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We show that some holographic dark energy models can lead to a future evolution of the universe in which the scale factor $a$ is asymptotically constant, while $\dot a \rightarrow 0$ and the corresponding energy and pressure densities also vanish. We provide specific examples of such models and general conditions that can lead to an asymptotically static universe, which we have called the ``long freeze." We show that in some cases, such evolution can follow an arbitrarily long exponential expansion essentially identical to the asymptotic evolution of $\Lambda$CDM., Comment: 5 pages with no figures, comments very welcome !

We examine the power spectrum $P_n$ of the $n$th harmonic for Garfinkle-Vachaspati cosmic strings, which correspond to planar rectangular loops. While these loop tractories are self-intersecting, a slightly perturbed non-self-intersecting form of these trajectories has been suggested as a generic end state of cosmic string loop fragmentation. We find that $P_n$ scales as $n^{-2} \ln n$, rather than the expected $n^{-2}$ for kink-kink collisions. This result is demonstrated analytically for even $n$ in square loops and numerically for all other cases. At lowest order, the effect of loop decays is to further enhance $P_n$ at large $n$ relative to its value at small $n$. The consequences for relic stochastic background radiation along with the caveats pertaining to our results are discussed., Comment: 11 pages, 7 figures

We explore the asymptotic future evolution of holographic dark energy (HDE) models, in which the density of the dark energy is a function of a cutoff scale $L$. We develop a general methodology to determine which models correspond to future big rip, little rip, and pseudo-rip (de Sitter) evolution, and we apply this methodology to a variety of well-studied HDE models. None of these HDE models display little rip evolution, and we are able to show, under very general assumptions, that HDE models with a Granda-Oliveros cutoff almost never evolve toward a future little rip. We extend these results to HDE models with nonstandard Friedman equations and show that a similar conclusion applies: little rip evolution is a very special case that is almost never realized in such models., Comment: v3, matches the published version in PRD, minor clarifications and typos corrected, 9 pages with no figures

We examine big bang nucleosynthesis (BBN) in models with a time-varying gravitational constant $G$, when this time variation is rapid on the scale of the expansion rate $H$, i.e, $\dot G/G \gg H$. Such models can arise naturally in the context of scalar-tensor theories of gravity and result in additional terms in the Friedman equation. We examine two representative models: a step-function evolution for $G$ and a rapidly-oscillating $G$. In the former case, the additional terms in the Friedman equation tend to cancel the effects of an initial value of $G$ that differs from the present-day value. In the case of deuterium, this effect is large enough to reverse the sign of the change in (D/H) for a given change in the initial value of $G$. For rapidly-oscillating $G$, the effect on the Friedman equation is similar to that of adding a vacuum energy density, and BBN allows upper limits to be placed on the product of the oscillation frequency and amplitude. The possibility that a rapidly oscillating $G$ could mimic a cosmological constant is briefly discussed., Comment: 8 pages, 8 figures, discussion and clarification added, including expanded discussion of the lithium problem, matches version to appear in Phys. Rev. D

We investigate the out-of-equilibrium dynamics of viscous fluids in a spatially flat Friedmann-Lema\^itre-Robertson-Walker cosmology using the most general causal and stable viscous energy-momentum tensor defined at first order in spacetime derivatives. In this new framework a pressureless viscous fluid having density $\rho$ can evolve to an asymptotic future solution in which the Hubble parameter approaches a constant while $\rho \rightarrow 0$, even in the absence of a cosmological constant (i.e., $\Lambda = 0$). Thus, while viscous effects in this model drive an accelerated expansion of the universe, the density of the viscous component itself vanishes, leaving behind only the acceleration. This behavior emerges as a consequence of causality in first-order theories of relativistic fluid dynamics and it is fully consistent with Einstein's equations., Comment: 14 pages, no figures, references updated, discussion added

When the expansion of the universe is dominated by a perfect fluid with equation of state parameter $w$ and a sound speed $c_s$ satisfying $w = c_s^2 \le 1$, the Hubble parameter $H$ and time $t$ satisfy the bound $Ht \ge 1/3$. There has been recent interest in "ultra-slow" expansion laws with $Ht < 1/3$ (sometimes described as "fast expanding" models). We examine various models that can produce ultra-slow expansion: scalar fields with negative potentials, barotropic fluids, braneworld models, or a loitering phase in the early universe. Scalar field models and barotropic models for ultra-slow expansion are unstable to evolution toward $w = 1$ or $w \rightarrow \infty$ in the former case and $w \rightarrow \infty$ in the latter case. Braneworld models can yield ultra-slow expansion but require an expansion law beyond the standard Friedman equation. Loitering early universe models can produce a quasi-static expansion phase in the early universe but require an exotic negative-density component. These results suggest that appeals to an ultra-slow expansion phase in the early universe should be approached with some caution, although the loitering early universe may be worthy of further investigation. These results do not apply to ultra-slow contracting models., Comment: 7 pages, 1 figure, references added

We examine astronomical observations that would be achievable over a future timeline corresponding to the documented history of human civilization so far, $\sim 10^4$ years. We examine implications for measurements of the redshift drift, evolution of the CMB, and cosmic parallax. A number of events that are rare on the scale of centuries will become easily observable on a timescale $\sim 10^4$ years. Implications for several measurements related to gravity are discussed., Comment: 4 pages, no figures, references updated and typos corrected

