Seshadri Nadathur

Beyond ΛCDM: Problems, solutions, and the road ahead

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

Observable gravitational waves from inflation with small field excursions

The detection of primordial gravitational waves, or tensor perturbations, would be regarded as compelling evidence for inflation. The canonical measure of this is the ratio of tensor to scalar perturbations, r. For single-field slow-roll models of inflation with small field excursions, the Lyth bound dictates that if the evolution of the slow-roll parameter epsilon is monotonic, the tensor-to-scalar ratio must be below observationally detectable levels. We describe how non-monotonic evolution of epsilon can evade the Lyth bound and generate observationally large r, even with small field excursions. This has consequences for the scalar power spectrum as it necessarily predicts an enhancement in the spectrum at very small scales and significant scale-dependent running at CMB scales. This effect has not been appropriately accounted for in previous analyses. We describe a mechanism that will generically produce the required behaviour in epsilon and give an example of this mechanism arising in a well-motivated small-field model. This model can produce r\geq0.05 while satisfying all current observational constraints.

The stacked ISW signal of rare superstructures in ΛCDM

A detection of the stacked integrated Sachs-Wolfe (ISW) signal in the CMB of rare superstructures identified in the SDSS Luminous Red Galaxy catalogue has been reported at very high statistical significance. The magnitude of the observed signal has previously been argued to be more than 3 sigma larger than the theoretical Lambda CDM expectation. However, this calculation was made in the linear approximation, and relied on assumptions that may potentially have caused the Lambda CDM expectation to be underestimated. Here we update the theoretical model calculation and compare it with an analysis of ISW maps obtained from N-body simulations of a Lambda CDM universe. The differences between model predictions and the map analyses are found to be small and cannot explain the discrepancy with observation, which remains at > 3 sigma significance. We discuss the cosmological significance of this anomaly and speculate on the potential of alternative models to explain it.

Can a supervoid explain the cold spot

The discovery of a void of size

Seeing patterns in noise: Gigaparsec-scale 'structures' that do not violate homogeneity

\sim200\;h^{-1}

The Jubilee ISW Project – II. Observed and simulated imprints of voids and superclusters on the cosmic microwave background

Mpc and average density contrast of

Cosmic voids and void lensing in the Dark Energy Survey Science Verification data

\sim-0.1

Self-similarity and universality of void density profiles in simulation and SDSS data

aligned with the Cold Spot direction has been recently reported. It has been argued that, although the first-order integrated Sachs-Wolfe (ISW) effect of such a void on the CMB is small, the second-order Rees-Sciama (RS) contribution exceeds this by an order of magnitude and can entirely explain the observed Cold Spot temperature profile. In this paper we examine this surprising claim using both an exact calculation with the spherically symmetric Lema\^itre-Tolman-Bondi metric, and perturbation theory about a background Friedmann-Robertson-Walker (FRW) metric. We show that both approaches agree well with each other, and both show that the dominant temperature contribution of the postulated void is an unobservable dipole anisotropy. If this dipole is subtracted, we find that the remaining temperature anisotropy is dominated by the linear ISW signal, which is orders of magnitude larger than the second-order RS effect, and that the total magnitude is too small to explain the observed Cold Spot profile. We calculate the density and size of a void that would be required to explain the Cold Spot, and show that the probability of existence of such a void is essentially zero in

Inflection point inflation: WMAP constraints and a solution to the fine tuning problem

\Lambda

The Jubilee ISW project – I. Simulated ISW and weak lensing maps and initial power spectra results

CDM. We identify the importance of \emph{a posteriori} selection effects in the identification of the Cold Spot, but argue that even after accounting for them, a supervoid explanation of the Cold Spot is always disfavoured relative to a random statistical fluctuation on the last scattering surface.