COSMO Seminars

The seminars take place at the auditorium of the 5th floor in the main CBPF building on Mondays at 10h30

Future Seminars:

To be announced

Next Seminar: Direct detection of dark matter: from WIMPS to dark photons​

When: 02/10/2017, at 10:30h

Speaker: João Torres de Mello Neto, Instituto de Física - UFRJ

Abstract:

The matter that we know makes up less than 5% of the Universe. From astronomical and cosmological observations we have strong evidence that dark matter exists and is the main component of the mass budget of the universe. The nature of dark matter is yet to be discovered and is one of the most pressing questions in physics. Direct detection experiments aim to detect the interaction of galactic dark matter with terrestrial nuclei. Weekly interacting massive particles (WIMP) are a likely candidate for dark matter, but axions, hidden photons and other candidates are also considered. In this talk we will review currently available major technologies being pursued by various collaborations, with special emphasis on the CCD technology used by the DAMIC (Dark Matter in CCDs) experiment and the measurements of radioactive contamination in the CCDs.

The latest results from the experiment on the search for WIMPs interactions in the bulk silicon will be discussed. We will also present constraints on the absorption of hidden-photon dark matter. The constraints are the best direct detection limits for low mass hidden-photon dark matter. We also discuss other exotic dark matter candidates.

Past Seminars

2017

Subjecting theories of dark matter to the cluster test

When: 04/9/2017, at 10:30h

Speaker: Theo M. Nieuwenhuizen, University of Amsterdam

Abstract:

Galaxy clusters were employed by Zwicky to argue for the existence of dark matter and still pose stringent tests. Modifications of Newtons law such as MOND, EG, MOG and f(R) theories are ruled out, unless $\sim 2$ eV neutrinos are added.

If dark matter is self-interacting with cross section $\sigma/m \sim 2 cm^2/gr$, MACHOs, primordial black holes and light axions are ruled out as cluster dark matter.

Recent strong lensing and X-ray gas data of the cluster A1835 allow to test thermal dark matter with classical, bosonic and fermionic distributions, next to (generalised) Navarro-Frenck-White profiles. Within the spherical approximation it is found that all these cases do not work well, except for the fermionic one. The interpretation in terms of (nearly) Dirac neutrinos with mass of $1.70^{+0.13}_{-0.17}$ eV/$c^2$ is consistent with earlier results on the cluster A1689, with the Planck and DES dark matter fractions and with the non-detection of neutrinoless double beta decay. The case will be tested in the 2018 KATRIN experiment.

H0 and s8 tensions and the scale invariant spectrum

When: 28/8/2017, at 10:30h

Speaker: Micol Benetti, ON

Abstract:

The currently most precise measurement of the local expansion rate H_0 by Riess et. al (2016) differs by ~ 3.4 standard deviations from the value reported by the Planck Collaboration (2015) assuming the standard cosmological model. Since the two approaches, namely using the geometric distance calibrations of Cepheids and the cosmic microwave background data, sample different epochs in cosmic evolution, and one of them uses the standard cosmological model as the fiducial cosmology, this tension in the value of H_0 has given rise to speculations. It has been argued that either new physics beyond the standard cosmology or the influence of local structures must be taken into account in order to reconcile these H_0 estimates. In this talk I introduce this H_0 tension and I present the results of our recent paper, where the effect of this discrepancy has been explored through a Bayesian analysis with the current Planck 2015 and Riess 2016 data. I present a way to alleviate such a tension studying the observational viability of the scale invariant spectrum and two of its extensions, i.e., assuming as free parameter the primordial helium mass fraction and the effective number of relativistic degrees of freedom (Neff). Also, I present analysis using Baryon Acoustic Oscillations measurements and Cluster Number counts and Weak Lensing data in order to take into account the tension on fluctuation amplitude parameter, σ8 , value which so far has always brought standard model with Neff extension to be discarded.

