Lorentz Center

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## New Directions in Modern Cosmology |

I will review the current status of distance to supernovae measurements, with an emphasis on the main systematic
uncertainties affecting these measurements and their impact on cosmological
measurements. I will then shortly review how the situation could be improved in
the near and more distant future.
Dark energy and inhomogeneity
The dimming of Type Ia supernovae could be the result of Hubble-scale inhomogeneity in the matter and spatial curvature, rather than signaling the presence of a dark energy component. Simple models which can fit current data are spherical void models - but with us near the centre of symmetry - and require a reworking of inflation instead of unknown physics today. Such a radical - and ridiculous - alternative to concordance cosmology is possible because we have not yet tested the assumption at the heart of our subject: the Copernican principle. The fact that LCDM is an excellent fit to the data is not synonymous with having tested the Copernican assumption on Hubble scales - especially while LCDM remains a phenomenological model. By investigating void models of dark energy we actually learn different ways we can go about verifying the concordance model in this fundamental way.
The COBE,
BOOMERANG and WMAP sky maps of the cosmic microwave background (CMB) provide an
excellent opportunity to test current practices in image reconstruction. All
three missions used image reconstruction to generate their sky maps in contrast
to the up coming Planck results which will provide
the first direct measurements in the CMB. There is growing evidence suggesting
the low l spherical harmonics of the WMAP sky maps may be inaccurate even
though they are consistent with the COBE sky maps. For example, for the 7 year
sky maps the WMAP team reported the quadrupole and octupole were so well aligned that there was only 1 part in
100,000 of the alignment happening by chance. The official WMAP image and
calibration pipeline has been replicated and is being examined for potential
reconstruction artefacts. This is the first time the calibration part of
official WMAPs image reconstruction has been examined other than by the WMAP
team. Image reconstruction is playing an ever increasing role in modern
cosmology. For example, estimating the distribution of dark matter from
gravitational lensing can be seen as a problem in
image reconstruction. The difference between data fitting reconstruction
algorithms, which require prior information, and data focusing ones, which do
not, should have particular relevance to cosmological problems when robust and
reliable image reconstructions are needed. Whether the Planck results agree
with COBE and WMAP’s at low l or not, there is a still a fundamental need of a
detailed review of the current image reconstruction practices employed in
modern cosmology.
In this talk we discuss the statistical properties of Harrison-Zeldovich like power spectra of the large scale mass
density fluctuations comparing it with different paradigmatic examples of
stochastic mass distributions. This leads us to introduce a complete classification of mass density fields depending on the large scale correlations of the fluctuations. In this way we show that the large scale mass distribution of stantard cosmological models falls in the class of "superhomogeneous" systems characterized by a fine balance between spatial correlations and anti-correlations, which does not have to be confused with the effect of integral constraints in finite volumes. Finally, we expose the main consequences on the clustering dynamics of such large scale property of the mass distribution.
Hydrogen-helium gas planets fragmented in clumps at the
plasma to gas phase transition, only 300,000 years after the big bang.
Earth-mass and million-star-clump masses are computed from the initial
conditions at the transition, reflecting the 10^7 plasma-to-gas decrease in kinematic
viscosity n and the density and rate-of-strain existing 30,000 years after the
big bang when photon-viscous forces first matched gravitational fragmentation
forces at the horizon scale
Most current tests of the cosmological model are large-scale and statistical. Upcoming large, high-precision astronomical surveys (especially LSST and Gaia) may make it possible for us to create a small-scale three-dimensional (or six-dimensional) map of the dark matter within the Milky Way. The leading cosmological model requires non-trivial structure even on these small scales. Ideally, the specific phase-space structure of the Milky Way could be used to test the cosmological model directly, in that it must be produced by gravitational collapse from reasonable initial conditions in a reasonable background. I discuss some observational methods that might pay off.
