produkcije makrozoobentosa reke Rače sa sedam lokaliteta, obavljeno je u periodu 2011. (april, jun, septembar, oktobar, decembar) i 2012. godine (februar i maj). Dominantne grupe u biomasi makrozoobentosa su Hirudinea (Annelida), Mollusca, Gammaridae (Crustacea) i Trichoptera (Insecta). Vrednosti biomase zoobentosa kretale su se u svim mesecima istraživanja u intervalu od 3,2001 g/m2, na lokalitetu RČ2 (u februaru) ,do 216,7120 g/m2, na lokalitetu RČ3 (u februaru). Biomasa faune dna najveća je u svim mesecima istraživanja na lokalitetu RČ3, koji je lociran nizvodno od pastrmskog ribnjaka. Na ovom lokalitetu biomasa makroinvertebrata se kretala od 87,8643 g/m2 (u aprilu 2011. godine) do 216,7120 g/m2 (u februaru 2012. godine)

Testing the Warm Dark Matter paradigm with large-scale structures

We explore the impact of a LWDM cosmological scenario on the clustering properties of large-scale structure in the Universe. We do this by extending the halo model. The new development is that we consider two components to the mass density: one arising from mass in collapsed haloes, and the second from a smooth component of uncollapsed mass. Assuming that the nonlinear clustering of dark matter haloes can be understood, then from conservation arguments one can precisely calculate the clustering properties of the smooth component and its cross-correlation with haloes. We then explore how the three main ingredients of the halo calculations, the mass function, bias and density profiles are affected by WDM. We show that, relative to CDM: the mass function is suppressed by ~50%, for masses ~100 times the free-streaming mass-scale; the bias of low mass haloes can be boosted by up to 20%; core densities of haloes can be suppressed. We also examine the impact of relic thermal velocities on the density profiles, and find that these effects are constrained to scales r<1 kpc/h, and hence of little importance for dark matter tests, owing to uncertainties in the baryonic physics. We use our modified halo model to calculate the non-linear matter power spectrum, and find significant small-scale power in the model. However, relative to the CDM case, the power is suppressed. We then calculate the expected signal and noise that our set of LWDM models would give for a future weak lensing mission. We show that the models should in principle be separable at high significance. Finally, using the Fisher matrix formalism we forecast the limit on the WDM particle mass for a future full-sky weak lensing mission like Euclid or LSST. With Planck priors and using multipoles l<5000, we find that a lower limit of 2.6 keV should be easily achievable.Comment: Replaced with version accepted for publication in PRD. Inclusion of: new figure showing dependence of predictions on cut-off mass; new discussion of mass function; updated refs. 18 pages, 10 Figure

Morfološki deformiteti usnog aparata roda chironomus (diptera: chironomidae) izazvani teškim metalima

U slatkovodnim ekosistemima larve Chironomidae su izložene svim prisutnim toksičnim materijama u vodi i sedimentu i kao takve mogu se koristiti kao idealni biotest organizmi. Kod larvi hironomida često se razvijaju deformacije usnih delova, posebno mentuma, za koje se pretpostavlja da su uglavnom prouzrokovani neorganskim hemikalijama. Kod gajenih hironomida (Chironomus) na sedimentu iz šaranskog ribnjaka utvrđen je stepen i tip deformiteta u zavisnosti od koncentracije i tipa teških metala. Larve Chironomus-a (prikupljene iz Jelenačkog potoka) bile su izložene olovo (II) acetat-3 hidratu i bakar (II) sulfat-pentahidratu u koncentracijama 30, 120 i 200 µg/g suve mase sedimenta. Za procenu stepena deformiteta korišćen je indeks toksičnosti TSI po Lenatu. Kod larvi Chironomus-a, izloženih olovu primećen je porast učestalosti deformiteta sa povećanjem koncentracija i to najčešće deformitet medijalno-lateralnih zuba, koji su kraći, i nedostatak zuba. Larve Chironomus-a izložene bakru pokazuju pad stope deformiteta od najniže ka najvišoj koncentarciji, a najčešći tipovi deformiteta su kraći i podeljeni medijalni zubi i kraći medijalno-lateralni zubi

Large-scale retrospective relative spectro-photometric self-calibration in space

We consider the application of relative self-calibration using overlap regions to spectroscopic galaxy surveys that use slit-less spectroscopy. This method is based on that developed for the SDSS by Padmanabhan at al. (2008) in that we consider jointly fitting and marginalising over calibrator brightness, rather than treating these as free parameters. However, we separate the calibration of the detector-to-detector from the full-focal-plane exposure-to-exposure calibration. To demonstrate how the calibration procedure will work, we simulate the procedure for a potential implementation of the spectroscopic component of the wide Euclid survey. We study the change of coverage and the determination of relative multiplicative errors in flux measurements for different dithering configurations. We use the new method to study the case where the flat-field across each exposure or detector is measured precisely and only exposure-to-exposure or detector-to-detector variation in the flux error remains. We consider several base dither patterns and find that they strongly influence the ability to calibrate, using this methodology. To enable self-calibration, it is important that the survey strategy connects different observations with at least a minimum amount of overlap, and we propose an "S"-pattern for dithering that fulfills this requirement. The final survey strategy adopted by Euclid will have to optimise for a number of different science goals and requirements. The large-scale calibration of the spectroscopic galaxy survey is clearly cosmologically crucial, but is not the only one.Comment: 23 pages, 19 figures, Accepted for publication in MNRAS, 201