``Do joint CMB and HST data support a scale invariant spectrum?,'' M. Benetti, L. L. Graef and J. S. Alcaniz, JCAP 1704, no. 04, 003 (2017)

(De)Confinement in Strong Interactions

When: 31/7/2017, at 10:30h

Speaker: Leticia F. Palhares, UERJ

Abstract:

Quantum Chromodynamics is the theory that describes the interaction between quarks and gluons, the current ultimate constituents of atomic nuclei. Even though very successful, it still presents fundamental open questions. One of the most prominent is related to the mechanism of confinement, that guarantees that quarks and gluons are never observed directly and are always confined in the interior of hadrons, like protons and neutrons. In this talk, I shall present a set of theoretical models that try to explain confinement, as well as what can we learn from observation of matter under extreme conditions, in which deconfinement could take place. In particular, this physics — usually associated with microscopic phenomena — will be shown to possibly play a role in astrophysical systems involving ultra-compact objects.

Asymptotic Symmetries in Gravity

When: 12/6/2017, at 10:30h

Speaker: Marc Henneaux, ULB, Belgium

Abstract:

The talk will review how asymptotic symmetries are defined and their generators constructed in theories with a gauge freedom, considering the specific case of gravity. The algebra of the conserved charges will be discussed. The analysis will be illustrated in various contexts (asymptotically anti-de Sitter spaces, asymptotically flat spaces).

Towards a measurement of the primordial CMB dipole

When: 08/5/2017, at 10:30h

Speaker: Miguel Quartin, UFRJ

Abstract:

It is believed that the solar system is moving with a velocity of 370 km/s compared to the CMB rest frame. The main effect of this is to produce the observed CMB dipole via the Doppler effect. There are nevertheless other physical consequences of not being on the CMB rest frame which are already important at the precision level of Planck. I will discuss this and the possibilities of independently measuring this velocity and thus also the primordial CMB dipole. I will also show that not properly taking it into account leads to systematic effects, for instance on measures of some of the so-called CMB anomalies and on the overall calibration of the Planck satellite. Finally, I will talk about the prospects of measuring the velocity using future 21cm surveys.

Searching for star-forming early-type galaxies: the role of the environment

When: 24/4/2017, at 10:30h

Speaker: Laurie Riguccini, Observatorio do Valongo/ UFRJ

Abstract:

ETGs were first believed to be read and dead but studies from the past decades have indicated that some ETGs show signs of star formation. Where does this star formation(SF) come from? Riguccini et al. (2015) unveiled a population of galaxies in transition: star-forming ETGs in the Coma cluster with a larger L24μm/LK ratio. They have shown that 70% of those star-forming ETGs are located in a the south-west part of the cluster, where a substructure is unfailing on the main cluster. In order to apply this mid-IR/near-IR selection to a wide sample of ETGs, we already checked the validity of using the WISE-all sky survey bands instead of the Sptizer-24μm/2MASS_K band ratio. We successfully found that using WISE 22μm/3.4μm is a reliable tool to select star-forming ETGs. We aim at understanding what happened in those ETGs to have a new burst of star formation and to built large samples of these objects. We aim to unveil star-forming ETGs in other clusters and explore the effect of the environment on their SF activity using the most populated clusters in the local Universe and expend it to wide survey such as the S-PLUS survey.

Quantum fields for higher spin particles without ghosts and indefinite metric - A program and some results

When: 17/4/2017, at 14:30h

Speaker: Jens Mund (UFJF)

Abstract:

Quantum field theories for interacting particles with higher spin are usually based on gauge theory. In this approach, one has to add unphysical degrees of freedom in the form of "negative norm" states and "ghost" fields before the construction, and divide them out afterwards by requiring gauge (or BRST) invariance of the S-matrix and observable fields.

In the talk an alternative approach is presented: Instead of Hilbert space positivity, the localization of the fields are weakened, in a way permitted by the principles of relativistic quantum field theory: The fields are localized on "Jordan-Mandelstam strings" extending to space-like infinity. They act in a true Hilbert space without ghosts.

The construction of interacting models is based only on the fundamental principles of unitarity, renormalizability, covariance, and locality of the observables. The latter implies a string-independence condition of the S-Matrix which has to be satisfied in every order and is quite restrictive. Any model complying with these principles must exhibit certain features -- which are thus derived from first principles. We conjecture that the following features are explained in this sense (which usually are put in by hand):

- The structure of gauge theories, namely: Lie algebra relations of the coupling constants in models with self-interacting vector bósons; minimal coupling principle; - The shape of the Higgs potential; - Chirality of electroweak interactions; and more...

