arXiv: 21 January 2011

Sudden Future Singularity models as an alternative to Dark Energy?

Hoda Ghodsi, Martin A. Hendry, Mariusz P. Dabrowski, Tomasz Denkiewicz

http://arxiv.org/abs/1101.3984v1

Current observational evidence does not yet exclude the possibility that dark energy could be in the form of phantom energy. A universe consisting of a phantom constituent will be driven toward a drastic end known as the `Big Rip' singularity where all the matter in the universe will be destroyed. Motivated by this possibility, other evolutionary scenarios have been explored by Barrow, including the phenomena which he called Sudden Future Singularities (SFS). In such a model it is possible to have a blow up of the pressure occurring at sometime in the future evolution of the universe while the energy density would remain unaffected. The particular evolution of the scale factor of the universe in this model that results in a singular behaviour of the pressure also admits acceleration in the current era. In this paper we will present the results of our confrontation of one example class of SFS models with the available cosmological data from high redshift supernovae, baryon acoustic oscillations (BAO) and the cosmic microwave background (CMB). We then discuss the viability of the model in question as an alternative to dark energy.

Effects of Explosion Asymmetry and Viewing Angle on the Type Ia Supernova Color and Luminosity Calibration

K. Maeda, G. Leloudas, S. Taubenberger, M. Stritzinger, J. Sollerman, N. Elias-Rosa, S. Benetti, M. Hamuy, G. Folatelli, P. A. Mazzali

http://arxiv.org/abs/1101.3935v1

Phenomenological relations exist between the peak luminosity and other observables of type Ia supernovae (SNe~Ia), that allow one to standardize their peak luminosities. However, several issues are yet to be clarified: SNe~Ia show color variations after the standardization. Also, individual SNe~Ia can show residuals in their standardized peak absolute magnitude at the level of $\sim 0.15$ mag. In this paper, we explore how the color and luminosity residual are related to the wavelength shift of nebular emission lines observed at $\gsim 150$ days after maximum light. A sample of 11 SNe Ia which likely suffer from little host extinction indicates a correlation ($3.3\sigma$) between the peak $B-V$ color and the late-time emission-line shift. Furthermore, a nearly identical relation applies for a larger sample in which only three SNe with $B-V \gsim 0.2$ mag are excluded. Following the interpretation that the late-time emission-line shift is a tracer of the viewing direction from which an off-centre explosion is observed, we suggest that the viewing direction is a dominant factor controlling the SN color and that a large part of the color variations is intrinsic, rather than due to the host extinction. We also investigate a relation between the peak luminosity residuals and the wavelength shift in nebular emission lines in a sample of 20 SNe. We thereby found a hint of a correlation (at $\sim 1.6 \sigma$ level). The confirmation of this will require a future sample of SNe with more accurate distance estimates. Radiation transfer simulations for a toy explosion model where different viewing angles cause the late-time emission-line shift are presented, predicting a strong correlation between the color and shift, and a weaker one for the luminosity residual.

Cosmic Lighthouses : Unveiling the nature of high-redshift galaxies

Pratika Dayal

Pratika Dayhttp://arxiv.org/abs/1101.3925v1al

We are in the golden age for the search for high-redshift galaxies, made possible by a combination of new instruments and innovative search techniques. One of the major aims of such searches is to constrain the epoch of reionization (EoR), which marks the second major change in the ionization state of the Universe. Understanding the EoR is difficult since whilst it is galaxy evolution which drives reionization, reionization itself influences galaxy evolution through feedback effects. Unraveling the interplay of reionization and galaxy evolution is further complicated by of a lack of understanding of the metal enrichment and dust distribution in high redshift galaxies. To this end, a class of galaxies called Lyman Alpha Emitters (LAEs) have been gaining enormous popularity as probes of all these three processes. In this thesis, we couple state of the art cosmological SPH simulations (GADGET-2) with a physically motivated, self-consistent model for LAEs, so as to be able to understand the importance of the intergalactic medium (IGM) ionization state, dust and peculiar velocities in shaping their observed properties. By doing so, the aim is to gain insight on the nature of LAEs, put precious constraints on their elusive physical properties and make predictions for future instruments such as the Atacama Large Millimeter Array (ALMA). Using our LAE model in conjunction with a code that builds the MW merger tree (GAMETE), we build a bridge between the high-redshift and the local Universe. We also use SPH simulations (GADGET-2) to study the nature of the earliest galaxies that have been detected as of yet, place constraints on their contribution to reionization, and predict their detectability using the next generation of instruments, such as the James Web Space Telescope (JWST).

Ph.D. thesis

Infrared Spectra and Spectral Energy Distributions for Dusty Starbursts and AGN

Lusine Sargsyan, Daniel Weedman, Vianney Lebouteiller, James Houck, Donald Barry, Ashot Hovhannisyan, Areg Mickaelian

http://arxiv.org/abs/1101.3876v1

We present spectroscopic results for all galaxies observed with the Spitzer Infrared Spectrograph (IRS) which also have total infrared fluxes f(ir) measured with the Infrared Astronomical Satellite (IRAS), also using AKARI photometry when available. Infrared luminosities and spectral energy distributions (SEDs) from 8 um to 160 um are compared to polycyclic aromatic hydrocarbon (PAH) emission from starburst galaxies or mid-infrared dust continuum from AGN at rest frame wavelengths ~ 8 um. A total of 301 spectra are analyzed for which IRS and IRAS include the same unresolved source, as measured by the ratio fv(IRAS 25 um)/fv(IRS 25 um). Sources have 0.004 < z < 0.34 and 42.5 < log L(IR) < 46.8 (erg per s) and cover the full range of starburst galaxy and AGN classifications. Individual spectra are provided electronically, but averages and dispersions are presented. We find that log [L(IR)/vLv(7.7 um)] = 0.74 +- 0.18 in starbursts, that log [L(IR)/vLv(7.7 um)] = 0.96 +- 0.26 in composite sources (starburst plus AGN), that log [L(IR)/vLv(7.9 um)] = 0.80 +- 0.25 in AGN with silicate absorption, and log [L(IR)/vLv(7.9 um)] = 0.51 +- 0.21 in AGN with silicate emission. L(IR) for the most luminous absorption and emission AGN are similar and 2.5 times larger than for the most luminous starbursts. AGN have systematically flatter SEDs than starbursts or composites, but their dispersion in SEDs overlaps starbursts. Sources with the strongest far-infrared luminosity from cool dust components are composite sources, indicating that these sources may contain the most obscured starbursts.