We derive exact general solutions (as opposed to attractor particular solutions) and corresponding first integrals for the evolution of a scalar field $\phi$ in a universe dominated by a background fluid with equation of state parameter $w_B$. In addition to the previously-examined linear [$V(\phi) = V_0 \phi$] and quadratic [$V(\phi) = V_0 \phi^2$] potentials, we show that exact solutions exist for the power law potential $V(\phi) = V_0 \phi^n$ with $n = 4(1+w_B)/(1-w_B) + 2$ and $n = 2(1+w_B)/(1-w_B)$. These correspond to the potentials $V(\phi) = V_0 \phi^6$ and $V(\phi) = V_0 \phi^2$ for matter domination and $V(\phi) = V_0 \phi^{10}$ and $V(\phi) = V_0 \phi^4$ for radiation domination. The $\phi^6$ and $\phi^{10}$ potentials can yield either oscillatory or non-oscillatory evolution, and we use the first integrals to determine how the initial conditions map onto each form of evolution. The exponential potential yields an exact solution for a stiff/kination ($w_B = 1$) background. We use this exact solution to derive an analytic expression for the evolution of the equation of state parameter, $w_\phi$, for this case., Comment: 13 pages, 3 figures, references added

We examine the simplest inflection point quintessence model, with a potential given by $V(\phi) = V_0 + V_3 \phi^3$. This model can produce either asymptotic de Sitter expansion or transient acceleration, and we show that it does not correspond to either pure freezing or thawing behavior. We derive observational constraints on the initial value of the scalar field, $\phi_i$, and $V_3/V_0$ and find that small values of either $\phi_i$ or $V_3/V_0$ are favored. While most of the observationally-allowed parameter space yields asymptotic de Sitter evolution, there is a small region, corresponding to large $V_3/V_0$ and small $\phi_i$, for which the current accelerated expansion is transient. The latter behavior is potentially consistent with a cyclic universe., Comment: 8 pages, 4 figures, added discussion of cyclic models

We examine nonrelativistic particles that decay into relativistic products in big rip, little rip, and pseudo-rip models for the future evolution of the universe. In contrast to decays that occur in standard $\Lambda$CDM, the evolution of the ratio $r$ of the energy density of the relativistic decay products to the energy density of the initially decaying particles can decrease with time in all of these models. In big rip and little rip models, $r$ always goes to zero asymptotically, while this ratio evolves to infinity or a constant in pseudo-rip models., Comment: 4 pages, 3 figures, discussion and asymptotic analytic solution added

Inhomogeneous big bang nucleosynthesis (BBN) produces a spatially inhomogeneous distribution of element abundances at $T \sim 10^9$ K, but subsequent element diffusion will tend to erase these inhomogeneities. We calculate the cosmological comoving diffusion length for the BBN elements. This diffusion length is limited by atomic scattering and is therefore dominated by diffusion when the atoms are neutral, between the redshifts of recombination and reionization. We find that the comoving diffusion length today is $d_{com} \approx 70$ pc for all of the elements of interest except $^7$Li, for which $d_{com}$ is an order of magnitude smaller because $^7$Li remains ionized throughout the relevant epoch. This comoving diffusion length corresponds to a substellar baryonic mass scale and is roughly equal to the horizon scale at BBN. These results lend support to the possibility that inhomogeneities on scales larger than the horizon at BBN could lead to a spatially inhomogeneous distribution of elements today, while purely subhorizon fluctuations at BBN can result only in a homogeneous element distribution at present., Comment: 5 pages, no figures

We examine the cosmological evolution of ultralight axionlike (ULA) scalar fields with potentials of the form $V(\phi) = m^2 f^2[1 - \cos(\phi/f)]^n$, with particular emphasis on the deviation in their behavior from the corresponding small$-\phi$ power-law approximations to these potentials: $V(\phi) \propto \phi^{2n}$. We show that in the slow-roll regime, when $\dot \phi^2/2 \ll V(\phi)$, the full ULA potentials yield a more interesting range of possibilities for quintessence than do the corresponding power law approximations. For rapidly oscillating scalar fields, we derive the equation of state parameter and oscillation frequency for the ULA potentials and show how they deviate from the corresponding power-law values. We derive an analytic expression for the equation of state parameter that better approximates the ULA value than does the pure power-law approximation., Comment: 9 pages, 6 figures

We examine the Swampland conjectures in the context of generic slow-roll thawing quintessence models. Defining $\lambda \equiv |V^{\prime}(\phi_i)/V(\phi_i)|$ and $K \equiv \sqrt{1 - 4V^{\prime \prime}(\phi_i)/3V(\phi_i)}$, where $\phi_i$ is the initial value of $\phi$, we find regions of parameter space consistent with both observational data and with the refined de Sitter conjecture, and we show that all such models satisfy the distance conjecture. We quantify the degree of fine-tuning on $\lambda$ needed to achieve these results., Comment: 8 pages, 5 figures, references added

Antimatter macroscopic dark matter (macros) refers to a generic class of antimatter dark matter candidates that interact with ordinary matter primarily through annihilation with large cross-sections. A combination of terrestrial, astrophysical, and cosmological observations constrain a portion of the anti-macro parameter space. However, a large region of the parameter space remains unconstrained, most notably for nuclear-dense objects., Comment: 13 pages, 1 figure

In the Ijjas-Steinhardt cyclic model, the universe passes through phases dominated by radiation, matter, and a dark energy scalar field, with the value of the scale factor increasing with each cycle. Since each cycle terminates in a finite time, it is straightforward to calculate the fraction of time that the universe spends in a state for which the matter and dark energy densities have comparable magnitudes; when this fraction is large, it can be taken as a solution of the coincidence problem. This solution of the coincidence problem requires a relatively short lifetime for each cycle, but unlike in the case of phantom models, there is no fixed upper bound on this lifetime. However, scalar field models satisfying the Swampland conjectures yield sufficiently short lifetimes to provide a satisfactory resolution of the coincidence problem., Comment: 5 pages, 1 figure, more detailed discussion of Swampland conjectures, references added