Our approach also allows for a direct (perturbative) construction of charged interacting fields which interpolate between the in- and outgoing fields, which the gauge theory approach does not provide. It also provides energy-momentum tensors for massless higher helicity particles, which for point-local fields are ruled out by the Weinberg-Witten Theorem.

Extension of renormalizability to higher spins: reconciling causal localization with positivity

When: 11/4/2017, at 14:30h

Speaker: Bert Schroer (Freie Universität Berlin)

Abstract:

The short distance scale dimension d{sd}=s+1 of spin s≥1 field can be lowered to the spin-independent value of the d{sd}=1 of its string-localized counterpart. The use of such fields in a new perturbation theory permits the preservation of positivity together with renormalizability. In contrast to the positivity-violating gauge theory (for which only gauge invariant operators preserve positivity), the new setting of QFT aims at a construction of a complete QFT in which all particles are described in terms of physical fields. Among its achievements are the construction of massless currents and energy-momentum tensors (the Weinberg-Witten problem), the correct degrees of freedom counting (the van Dam-Veltman-Zakaharov discontinuity problem) and a more profound understanding of the Higgs mechanism.

Leading CFT constraints on multi-critical models in d > 2

When: 10/4/2017, at 14:30h

Speaker: Alessandro Codello (USD, Denmark and INFN-Bologna, Italy)

Abstract:

I'll describe the family of renormalizable scalar QFTs with self-interacting potentials of highest monomial φ^m below their upper critical dimensions dc = 2m/(m−2), and study them using a combination of CFT constraints, Schwinger-Dyson equation and the free theory behavior at the upper critical dimension.

For even integers m ≥ 4 these theories coincide with the Landau-Ginzburg description of multi-critical phenomena and interpolate with the unitary minimal models in d = 2, while for odd m the theories are non-unitary and start at m = 3 with the Lee-Yang universality class.

For all the even potentials and for the Lee-Yang universality class, I show how the assumption of conformal invariance is enough to compute the scaling dimensions of the local operators φ^k and of some families of structure constants at leading order.

Related paper: 1703.04830

f(R): a tale of two frames

When: 10/4/2017, at 10:30h

Speaker: Sergio E. Joras (UFRJ, Rio de Janeiro)

Abstract:

Modified gravity theories know as f(R) are used to generate an accelerated expansion of the universe, both in its early ages (inflation) and in more recent epochs (instead of assuming the existence of the dark energy). It is well known that any f(R) can be recast as General Relativity (GR) with an extra scalar field upon a suitable conformal transformation, i.e, in a different frame. Here we take the opposite direction: starting from GR with a scalar field, what is the corresponding f(R) in the other frame? Here we apply this procedure — know for a while [Magnano et al, 1987] — with unexpected results.

Aberration and Doppler effect on the CMB: kinematical and intrinsic contributions

When: 27/3/2017, at 10:30h

Speaker: Omar Roldan (UFRJ, Rio de Janeiro)

Abstract:

The aberration and Doppler coupling effects of the Cosmic Microwave Background (CMB) were recently measured by the Planck satellite. The most straightforward interpretation leads to a direct detection of our peculiar velocity, consistent with the measurement of the well-known dipole. We show however, that in some cases a primordial dipolar potential can also produce such effects. But in addition it produces other signatures due to integrated terms which we will discuss in some detail. We also discuss how aberration measurements can be used to constraint some models which introduce super-horizon perturbations to explain the hemispherical asymmetry.

Related paper: 10.1088/1475-7516/2016/06/026, https://arxiv.org/abs/1603.02664

Non-Linear Quantum Mechanics and de Broglie's Double Solution Program

When: 13/3/2017, at 10:30h

Speaker: Thomas Durt (Aix Marseille University, France) Abstract:

Recently, non-linear modifications of the Schroedinger equation, like e.g. the Schroedinger-Newton equation have been studied in relation with the measurement problem.

