Projects
Here are brief descriptions of just some of our active void science efforts. Interested in collaborating with us to gain access to pre-release catalogs and mock data sets? Please contact us!
Absorption in the Cosmic Web
Collaborators: Simon Morris (Durham), Neil Crighton (MPA)
Project website (requires login)
Combining void catalogs with inter-galactic medium (IGM) measurements from quasi-stellar objects (QSOs) absorption lines it is possible to investigate the properties of the IGM clouds at different large-scale structure (LSS) environments. Tejos et al. 2012 have shown that despite their extremely low densities, galaxy voids are not empty and contain a significant fraction of Ly-alpha absorption systems. The number of Ly-alpha systems inside galaxy voids is consistent with the random expectation but there is an excess of such systems at the edges of voids. This excess spans over ~5 Mpc scales, the typical scales of the `cosmic-web' as predicted by cosmological simulations.
The distribution of column densities and Doppler parameters of these systems are different depending on their LSS environment. Low-density environments (voids) have HI systems with lower column density and Doppler parameter than higher density ones (edges of voids). Results obtained at the very large scales (>~ 10 Mpc) are a good complement to galaxy-IGM studies obtained at smaller scales (<~ 2 Mpc). Focusing on the distribution and properties of gas within and around galaxy voids offer important observational constrains towards a better understanding of the origin of Ly-alpha absorption systems and their connection with galaxies. Please see Tejos et al. 2012 for further details.
Alcock-Paczynski Effect
Collaborators: Guilhem Lavaux (Perimeter/IAP), Ben Wandelt (IAP/UPMC/Illinois), David Weinberg (OSU)
Project website (requires login)
The Alcock-Paczynski effect tells us that if we have a populations of standard spheres in the Universe, then their ellipticity as a function of redshift will tell us about cosmology. Specifically, the ratio of their extent along the line of site to their angular extent is proportional to the angular diameter distance times the expansion rate. While any particular void is certainly not a sphere, they should not have any particular orientation or preferred shape. Hence, on average voids should be spherical, and we can exploit them to derive cosmological parameters via the Alock-Paczynski effect.
We have applied this analysis to stacked voids in several redshift bins of the the Sloan Digital Sky Survey Data Release 7, and while we did not reach enough statistical significance to claim a detection, our method of shape estimation proved reliable enough that we are confident that we can successfully use voids to constrain cosmology with future galaxy surveys.
We are currently applying the void analysis algorithm to SDSS DR9.
Anti-Lensing of Galaxies
Collaborators: Erin Sheldon (Brookhaven), Elisabeth Krause (UPenn)
Project website (requires login)
We seek to perform the first measurement of the diminutive lensing signal arising from matter underdensities associated with cosmic voids. While undetectable individually, by stacking the weak gravitational shear estimates around 901 voids detected in SDSS DR7 by Sutter et al. (2012b), we find substantial evidence for a depression of the lensing signal compared to the cosmic mean. This depression is most pronounced at the void radius, in agreement with analytical models of void matter profiles. We invite independent investigations of our findings by releasing data and analysis code to the public at https://github.com/pmelchior/void-lensing.
Contents of Voids
PI: Danny Pan (Shanghai Astronomical Observatory)
Collaborators: Charles Danforth (U. Colorado), Michael Vogeley (Drexel)
Project website (requires login)
We use a void sample from Sloan Digital Sky Survey Data Release 7 and compare it with QSO absorption spectra from HST and FUSE to detect the gas content in voids. Using absorption spectrum allows us to detect lower column densities of neutral hydrogen and compare the environments of galaxies that reside in different large scale structure. Galaxy and halo evolution are expected to be quite different in clusters and filaments compared to voids, and the IGM is expected to behave differently as well. These results can be seen in the absorption in the Lyman alpha forest of background QSOs as it traverses through the halos of galaxies that reside in different LSS. In preliminary results from STIS, we see that the gas content and extent in voids are significantly different than the walls.
We are currently extending this research to more recent COS QSOs.