Macroscopic dark matter refers to a variety of dark matter candidates that would be expected to (elastically) scatter off of ordinary matter with a large geometric cross-section. A wide range of macro masses $M_X$ and cross-sections $\sigma_X$ remain unprobed. We show that over a wide region within the unexplored parameter space, collisions of a macro with a human body would result in serious injury or death. We use the absence of such unexplained impacts with a well-monitored subset of the human population to exclude a region bounded by $\sigma_X \geq 10^{-8} - 10^{-7}$ cm$^2$ and $M_X < 50$ kg. Our results open a new window on dark matter: the human body as a dark matter detector., Comment: 9 pages, 2 figure

We examine the thermal evolution of particle number densities in the early universe when the particles have a finite diffusion length. Assuming that annihilations are impossible when the mean separation of the particles is larger than their diffusion length, we derive a version of the Boltzmann equation for freeze out in this scenario and an approximate solution, accurate to better than 2\%. The effect of a finite diffusion length is to increase the final relic freeze-out abundance over its corresponding value when diffusion effects are ignored. When diffusion is limited only by scattering off of the thermal background, and the annihilation cross section is bounded by unitarity, a significant effect on the freeze-out abundance requires a scattering cross section much larger than the annihilation cross section. A similar effect is demonstrated when the relic particles are produced via the freeze-in mechanism, but in this case the finite diffusion length is due to the scattering of particles that annihilate into the relic particle of interest. For freeze in, the effect of a finite diffusion length is to reduce the final relic particle abundance. The effects of a finite diffusion length are most important when the scattering cross section or the relic mass are very large. While we have not found a particularly compelling example where this would affect previous results, with the current interest in new dark matter candidates it could become an important consideration., Comment: 10 pages, 3 figures, discussion generalized to p-wave and higher-order annihilations, references added

We reexamine $k$-essence dark energy models with a scalar field $\phi$ and a factorized Lagrangian, $\mathcal L = V(\phi)F(X)$, with $X = \frac{1}{2} \nabla_\mu \phi \nabla^\mu \phi.$ A value of the equation of state parameter, $w$, near $-1$ requires either $X \approx 0$ or $dF/dX \approx 0$. Previous work showed that thawing models with $X \approx 0$ evolve along a set of unique trajectories for $w(a)$, while those with $dF/dX \approx 0$ can result in a variety of different forms for $w(a)$. We show that if $dV/d\phi$ is small and $(1/V)(dV/d\phi)$ is roughly constant, then the latter models also converge toward a single unique set of behaviors for $w(a)$, different from those with $X \approx 0$. We derive the functional form for $w(a)$ in this case, determine the conditions on $V(\phi)$ for which it applies, and present observational constraints on this new class of models. We note that $k$-essence models with $dF/dX \approx 0$ correspond to a dark energy sound speed $c_s^2 \approx 0$., Comment: 7 pages, 2 figures, discussion and references added

We examine the case of a random isotropic velocity field, in which one of the velocity components (the "radial" component, with magnitude $v_z$) can be measured easily, while measurement of the velocity perpendicular to this component (the "transverse" component, with magnitude $v_T$) is more difficult and requires long-time monitoring. Particularly important examples are the motion of galaxies at cosmological distances and the interpretation of Gaia data on the proper motion of stars in globular clusters and dwarf galaxies. We address two questions: what is the probability distribution of $v_T$ for a given $v_z$, and for what choice of $v_z$ is the expected value of $v_T$ maximized? We show that, for a given $v_z$, the probability that $v_T$ exceeds some value $v_0$ is $p(v_T \ge v_0 | v_z) = {p_z(\sqrt{v_0^2 + v_z^2}})/{p_z(v_z)}$, where $p_z(v_z)$ is the probability distribution of $v_z$. The expected value of $v_T$ is maximized by choosing $v_z$ as large as possible whenever $\ln p_z(\sqrt{v_z})$ has a positive second derivative, and by taking $v_z$ as small as possible when this second derivative is negative., Comment: 6 pages, 4 figures, references and discussion added

If the dark energy density asymptotically approaches a nonzero constant, $\rho_{DE} \rightarrow \rho_0$, then its equation of state parameter $w$ necessarily approaches $-1$. The converse is not true; dark energy with $w \rightarrow -1$ can correspond to either $\rho_{DE} \rightarrow \rho_0$ or $\rho_{DE} \rightarrow 0$. This provides a natural division of models with $w \rightarrow -1$ into two distinct classes: asymptotic $\Lambda$ ($\rho_{DE} \rightarrow \rho_0$) and pseudo-$\Lambda$ ($\rho_{DE} \rightarrow 0$). We delineate the boundary between these two classes of models in terms of the behavior of $w(a)$, $\rho_{DE}(a)$, and $a(t)$. We examine barotropic and quintessence realizations of both types of models. Barotropic models with positive squared sound speed and $w \rightarrow -1$ are always asymptotically $\Lambda$; they can never produce pseudo-$\Lambda$ behavior. Quintessence models can correspond to either asymptotic $\Lambda$ or pseudo-$\Lambda$ evolution, but the latter is impossible when the expansion is dominated by a background barotropic fluid. We show that the distinction between asymptotic $\Lambda$ and pseudo-$\Lambda$ models for $w> -1$ is mathematically dual to the distinction between pseudo-rip and big/little rip models when $w < -1$., Comment: 7 pages, no figures, references added