In particular, the presence of a non-linearity would explain why the wave packet associated to a quantum particle does not spread with time. In other words, particles behave as solitons, which is strongly reminiscent of de Broglie's Double Solution Program, elaborated by Louis de Broglie in the twenties, of which the de Broglie-Bohm pilot wave dynamics is a simplified version. It is also reminiscent of Poincare's pressure, invoked by Poincare in 1905 in order to solve the wave-particle duality in the context of classical field theory. We shall describe recent attempts to derive the de Broglie-Bohm pilot wave dynamics from non-linear wave dynamics, and to apply them to quantum corpuscles at one side, and to so-called bouncing oil droplets at the other side.

Related papers:

http://iopscience.iop.org/article/10.1209/0295-5075/114/10004/meta Thomas Durt, Generalized guidance equation for peaked quantum solitons and effective gravity, EPL (Europhysics Letters), Volume 114, Number 1.

https://arxiv.org/abs/1602.03133 Thomas Durt, Generalized guidance equation for peaked quantum solitons: the single particle case.

Cores in Dwarf Galaxies from Fermi Repulsion

When: 20/2/2017 at 10:30h

Speaker: James Unwin (University of Illinois at Chicago, U.S.A.)

Abstract:

Cold dark matter provides a remarkably good description of cosmology and astrophysics. However, observations connected with small scales might be in tension with this framework. In particular, structure formation simulations suggest that the density profiles of dwarf spheroidal galaxies should exhibit cusps, in contrast to observations. I will show that Fermi repulsion can explain the observed cored density profiles in dwarf galaxies for sub-keV fermionic dark matter. While in conventional dark matter scenarios, such sub-keV thermal dark matter would be excluded by free streaming bounds, I will argue that these constraints are ameliorated in models with dark matter at lower temperature than conventional thermal scenarios. Finally, I will outline a class in which the dark matter typically has a lower temperature than the thermal expectation, dubbed Flooded Dark Matter, and discuss aspects of model building.

2016

Bariogênese gravitacional / Gravitational baryogenesis

When: 5/12/2016 at 10:30h

Speaker: Vicente Antunes (CBPF)

Abstract:

Uma das questões cruciais na cosmologia moderna está relacionada à observação da assimetria de barions no universo. Nos modelos usuais de bariogênese a gravitação desempenha papel secundário, embora o processo tenha muito provavelmente ocorrido nos primórdios da atual fase de expansão do universo onde o campo gravitacional era extremamente forte. Nesse trabalho reexaminamos a proposta original de Sakharov no qual a gravitação está no centro da bariogênese. Veremos que dependendo do modo como a matéria se acopla com a gravitação ela pode implicar na não conservação do número bariônico e concomitantemente à violação de CP. Ademais, acoplamento não-mínimo com a gravitação é capaz de induzir massa efetiva reduzida dos quarks favorecendo transição de fase na QCD. Como consequência, uma eficiente assimetria bariônica ocorre na transição da época dos quarks para a época hadrônica. Quando quarks e gluons se tornam confinados o presente mecanismo evanesce e a assimetria se congela.

One of the biggest puzzles in modern cosmology is the observed baryon asymmetry in the universe. In current models of baryogenesis gravity plays a secondary role, although the process is believed to have happened in the early universe, under the influence of an intense gravitational field. In the present work we resume Sakharov's original program for baryogenesis and propose a central role for gravity in the process. This is achieved through a non-minimal coupling (NMC) between the gravitational field and both the strong interaction field and the quark fields. When in action, the present mechanism leads to baryon number non-conservation and CP violation. Moreover, the NMC induces reduced effective quark masses, which favours a first order QCD phase transition. As a consequence, a baryon asymmetry can be attained in the transition from the quark epoch to the hadron epoch.

Breaking of Spatial Diffeomorphism Invariance, Inflation and the Spectrum of Cosmological Perturbations

When: 18/11/2016 at 10h30

Speaker: Leila Graef (Observatório Nacional, Rio de Janeiro)

Abstract:

Standard inflationary models yield a characteristic signature of a primordial power spectrum with a red tensor and scalar tilt. Nevertheless, it was recently suggested that, by breaking the assumption of spatial diffeomorphism invariance in the context of the effective field theory of inflation, a blue tensor spectrum can be achieved without violating the Null Energy Condition. In this context, we explore in which cases a blue tensor tilt can be obtained along with a red tilt in the scalar spectrum. Ultimately, we analyze under which conditions this model can reproduce the specific consistency relation of String Gas Cosmology.