Nano-Hertz Gravitational Waves Searches with Interferometric Pulsar Timing Experiments

Massimo Tinto

http://arxiv.org/abs/1101.3990v1

We estimate the sensitivity to nano-Hertz gravitational waves of pulsar timing experiments in which two highly-stable millisecond pulsars are tracked simultaneously with two neighboring radio telescopes that are referenced to the same time-keeping subsystem (i.e. "the clock"). By taking the difference of the two time-of-arrival residual data streams we can exactly cancel the clock noise in the combined data set, thereby enhancing the sensitivity to gravitational waves. We estimate that, in the band ($10^{-9} - 10^{-8}$) Hz, this "interferometric" pulsar timing technique can potentially improve the sensitivity to gravitational radiation by almost two orders of magnitude over that of single-telescopes. Interferometric pulsar timing experiments could be performed with neighboring pairs of antennas of the forthcoming large arraying projects.

Palatini Approach to Modified Gravity: f(R) Theories and Beyond

Gonzalo J. Olmo

http://arxiv.org/abs/1101.3864v1

We review the recent literature on modified theories of gravity in the Palatini approach. After discussing the motivations that lead to consider alternatives to Einstein's theory and to treat the metric and the connection as independent objects, we review several topics that have been recently studied within this framework. In particular, we provide an in-depth analysis of the cosmic speedup problem, laboratory and solar systems tests, the structure of stellar objects, the Cauchy problem, and bouncing cosmologies. We also discuss the importance of going beyond the f(R) models to capture other phenomenological aspects related with dark matter/energy and quantum gravity.

Accelerating Universe: Dark Energy vs Modified Gravity / Seminar Series (2)

Speaker:Shant Baghram
Affiliation:Sharif University of Technology
Title: Accelerating Universe: Dark Energy vs Modified Gravity
Date: 21 January 2011
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:

First, I will give a very brief description of the accelerating universe state and its cosmological evidence like SNIa, CMB and the LSS. Then I propose the Cosmological constants (LCDM-model) and its alternative Dark energy (DE) and Modified gravity (MG) models as plausible candidates for describing the accelerating Universe. Some cosmological observations, especially large scale structure probes such as matter power spectrum, the ISW effect and growth index are discussed later, as useful tools to distinguish between viable DE and MG models which are equivalent in predicting the background dynamics of  Universe. Finally, the reconstruction of the dynamics method as a probable way for investigating the problem is introduced, and the future prospects on the issue is discussed. 

arXiv: 20 January 2011

On the connection between Newtonian simulations and General Relativity

Nora Elisa Chisari, Matias Zaldarriaga

http://arxiv.org/abs/1101.3555v1

On large scales, comparable to the horizon, the observable clustering properties of galaxies are affected by various General Relativistic effects. To calculate these effects one needs to consistently solve for the metric, densities and velocities in a specific coordinate system or gauge. The method of choice for simulating large scale structure is numerical N-body simulations which are performed in the Newtonian limit. Even though one might worry that the use of the Newtonian approximation would make it impossible to use these simulations to compute properties on very large scales we show that the simulations are still solving the dynamics correctly even for long modes and give formulas to obtain the position of particles in the Newtonian gauge given the positions computed in the simulation. We also give formulas to convert from the output coordinates of N-body simulations to the observable coordinates of the particles.

Nonlocal Gravity

Bahram Mashhoon

http://arxiv.org/abs/1101.3752v1

The analysis of measurements of accelerated observers in Minkowski spacetime has led to the development of nonlocal special relativity theory. Inertia and gravitation are intimately connected in accordance with the principle of equivalence. We therefore seek a nonlocal generalization of the theory of gravitation such that in the new theory the field equations are integro-differential equations for the local gravitational field. We show that it is possible to develop a nonlocal generalization of Einstein's theory of gravitation via the introduction of a scalar "constitutive" kernel in the teleparallel equivalent of general relativity. The resulting nonlocal theory is essentially equivalent to Einstein's theory plus "dark matter". That is, nonlocality simulates dark matter by introducing a new source term into general relativity. In the linear approximation for the nonlocal modification of Newtonian gravity, we recover the theoretical basis for the phenomenological Tohline-Kuhn modified gravity approach to the explanation of the astrophysical evidence for dark matter.

Pulsar Timing Sensitivity to Very-Low-Frequency Gravitational Waves

Fredrick A. Jenet, J.W. Armstrong, Massimo Tinto

http://arxiv.org/abs/1101.3759v1

At nanohertz frequencies gravitational waves (GWs) cause variations in time-of-arrival of pulsar signals potentially measurable via precision timing observations. Here we compute very-low-frequency GW sensitivity constrained by instrumental, propagation, and other noises fundamentally limiting pulsar timing observations. Reaching expected GW signal strengths will require estimation and removal of $\simeq$99% of time-of-arrival fluctuations caused by typical interstellar plasma turbulence and a reduction of white rms timing noise to $\sim$100 nsec or less. If these were achieved, single-pulsar signal-to-noise ratio (SNR) = 1 sensitivity is then limited by the best current terrestrial time standards at $h_{rms} \sim$2 $\times 10^{-16}$ [f/(1 cycle/year)]$^{-1/2}$ for $f < 3 \times 10^{-8}$ Hz, where f is Fourier frequency and a bandwidth of 1 cycle/(10 years) is assumed. This sensitivity envelope may be optimistic in that it assumes negligible intrinsic pulsar rotational noise, perfect time transfer from time standard to observatory, and stable pulse profiles. Nonetheless it can be compared to predicted signal levels for a broadband astrophysical GW background from supermassive black hole binaries. Such a background is comparable to timekeeping-noise only for frequencies lower than about 1 cycle/(10 years), indicating that reliable detections will require substantial improvements in signal-to-noise ratio through pulsar array signal processing.

arXiv: 19 January 2011

The Statefinder hierarchy: An extended null diagnostic for concordance cosmology

Maryam Arabsalmani, Varun Sahni

http://arxiv.org/abs/1101.3436v1

We show how higher derivatives of the expansion factor can be developed into a null diagnostic for concordance cosmology (LCDM). It is well known that the Statefinder -- the third derivative of the expansion factor written in dimensionless form, a^{(3)}/aH^3, equals unity for LCDM. We generalize this result to higher derivatives of the expansion factor and demonstrate that the hierarchy, a^{(n)}/aH^n, can be converted to a form that stays pegged at unity in concordance cosmology. This remarkable property of the Statefinder hierarchy enables it to be used as an extended null diagnostic for the cosmological constant. The Statefinder hierarchy combined with the growth rate of matter perturbations defines a composite null diagnostic which can distinguish evolving dark energy from LCDM.

arXiv: 18 January 2011

Are Dwarf Galaxies Dominated by Dark Matter?