We reexamine big bang nucleosynthesis with large-scale baryon density inhomogeneities when the length scale of the density fluctuations exceeds the neutron diffusion length ($\sim 10^7-10^8$ cm at BBN), and the amplitude of the fluctuations is sufficiently small to prevent gravitational collapse. In this limit, the final light element abundances can be determined by simply mixing the abundances from regions with different baryon/photon ratios without interactions. We examine gaussian, lognormal, and gamma distributions for the baryon/photon ratio, $\eta $. We find that the deuterium and lithium-7 abundances increase with the RMS fluctuation in $\eta $, while the effect on helium-4 is much smaller. We show that these increases in the deuterium and lithium-7 abundances are a consequence of Jensen's inequality, and we derive analytic approximations for these abundances in the limit of small RMS fluctuations. Observational upper limits on the primordial deuterium abundance constrain the RMS fluctuation in $\eta $ to be less than $17\%$ of the mean value of $\eta $. This provides us with a new limit on the graininess of the early universe., Comment: 8 pages, 2 figures, reaction rates updated, minor changes to figures and limits, added analytic approximations for element abundances and discussion of Jensen's inequality, to appear in Phys. Rev. D

The asymmetry between matter and antimatter is key to the existence and nature of our Universe. A measure of the matter - antimatter asymmetry of the Universe is provided by the present value of the universal ratio of baryons (baryons minus antibaryons) to photons (or the ratio of baryons to entropy). The baryon asymmetry parameter is an important physical and cosmological parameter. But how fine tuned is it? A "natural" value for this parameter is zero, corresponding to equal amounts of matter and antimatter. Another, also possibly natural, choice for this dimensionless parameter would be of order unity, corresponding to nearly equal amounts (by number) of matter (and essentially no antimatter) and photons in every comoving volume. However, observations suggest that in the Universe we inhabit the value of this parameter is nonzero, but smaller than this natural value by some nine to ten orders of magnitude. In this contribution we review the evidence that our Universe does not contain equal amounts of matter and antimatter. Any change in the magnitude of the baryon asymmetry parameter necessarily leads to a universe with physical characteristics different from those in our own. The degree of fine tuning in the baryon asymmetry parameter is determined by the width of the range over which it can be varied and still allow for the existence of life. Our results suggest that the baryon asymmetry parameter can be varied over a very wide range without impacting the prospects for life; this result is not suggestive of fine tuning. [abridged], Comment: Chapter for the book, Consolidation of Fine Tuning; 38 pages, 1 figure

We study a rapidly-oscillating scalar field with potential $V(\phi) = k|\phi|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form $(1- 8 \pi G_0 \xi \phi^2) R$ in the action. In the weak coupling limit, we calculate the effect of the nonminimal coupling on the time-averaged equation of state parameter $\gamma = (p + \rho)/\rho$. The change in $\langle \gamma \rangle$ is always negative for $n \ge 2$ and always positive for $n < 0.71$ (which includes the case where the oscillating scalar field could serve as dark energy), while it can be either positive or negative for intermediate values of $n$. Constraints on the time-variation of $G$ force this change to be infinitesimally small at the present time whenever the scalar field dominates the expansion, but constraints in the early universe are not as stringent. The rapid oscillation induced in $G$ also produces an additional contribution to the Friedman equation that behaves like an effective energy density with a stiff equation of state, but we show that, under reasonable assumptions, this effective energy density is always smaller than the density of the scalar field itself., Comment: 14 pages

A change in the fundamental constants of nature or plasma effects in the early universe could stabilize $^8$Be against decay into two $^4$He nuclei. Coc et al. examined this effect on big bang nucleosynthesis as a function of $B_8$, the mass difference between two $^4$He nuclei and a single $^8$Be nucleus, and found no effects for $B_8 \le 100$ keV. Here we examine stable $^8$Be with larger $B_8$ and also allow for a variation in the rate for $^4$He + $^4$He $\longrightarrow$ $^8$Be to determine the threshold for interesting effects. We find no change to standard big bang nucleosynthesis for $B_8 < 1$ MeV. For $B_8 \gtrsim 1$ MeV and a sufficiently large reaction rate, a significant fraction of $^4$He is burned into $^8$Be, which fissions back into $^4$He when $B_8$ assumes its present-day value, leaving the primordial $^4$He abundance unchanged. However, this sequestration of $^4$He results in a decrease in the primordial $^7$Li abundance. Primordial abundances of $^7$Li consistent with observationally-inferred values can be obtained for reaction rates similar to those calculated for the present-day (unbound $^8$Be) case. Even for the largest binding energies and largest reaction rates examined here, only a small fraction of $^8$Be is burned into heavier elements, consistent with earlier studies. There is no change in the predicted deuterium abundance for any model we examined., Comment: 7 pages, 2 figures, expanded discussion of 8Be binding energy, added reference, to appear in Phys. Rev. D

We consider Big Bang nucleosynthesis and the cosmic microwave background when both the neutrino temperature and neutrino number are allowed to vary from their standard values. The neutrino temperature is assumed to differ from its standard model value by a fixed factor from Big Bang nucleosynthesis up to the present. In this scenario, the effective number of relativistic degrees of freedom, $N_{\rm eff}^{\rm CMB}$, derived from observations of the cosmic microwave background is not equal to the true number of neutrinos, $N_\nu$. We determine the element abundances predicted by Big Bang nucleosynthesis as a function of the neutrino number and temperature, converting the latter to the equivalent value of $N_{\rm eff}^{\rm CMB}$. We find that a value of $N_{\rm eff}^{\rm CMB} \approx 3$ can be made consistent with $N_\nu = 4$ with a decrease in the neutrino temperature of $\sim 5\%$, while $N_\nu = 5$ is excluded for any value of $N_{\rm eff}^{\rm CMB}$. No observationally-allowed values for $N_{\rm eff}^{\rm CMB}$ and $N_\nu$ can solve the lithium problem., Comment: 8 pages, 5 figures, replacement corresponding to version accepted to Phys Rev D

