Found 17 talks width keyword dark energy

Tuesday September 13, 2022
Dr. Martín López Corredoira


Cosmological observations (redshifts, cosmic microwave background radiation, abundance of light elements, formation and evolution of galaxies, large-scale structure) find explanations within the standard Lambda-CDM model, although many times after a number of ad hoc corrections. Nevertheless, the expression ‘crisis in cosmology’ stubbornly reverberates in the scientific literature: the higher the precision with which the standard cosmological model tries to fit the data, the greater the number of tensions that arise. Moreover, there are alternative explanations for most of the observations. Therefore, cosmological hypotheses should be very cautiously proposed and even more cautiously received.

There are also sociological and philosophical arguments to support this scepticism. Only the standard model is considered by most professional cosmologists, while the challenges of the most fundamental ideas of modern cosmology are usually neglected. Funding, research positions, prestige, telescope time, publication in top journals, citations, conferences, and other resources are dedicated almost exclusively to standard cosmology. Moreover, religious, philosophical, economic, and political ideologies in a world dominated by anglophone culture also influence the contents of cosmological ideas.

Thursday February 10, 2022
Dr. Marcos Pellejero


A key problem that we are facing in cosmology nowadays is that we cannot make accurate predictions with our current theoretical models. We have all of the pieces of the standard model but it doesn't have an analytical solution. The only way to have accurate predictions is to run a cosmological simulation. Then, why not use these simulations as the theory model? Well, for one main reason, if we want to explore the full parameter space comprised in the standard model, we need thousands of such simulations, and they are terribly computationally expensive. We wouldn't be able to do it in years! In this talk, I will tell you how in the last few years we have come up with a way to circumvent this problem.

Thursday November 4, 2021
Dr. Sergio Contreras



On the LCDM cosmology, dark matter collapses into virialised objects called haloes. The abundance and distribution of these haloes are a direct consequence of the cosmology of the Universe. By constraining the dark matter halo clustering, we could also constraint the cosmology from our Universe. Since dark matter haloes can not be observed, we need to use galaxies to trace them.

In this talk, I will present a new method that we develop capable of constraining cosmological information from the redshift space galaxy clustering.  We use the scaling of cosmological simulations and the SubHalo Abundance Matching extended (SHAMe) empirical model to produce realistic galaxy clustering measurements over a wide range of cosmologies. We generate more than 500,000 clustering measurements at different cosmological and SHAMe parameters to build an emulator capable of reproducing the projected correlation function, monopole and quadrupole of the galaxies. We run an MCMC using this emulator to constrain the cosmology of the TNG300 hydrodynamic simulation. We correctly predicted the cosmology of the TNG300 simulation constraining sigma8 between [0.75,0.83] and Omega matter h^2 between [0.127,0.162]. The best constraints are obtained when including scales below 2 Mpc/h and when combining all different clustering statistics. We conclude that our approach can be used to constrain cosmological and galaxy formation parameters from the galaxy clustering of galaxy surveys.

Thursday September 16, 2021
Dr. Siddharth Mishra-Sharme


The next decade will see a deluge of new cosmological data that will enable us to accurately map out the distribution of matter in the local Universe, image billions of stars and galaxies to unprecedented precision, and create high-resolution maps of the Milky Way. Signatures of new physics as well as astrophysical processes of interest may be hiding in these observations, offering significant discovery potential. At the same time, the complexity of astrophysical data provides significant challenges to carrying out these searches using conventional methods. I will describe how overcoming these issues will require a qualitative shift in how we approach modeling and inference in cosmology, bringing together several recent advances in machine learning and simulation-based (or likelihood-free) inference. I will ground the talk through examples of proposed analyses that use machine learning-enabled simulation-based inference with an aim to uncover the identity of dark matter, while at the same time emphasizing the generality of these techniques to a broad range of problems in astrophysics, cosmology, and beyond.