Cosmic Magnetization
Collaborators: Vladimir V. Vassiliev (UCLA), Thomas Weisgarber (U. Wisconsin-Madison)
Project website (requires login)
Numerous observations have established the presence of magnetic fields in Galaxies, Galaxy Clusters, and Filaments that comprise the large scale structure of the Universe. However, an unambiguous detection of the intergalactic magnetic field (IGMF) presumed to exist in Cosmic Voids, remains elusive. Many models regarding the origination of a primordial field suggests that large scale magnetic fields could arise through a Biermann battery mechanism operating during phase transitions in the early universe. The existence of such a field is theoretically well-motivated, because models that are commonly invoked to explain the properties of the present day magnetic fields in galaxies and clusters, require a "seed" field for amplification. Thus, detection of the IGMF in Cosmic Voids could provide important constraints on outstanding problems of its origin and role in structure formation.
Because standard techniques of detecting weak magnetic fields in the universe (Faraday Rotation measurements, Zeeman splitting, the polarization of optical starlight, etc.) are not sufficiently sensitive to detect the IGMF in Cosmic Voids, a new observational window is needed. Recently, it has been shown that gamma-ray observations from extragalactic sources may provide a way to better constrain or detect these fields. Gamma-rays with energy > 100 GeV propagating from extragalactic sources interact with photons of the UV - far IR Extragalactic Background Light and create electron/positron pairs. These pairs then undergo inverse Compton scattering on the CMB radiation, producing secondary gamma-rays at lower energies than the primary. As a result, an electromagnetic cascade develops. Because the pairs' trajectories are affected by Lorentz-force interactions with the magnetic field, the cascade emission at the ~ GeV scale carries information regarding the properties of the Magnetic Field in which it was created. If the cascade from a given source develops within a region of Cosmic Voids, then gamma ray observations of this source may provide a way to constrain or detect the influence of the magnetic field.
Our paper, recently submitted to the Astrophysical Journal, summarizes the present status of lower limits derived from gamma-ray observations placed on the IGMF in Voids, and argues that these limits are premature, because a better grasp of many systematic uncertainties is required to sustain such a limit. One of the major sources of uncertainty, is the location of Voids in the vicinity of the gamma ray sources.
Coupled dark matter-dark energy
PI: Edoardo Carlesi (UAM)
Collaborators: Alexander Knebe (UAM)
Project website (requires login)
Quintessence models are dynamical dark energy cosmologies where the cosmological constant is replaced by a scalar field, which, in the most general case, is non-minimally coupled to the dark matter sector, where it acts as an effective long range fifth force. These models were devised to address the fine tuning problems of LCDM, as the late-time dark energy domination phase arises naturally, without depending on the initial conditions. These cosmologies are able to explain a wide array of observational data (such as CMB,SNe Ia, BAO) so that testing the non-linear regime of the theories is an important step in testing their viability even at smaller scales. To do this we modified the standard N-body / hydro code GADGET-2 to run a series of high-resolution simulations, which allow us to analyze the behavior of these cosmologies on non linear scales, down to the internal structure of galaxy clusters. The simulations were run using different values of the quintessence interaction strength, to be able to quantify the dependence of observables on the magnitude of the fifth force and constrain it.
In this project we aim at characterizing different cosmological models and in particular finding signatures of long range fifth-forces in the dark sector using void properties.
Extracting Primordial Non-Gaussianity
Collaborators: Benjamin Wandelt (IAP/CNRS/Illinois)
Project website (requires login)
Redshift surveys measure the location of millions of galaxies in the observable Universe, thereby constructing a three-dimensional map of its large-scale structure. This structure is characterized by dense clusters of galaxies, connected by filaments and walls of lower number density. The remaining and dominant volume within this cosmic web is taken up by voids, vast regions of relatively empty space. While clusters, filaments and walls have all entered different stages of nonlinear evolution during cosmic history, voids represent structures that are still close to linear and therefore more easy to relate with the initial conditions of the Universe. The characteristics of these initial conditions are generated shortly after the big bang, most likely during a stage of inflation, and carry information about the fundamental physical processes that govern this very early phase of expansion. One such characteristic is the degree of non-Gaussianity in the initial fluctuations, which can discriminate between single- and multifield inflationary models. In this project we want to analyze the clustering properties of the observed voids. Recent studies have emphasized possible signatures of primordial non-Gaussianity in the clustering statistics of large-scale structure. While most of these studies have focused on the clustering of dark matter and dark matter halos in simulations, we want to extend this analysis to voids. Our plan is to investigate the clustering properties of halos and voids in the presence of primordial non-Gaussianity using simulations, and to develop optimal methods to extract signatures from non-Gaussianity in a combined analysis of halos and voids. Once we have identified useful techniques, we can directly apply them to the observational void data at hand.