We examine a quintessence model with a modified exponential potential given by $V(\phi) = V_0(1+e^{-\lambda \phi})$. Unlike quintessence with a standard exponential potential, our model can yield an acceptable accelerated expansion at late times, while producing a distinct "early dark energy" signature at high redshift. We determine the evolution of the equation of state parameter, $w_\phi$, and the density parameter, $\Omega_\phi$, as a function of the scale factor. The strongest constraints on the model come from cosmic microwave background observations rather than supernova data. The former give the limit $\lambda > 13$. This model predicts a value of the effective neutrino number during Big Bang nucleosynthesis larger than the standard model value. It also provides a partial solution to the coincidence problem, in that the ratio of the quintessence energy density is always within a few orders of magnitude of the background radiation or matter density from the early universe up to the present, but it does not explain why the accelerated expansion is beginning near the present day, suggesting that these two different ways of characterizing the coincidence problem are not entirely equivalent., Comment: 5 pages, 2 figures, reference and discussion added

We apply the Alcock-Paczynski (AP) test to the stacked voids identified using the large-scale structure galaxy catalog from the Baryon Oscillation Spectroscopic Survey (BOSS). This galaxy catalog is part of the Sloan Digital Sky Survey (SDSS) Data Release 12 and is the final catalog of SDSS-III. We also use 1000 mock galaxy catalogs that match the geometry, density, and clustering properties of the BOSS sample in order to characterize the statistical uncertainties of our measurements and take into account systematic errors such as redshift space distortions. For both BOSS data and mock catalogs, we use the ZOBOV algorithm to identify voids, we stack together all voids with effective radii of 30-100Mpc/h in the redshift range 0.43-0.7, and we accurately measure the shape of the stacked voids. Our tests with the mock catalogs show that we measure the stacked void ellipticity with a statistical precision of 2.6%. We find that the stacked voids in redshift space are slightly squashed along the line of sight, which is consistent with previous studies. We repeat this measurement of stacked void shape in the BOSS data assuming several values of Omega_m within the flat LCDM model, and we compare to the mock catalogs in redshift space in order to perform the AP test. We obtain a constraint of $\Omega_m = 0.38^{+0.18}_{-0.15}$ at the 68% confidence level from the AP test. We discuss the various sources of statistical and systematic noise that affect the constraining power of this method. In particular, we find that the measured ellipticity of stacked voids scales more weakly with cosmology than the standard AP prediction, leading to significantly weaker constraints. We discuss how AP constraints will improve in future surveys with larger volumes and densities., Comment: 10 pages, 7 figures. Submitted to the ApJ. Replaced with updated version

We derive general conditions for the existence of stable scaling solutions for the evolution of noncanonical quintessence, with a Lagrangian of the form $\mathcal{L}(X,\phi)=X^{\alpha}-V(\phi)$, for power-law and exponential potentials when the expansion is dominated by a background barotropic fluid. Our results suggest that in most cases, noncanonical quintessence with such potentials does not yield interesting models for the observed dark energy. When the scaling solution is not an attractor, there is a wide range of model parameters for which the evolution asymptotically resembles a zero-potential solution with equation of state parameter $w = 1/(2\alpha -1)$, and oscillatory solutions are also possible for positive power-law potentials; we derive the conditions on the model parameters which produce both types of behavior. We investigate thawing noncanonical models with a nearly-flat potential and derive approximate expressions for the evolution of $w(a)$. These forms for $w(a)$ differ in a characteristic way from the corresponding expressions for canonical quintessence., Comment: 6 pages, 1 figure, minor clarifications and corrections

We present a cosmic void catalog using the large-scale structure galaxy catalog from the Baryon Oscillation Spectroscopic Survey (BOSS). This galaxy catalog is part of the Sloan Digital Sky Survey (SDSS) Data Release 12 and is the final catalog of SDSS-III. We take into account the survey boundaries, masks, and angular and radial selection functions, and apply the ZOBOV void finding algorithm to the galaxy catalog. We identify a total of 10,643 voids. After making quality cuts to ensure that the voids represent real underdense regions, we obtain 1,228 voids with effective radii spanning the range 20-100Mpc/h and with central densities that are, on average, 30% of the mean sample density. We release versions of the catalogs both with and without quality cuts. We discuss the basic statistics of voids, such as their size and redshift distributions, and measure the radial density profile of the voids via a stacking technique. In addition, we construct mock void catalogs from 1000 mock galaxy catalogs, and find that the properties of BOSS voids are in good agreement with those in the mock catalogs. We compare the stellar mass distribution of galaxies living inside and outside of the voids, and find no significant difference. These BOSS and mock void catalogs are useful for a number of cosmological and galaxy environment studies., Comment: 7 pages, 7 figures, submitted to the ApJ

