# One Hundred Years of the Cosmological Constant: from 'Superfluous Stunt' to Dark Energy [CL]

We present a centennial review of the history of the term known as the cosmological constant. First introduced to the general theory of relativity by Einstein in 1917 in order to describe a universe that was assumed to be static, the term fell from favour in the wake of the discovery of cosmic the expanding universe, only to make a dramatic return in recent times. We consider historical and philosophical aspects of the cosmological constant over four main epochs: (i) the use of the term in static cosmologies (both Newtonian and relativistic; (ii) the marginalization of the term following the discovery of cosmic expansion; (iii) the use of the term to address specific cosmic puzzles such as the timespan of expansion, the formation of galaxies and the redshifts of the quasars; (iv) the re-emergence of the term in today’s Lamda-CDM cosmology. We find that the cosmological constant was never truly banished from theoretical models of the universe, but was sidelined by astronomers for reasons of convenience. We also find that the return of the term to the forefront of modern cosmology did not occur as an abrupt paradigm shift due to one particular set of observations, but as the result of a number of empirical advances such as the measurement of present cosmic expansion using the Hubble Space Telescope, the measurement of past expansion using type SN 1a supernovae as standard candles, and the measurement of perturbations in the cosmic microwave background by balloon and satellite. We give a brief overview of contemporary interpretations of the physics underlying the cosmic constant and conclude with a synopsis of the famous cosmological constant problem.

C. ORaifeartaigh, M. OKeeffe, W. Nahm, et. al.
Tue, 21 Nov 17
1/79

Comments: 60 pages, 5 figures. Submitted to the European Physical Journal (H)

# Synthetic observations of protostellar multiple systems [SSA]

Observations of protostars are often compared with synthetic observations of models in order to infer the underlying physical properties of the protostars. The majority of these models have a single protostar, attended by a disc and an envelope. However, observational and numerical evidence suggests that a large fraction of protostars form as multiple systems. This means that fitting models of single protostars to observations may be inappropriate. We produce synthetic observations of protostellar multiple systems undergoing realistic, non-continuous accretion. These systems consist of multiple protostars with episodic luminosities, embedded self-consistently in discs and envelopes. We model the gas dynamics of these systems using smoothed particle hydrodynamics and we generate synthetic observations by post-processing the snapshots using the \textsc{spamcart} Monte Carlo radiative transfer code. We present simulation results of three model protostellar multiple systems. For each of these, we generate $4\times10^4$ synthetic spectra at different points in time and from different viewing angles. We propose a Bayesian method, using similar calculations to those presented here, but in greater numbers, to infer the physical properties of protostellar multiple systems from observations.

O. Lomax and A. Whitworth
Tue, 21 Nov 17
2/79

Comments: 13 pages. Accepted by MNRAS

# Relieving the Tension between Weak Lensing and Cosmic Microwave Background with Interacting Dark Matter and Dark Energy Models [CEA]

We constrain interacting dark matter and dark energy (IDMDE) models using a 450-degree-square cosmic shear data from the Kilo Degree Survey (KiDS) and the angular power spectra from Planck’s latest cosmic microwave background measurements. We revisit the discordance problem in the standard Lambda cold dark matter ($\Lambda$CDM) model between weak lensing and Planck datasets and extend the discussion by introducing interacting dark sectors. The IDMDE models are found to be able to alleviate the discordance between KiDS and Planck as previously inferred from the $\Lambda$CDM model, and moderately favored by a combination of the two datasets.

R. An, C. Feng and B. Wang
Tue, 21 Nov 17
3/79

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# Covariance of the galaxy angular power spectrum with the halo model [CEA]

As the determination of density fluctuations becomes more precise with larger surveys, it becomes more important to account for the increased covariance due to the non-linearity of the field. Here I focus on the galaxy density, with analytical prediction of the non-Gaussianity using the halo model coupled with standard perturbation theory. I carry out an exact and exhaustive derivation of all tree-level terms of the non-Gaussian covariance of the galaxy $C_\ell$, with the computation developed up to third order in perturbation theory and local halo bias, including the non-local tidal tensor effect. A diagrammatic method is used to derive the involved galaxy 3D trispectra, including shot-noise contributions. The projection to the angular covariance is derived in all trispectra cases with and without Limber’s approximation, with the formulae being of potential interest for other observables than galaxies. The effect of substracting shot-noise from the measured spectrum is also discussed, and does simplify the covariance, though some non-Gaussian shot-noise terms still remain. I make the link between this complete derivation and partial terms which have been used previously in the literature, including super-sample covariance (SSC). I uncover a wealth of additional terms which were not previously considered, include a whole new class which I dub braiding terms as it contains multipole-mixing kernels. The importance of all these new terms is discussed with analytical arguments. I find that they can become comparable to, if not bigger than, SSC if the survey is large or deep enough to probe scales comparable with the matter-radiation equality $k_\mathrm{eq}$. A short self-contained summary of the equations is provided in Section 9 for the busy reader, ready to be implemented numerically for analysis of current and future galaxy surveys.

F. Lacasa
Tue, 21 Nov 17
4/79

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# On the patchiness of the individual pulse spectra at the very low radio frequencies [HEAP]

We have used sensitive LOw Frequency ARray (LOFAR) observations of PSR B0809+74 at 15–62 MHz to study the anomalously intensive pulses, first reported by Ulyanov et al. (2006) at 18–30MHz. Similarly to Ulyanov et al., we found that the spectra of strong pulses consist of distinct bright patches. Moreover, these spectral patches were spotted to drift upwards in frequency over the course of several pulse sequences. We established that this drift is not pulsar-intrinsic, but is caused by the broadband ~20 second-long enhancements of recorded signal, which influenced the dispersed tracks of several pulses at once. We speculate on the cause of such enhancements (i.e. propagation or telescope-related) and the ramifications they bring to the single-pulse studies at the very low radio frequencies. Depending on the origin, the phenomenon may also affect the analysis of highly dispersed single pulses at higher radio frequencies, e.g. Fast Radio Bursts.

X. Song, V. Kondratiev and A. Bilous
Tue, 21 Nov 17
5/79

Comments: 2 pages, 2 figures, to appear in the proceedings of “IAUS 337: Pulsar Astrophysics – The Next 50 Years” eds: P. Weltevrede, B.B.P. Perera, L. Levin Preston & S. Sanidas. Poster available at: this http URL

# On the collisionless asymmetric magnetic reconnection rate [CL]

A prediction of the steady-state reconnection electric field in asymmetric reconnection is obtained by maximizing the reconnection rate as a function of the opening angle made by the upstream magnetic field on the weak magnetic field (magnetosheath) side. The prediction is within a factor of two of the widely examined asymmetric reconnection model [Cassak and Shay, Phys. Plasmas 14, 102114, 2007] in the collisionless limit, and they scale the same over a wide parameter regime. The previous model had the effective aspect ratio of the diffusion region as a free parameter, which simulations and observations suggest is on the order of 0.1, but the present model has no free parameters. In conjunction with the symmetric case [Liu et al., Phys. Rev. Lett. 118, 085101, 2017], this work further suggests that this nearly universal number 0.1, essentially the normalized fast reconnection rate, is a geometrical factor arising from maximizing the reconnection rate within magnetohydrodynamic (MHD)-scale constraints.

Y. Liu, M. Hesse, P. Cassak, et. al.
Tue, 21 Nov 17
6/79