In the standard cosmological model power-law correlations observed in galaxy distributions over a significant range of scales are the result of a combination of fortuitous circumstances, rather than indicative of scale-invariant properties in the underlying dark matter distribution. Indeed the use of halo models to describe numerical simulations suggests that dark matter clustering can never be scale invariant. We discuss here a simplified one-dimensional model of structure formation in the universe, which has the particular interest that its numerical integration may be performed ``exactly''. This model does produce scale-invariant (fractal) clustering from a simple broad class of initial conditions, in a spatial range which grows monotonically in time. We study both how this range and the exponents characterizing it are determined, as well as the description of the mass distribution in terms of "halo" structures. Our results suggest that it may simply be because of poor numerical resolution that scale-invariant properties have been obscured in three dimensional simulations of cold dark matter.
Contemporary cosmology assumes the validity of the General Theory of Relativity which means that Newtonian dynamics is valid on the scales of galaxies. But this comes at a price: in order to account for the observed motions of stars and gas in and around galaxies the existence of exotic dark matter must be postulated. The resulting very popular concordance cosmological model then allows precise calculations of the matter distribution within and in the surroundings of galaxies. A very large volume of work has accumulated to date in an attempt to account for the existence and properties of galaxies such as our Milky Way, which must have evolved through a complex history of hierarchical structure growth driven by dark matter. The existing galaxies in the Local Group are compared to these theoretical results finding very significant disagreement in their overall properties. Furthermore, the concordance cosmological model implies that new dwarf galaxies be born during galaxy encounters. While this important process is usually ignored by the cosmological community, it appears to lead to a serious disagreement with the observed number of dwarf galaxies. Nearby as well as distant galaxies indeed look as if they were purely the result of baryonic processes. The observed phase-space distribution and the internal properties of the Milky Way satellite galaxies as well as the impressive similarity of large star-forming galaxies appear to lead, in view of the theoretical results, to the inescapable implication that the concordance cosmological model needs to be abandoned in favour of a model which relies mostly on baryon-driven processes, and thus to dynamics on galaxy scales being non-Newtonian.
Weak lensing has several important effects on the CMB, including modifying the power spectra and generating a non-zero bispectrum and trispectrum. Accounting for these effects will be very important for the analysis of the CMB power spectrum and searches for primordial non-Gaussianity. Measurements of the lensing potential can be used to break the degeneracies in the linear CMB data, significantly improving constraints on several cosmological parameters. CMB lensing should play a major role in future measurements of cosmological parameters and tests of non-standard cosmology, and may also limit the detection of primordial gravitational waves. I will review the nature of the various effects and discuss recent work on the CMB bispectrum and the use of statistical anisotropy estimators to reconstruct the lensing potential.
Our ApJ 2006 finding of too weak a Sunyaev-Zel'dovich signal in WMAP is now officially confirmed by the WMAP7 cosmology paper of Komatsu et al 2010. The anomaly could indeed be due in part to cluster physics, but there does appear a significant component of it that cannot be explained in this way. I shall venture to connect it with our ApJ 2010 paper on the existence of 7 micro-K (10% of the first acoustic peak), degree-scale, foreground unrelated, non-blackbody anisotropy in WMAP5, by suggesting that both are pointing to significant remaining issues in the beam-size of WMAP. There just does not seem to be another way of understanding these serious discrepancies.
The properties of a preferred time coordinate, homogeneous, isotropic space slices and matter velocity purely in the time direction make FLRW universes seem too simplistic; furthermore to fit with observations they necessitate inflation, dark matter and dark energy. It seems more appropriate to us to suppose a general space-time, with luminous matter and the earth moving along some selection of geodesics (except perhaps at some shocks), and to ask what features of the distribution of luminous matter positions and velocities would give rise to a Hubble law, and to explain the near isotropy of the cosmic microwave background by mixing of null geodesics. Some preliminary results in these directions will be sketched.