R.A. Swaters, R. Sancisi, T.S. van Albada, J.M. van der Hulst

http://arxiv.org/abs/1101.3120v1

Mass models for a sample of 18 late-type dwarf and low surface brightness galaxies show that in almost all cases the contribution of the stellar disks to the rotation curves can be scaled to explain most of the observed rotation curves out to two or three disk scale lengths. The concept of a maximum disk, therefore, appears to work as well for these late-type dwarf galaxies as it does for spiral galaxies. Some of the mass-to-light ratios required in our maximum disk fits are high, however, up to about 15 in the R-band, with the highest values occurring in galaxies with the lowest surface brightnesses. Equally well-fitting mass models can be obtained with much lower mass-to-light ratios. Regardless of the actual contribution of the stellar disk, the fact that the maximum disk can explain the inner parts of the observed rotation curves highlights the similarity in shapes of the rotation curve of the stellar disk and the observed rotation curve. This similarity implies that the distribution of the total mass density is closely coupled to that of the luminous mass density in the inner parts of late-type dwarf galaxies.

arXiv: 17 January 2011

Time-dependent matter instability and star singularity in $F(R)$ gravity

Kazuharu Bamba, Shin'ichi Nojiri, Sergei D. Odintsov

http://arxiv.org/abs/1101.2820v1

We investigate a curvature singularity appearing in the star collapse process in $F(R)$ gravity. In particular, we propose an analytical understanding of the mechanism to produce the curvature singularity. Moreover, we explicitly demonstrate that $R^\alpha$ ($1 < \alpha \leq 2$) term addition could cure the curvature singularity and viable $F(R)$ gravity models could become free of such a singularity. Furthermore, we discuss the realization process of the curvature singularity and estimate the time scale of its appearance. For exponential gravity, it is shown that in case of the star collapse, the time scale is much shorter than the age of the universe, whereas in cosmological circumstances, it is as long as the cosmological time.

Palatini Actions and Quantum Gravity Phenomenology

Gonzalo J. Olmo

http://arxiv.org/abs/1101.2841v1

We show that a quadratic gravitational Lagrangian in the Palatini formulation is able to capture different aspects of quantum gravity phenomenology in a single framework. In particular, we show that in this theory field excitations propagating with different energy-densities perceive different background metrics, which is a fundamental characteristic of the DSR and Rainbow Gravity approaches. Also, the resulting isotropic and anisotropic cosmologies are free from the big bang singularity. This singularity avoidance occurs non-perturbatively and shares some similitudes with the effective dynamics of loop quantum cosmology.

arXiv: 14 January 2011

Testing a Phenomenologically Extended DGP Model with Upcoming Weak Lensing Surveys

Stefano Camera, Antonaldo Diaferio, Vincenzo F. Cardone

http://arxiv.org/abs/1101.2560v1

A phenomenological extension of the well-known brane-world cosmology of Dvali, Gabadadze and Porrati (eDGP) has recently been proposed. In this model, a cosmological-constant-like term is explicitly present as a non-vanishing tension sigma on the brane, and an extra parameter alpha tunes the cross-over scale r_c, the scale at which higher dimensional gravity effects become non negligible. Since the Hubble parameter in this cosmology reproduces the same LCDM expansion history, we study how upcoming weak lensing surveys, such as Euclid and DES (Dark Energy Survey), can confirm or rule out this class of models. We perform Markov Chain Monte Carlo simulations to determine the parameters of the model, using Type Ia Supernov\ae, H(z) data, Gamma Ray Bursts and Baryon Acoustic Oscillations. We also fit the power spectrum of the temperature anisotropies of the Cosmic Microwave Background to obtain the correct normalisation for the density perturbation power spectrum. Then, we compute the matter and the cosmic shear power spectra, both in the linear and non-linear regimes. The latter is calculated with the two different approaches of Hu and Sawicki (2007) (HS) and Khoury and Wyman (2009) (KW). With the eDGP parameters coming from the Markov Chains, KW reproduces the LCDM matter power spectrum at both linear and non-linear scales and the LCDM and eDGP shear signals are degenerate. This result does not hold with the HS prescription: Euclid can distinguish the eDGP model from LCDM because their expected power spectra roughly differ by the 3sigma uncertainty in the angular scale range 700<l<3000; on the contrary, the two models differ at most by the 1sigma uncertainty over the range 500<l<3000 in the DES experiment and they are virtually indistinguishable.

Properties of the HII region populations of M51 and NGC 4449 from Halpha images with ACS on HST

Leonel Gutiérrez, John E. Beckman, Valeria Buenrostro

http://arxiv.org/abs/1101.2614v1

We have used the images from the ACS on HST in Halpha, and in the neighboring continuum, to produce flux calibrated images of the large spiral galaxy M51, and the dwarf irregular NGC 4449. From these images we have derived the absolute luminosities in Halpha, the areas, and the positions with respect to the galactic centers as reference points, of over 2600 HII regions in M51 and over 270 HII regions in NGC 4449. Using this database we have derived luminosity (L)--volume (V) relations for the regions in the two galaxies, showing that within the error limits these obey the equation L ~ V^(2/3), which differs from the linear relation expected for regions of constant uniform electron density. We discuss briefly possible models which would give rise to this behavior, notably models with strong density inhomogeneities within the regions. Plotting the luminosity functions for the two galaxies we find a break in the slope for M51 at log(L) = 38.5 dex (units in erg s^(-1)) for M51 in good agreement with the previous ground-based study by Rand, and above this luminosity NGC 4449 also shows a sharp decline in its luminosity function, although the number of regions is too small to plot the function well at higher luminosities. The cumulative diameter distribution for the HII regions of M51 shows dual behaviour, with a break at a radius close to 100 pc, the radius of regions with the break luminosity. Here too we indicate the possible physical implications.