We consider a first order formulation of relativistic fluids with bulk viscosity based on a stress-energy tensor introduced by Lichnerowicz. Choosing a barotropic equation of state, we show that this theory satisfies basic physical requirements and, under the further assumption of vanishing vorticity, that the equations of motion are causal, both in the case of a fixed background and when the equations are coupled to Einstein's equations. Furthermore, Lichnerowicz's proposal does not fit into the general framework of first order theories studied by Hiscock and Lindblom, and hence their instability results do not apply. These conclusions apply to the full-fledged non-linear theory, without any equilibrium or near equilibrium assumptions. Similarities and differences between the approach here explored and other theories of relativistic viscosity, including the Mueller-Israel-Stewart formulation, are addressed. Cosmological models based on the Lichnerowicz stress-energy tensor are studied. Finally, it is also shown how the present model can be generalized to a second order formulation., Comment: 43 pages, 1 figure, discussion added, references updated, to appear in IJMPD

We consider cosmological solutions to general relativity with a single barotropic fluid, where the pressure is a general function of the density, $p = f(\rho)$. We derive conditions for static and oscillating solutions and provide examples, extending earlier work to these simpler and more general single-fluid cosmologies. Generically we expect such solutions to suffer from instabilities, through effects such as quantum fluctuations or tunneling to zero size. We also find a classical instability ("no-go" theorem) for oscillating solutions of a single barotropic perfect fluid due to a necessarily negative squared sound speed., Comment: 5 pages; v2: additional references, minor clarification in Sec. IIC, matches version published in JCAP

Grand unified theories based on large groups (with rank greater or equal to 6 are a natural context for dark matter models. They contain Standard-Model-singlet fermions that could be dark matter candidates, and can contain new non-abelian interactions whose sphalerons convert baryons, leptons, and dark matter into each other, "cogenerating" a dark matter asymmetry comparable to the baryon asymmetry. In this paper it is shown that the same non-abelian interactions can "pre-annihilate" the symmetric component of heavy dark matter particles, which then decay late into light stable dark matter particles that inherit their asymmetry. It is shown that such decays can come from d=5 operators that are Planck or GUT suppressed. We derive constraints on such models and present a simple realization based on the group SU(7)., Comment: 14 pages, a fully unified model presented in much greater detail

We examine the Chevallier-Polarski-Linder (CPL) parametrization, in the context of quintessence and barotropic dark energy models, to determine the subset of such models to which it can provide a good fit. The CPL parametrization gives the equation of state parameter $w$ for the dark energy as a linear function of the scale factor $a$, namely $w = w_0 + w_a(1-a)$. In the case of quintessence models, we find that over most of the $w_0$, $w_a$ parameter space the CPL parametrization maps onto a fairly narrow form of behavior for the potential $V(\phi)$, while a one-dimensional subset of parameter space, for which $w_a = \kappa (1+w_0)$, with $\kappa$ constant, corresponds to a wide range of functional forms for $V(\phi)$. For barotropic models, we show that the functional dependence of the pressure on the density, up to a multiplicative constant, depends only on $w_i = w_a + w_0$ and not on $w_0$ and $w_a$ separately. Our results suggest that the CPL parametrization may not be optimal for testing either type of model., Comment: 11 pages, 5 figures, typo corrected in Eq. (17), to appear in Phys. Rev. D

We calculate finite-temperature corrections to the decay rate of a generic neutral (pseudo)scalar particle that decays into (pseudo)scalars or fermion-antifermion pairs. The ratio of the finite-temperature decay rate to the zero-temperature decay rate is presented. Thermal effects are largest in the limit where the decaying particle is nonrelativistic but with a mass well below the background temperature, but significant effects are possible even when we relax the former assumption. Thermal effects are reduced for the case of nonzero momentum of the decaying particle. We discuss cosmological scenarios under which significant finite-temperature corrections to the decay rate can be achieved., Comment: 21 pages, 5 figures, v2: version to appear in PRD

We examine the cosmological consequences of an alternative to the standard expression for bulk viscosity, one which was proposed to avoid the propagation of superluminal signals without the necessity of extending the space of variables of the theory. The Friedmann equation is derived for this case, along with an expression for the effective pressure. We find solutions for the evolution of the density of a viscous component, which differs markedly from the case of conventional Eckart theory; our model evolves toward late-time phantom-like behavior with a future singularity. Entropy production is addressed, and some similarities and differences to approaches based on the Mueller-Israel-Stewart theory are discussed., Comment: Text and references expanded. To appear in Physical Review D

We examine quintessence models for dark energy in which the scalar field, $\phi$, evolves near the vicinity of a local maximum or minimum in the potential $V(\phi)$, so that $V(\phi)$ be approximated by a quadratic function of $\phi$ with no linear term. We generalize previous studies of this type by allowing the initial value of $d \phi/dt$ to be nonzero. We derive an analytic approximation for $w(a)$ and show that it is in excellent agreement with numerical simulations for a variety of scalar field potentials having local minima or maxima. We derive an upper bound on the present-day value of $w$ as a function of the other model parameters and present representative limits on these models from observational data. This work represents a final generalization of previous studies using linear or quadratic approximations for $V(\phi)$., Comment: 16 pages, 12 figures, added discussion of observational constraints on these models

We use cosmological N-body simulations to investigate whether measurements of the moments of large-scale structure can yield constraints on primordial non-Gaussianity. We measure the variance, skewness, and kurtosis of the evolved density field from simulations with Gaussian and three different non-Gaussian initial conditions: a local model with f_NL=100, an equilateral model with f_NL=-400, and an orthogonal model with f_NL=-400. We show that the moments of the dark matter density field differ significantly between Gaussian and non-Gaussian models. We also make the measurements on mock galaxy catalogs that contain galaxies with clustering properties similar to those of luminous red galaxies (LRGs). We find that, in the case of skewness and kurtosis, galaxy bias reduces the detectability of non-Gaussianity, though we can still clearly discriminate between different models in our simulation volume. However, in the case of the variance, galaxy bias greatly amplifies the detectability of non-Gaussianity. In all cases we find that redshift distortions do not significantly affect the detectability. When we restrict our measurements to volumes equivalent to the Sloan Digital Sky Survey II (SDSS-II) or Baryon Oscillation Spectroscopic Survey (BOSS) samples, the probability of detecting a departure from the Gaussian model is high by using measurements of the variance, but very low by using only skewness and kurtosis measurements. We find that skewness and kurtosis measurements are never likely to yield useful constraints on primordial non-Gaussianity, but future surveys should be large enough to place meaningful constraints using measurements of the galaxy variance.(Abridged)

