Promise of Future Searches for Cosmic Topology.

The shortest distance around the Universe through us is unlikely to be much larger than the horizon diameter if microwave background anomalies are due to cosmic topology. We show that observational constraints from the lack of matched temperature circles in the microwave background leave many possibilities for such topologies. We evaluate the detectability of microwave background multipole correlations for sample cases.

Gravitational Glint: Detectable Gravitational Wave Tails from Stars and Compact Objects.

Observations of a merging neutron star binary in both gravitational waves, by the Laser Interferometer Gravitational-Wave Observatory (LIGO), and across the spectrum of electromagnetic radiation, by myriad telescopes, have been used to show that gravitational waves travel in vacuum at a speed that is indistinguishable from that of light to within one part in a quadrillion. However, it has long been expected mathematically that, when electromagnetic or gravitational waves travel through vacuum in a curved spacetime, the waves develop tails that travel more slowly. The associated signal has been thought to be undetectably weak.

Galaxy Correlation Functions Provide a More Robust Cosmological Standard Ruler.

We show how a characteristic length scale imprinted in the galaxy two-point correlation function, dubbed the "linear point," can serve as a comoving cosmological standard ruler. In contrast to the baryon acoustic oscillation peak location, this scale is constant in redshift and is unaffected by nonlinear effects to within 0.5 percent precision.

Departures from the Friedmann-Lemaitre-Robertston-Walker Cosmological Model in an Inhomogeneous Universe: A Numerical Examination.

While the use of numerical general relativity for modeling astrophysical phenomena and compact objects is commonplace, the application to cosmological scenarios is only just beginning. Here, we examine the expansion of a spacetime using the Baumgarte-Shapiro-Shibata-Nakamura formalism of numerical relativity in synchronous gauge. This work represents the first numerical cosmological study that is fully relativistic, nonlinear, and without symmetry.

Using quasars as standard clocks for measuring cosmological redshift.

We report hitherto unnoticed patterns in quasar light curves. We characterize segments of the quasar's light curves with the slopes of the straight lines fit through them. These slopes appear to be directly related to the quasars' redshifts.

Modifying gravity: you cannot always get what you want.

The combination of general relativity (GR) and the Standard Model of particle physics disagrees with numerous observations on scales from our Solar System up. In the canonical concordance model of Lambda cold dark matter (ΛCDM) cosmology, many of these contradictions between theory and data are removed or alleviated by the introduction of three completely independent new components of stress energy--the inflaton, dark matter and dark energy. Each of these in its turn is meant to have dominated (or to currently dominate) the dynamics of the Universe.

Einstein's theory of gravity and the problem of missing mass.

The observed matter in the universe accounts for just 5% of the observed gravity. A possible explanation is that Newton's and Einstein's theories of gravity fail where gravity is either weak or enhanced. The modified theory of Newtonian dynamics (MOND) reproduces, without dark matter, spiral-galaxy orbital motions and the relation between luminosity and rotation in galaxies, although not in clusters.

Why anthropic reasoning cannot predict Lambda.

We revisit anthropic arguments purporting to explain the measured value of the cosmological constant. We argue that different ways of assigning probabilities to candidate universes lead to totally different anthropic predictions. As an explicit example, we show that weighting different universes by the total number of possible observations leads to an extremely small probability for observing a value of Lambda equal to or greater than what we now measure.

Why black hole production in scattering of cosmic ray neutrinos is generically suppressed.

It has been argued that neutrinos originating from ultrahigh energy cosmic rays can produce black holes deep in the atmosphere in models with TeV-scale quantum gravity. Such black-hole events could be observed at the Auger Observatory. However, any phenomenologically viable model with a low scale of quantum gravity must explain how to preserve protons from rapid decay.

Is the low-l microwave background cosmic?

The large-angle (low-l) correlations of the cosmic microwave background exhibit several statistically significant anomalies compared to the standard inflationary cosmology. We show that the quadrupole plane and the three octopole planes are far more aligned than previously thought (99.9% C.

Constraining the topology of the universe.

The first year data from the Wilkinson Microwave Anisotropy Probe are used to place stringent constraints on the topology of the Universe. We search for pairs of circles on the sky with similar temperature patterns along each circle. We restrict the search to back-to-back circle pairs, and to nearly back-to-back circle pairs, as this covers the majority of the topologies that one might hope to detect in a nearly flat universe.

Squeezing MOND into a Cosmological Scenario.

Explaining the effects of dark matter using modified gravitational dynamics (MOND) has for decades been both an intriguing and controversial possibility. By insisting that the gravitational interaction that accounts for the Newtonian force also drives cosmic expansion, one may kinematically identify which cosmologies are compatible with MOND, without explicit reference to the underlying theory so long as the theory obeys Birkhoff's law. We find that the critical acceleration a(0) must have a slight source-mass dependence (a(0) approximately M(1/3)) and that MOND cosmologies are naturally compatible with observed late-time expansion history.

Homogeneity, flatness, and "large" extra dimensions.

We consider a model in which the Universe is the direct product of an ordinary (3+1)-dimensional space--a brane where all the standard model fields are confined-and a compact hyperbolic manifold. The decay of massive Kaluza-Klein modes leads to the injection of bulk entropy into the observable Universe. The large statistical averaging inherent in the collapse of the initial entropy onto the brane smoothes out any initial inhomogeneities in the distribution of matter and of three-curvature.

Compact hyperbolic extra dimensions: branes, kaluza-klein modes, and cosmology.

We reconsider theories with low gravitational (or string) scale M(*) where Newton's constant is generated via new large-volume spatial dimensions, while standard model states are localized to a 3-brane. Utilizing compact hyperbolic manifolds we show that the spectrum of Kaluza-Klein modes is radically altered. This allows the early Universe to evolve normally up to substantial temperatures, and completely negates the astrophysical constraints on M(*).

Measuring the topology of the universe.

Observations of microwave background fluctuations can yield information not only about the geometry of the universe but potentially about the topology of the universe. If the universe is negatively curved, then the characteristic scale for the topology of the universe is the curvature radius. Thus, if we are seeing the effects of the geometry of the universe, we can hope to soon see signatures of the topology of the universe.