Growth factor and galaxy bias from future redshift surveys: a study on parametrizations

Cinzia Di Porto, Luca Amendola, Enzo Branchini

http://arxiv.org/abs/1101.2453v1

Many experiments in the near future will test dark energy through its effects on the linear growth of matter perturbations. In this paper we discuss the constraints that future large-scale redshift surveys can put on three different parameterizations of the linear growth factor and how these constraints will help ruling out different classes of dark energy and modified gravity models. We show that a scale-independent bias can be estimated to a few percent per redshift slice by combining redshift distortions with power spectrum amplitude, without the need of an external estimation. We find that the growth rate can be constrained to within 2-4% for each $\Delta z=0.2$ redshift slice, while the equation of state $w$ and the index $\gamma$ can be simultaneously estimated both to within 0.02. We also find that a constant dimensionless coupling between dark energy and dark matter can be constrained to be smaller than 0.14.

Towards a Cosmological Dual to Inflation

Justin Khoury, Godfrey E. J. Miller

http://arxiv.org/abs/1012.0846v1

We derive all single-field cosmologies with unit sound speed that generate scale invariant curvature perturbations on a dynamical attractor background. We identify three distinct phases: slow-roll inflation; a slowly contracting adiabatic ekpyrotic phase, described by a rapidly-varying equation of state; and a novel adiabatic ekpyrotic phase on a slowly expanding background. All of these yield identical power spectra. The degeneracy is broken at the 3-point level: unlike the nearly gaussian spectrum of slow-roll inflation, adiabatic ekpyrosis predicts large non-gaussianities on small scales. We briefly comment on extending the analysis to the case of a time-dependent sound speed.

arXiv: 13 January 2011

Cosmological magnetic field survival

John D. Barrow, Christos G. Tsagas

http://arxiv.org/abs/1101.2390v1

It is widely believed that primordial magnetic fields are dramatically diluted by the expansion of the universe. As a result, cosmological magnetic fields with residual strengths of astrophysical relevance are generally sought by going outside standard cosmology, or by extending conventional electromagnetic theory. Nevertheless, the survival of strong B-fields of primordial origin is possible in spatially open Friedmann universes without changing conventional electromagnetism. The reason is the hyperbolic geometry of these spacetimes, which slows down the adiabatic magnetic decay-rate and leads to their superadiabatic amplification on large scales. So far, the effect has been found to operate on Friedmannian backgrounds containing either radiation or a slow-rolling scalar field. We show here that the superadiabatic amplification of large-scale magnetic fields, generated by quantum fluctuations during inflation, is essentially independent of the type of matter that fills the universe and appears to be a generic feature of open Friedmann spacetimes. We estimate the late-time strength of any residual field in a marginally open universe and show that it can easily meet the requirements for the dynamo generation of the magnetic fields observed in galaxies today.

HST/NICMOS Imaging of Bright High-Redshift 24μm-selected Galaxies: Merging Properties

http://arxiv.org/abs/1101.2368v1

We present new results on the physical nature of infrared-luminous sources at 0.5<z<2.8 as revealed by HST/NICMOS imaging and IRS mid-infrared spectroscopy. Our sample consists of 134 galaxies selected at 24\mum with a flux of S(24\mum) > 0.9 mJy. We find many (~60%) of our sources to possess an important bulge and/or central point source component, most of which reveal additional underlying structures after subtraction of a best-fit sersic (or sersic+PSF) profile. Based on visual inspection of the NIC2 images and their residuals, we estimate that ~80% of all our sources are mergers. We calculate lower and upper limits on the merger fraction to be 62% and 91% respectively. At z < 1.5, we observe objects in early (pre-coalescence) merging stages to be mostly disk and star formation dominated, while we find mergers to be mainly bulge-dominated and AGN-starburst composites during coalescence and then AGN-dominated in late stages. This is analogous to what is observed in local ULIRGs. At z \geq 1.5, we find a dramatic rise in the number of objects in pre-coalescence phases of merging, despite an increase in the preponderance of AGN signatures in their mid-IR spectra and luminosities above 10^12.5 L_sun. We further find the majority of mergers at those redshifts to retain a disk-dominated profile during coalescence. We conclude that, albeit still driven by mergers, these high-z ULIRGs are substantially different in nature from their local counterparts and speculate that this is likely due to their higher gas content. Finally, we observe obscured ({\tau}_{9.7\mum} > 3.36) quasars to live in faint and compact hosts and show that these are likely high-redshift analogs of local dense-core mergers. We find late-stage mergers to show predominantly unobscured AGN spectra, but do not observe other morphological classes to occupy any one specific region in the Spoon diagram. [abridged]

Are small-scale sub-structures a universal property of galaxy halos? The case of the giant elliptical NGC~5128

M. Mouhcine, R. Ibata, M. Rejkuba

http://arxiv.org/abs/1101.2325v1

We present an analysis of the spatial and chemical sub-structures in a remote halo field in the nearby giant elliptical galaxy Centaurus A (NGC~5128), situated at about 38 kpc from the centre of the galaxy. The observations were taken with the Advanced Camera for Surveys instrument on board the Hubble Space Telescope, and reach down to the horizontal branch. In this relatively small 3.8 kpc by 3.8 kpc field, after correcting for Poisson noise, we do not find any statistically strong evidence for the presence of small-scale sub-structures in the stellar spatial distribution on scales greater than 100 pc. However, we do detect the presence of significant small spatial-scale inhomogeneities in the stellar median metallicity over the surveyed field. We argue that these localized chemical substructures could be associated with not-fully mixed debris from the disruption of low mass systems. NGC 5128 joins the ranks of the late-type spiral galaxies the Milky Way, for which the stellar halo appears to be dominated by small-scale spatial sub-structures, and NGC~891, where localized metallicity variations have been detected in the inner extra-planar regions. This suggests that the presence of small-scale sub-structures may be a generic property of stellar halos of large galaxies.

arXiv: 12 January 2010

A Different Look at Dark Energy and the Time Variation of Fundamental Constants
Marvin Weinstein
http://arxiv.org/abs/1101.2177v1
This paper makes the simple observation that a fundamental length, or
cutoff, in the context of Friedmann-Lema\^itre-Robertson-Walker (FRW)
cosmology implies very different things than for a static universe. It
is argued that it is reasonable to assume that this cutoff is
implemented by fixing the number of quantum degrees of freedom per
co-moving volume (as opposed to a Planck volume) and the relationship
of the vacuum-energy of all of the fields in the theory to the
cosmological constant (or dark energy) is re-examined. The
restrictions that need to be satisfied by a generic theory to avoid
conflicts with current experiments are discussed, and it is shown that
in any theory satisfying these constraints knowing the difference
between $w$ and minus one allows one to predict $\dot{w}$. It is
argued that this is a robust result and if this prediction fails the
idea of a fundamental cutoff of the type being discussed can be ruled
out. Finally, it is observed that, within the context of a specific
theory, a co-moving cutoff implies a predictable time variation of
fundamental constants. This is accompanied by a general discussion of
why this is so, what are the strongest phenomenological limits upon
this predicted variation, and which limits are in tension with the
idea of a co-moving cutoff. It is pointed out, however, that a careful
comparison of the predicted time variation of fundamental constants is
not possible without restricting to a particular model field-theory
and that is not done in this paper.