Meeting ID: 831 9395 9785
Passcode: 343950O


Thursday September 9, 2021
Dr. Carlos Hernández-Monteagudo


In cosmology, it is customary to convert observed redshifts into distances in order to study the large scale distribution of matter probes like galaxies and quasars, and to obtain cosmological constraints thereof. In this talk, I describe a new approach which bypasses such conversion and studies the "field of redshifts" as a new cosmological observable, dubbed thereafter as angular redshift fluctuations (ARF). By comparing linear theory predictions to the output of N-body cosmological simulations, I will show how the ARF are actually sensitive to both the underlying density and radial peculiar velocity fields in the universe, and how one can obtain cosmological and astrophysical constraints from them. And since "the prove of the pudding is in the eating", I will demonstrate how ARF provide, under a very simple setup, competitive constraints on the nature of peculiar velocities and gravity from BOSS DR13 data. Furthermore, I will also show that by combining ARF with maps of the cosmic microwave background (CMB), we can unveil the signature of the missing (and moving) baryons, doubling the amount of detected baryons in disparate cosmic epochs ranging from z=0 up to z=5, and providing today's most precise description of the spatial distribution of baryons in the universe.


Tuesday May 25, 2021
Drs. Olga Mena


In this talk, we shall review the impact of the neutrino properties on the different cosmological observables. We shall also present the latest cosmological constraints on the neutrino masses and on the effective number of relativistic species. Special attention would be devoted to the role of neutrinos in solving the present cosmological tensions.

Thursday April 29, 2021
Prof. Steen Hansen
COpenhagenUNi / DARK cosmology center


The expansion of the Universe is in an accelerated phase. This
acceleration was first estabilished by observations of SuperNovae, and
has since been confirmed through a range of independent observations.

The physical cause of this acceleration is coined Dark Energy, and
most observations indicate that Einsteins cosmological constant
provides a very good fit. In that case, approximately 70% of the
energy of the Universe presently consists of this cosmological

I will in this talk address the possibility that there may exist other
possible causes of the observed acceleration. In particular will I
discuss a concrete model, inspired by the well-known Lorentz force in
electromagnetism, where Dark Matter causes the acceleration.  With a
fairly simple numerical simulation we find that the model appears
consistent with all observations.

In such a model, where Dark Matter properties causes the acceleration
of the Universe, there is no need for a cosmological constant.

Tuesday October 30, 2012
Dr. Alexander Unzicker
Pestalozzi-Gymnasium, Munich, Germany


The concordance model of cosmology with its constituents dark matter and dark Energy is an established description of some anomalous observations. However, a series of additional contradictions indicate that the current view is far from satisfactory. Rather than describing observations with new numbers, it is argued that science should reflect its method, considering the fact that real progress was usually achieved by simplification. History, not only with the example of the epicycles, has shown many times that creating new ad-hoc concepts dominated over putting in doubt what had been established earlier. Also critical astrophysicists often believe that lab-tested particle physics has reliable evidence for its model. It is argued instead that the very same sociological and psychological mechanisms have been at work and brought particle physics in a still more deperate situation long ago. As an example, a couple of absurdities of the recent Higgs boson announcements are outlined. It seems inevitable that physics needs a new culture of data evaluation, raw data and source code must become equally transparent and openly accessible.

Thursday February 16, 2012
Dr. Daniel Eisenstein
SLOAN Digital Sky Survey, CfA Harvard, USA


I will discuss how the acoustic oscillations that propagate in the photon-baryon fluid during the first million years of the Universe provide a robust method for measuring the cosmological distance scale. The distance that the sound can travel can be computed to high precision and creates a signature in the late-time clustering of matter that serves as a standard ruler. Galaxy clustering results from the Sloan Digital Sky Survey reveal this feature, giving a geometric distance to a redshift of 0.3 and an accurate measurement of Omega_matter. I will review our recent work on the theory and practice of the acoustic oscillation method and our latest cosmology results from SDSS-II. I will then present SDSS-III, which will use the acoustic method to produce 1% distance measurements in order to map the curvature and expansion history of the Universe and measure the evolution of dark energy.

Thursday December 1, 2011
Dr. Carlos Hernandez-Monteagudo


I revisit the claim of Dark Energy detection after stacking CMB data on the angular position of voids and superclusters in Sloan Data. I examine the theoretically expected amplitude for the ISW-induced signal and explore its scale dependence. I next confront these predictions with results obtained from real WMAP data, and evaluate the degree of agreement and the possible presence of contaminants. In a more general context, I address the possibility of unveiling the signature of Dark Energy on the CMB by looking at isolated regions on the sky hosting high-threshold projected under/over-densities: this constitutes a novel approach since it is less sensitive to large angle systematics commonly present in large scale structure surveys.

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