The anapole moment is the only allowed electromagnetic moment for Majorana fermions. Fermionic dark matter acquiring an anapole can have a standard thermal history and be consistent with current direct detection experiments. In this paper, we calculate the collider monojet signatures of anapole dark matter and show that the current LHC results exclude anapole dark matter with mass less than 100 GeV, for an anapole coupling that leads to the correct thermal relic abundance., Comment: 11 pages, 3 figures, v2: version to appear in PRD

We examine models in which the accelerated expansion of the universe is driven by a scalar field rolling near an inflection point in the potential. For the simplest such models, in which the potential is of the form V(\phi) = V_0 + V_3 (\phi-\phi_0)^3, the scalar field can either evolve toward \phi = \phi_0 at late times, yielding an asymptotic de Sitter expansion, or it can transition through the inflection point, producing a transient period of acceleration. We determine the parameter ranges which produce each of these two possibilities and also map out the region in parameter space for which the equation of state of the scalar field is close to -1 at all times up to the present, mimicking \LambdaCDM. We show that the latter can be consistent with either eternal or transient acceleration. More complicated inflection point models are also investigated., Comment: 5 pages, 2 figures, clarification and additional figure added, to appear in Phys. Rev. D

Short baseline neutrino experiments such as LSND and MiniBooNE seem to suggest the existence of light sterile neutrinos. Meanwhile, current cosmic microwave background (CMB) and big bang nucleosynthesis (BBN) measurements place an upper bound on the effective number of light neutrinos, $N_{eff}$ and the PLANCK satellite will measure $N_{eff}$ to a much higher accuracy and further constrain the number of sterile neutrinos allowed. We demonstrate that if an MeV dark matter particle couples more strongly to electrons and/or photons than to neutrinos, then p-wave annihilation after neutrino decoupling can reduce the value of $N_{eff}$ inferred from BBN and PLANCK. This mechanism can accommodate two eV sterile neutrinos even if PLANCK observes $N_{eff}$ as low as the standard model theoretical value of 3.046, and a large neutrino asymmetry is not needed to obtain the correct primordial element abundances. The dark matter annihilation also weakens the cosmological upper bounds on the neutrino masses, and we derive a relationship between the change in these bounds and the corresponding change in $N_{eff}$. Dark matter with an electric dipole moment or anapole moment is a natural candidate that exhibits the desired properties for this mechanism. Coincidentally, a dark matter particle with these properties and lighter than 3 MeV is precisely one that can explain the 511 keV gamma-ray line observed by INTEGRAL. We show that the addition of two eV sterile neutrinos allows this kind of dark matter to be lighter than 3 MeV, which is otherwise ruled out by the CMB bound on $N_{eff}$ if only active neutrinos are considered., Comment: 21 pages, 1 figure, v3: version to appear in PRD with updated references; v2: updated with a detailed discussion about cosmological upper bounds on neutrino masses

We consider dark matter (DM) that interacts with ordinary matter exclusively through an electromagnetic anapole, which is the only allowed electromagnetic form factor for Majorana fermions. We show that unlike DM particles with an electric or magnetic dipole moment, anapole dark matter particles annihilate exclusively into fermions via purely p-wave interactions, while tree-level annihilations into photons are forbidden. We calculate the anapole moment needed to produce a thermal relic abundance in agreement with cosmological observations, and show that it is consistent with current XENON100 detection limits on the DM-nucleus cross-section for all masses, while lying just below the detection threshold for a mass ~ 30-40 GeV., Comment: 7 pages, 5 figures, v3: version to appear in PLB

Thermal dark matter that couples more strongly to electrons and photons than to neutrinos will heat the electron-photon plasma relative to the neutrino background if it becomes nonrelativistic after the neutrinos decouple from the thermal background. This results in a reduction in N_eff below the standard-model value, a result strongly disfavored by current CMB observations. Taking conservative lower bounds on N_eff and on the decoupling temperature of the neutrinos, we derive a bound on the dark matter particle mass of m_\chi > 3-9 MeV, depending on the spin and statistics of the particle. For p-wave annihilation, our limit on the dark matter particle mass is stronger than the limit derived from distortions to the CMB fluctuation spectrum produced by annihilations near the epoch of recombination., Comment: 5 pages, 1 figure, discussion added, references added and updated, labels added to figure, to appear in Phys. Rev. D

We examine cyclic phantom models for the universe, in which the universe is dominated sequentially by radiation, matter, and a phantom dark energy field, followed by a standard inflationary phase. Since this cycle repeats endlessly, the Universe spends a substantial portion of its lifetime in a state for which the matter and dark energy densities have comparable magnitudes, thus ameliorating the coincidence problem. We calculate the fraction of time that the universe spends in such a coincidental state and find that it is nearly the same as in the case of a phantom model with a future big rip. In the limit where the dark energy equation of state parameter, w, is close to -1, we show that the fraction of time, f, for which the ratio of the dark energy density to the matter density lies between r_1 and r_2, is f = -(1+w) ln [(\sqrt{r_2} + \sqrt{1+r_2})/(\sqrt{r_1} + \sqrt{1+r_1})]., Comment: 4 pages, no figures, discussion and references added