The Cosmogrid Simulation: Statistical Properties of Small Dark Matter Halos
Tomoaki Ishiyama, Junichiro Makino, Simon Portegies Zwart, Derek
Groen, Keigo Nitadori, Steven Rieder, Cees de Laat, Stephen McMillan,
Kei Hiraki, Stefan Harfst
http://arxiv.org/abs/1101.2020v1
We present the results of the "Cosmogrid" cosmological N-body
simulation suites based on the concordance LCDM model. The Cosmogrid
simulation was performed in a 30Mpc box with 2048^3 particles. The
mass of each particle is 1.28x10^5 Msun which is sufficient to resolve
ultra-faint dwarfs. We found that the halo mass function shows good
agreement with the Sheth and Tormen (1999) fitting function down to
~10^7 Msun. We have analyzed the spherically averaged density profiles
of the three most massive halos which are of galaxy group size and
contain at least 170 million particles. The slopes of these density
profiles become shallower than -1 at the inner most radius. We also
find a clear correlation of halo concentration with mass. The mass
dependence of the concentration parameter cannot be expressed by a
single power law, however a simple model based on the Press-Schechter
theory gives reasonable agreement with this dependence. The spin
parameter does not show a correlation with the halo mass. The
probability distribution functions for both concentration and spin are
well fitted by the log-normal distribution for halos with the masses
larger than ~10^8 Msun.

Planck Early Results: The Power Spectrum Of Cosmic Infrared Background
Anisotropies
Planck Collaboration: P. A. R. Ade, N. Aghanim, M. Arnaud, M. Ashdown,
J. Aumont, C. Baccigalupi, A. Balbi, A. J. Banday, R. B. Barreiro, J.
G. Bartlett, E. Battaner, K. Benabed, A. Benoit, J.-P. Bernard, M.
Bersanelli, R. Bhatia, K. Blagrave, J. J. Bock, A. Bonaldi, L.
Bonavera, J. R. Bond, J. Borrill, F. R. Bouchet, M. Bucher, C.
Burigana, P. Cabella, J.-F. Cardoso, A. Catalano, L. Cayon, A.
Challinor, A. Chamballu, L.-Y Chiang, C. Chiang, P. R. Christensen, D.
L. Clements, S. Colombi, F. Couchot, A. Coulais, B. P. Crill, F.
Cuttaia, L. Danese, R. D. Davies, R. J. Davis, P. de Bernardis, G. de
Gasperis, A. de Rosa, G. de Zotti, J. Delabrouille, J.-M. Delouis,
F.-X. Desert, H. Dole, S. Donzelli, O. Dore, U. Dorl, M. Douspis, X.
Dupac, G. Efstathiou, T. A. Ensslin, H. K. Eriksen, F. Finelli,
et al. (144 additional authors not shown)
http://arxiv.org/abs/1101.2028v1
Using Planck maps of six regions of low Galactic dust emission with a
total area of about 140 square degrees, we determine the angular power
spectra of Cosmic Infrared Background (CIB) anisotropies from
multipole l = 200 to l = 2000 at 217, 353, 545 and 857 GHz. We use
observations of HI emission as a tracer of thermal dust emission in
order to reduce the already low level of Galactic dust emission and
use the 143 GHz Planck maps in these fields to clean out cosmic
microwave background anisotropies. Both of these cleaning processes
are necessary in order to avoid significant contamination of the CIB
signal. We measure correlated CIB structure across frequencies. As
expected, the correlation decreases with increasing frequency
separation as the contribution of high-redshift galaxies to CIB
anisotropies increases with wavelengths. We find no significant
difference between the frequency spectrum of the CIB anisotropies and
the CIB mean, with dI/I=15% from 217 to 857 GHz. In terms of
clustering properties, the Planck data alone ruled out the linear
scale- and redshift- independent bias model. Non-linear corrections
are important. Consequently, we develop an alternative model that
couples a dusty galaxy, parametric evolution model with a simple halo
model approach. It provides an excellent fit to the measured
anisotropy angular power spectra and suggests that a different halo
occupation distribution is required at each frequency, which is
consistent with the fact that we expect each frequency to be dominated
by contributions from different redshifts. In our best-fit model, half
of the anisotropies power at l=2000 comes from redshifts z<0.8 at 857
GHz and z<0.9 at 545 GHz, while about 1/5 and 2/3 come from redshifts
z>3.5 at 353 and 217 GHz, respectively.

A Deep Dive into f(R) Gravity Theory
Solmaz Asgari, Reza Saffari
http://arxiv.org/abs/1101.2132v1
In this paper we derive behavior of deceleration parameter with
respect to redshift in context of f(R) gravity in vacuum. Here we show
that f(R) gravity will cover all the dynamical history of the Universe
from the beginning to the late time accelerating phase transition.
Here we obtained a continues inflationary behavior of the Universe
before the main inflationary period.

arXiv: 11 January 2010

Modified Gravity or Dark Matter?
J. W. Moffat
http://arxiv.org/abs/1101.1935v1
Modified Gravity (MOG) has been used successfully to explain the
rotation curves of galaxies, the motion of galaxy clusters, the Bullet
Cluster, and cosmological observations without the use of dark matter
or Einstein's cosmological constant. We review the main theoretical
ideas and applications of the theory to astrophysical and cosmological
data.