Varying speed of light and cosmic
structure I review varying speed of light theories resulting from having different metrics for matter and for gravity. In these theories there are 2 light cones at any point and the speed of light with respect to that of gravity changes. In the minimal theory the action maps into a (anti-)DBI action in the Einstein frame, and scale-invariant fluctuations are produced. I explain how this basic prediction comes about and can be modified in non-minimal theories. The predictive value of the theory is then in its non-Gaussian predictions which have a unique form for each tilt.
We have investigated the odd-parity preference of the WMAP 7
year power spectrum. Comparison with simulation shows the odd-parity preference
of WMAP data (2 ? l ? 22) is anomalous at 4-in-1000
level. We have investigated its origins, and ruled out some
of non-cosmological origins such as asymmetric beams, noise and cut-sky effect.
Multipoles associated with the odd-parity preference
happen to coincide with some of other CMB anomalies. Therefore, there may exist a common origin, whether cosmological or not. Besides,
we find it likely that low quadrupole power is the
part
The observed properties of the primordial fluctuations in the cosmic microwave background (CMB) can provide constraints on physical theories in regimes otherwise inaccessible to experiment. A "concordance" picture of nearly-scale-invariant, adiabatic, Gaussian fluctuations obeying statistical isotropy and homogeneity has emerged in recent years. I will summarize recent progress in testing with CMB data the inflationary hypothesis for describing the very early universe, and the advances we can expect in the near future.
Cosmology
is undergoing a revolution due to the many new data which are becoming
available and which are expected in the near future. For several decades this
field has been essentially based on conjectures strongly based on the only
exactly solvable model available (Friedmann). The
observation of large structures in the galaxy distribution has challenged this
model already long ago and the very recent data from the SDSS project have
given further evidence for a complex structure. In addition other data have
posed additional problems with respect to the standard model. The large scale
flow of galaxies and the observation (by gravitational lensing)
that also dark matter is very clumpy represent important new elements. A new
picture emerges in which the complexity of the universe should be seen as an
interesting fact and not just as a technical difficulty to avoid. For example
the inhomogeneity and scaling properties could lead
to the acceleration effects which led instead to the introduction of dark
energy. In this perspective also the connection between a highly structured
matter distribution with a smooth cosmic background
radiation (CMBR) could be seen in a novel framework. This rapidly evolving
situation will lead, in our opinion, to a new picture of the universe in which
complex structures should become a central point of its description. We provide
a survey of these points and outline the possible developments.
I will review some of the recent work on primordial nongaussianity. Next I will present a phase space path integral formalism for cosmological perturbations, which allows for a straighforward calculation of Feynam rules. As two examples, I will sketch how to calculate the three point and four point functions for the curvature perturbation during inflation.
Cosmologists measure large scale structure via CMB fluctuations and via galaxy clustering. We first review the successes in explaining these observations of the standard LCDM cosmology. In particular, we shall look at QSO clustering which provides a more powerful probe of the Universe on the very largest scales than do galaxies. We shall then summarise the fundamental issues for the LCDM model, including its difficulties at smaller scales. This prompts a re-inspection of the large-scale structure observations. We point out that galaxies and QSOs are well known to be biased tracers of mass and that this may give an easy escape route for other cosmological models. The CMB is a tougher nut to crack but we review recent evidence that there may be observational issues with even the gold-standard WMAP CMB results.