Gravitational Delensing
Collaborators: Joe Silk (Oxford/IAP)
Project website (requires login)
The gravitational field around an inhomogeneous matter distribution can deflected light rays when they are propagated though the field. The effect had been speculated well before the advent of Einstein's general theory of gravity. The lensing effect was final confirmed in 1920. Having voids as highly underdense regions, they could, in principle, act as a source of de-magnification of the background sources behind. Looking behind the region with large void volume could have an observable effects either in the cosmic microwave background or galaxies. In addition, this is another way to test the theory of gravitation given by Einstein.
ISW from Stacked Voids
PI: Yan-Chuan Cai (ICC, Durham)
Collaborators: Mark Neyrinck (JHU), Istvan Szapudi (IfA Hawaii), Shaun Cole (ICC, Durham), Carlos Frenk (ICC, Durham)
Project website (requires login)
The late time Integrated Sachs-Wolfe effect (ISW) serves as a direct constraint for dark energy and modified gravity. The detected ISW signal from the stacking of 4-deg^2-size regions of the CMB corresponding to the SDSS super clusters and super voids (Granett et al. 2008) is found to be 2 to 3-sigma higher than estimations from simulations, indicating tension with the concordance cosmology. We propose to re-investigate this issue using the new, independent SDSS void catalogue. This is interesting in that A.) The catalogue is at lower redshift (z<0.4), where the expected ISW signal should be higher. The detection of the signal at a different redshift range will enable us to check for the consistency of cosmology and potential systematics. B.) The void catalogue is constructed using spectroscopic redshift data, which should be more reliable. C.) Stacking using a larger sample, we expect to beat down the CMB noise more effectively and a more significant signal is expected. We will apply matched filters to each void on the WMAP sky map, and compare the stacked filtered temperature to theoretical calculations.
Large-Scale Anisotropies
Collaborators: Ravi Sheth (UPenn, ICTP)
Project website (requires login)
Recently, a strong directional dependence in the clustering around haloes on BAO scales has been measured in N-body simulations. We quantified this anisotropy based on linear theory and ingredients from the triaxial collapse models of halo formation: the orientation of a halo is strongly correlated with the local Lagrangian shear. The theory is found to match simulations well. The possible use of this model is to test modified gravity scenarios in which the functional form of the anisotropy is different from LCDM, also models of structure formation through the amplitude of the anisotropy.
Similarly to haloes, we predict an anisotropy in the matter distribution around voids. Next, the model is to be tested on simulated and real void data as well.
Signal Processing Methods
PI: Boris Leistedt (UCL)
Collaborators: Hiranya Peiris (UCL), Jason McEwen (UCL)
Project website (requires login)
Inflation is an elegant paradigm for seeding cosmic structure. Primordial non-Gaussianity (NG) is a sensitive probe of inflation interactions, which is difficult to access by any other means. Cosmic voids have recently been shown to be a useful observable for constraining primordial NG, but in order to infer robust constraints one must account for systematic uncertainties. In this project, we will use novel techniques from information theory and signal processing to improve the detection of voids in galaxy surveys, and assemble large catalogues that can be used to constrain the physics of the early universe. We will design a robust Bayesian pipeline to constrain primordial NG while propagating the systematic uncertainties associated with the data.
Velocity Profile Reconstruction
PI: Alice Pisani (IAP)
Collaborators: Guilhem Lavaux (Perimeter/IAP), P.M. Sutter (Illinois/IAP/OSU), Ben Wandelt(IAP/UPMC/Illinois)
Project website (requires login)
The study of voids promises to yield important constraints on the most elusive component of the universe, dark energy. Using the expected spherical symmetry of stacked voids it is possible to determine the radial velocity profiles of void stacks. Large galaxy spectroscopic redshift surveys, such as the Sloan Digital Sky Survey probe the three-dimensional representation of the large-scale structure. We constructed an algorithm that allows the reconstruction of peculiar velocity profiles of void stacks. We are currently using simulations to test the match between the reconstructed and input velocity profiles. Reconstructed velocity profiles from real voids promise to add dynamical constraints to the cosmological information obtained from stacked voids.