The Atacama Cosmology Telescope: Detection of Sunyaev-Zel'dovich
Decrement in Groups and Clusters Associated with Luminous Red Galaxies
Nick Hand, John William Appel, Nick Battaglia, J Richard Bond, Sudeep
Das, Mark J. Devlin, Joanna Dunkley, Rolando Dunner, Thomas
Essinger-Hileman, Joseph W. Fowler, Amir Hajian, Mark Halpern, Matthew
Hasselfield, Matt Hilton, Adam D. Hincks, Renee Hlozek, John P.
Hughes, Kent D. Irwin, Jeff Klein, Arthur Kosowsky, Yen-Ting Lin,
Tobias A. Marriage, Danica Marsden, Mike McLaren, Felipe Menanteau,
Kavilan Moodley, Michael D. Niemack, Michael R. Nolta, Lyman A. Page,
Lucas Parker, Bruce Partridge, Reed Plimpton, Erik D. Reese, Felipe
Rojas, Neelima Sehgal, Blake D. Sherwin, Jonathan L. Sievers, David N.
Spergel, Suzanne T. Staggs, Daniel S. Swetz, Eric R. Switzer, Robert
Thornton, Hy Trac, Katerina Visnjic, Ed Wollack
http://arxiv.org/abs/1101.1951v1
We present a detection of the Sunyaev-Zel'dovich (SZ) decrement
associated with the Luminous Red Galaxy (LRG) sample of the Sloan
Digital Sky Survey. The SZ data come from 148 GHz maps of the
equatorial region made by the Atacama Cosmology Telescope (ACT). The
LRG sample is divided by luminosity into four bins, and estimates for
the central Sunyaev-Zel'dovich (SZ) temperature decrement are
calculated through a stacking process. We detect and account for a
bias of the SZ signal due to weak radio sources. We use numerical
simulations to relate the observed decrement to Y200 and clustering
properties to relate the galaxy luminosity bins to mass. We also use a
relation between BCG luminosity and cluster mass based on stacked
gravitational lensing measurements to estimate the characteristic halo
masses. The masses are found to be in the range 1e13 - 1e14 M_sun, a
lower range than has been previously probed.

Exploring a string-like landscape
Jonathan Frazer, Andrew R Liddle
http://arxiv.org/abs/1101.1619v1
We explore inflationary trajectories within randomly-generated
two-dimensional potentials, considered as a toy model of the string
landscape. Both the background and perturbation equations are solved
numerically, the latter using the two-field formalism of Peterson and
Tegmark which fully incorporates the effect of isocurvature
perturbations. Sufficient inflation is a rare event, occurring for
only roughly one in $10^5$ potentials. For models generating
sufficient inflation, we find that the majority of runs satisfy
current constraints from WMAP. The scalar spectral index is less than
1 in all runs. The tensor-to-scalar ratio is below the current limit,
while typically large enough to be detected by next-generation CMB
experiments and perhaps also by Planck. In many cases the inflationary
consistency equation is broken by the effect of isocurvature modes.

Robustness to systematics for future dark energy probes
M. C. March (Imperial), R. Trotta (Imperial), L. Amendola
(Heidelberg), D. Huterer (U. of Michigan)
http://arxiv.org/abs/1101.1521v1
We extend the Figure of Merit formalism usually adopted to quantify
the statistical performance of future dark energy probes to assess the
robustness of a future mission to plausible systematic bias. We
introduce a new robustness Figure of Merit which can be computed in
the Fisher Matrix formalism given arbitrary systematic biases in the
observable quantities. We argue that robustness to systematics is an
important new quantity that should be taken into account when
optimizing future surveys. We illustrate our formalism with toy
examples, and apply it to future type Ia supernova (SNIa) and baryonic
acoustic oscillation (BAO) surveys. For the simplified systematic
biases that we consider, we find that SNIa are a somewhat more robust
probe of dark energy parameters than the BAO. We trace this back to a
geometrical alignement of systematic bias direction with statistical
degeneracy directions in the dark energy parameter space.

Extension of loop quantum gravity to $f(R)$ theories
Xiangdong Zhang, Yongge Ma
http://arxiv.org/abs/1101.1752v1
The 4-dimensional metric $f(\R)$ theories of gravity are cast into
connection-dynamical formalism with real $\SU(2)$-connections as
configuration variables. Through this formalism, the classical metric
$f(\R)$ theories are quantized by extending the loop quantization
scheme of general relativity. Our results imply that the
non-perturbative quantization procedure of loop quantum gravity is
valid not only for general relativity but also for a rather general
class of 4-dimensional metric theories of gravity.

Distant star clusters of the Milky Way in MOND
Hossein Haghi (IASBS, Zanjan), Holger Baumgardt (Queensland), Pavel
Kroupa (AIfA, Bonn)
http://arxiv.org/abs/1101.1952v1
We determine the mean velocity dispersion of six Galactic outer halo
globular clusters, AM 1, Eridanus, Pal 3, Pal 4, Pal 15, and Arp 2 in
the weak acceleration regime to test classical vs. modified Newtonian
dynamics (MOND). Owing to the non-linearity of MOND's Poisson
equation, beyond tidal effects, the internal dynamics of clusters is
affected by the external field in which they are immersed. For the
studied clusters, particle accelerations are much lower than the
critical acceleration a_0 of MOND, but the motion of stars is neither
dominated by internal accelerations (a_i >> a_e) nor external
accelerations (a_e >> a_i). We use the N-body code N-MODY in our
analysis, which is a particle-mesh-based code with a numerical MOND
potential solver developed by Ciotti, Londrillo, and Nipoti (2006) to
derive the line-of-sight velocity dispersion by adding the external
field effect. We show that Newtonian dynamics predicts a low-velocity
dispersion for each cluster, while in modified Newtonian dynamics the
velocity dispersion is much higher. We calculate the minimum number of
measured stars necessary to distinguish between Newtonian gravity and
MOND with the Kolmogorov-Smirnov test. We also show that for most
clusters it is necessary to measure the velocities of between 30 to 80
stars to distinguish between both cases. Therefore the observational
measurement of the line-of-sight velocity dispersion of these clusters
will provide a test for MOND.

arXiv: 10 January 2010

Correlations in the (Sub)millimeter background from ACTxBLAST

Amir Hajian, Marco P. Viero, Graeme Addison, Paula Aguirre, John
William Appel, Nick Battaglia, James J. Bock, J. Richard Bond, Sudeep
Das, Mark J. Devlin, Simon R. Dicker, Joanna Dunkley, Rolando Dunner,
Thomas Essinger-Hileman, John P. Hughes, Joseph W. Fowler, Mark
Halpern, Matthew Hasselfield, Matt Hilton, Adam D. Hincks, Renee
Hlozek, Kent D. Irwin, Jeff Klein, Arthur Kosowsky, Yen-Ting Lin,
Tobias A. Marriage, Danica Marsden, Gaelen Marsden, Felipe Menanteau,
Lorenzo Moncelsi, Kavilan Moodley, Calvin B. Netterfield, Michael D.
Niemack, Michael R. Nolta, Lyman A. Page, Lucas Parker, Douglas Scott,
Neelima Sehgal, Jon Sievers, David N. Spergel, Suzanne T. Staggs,
Daniel S. Swetz, Eric R. Switzer, Robert Thornton, Ed Wollack