Related paper: arXiv: 1506.00896v1 [astro-ph.CO] (JCAP v. 2015, p. 009-009, 2015); http://iopscience.iop.org/article/10.1088/1475-7516/2015/10/009/pdf

Numerical Relativity and the First Detections of Gravitational Waves

When: 3/11/2016 at 14h

Speaker: Deirdre Shoemaker (Georgia Institute of Technology, U.S.A.)

Abstract:

For decades, both the gravitational wave and numerical relativity communities have worked to prepare for the era of gravitational wave astronomy. With the first observation of two black holes merging (GW150914), quickly followed by a second (GW151226), the new era of gravitational wave astronomy has begun. I will present the role that numerical relativity played in the unveiling of the gravitational wave sky and anticipate how it might improve our understanding of astrophysics.

Effective models of quantum gravity induced by Planck scale modifications in the covariant quantum algebra

When: 10/10/2016 at 10h30

Speaker: Gustavo Pazzini de Brito (CBPF)

Abstract:

In this talk we introduce a modified covariant quantum algebra based in the so-called Quesne-Tkachuk algebra. By means of a deformation procedure we arrive at a class of higher derivative models of gravity. The study of the particle spectra of these models reveals an equivalence with the physical content of the well-known renormalizable and super-renormalizable higher derivative gravities. The particle spectrum exhibits the presence of spurious complex ghosts and, in light of this problem, we suggest an interesting interpretation in the context of minimal length theories. Also, a discussion regarding the non-relativistic potential energy is proposed.

Related paper: https://arxiv.org/abs/1610.01480

Gravitational Wave Astronomy and Black Hole Perturbation Theory

When: 10/10/2016 at 10h30

Speaker: Niels Warburton (Massachusetts Institute of Technology, U.S.A., and University College Dublin, Ireland)

Abstract:

Gravitational waves have opened a new window on our Universe. Just over a year ago we did not know that black holes exist in binaries; that the components of such binaries could be very heavy, implying interesting astrophysics in the environment in which the black holes formed; and nor had Einstein's theory of General Relativity ever been tested in the highly dynamical, strong-field regime. These insights and more were gleaned from just two signals that each lasted less than a second and contained a few tens of GW cycles. Imagine what we will learn about the dark side of the cosmos from signals that last minutes or even years?

The gravitational waves from the inspiral of a stellar-mass compact object into a larger black hole will provide such long lasting signals. In this talk I will briefly review the recent detection of gravitational waves before discussing how black hole perturbation theory can be used to model small mass-ratio binaries. I will concentrate on rapidly rotating black holes, the subject of work we recently published in Classical and Quantum Gravity.

Related papers: http://iopscience.iop.org/article/10.1088/0264-9381/33/15/155002/meta or arXiv: http://arxiv.org/abs/1603.01221

Speaker's website: nielswarburton.net

NOTE: The seminar will be broadcast on the weblink https://hangouts.google.com/hangouts/_/ucd.ie/seminardrnwarburton It is via Google Hangouts, so if you connect, please make sure to turn off your microphone sound and camera so that we do not hear you in the seminar room.

Extending CAMB and Coyote spectra

When: 14/04/2016 at 10h30

Speaker: Luciano Casarini (Visiting Professor at UFES, Vitoria)

Abstract:

We discuss an extension of the Coyote emulator to predict non-linear matter power spectra of dark energy models with a scale factor dependent equation of state of the form w = w_0 + ( 1 - a )w_a . The extension is based on the mapping rule between non-linear spectra of DE models with constant equation of state and those with time varying one.

http://www.cosmo-ufes.org/luciano-casarini.html

Piercing the Vainshtein screen with anomalous gravitational wave speed: Constraints on modified gravity from binary pulsars

When: 07/04/2016 at 10h30

Speaker: Hermano Velten (CPT, Marseille and UFES, Vitoria)

Abstract:

By using observations of the Hulse-Taylor pulsar we constrain the gravitational wave (GW) speed to the level of 10−2. We apply this result to scalar-tensor theories that generalize Galileon 4 and 5 models, which display anomalous propagation speed and coupling to matter for GWs. We argue that this effect survives conventional screening due to the persistence of a scalar field gradient inside virialized overdensities, which effectively "pierces" the Vainshtein screening. In specific branches of solutions, our result allows to directly constrain the cosmological couplings in the effective field theory of dark energy formalism.

http://arxiv.org/abs/1507.05047

CMB temperature fluctuations and polarization seeded by cosmic strings

When: 24/02/2016 at 10h30

Speaker: Martin Landriau (The University of Texas - Mc Donald Observatory)

Abstract:

After reviewing the basics of cosmic string formation and evolution, I will describe the cosmological consequences of their formation, focusing on their effects on the cosmic microwave background and presenting some new results. I will conclude by discussing future prospects for detection in the light of current observational constraints.