Kvalitet vode u reci Raškoj na osnovu organizama zoobentosa i zooplanktona kao bioindikatora

U cilju ispitivnja kvaliteta vode reke Raške, organizmi zoobentosa i zooplanktona korišćeni su kao bioindikatori. Istraživanje je sprovedeno od aprila 2011. godine do maja 2012. godine, u vremenskim intervalima od dva meseca. Odabrano je ukupno pet lokaliteta na dužinini toka od 2.5 km. Između drugog i trećeg lokaliteta lociran je pastrmski ribnjak, čiji je uticaj na zajednice organizama praćen. Na lokalitetima koji su obuhvaćeni istraživanjima konstatovano je 57 taksona makrozoobentosa (34 vrste, 21 roda, 1 familija i 1 klasa) i 75 taksona zooplanktona (58 vrsta, 15 rodova, 1 red i 1 klasa). Srednje vrednosti indeksa saprobnosti ukazivale su na manje razlike u dobijenim vrednostima korišćenjem zoobentosa i zooplanktona. Na osnovu saprobioloških analiza, kada su kao bioindikatori korišćeni organizmi bentosa, kvalitet vode u reci Raškoj je druge klase kvaliteta, ili na prelazu između prve i druge klase, uglavnom na lokalitetima iznad ribnjaka. Organizmi zooplanktona su pokazali da voda celom dužinom toka pripada prvoj klasi kvaliteta. Najmanje vrednosti indeksa saprobnosti zabeležene su na prvom, referentnom lokalitetu 1.453±0.098 (zooplankton), a najviše na četvrtom lokalitetu 1.88±0.021 (zoobentos). Na lokalitetu ispod ribnjka, gde je zabeležen pad u koncentraciji kiseonika, dominiraju organizmi bentosa koji tolerišu veći stepen organskog zagađenja (Chironomidae, Oligochaeta, Simuliidae). Iako su koncentracije ukupnog fosfora i ortofosfata rasle od trećeg ka petom lokalitetu, njihove koncentracije nisu premašivale dozvoljene vrednosti za salmonidne vode. Međutim, srednje vrednosti koncentracije nejonizovanog amonijaka (NH4+) su iznad referentnih vrednosti na trećem i četvrtom lokalitetu. Razlog nepodudaranja vrednosti indekasa saprobnosti za dve istraživane grupe, može biti usled nestabilnih zooplanktonskih zajednica zbog izraženog efekta drifta, kao i usled činjenice da će zbog procesa sedimentacije dospelih organskih materija, organizmi dna biti izloženiji njihovom uticaju. Zajednice bentosa, kao znatno stabilnije i u vremenu i u prostoru, pogodnije su za procenu kvaliteta tekućih voda

Larve chironomus plumosus (diptera, insecta) izvor esencijalnih masnih kiselina za ishranu šaranske mlađi

U cilju ispitivanja koliko su larve Chironomus plumosus-a pogodne za ishranu gajenih slatkovodnih riba, pre svega šarana, ižvršena je analiza hemijskog i masnokiselinskog sastava larvi koje su prikupljene na kraju tromesečnog perioda gajenja šaranske mlađi u dva eksperimentalna ribnjačka bazena u Centru za ribarstvo i primenjenu hidrobiologiju Poljoprivrednog fakulteta Univerziteta u Beogradu. Udeo sirovih proteina u larvama Chironomus plumosus-a iznosio je 6,61 u jezeru L1 odnosno 6,18% u jezeru L2, što predstavlja vrednost adekvatnu za rast svih slatkovodnih vrsta riba. Sadržaj lipida bio je 0,49 odnosno 0,73%, što je energetski povoljno za sve ribe koje žive u toplim vodama. Prirodnu hranu (larve Chironomus plumosus) karakteriše i visok sadržaj vode: 88,95 u L1 i 89,62% u L2, a što ih čini pogodnom za ishranu šaranske malađi. Lipidnu frakciju larvi Chironomidae u jezeru L1 je činilo 45.36% zasićenih i 53.96 nezasićenih masnih kiselina. U lipidnoj frakciji larvi Chironomidae iz jezera L2 nađeno je 53.47% zasićenih i 46.42% nezasićenih masnih kiselina. Od polinezasićenih (esencijalnih) masnih kiselina nađenih u hironomidama u jezeru L1, najveći deo je pripadao ω-6 linolnoj kiselini (21,37%), zatim ω-3 linolenskoj (3,21%) i eikozopentanskoj ω-3 kiselini (1,27%). Sadržaj linolne kiseline u larvama Chironomidae iz jezera L2 je bio niži i iznosio je 9,78%, eikozopentanska ω-3 kiselina je zastupljena sa 0,45%, a sadržaj linoleinske kiseline je bio viši i iznosio 7,78%. Nedostatak PUFA sa 22 C atoma je verovatno povezan sa slabom enzimatskom sposobnošću larvi Chironomidae za sintezu ovih kiselna iz njihovih prekursora PUFA sa 18 C atoma. Izmerena količina ω-3 i ω-6 masnih kiselina u larvama Chironomus plumosus je iznosila 0,21 u L1, a u jezeru L2 0,81, zadovoljava nutritivne zahteve šarana