http://arxiv.org/abs/1101.1517v1
We present measurements of the auto- and cross-frequency correlation
power spectra of the cosmic (sub)millimeter background at: 250, 350,
and 500 um (1200, 860, and 600 GHz) from observations made with the
Balloon-borne Large Aperture Submillimeter Telescope, BLAST; and at
1380 and 2030 um (218 and 148 GHz) from observations made with the
Atacama Cosmology Telescope, ACT. The overlapping observations cover
8.6 deg^2 in an area relatively free of Galactic dust near the south
ecliptic pole (SEP). The ACT bands are sensitive to radiation from the
CMB, the Sunyaev-Zel'dovich (SZ) effect from galaxy clusters, and to
emission by radio and dusty star-forming galaxies (DSFGs), while the
dominant contribution to the BLAST bands is from DSFGs. We confirm and
extend the BLAST analysis of clustering with an independent pipeline,
and also detect correlations between the ACT and BLAST maps at over
25sigma significance, which we interpret as a detection of the DSFGs
in the ACT maps. In addition to a Poisson component in the
cross-frequency power spectra, we detect a clustered signal at
>4sigma, and using a model for the DSFG evolution and number counts,
we successfully fit all our spectra with a linear clustering model and
a bias that depends only on redshift and not on scale. Finally, the
data are compared to, and generally agree with, phenomenological
models for the DSFG population. This study represents a first of its
kind, and demonstrates the constraining power of the cross-frequency
correlation technique to constrain models for the DSFGs. Similar
analyses with more data will impose tight constraints on future
models.

Primordial non-Gaussianities from inflation
Hael Collins (The Niels Bohr International Academy)
http://arxiv.org/abs/1101.1308v1
These notes present a detailed introduction to Maldacena's
calculation of the cubic terms in the inflationary action. These
interactions are important since they produce the most readily
observable evidence for a non-Gaussian component in the pattern of
primordial fluctuations produced by inflation. In the simplest class
of inflationary theories, those with only a single scalar field
participating in the inflationary era, these non-Gaussianities are
predicted to be extremely small, as will be reviewed here.

Galilean-invariant scalar fields can strengthen gravitational lensing
Mark Wyman
http://arxiv.org/abs/1101.1295v1
The mystery of dark energy suggests that there is new gravitational
physics at low energies and on long length scales. On the other hand,
low mass degrees of freedom in gravity are strictly limited by
observations within the solar system. A compelling way to resolve this
apparent contradiction is to add a galilean-invariant scalar field to
gravity. Called galileons, these scalars have strong self interactions
near overdensities, like the solar system, that suppress their effects
on the motion of massive particles. These non-linearities are weak on
cosmological scales, permitting new physics to operate. In this note,
we point out that extending galilean invariance to the coupling of
galileons to stress-energy -- as was first done in the case of massive
gravity -- can have a surprising phenomenological consequence:
enhanced gravitational lensing. Weak lensing observations should be
able to detect or rule out this effect.

arXiv: 7 January 2011

Discovery and Cosmological Implications of SPT-CL J2106-5844, the Most Massive Known Cluster at z > 1

R. J. Foley, K. Andersson, G. Bazin, T. de Haan, J. Ruel, P. A. R. Ade, K. A. Aird, R. Armstrong, M. L. N. Ashby, M. Bautz, B. A. Benson, L. E. Bleem, M. Bonamente, M. Brodwin, J. E. Carlstrom, C. L. Chang, A. Clocchiatti, T. M. Crawford, A. T. Crites, S. Desai, M. A. Dobbs, J. P. Dudley, G. G. Fazio, W. R. Forman,G. Garmire, E. M. George, M. D. Gladders, A. H. Gonzalez, N. W. Halverson, F. W. High, G. P. Holder, W. L. Holzapfel, S. Hoover, J. D. Hrubes, C. Jones, M. Joy, R. Keisler, L. Knox, A. T. Lee, E. M. Leitch, M. Lueker, D. Luong-Van, D. P. Marrone, J. J. McMahon, J. Mehl, S. S. Meyer, J. J. Mohr, T. E. Montroy, S. S. Murray, S. Padin, T. Plagge, C. Pryke, C. L. Reichardt, A. Rest, J. E. Ruhl, B. R. Saliwanchik, A. Saro, K. K. Schaffer, L. Shaw, E. Shirokoff, J. Song, H. G. Spieler, et al. (11 additional authors not shown)

http://arxiv.org/abs/1101.1286v1

Using the South Pole Telescope (SPT), we have discovered the most massive known galaxy cluster at z > 1, SPT-CL J2106-5844. In addition to producing a strong Sunyaev-Zel'dovich effect signal, this system is a luminous X-ray source and its numerous constituent galaxies display spatial and color clustering, all indicating the presence of a massive galaxy cluster. VLT and Magellan spectroscopy of 18 member galaxies shows that the cluster is at z = 1.132^+0.002_-0.003. Chandra observations obtained through a combined HRC-ACIS GTO program reveal an X-ray spectrum with an Fe K line redshifted by z = 1.18 +/- 0.03. These redshifts are consistent with galaxy colors in extensive optical, near-infrared, and mid-infrared imaging. SPT-CL J2106-5844 displays extreme X-ray properties for a cluster, having a core-excluded temperature of kT = 11.0^+2.6_-1.9 keV and a luminosity (within r_500) of L_X (0.5 - 2.0 keV) = (13.9 +/- 1.0) x 10^44 erg/s. The combined mass estimate from measurements of the Sunyaev-Zel'dovich effect and X-ray data is M_200 = (1.27 +/- 0.21) x 10^15 M_sun. The discovery of such a massive gravitationally collapsed system at high redshift provides an interesting laboratory for galaxy formation and evolution, and is a powerful probe of extreme perturbations of the primordial matter density field. We discuss the latter, determining that, under the assumption of LambdaCDM cosmology with only Gaussian perturbations, there is only a 7% chance of finding a galaxy cluster similar to SPT-CL J2106-5844 in the 2500 deg^2 SPT survey region, and that only one such galaxy cluster is expected in the entire sky.