Martin's webpage http://www.as.utexas.edu/~landriau/Site/Home.html

2015

Arrow of time in scalar field cosmology with bounces

When: 02/12/2015 at 10h30

Speaker: Aleksey Toporensky (Lomonosov Moscow State University)

Abstract:

A Universe filled with a homogeneous scalar field exhibits `Cosmological hysteresis'. Cosmological hysteresis is caused by the asymmetry in the equation of state during expansion and contraction. This asymmetry results in the formation of a hysteresis loop whose value can be non-vanishing during each oscillatory cycle. For flat potentials, a negative value of the hysteresis loop leads to the increase in amplitude of consecutive cycles and to a universe with older and larger successive cycles. Such a universe appears to possess an arrow of time even though entropy production is absent and all of the equations respect time-reversal symmetry ! Cosmological hysteresis appears to be widespread and exists for a large class of scalar field potentials and mechanisms for making the universe bounce.

What is Dynamics in Quantum Gravity?

When: 21/10/2015 at 10h30

Speaker: Przemyslaw Malkiewicz (APC - Paris)

Abstract:

Dynamics of general relativistic systems is given with respect to internal clocks. I investigate the extent to which the choice of internal clock in quantum description of the gravitational field determines its quantum dynamics. Within a fixed quantization I find the abundance of possible semiclassical extensions to general relativity just by switching between clocks. It follows that quantities like minimal volume, maximal curvature and even a number of quantum bounces, often used to describe quantum effects in gravity, are not well-defined.

Primordial quantum non-equilibrium and large-scale cosmic anomalies

When: 14/10/2015 at 10h30

Speaker: Samuel Colin (New Postdoc at ICRA CBPF)

Abstract:

According to inflationary cosmology, the cosmic structure, in particular that of the cosmic microwave background, is seeded by the primordial vacuum fluctuations of the inflation field. Recent results reported by the Planck Team suggest that there is an anomalous large-scale power deficit in the cosmic microwave background. During this talk, I will show that such a deficit occurs naturally in the de Broglie Bohm pilot-wave theory, which is a deterministic formulation of quantum theory in which the Born law arises dynamically (a process referred to as relaxation to quantum equilibrium), provided that the inflation field is initially in a state of quantum non-equilibrium (the Born law is not satisfied). More specifically, for a radiation-dominated pre-inflationary era, the deficit will have a characteristic shape (an inverse-tangent dependence on k, the wave-number, with oscillations), which is compatible with the observed data for an appropriate choice of cosmological parameters.

This talk is based on recent works done in collaboration with Antony Valentini (Clemson University):

http://arxiv.org/abs/1306.1579 and http://arxiv.org/abs/1407.8262.

A Discussion on Metric-Affine Theories of Gravity: Some Theoretical and Experimental aspects

When: 24/09/2015 at 10h30

Speaker: Salvatore Capozziello (University of Naples "Federico II", Italy)

Abstract:

We discuss the metric and metric-affine theories of gravity in
view of addressing open problems like dark energy and dark matter in
cosmology. In particular we discuss geodesic and metric structures with
the aim to identify the fundamental variables capable of describing the
gravitational field. The role of Equivalence Principle is considered. Some
experimental proposals are reviewed.

http://www.researchgate.net/profile/Salvatore_Capozziello

Primordial Non-gaussianities of inflationary step-like models

When: 16/09/2015 at 10h30

Speaker: Camila Novaes (ON - Rio de Janeiro)

Abstract:

We use Minkowski Functionals to explore the presence of non-Gaussian signatures in simulated cosmic microwave background (CMB) maps. Precisely, we analyse the non-Gaussianities produced from the angular power spectra emerging from a class of inflationary models with a primordial step- like potential. This class of models are able to perform the best-fit of the low-l ‘features’, revealed first in the CMB angular power spectrum by the WMAP experiment and then confirmed by the Planck collaboration maps. Indeed, such models generate oscillatory features in the primordial power spectrum of scalar perturbations, that are then imprinted in the large scales of the CMB field. Interestingly, we discover Gaussian deviations in the CMB maps simulated from the power spectra produced by these models, as compared with Gaussian ΛCDM maps. Moreover, we also show that the kind and level of the non-Gaussianities produced in these simulated CMB maps are compatible with that found in the four foreground-cleaned Planck maps. Our results indicate that inflationary models with a step-like potential are not only able to improve the best-fit respect to the ΛCDM model accounting well for the ‘features’ observed in the CMB angular power spectrum, but also suggesting a possible origin for certain non-Gaussian signatures observed in the Planck data.

The self-force approach to the general relativistic two-body problem

When: 19/08/2015 at 10h30

Speaker: Niels Warburton (MIT)

Abstract:

In this talk I shall review progress using black hole perturbation theory techniques to understand the two-body problem in the general relativistic context. There is mounting evidence that techniques originally envisioned to be solely applicable to modeling sources for space-based gravitational-wave detectors will find utility across a wide range of mass ratios. I will concentrate on the self-force approach whereby the inspiral of a compact object into a massive black hole is computed by calculating the local `self-force’ that the particle experiences due to its interaction with its own metric perturbation. I will discuss a number of recent results and future research directions being taken by the self-force community.

http://www.nielswarburton.net

The cosmic acceleration and the Cascading DGP model

When: 22/07/2015 at 10h30

Speaker: Fulvio Sbisa (University of Portsmooth)

Abstract:

The discovery of the late time accelerated expansion of the universe remains puzzling fifteen years after its discovery. Despite the acceleration may be due to vacuum energy or to a "dark" source of energy, it may be that the observations are just signalling a breakdown of the validity of GR on very large scales. I will present a model based on the braneworld paradigm, the Cascading DGP model, which aims to provide a consistent modification of General Relativity on ultra large distances, and may shed light on the cosmological acceleration and the cosmological constant problems. I will in particular discuss the presence/absence of a ghost instability depending on the amount of vacuum energy present in our 4D world.

Propriedades dinâmicas e mecânica estatística da matéria escura em halos esféricos

When: 17/06/2015 at 10h30

Speaker: Leandro Beraldo e Silva (IF, USP)

Abstract:

A determinação do estado de equilíbrio (perfil de densidades e distribuição de velocidades) de sistemas auto-gravitantes como aglomerados globulares, galáxias e halos de matéria escura é um problema antigo e ainda sem solução completa. Nos últimos anos, simulações numéricas têm fornecido informações importantes sobre estas funções, mas sua explicação nos termos da mecânica estatística ainda não foi satisfatoriamente dada, embora algumas propostas teóricas tenham sido formuladas recentemente. Nesteseminário, introduziremos este problema e discutiremos testes observacionais de diferentes modelos (fenomenológicos e teóricos) para os perfis de densidade, utilizando dados de aglomerados de galáxias obtidos via lentes gravitacionais. Ao fim, apresentaremos um modelo teórico proposto por nós (em que o conceito de indistinguibilidade é associado ao nível de mistura do sistema), mostrando as implicações para o perfil de densidades e como esta associação levanta dúvidas sobre a validade da equação de Vlasov durante o processo de colapso dos halos de matéria escura.

Analog gravity and analog Hawking radiation: the experimental search

When: 27/04/2015 at 10h30

Speaker: Dr. Alessandro Fabbri, Centro Fermi and Univ. Bologna (Italy), IFIC-CSIC, Univ. Valencia (Spain)

Abstract:

The Hawking effect (quantum particle creation by black holes) is not specific to gravity. An analog of it can be found in fluids undergoing supersonic flow (called acoustic black holes). We review ongoing attempts to observe it in condensed matter systems, and focus on a proposal to detect it through correlation measurements in Bose-Einstein condensates.

http://arxiv.org/pdf/1411.7842.pdf

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