Dark energy models with various scenarios of evolution are considered from the viewpoint of the formalism for the equation of state. It is shown that these models are compatible with current astronomical data. Some of the models presented here evolve arbitrarily close to $\Lambda$CDM up to the present, but diverge in the future into a number of different possible asymptotic states, including asymptotic de-Sitter (pseudo-rip) evolution, little rips with disintegration of bound structures, and various forms of finite-time future singularities. Therefore it is impossible from observational data to determine whether the universe will end in a future singularity or not. We demonstrate that the models under consideration are stable for a long period of time (billions of years) before entering a Little Rip/Pseudo-Rip induced dissolution of bound structures or before entering a soft finite-time future singularity. Finally, the physical consequences of Little Rip, Type II, III and Big Crush singularities are briefly compared., Comment: 15 pages, 1 figure, version to appear in Physics Letters B

If we assume that the cosmic energy density will remain constant or strictly increase in the future, then the possible fates for the universe can be divided into four categories based on the time asymptotics of the Hubble parameter H(t): the cosmological constant, for which H(t) = constant, the big rip, for which H(t) goes to infinity at finite time, the little rip, for which H(t) goes to infinity as time goes to infinity, and the pseudo-rip, for which H(t) goes to a constant as time goes to infinity. Here we examine the last of these possibilities in more detail. We provide models that exemplify the pseudo-rip, which is an intermediate case between the cosmological constant and the little rip. Structure disintegration in the pseudo-rip depends on the model parameters. We show that pseudo-rip models for which the density and Hubble parameter increase monotonically can produce an inertial force which does not increase monotonically, but instead peaks at a particular future time and then decreases., Comment: 4 pages, 2 figures, title changed to agree with published version

Cosmic microwave background (CMB) observations suggest the possibility of an extra dark radiation component, while the current evidence from big bang nucleosynthesis (BBN) is more ambiguous. Dark radiation from a decaying particle can affect these two processes differently. Early decays add an additional radiation component to both the CMB and BBN, while late decays can alter the radiation content seen in the CMB while having a negligible effect on BBN. Here we quantify this difference and explore the intermediate regime by examining particles decaying during BBN, i.e., particle lifetimes \tau_X satisfying 0.1 sec < \tau_X < 1000 sec. We calculate the change in the effective number of neutrino species, N_{eff}, as measured by the CMB, \Delta N_{CMB}, and the change in the effective number of neutrino species as measured by BBN, \Delta N_{BBN}, as a function of the decaying particle initial energy density and lifetime, where \Delta N_{BBN} is defined in terms of the number of additional two-component neutrinos needed to produce the same change in the primordial helium-4 abundance as our decaying particle. As expected, for short lifetimes (\tau_X < 0.1 sec), the particles decay before the onset of BBN, and \Delta N_{CMB} = \Delta N_{BBN}, while for long lifetimes (\tau_X > 1000 sec), \Delta N_{BBN} is dominated by the energy density of the nonrelativistic particles before they decay, so that \Delta N_{BBN} remains nonzero and becomes independent of the particle lifetime. By varying both the particle energy density and lifetime, one can obtain any desired combination of \Delta N_{BBN} and \Delta N_{CMB}, subject to the constraint that \Delta N_{CMB} >= \Delta N_{BBN}. We present limits on the decaying particle parameters derived from observational constraints on \Delta N_{CMB} and \Delta N_{BBN}., Comment: 6 pages, 3 figures, added figure and discussion of observational constraints, to appear in Phys. Rev. D

We examine in more detail specific models which yield a little rip cosmology, i.e., a universe in which the dark energy density increases without bound but the universe never reaches a finite-time singularity. We derive the conditions for the little rip in terms of the inertial force in the expanding universe and present two representative models to illustrate in more detail the difference between little rip models and those which are asymptotically de Sitter. We derive conditions on the equation of state parameter of the dark energy to distinguish between the two types of models. We show that coupling between dark matter and dark energy with a little rip equation of state can alter the evolution, changing the little rip into an asymptotic de Sitter expansion. We give conditions on minimally-coupled phantom scalar field models and on scalar-tensor models that indicate whether or not they correspond to a little rip expansion. We show that, counterintuitively, despite local instability, a little-rip cosmology has an infinite lifetime., Comment: LaTeX, 10 pages, no figure, version to appear in Phys.Lett B

We examine models in which the dark energy density increases with time (so that the equation-of-state parameter w satisfies w < -1), but w approaches -1 asymptotically, such that there is no future singularity. We refine previous calculations to determine the conditions necessary to produce this evolution. Such models can display arbitrarily rapid expansion in the near future, leading to the destruction of all bound structures (a "little rip"). We determine observational constraints on these models and calculate the point at which the disintegration of bound structures occurs. For the same present-day value of w, a big rip with constant w disintegrates bound structures earlier than a little rip., Comment: 5 pages, 1 figure

We examine the evolution of quintessence models with potentials satisfying (V'/V)^2<<1 and V"/V<<1, in the case where the initial field velocity is nonzero. We derive an analytic approximation for the evolution of the equation of state parameter, w, for the quintessence field. We show that such models are characterized by an initial rapid freezing phase, in which the equation of state parameter w decreases with time, followed by slow thawing evolution, for which w increases with time. These models resemble constant-V models at early times but diverge at late times. Our analytic approximation gives results in excellent agreement with exact numerical evolution., Comment: 6 pages, 5 figures, references added