Sub-millimetre galaxies reside in dark matter halos with masses greater than 3x10^11 solar masses

Alexandre Amblard, Asantha Cooray, Paolo Serra (UC Irvine), B. Altieri, V. Arumugam, H. Aussel, A. Blain,J. Bock, A. Boselli, V. Buat, N. Castro-Rodriguez, A. Cava, P. Chanial, E. Chapin, D.L. Clements, A. Conley, L. Conversi, C.D. Dowell, E. Dwek, S. Eales, D. Elbaz, D. Farrah, A. Franceschini, W. Gear, J. Glenn, M. Griffin, M. Halpern, E. Hatziminaoglou, E. Ibar, K. Isaak, R.J. Ivison, A.A. Khostovan, G. Lagache,L. Levenson, N. Lu, S. Madden, B. Maffei, G. Mainetti, L. Marchetti, G. Marsden, K. Mitchell-Wynne, H.T. Nguyen, B. O'Halloran, S.J. Oliver, A. Omont, M.J. Page, P. Panuzzo, A. Papageorgiou, C.P. Pearson, I. Perez-Fournon, M. Pohlen, N. Rangwala, I.G. Roseboom, M. Rowan-Robinson, M. Sanchez Portal, B. Schulz, Douglas Scott, N. Seymour, D.L. Shupe, A.J. Smith, J.A. Stevens, et al. (13 additional authors not shown)

http://arxiv.org/abs/1101.1080v1

The extragalactic background light at far-infrared wavelengths originates from optically-faint, dusty, star-forming galaxies in the universe with star-formation rates at the level of a few hundred solar masses per year. Due to the relatively poor spatial resolution of far-infrared telescopes, the faint sub-millimetre galaxies are challenging to study individually. Instead, their average properties can be studied using statistics such as the angular power spectrum of the background intensity variations. A previous attempt at measuring this power spectrum resulted in the suggestion that the clustering amplitude is below the level computed with a simple ansatz based on a halo model. Here we report a clear detection of the excess clustering over the linear prediction at arcminute angular scales in the power spectrum of brightness fluctuations at 250, 350, and 500 microns. From this excess, we find that sub-millimetre galaxies are located in dark matter halos with a minimum mass of log[M_min/M_sun ]= 11.5^+0.7_-0.2 at 350 microns. This minimum dark matter halo mass corresponds to the most efficient mass scale for star formation in the universe, and is lower than that predicted by semi-analytical models for galaxy formation.

General Covariance in Gravity at a Lifshitz Point

Petr Horava

http://arxiv.org/abs/1101.1081v1

This paper is based on the invited talks delivered by the author at GR 19: the 19th International Conference on General Relativity and Gravitation, Ciudad de M\'exico, M\'exico, July 2010. In Part 1, we briefly review some of the main features of quantum gravity with anisotropic scaling, and comment on its possible relation to the causal dynamical triangulations (CDT) approach to lattice quantum gravity. Part 2 explains the construction of gravity with anisotropic scaling with an extended gauge symmetry -- essentially a nonrelativistic version of general covariance. This extra symmetry eliminates the scalar graviton polarization, and thus brings the theory closer to general relativity at long distances.

arXiv: 6 January 2011

Gravitational microlensing in modified gravity theories: Inverse-square theorem

Hideki Asada

http://arxiv.org/abs/1101.0864v1

Microlensing studies are usually based on the lens equation that is valid only to the first order in the gravitational constant G and lens mass M. We consider corrections to the conventional lens equation in terms of differentiable functions, so that they can express not only the second-order effects of GM in general relativity but also modified gravity theories. As a generalization of Ebina et al. (Prog. Theor. Phys. 104 (2000) 1317), we show that, provided that the spacetime is static, spherically symmetric and asymptotically flat, the total amplification by microlensing remains unchanged at the linear order of the correction to the deflection angle, if and only if the correction takes a particular form as the inverse square of the impact parameter, whereas the magnification factor for each image is corrected. It is concluded that the light curve shape by microlensing is inevitably changed and will thus allow us to probe modified gravity, unless a modification to the deflection angle takes the particular form. No systematic deviation in microlensing observations has been reported. For instance, therefore, the Yukawa-type correction is constrained as the characteristic length > $10^{14}$ m.

Cosmological structure formation with clustering quintessence

Emiliano Sefusatti, Filippo Vernizzi

http://arxiv.org/abs/1101.1026v1

We study large-scale structure formation in the presence of a quintessence component with zero speed of sound in the framework of Eulerian Perturbation Theory. Due to the absence of pressure gradients, quintessence and dark matter are comoving and can be studied as a unique fluid in terms of the total energy density contrast and the common velocity. In this description the clustering of quintessence enhances the linear term proportional to the velocity divergence in the continuity equation by a factor (1+w) Omega_Q / Omega_m. This is responsible for a rapid evolution of the growth rate at low redshifts, and modifies the standard relation between the velocity divergence and the growth factor. For the total fluid, the solutions for the linear growth function and growth rate can be written in integral forms and admit simple fitting formulae, as in the LambdaCDM case. At second order in perturbation theory, we derive an explicit expression for the kernels F_2 and G_2. They receive modifications of the order of the ratio between quintessence and total energy density perturbations, which affect the corresponding tree-level bispectra. We finally compute the cumulative signal-to-noise in the power spectrum, bispectrum and reduced bispectrum, expected for departures from a LambdaCDM cosmology both in the clustering and smooth quintessence scenarios. The reduced bispectrum, in particular, receives sensible modifications only in the clustering case and can potentially be used to detect or rule out the model.

The evolution of white dwarfs with a varying gravitational constant

L. G. Althaus, A. H. Corsico, S. Torres, P. Loren-Aguilar, J. Isern, E. Garcia-Berro

http://arxiv.org/abs/1101.0986

Within the theoretical framework of some modern unification theories the constants of nature are functions of cosmological time. White dwarfs offer the possibility of testing a possible variation of G and, thus, to place constraints to these theories. We present full white dwarf evolutionary calculations in the case that G decreases with time. White dwarf evolution is computed in a self-consistent way, including the most up-to-date physical inputs, non-gray model atmospheres and a detailed core chemical composition that results from the calculation of the full evolution of progenitor stars. We find that the mechanical structure and the energy balance of white dwarfs are strongly modified by the presence of a varying G. In particular, for certain values of the rate of change of G, the evolution of cool white dwarfs is markedly affected. The impact of a varying G is more notorious in the case of more massive white dwarfs. In view of the recent results reporting that a very accurate white dwarf cooling age can be derived for the old and metal-rich open cluster NGC 6791, our study suggests that this cluster could be a potential target to constrain or detect a ypothetical secular variation of G