Since the first optical identificaton of gamma-ray bursts (GRBs) in 1997, they became a new direction in the study of the universe at large redshifts. In particular, on April, 23, 2009 the highest spectroscopic redshift was measured z=8.2, and this happend to be of GRB 090423. Currently the state of the GRB problem and progress in this field can be fixed in the following way: 1.GRBs belong to the most distant observable objects with measurable red shifts in the universe. 2.GRBs are related to star forming in distant (and very distant) galaxies. 3.GRBs and their afterglows allow us seeing also the most distant explosions of massive stars at the end of their evolution. 4.This is confirmed by observations of long bursts, but, most probably short GRBs are also related to some very old compact objects (neutron and quark stars) resulting from evolution of the same massive stars. 5.And at what z (> 10-20?) are GRBs unobservable already? Now this can be the main cosmological GRB-test. The talk concerns: GRB rate, galaxies and star forming at large redshifts, model-independent observational tests, and the statement of problems in the NIR range. (Is evolution of anything observed as z increases?) In particular, we will consider the relation between GRB rate and star-forming rate (GRBR vs. SFR). In the talk some papers on GRBs and SFR in galaxies with large redshifts will be commented. The questions are: Is a fast decrease of SRF at z > 4 observed indeed, which is to be observed in standard (and not very standard) cosmologies? Is there any difference between GRBR and SFR beyond z~4?
We analyzed magnitudes and Petrosian radii at small redhifts from the Sloan Digital Sky Survey (data release 7). Galaxy sizes calculated by means of Petrosian radii were found to increase unexpectedly with redshift z. A variety of possible reasons was investigated: A Malmquist bias was excluded by volume limited samples, other selection effects due to SDSS data peculiarities and a K-correction were taken into account. The anomly is not affected by the value of the Hubble constant and is stable across a wide range of galaxy luminosities. We determined the relative increase of the average size and found it to be about twice as much as the respective increase of wavelengths due to the cosmological redshift. Since the effect is beyond any statistical noise, only systematical errors can explain it within standard cosmology. To facilitate further investigatons, the complete code with comments and instructions is published.
Different methods are used to investigate properties of cosmic microwave background observed with the NASA mission WMAP with the main goal of search for peculiar axes. There are several possible explanations for existance of peculiar directions in WMAP maps both having physical reasons or being systematic effects. In our work, we implemented new mosaic correlation methods to study the statistical properties of foreground components and a restored background signal map for different ranges of multipoles. Using new approach, we detected in the spherical distribution of correlation coefficients, at least, three standard astronomical coordinate system which are reflected in positions of cold and hot spots of multipoles on the WMAP CMB map. We discuss the probable reasons of the detected observable phenomena. This study was partly supported by the Russian Foundation of Basic Researches (grant No 09-02-00298).
Cosmological backreaction suggests a link between structure formation and the expansion history of the Universe. In order to quantitatively examine this connection we dynamically investigate a volume partition of the Universe into over– and underdense regions. This allows to trace structure formation using the volume fraction of the overdense regions lambda_M as its characterizing parameter. Employing results from cosmological perturbation theory, and under the assumption of an initial near to homogeneous Gaussian density field, we construct a three–parameter model for the effective cosmic expansion history, involving lambda_M_0, the matter density and the Hubble rate of today’s Universe. The talk presents basic issues of constructing models for the average evolution of cosmological variables together with their volume-partitioning, and discusses first tests of the multiscale model to explain what we know about the evolution of the Universe in the backreaction context. Furthermore, the possible benefits of an application of the partitioning approach to more general cases will be discussed.
Below scales of about 100/h Mpc our universe displays a complex inhomogeneous structure dominated by voids, with clusters of galaxies in sheets and filaments. The coincidence that cosmic expansion appears to start accelerating at the epoch when such structures form has prompted a number of researchers to question whether dark energy is a signature of a failure of the standard cosmology to properly account, on average, for the distribution of matter we observe. Several ideas have been put forward, with intense debate and controversy. I will discuss the debate and the key issues which involve both the change to average cosmic evolution from backreaction, and the solution to the "fitting problem": How do our own observations relate to average quantities when the variance of local geometry becomes significant? I suggest that a viable alternative to a smooth dark energy may exist purely in general relativity, and that it involves understanding foundational issues - in particular, quasilocal gravitational energy. I will also discuss the observational signatures which will allow such a scenario to be tested in future and distinguished from the standard Lambda CDM model. [Back] |