Exploring Gravitational Lensing

Einstein’s derivation of the lensing equation, solution, and amplification in AEA 62-275 (Albert Einstein Archives, The Hebrew University of Jerusalem, Israel)

Tilman Sauer, Tobias Schütz
In this article, we discuss the idea of gravitational lensing, from a systematic, historical and didactic point of view. We show how the basic lensing equation together with the concepts of geometrical optics opens a space of implications that can be explored along different dimensions. We argue that Einstein explored the idea along different pathways in this space of implication, and that these explorations are documented by different calculational manuscripts. The conceptualization of the idea of gravitational lensing as a space of exploration also shows the feasibility of discussing the idea in the classroom using some of Einstein’s manuscripts.
Read more https://arxiv.org/pdf/1905.07174.pdf

Is The Starry Night Turbulent?

The Starry Night
Vincent van Gogh (1889)


James Beattie, Neco Kriel
Vincent van Gogh’s painting, The Starry Night, is an iconic piece of art and cultural history. The painting portrays a night sky full of stars, with eddies (spirals) both large and small. Kolmogorov1941’s description of subsonic, incompressible turbulence gives a model for turbulence that involves eddies interacting on many length scales, and so the question has been asked: is The Starry Night turbulent? To answer this question, we calculate the azimuthally averaged power spectrum of a square region (1165×1165 pixels) of night sky in The Starry Night. We find a power spectrum, P(k), where k is the wavevector, that shares the same features as supersonic turbulence. It has a power-law P(k)∝k2.1±0.3 in the scaling range, 34≤k≤80. We identify a driving scale, kD=3, dissipation scale, kν=220 and a bottleneck. This leads us to believe that van Gogh’s depiction of the starry night closely resembles the turbulence found in real molecular clouds, the birthplace of stars in the Universe.

Read more at https://arxiv.org/pdf/1902.03381.pdf

Aside

Gravitational Waves: A New Astronomy


Luc Blanchet
Contemporary astronomy is undergoing a revolution, perhaps even more important than that which took place with the advent of radioastronomy in the 1960s, and then the opening of the sky to observations in the other electromagnetic wavelengths. The gravitational wave detectors of the LIGO/Virgo collaboration have observed since 2015 the signals emitted during the collision and merger of binary systems of massive black holes at a large astronomical distance. This major discovery opens the way to the new astronomy of gravitational waves, drastically different from the traditional astronomy based on electromagnetic waves. More recently, in 2017, the detection of gravitational waves emitted by the inspiral and merger of a binary system of neutron stars has been followed by electromagnetic signals observed by the γ and X satellites, and by optical telescopes. A harvest of discoveries has been possible thanks to the multi-messenger astronomy, which combines the information from the gravitational wave with that from electromagnetic waves. Another important aspect of the new gravitational astronomy concerns fundamental physics, with the tests of general relativity and alternative theories of gravitation, as well as the standard model of cosmology.
Read more at https://arxiv.org/pdf/1805.08563.pdf

Neutrino Astronomy with IceCube and Beyond

Kevin J. Meagher on behalf of the IceCube Collaboration
The IceCube Neutrino Observatory is a cubic kilometer neutrino telescope located at the geographic South Pole. Cherenkov radiation emitted by charged secondary particles from neutrino interactions is observed by IceCube using an array of 5160 photomultiplier tubes embedded between a depth of 1.5 km to 2.5 km in the Antarctic glacial ice. The detection of astrophysical neutrinos is a primary goal of IceCube and has now been realized with the discovery of a diffuse, high-energy flux consisting of neutrino events from tens of TeV up to several PeV. Many analyses have been performed to identify the source of these neutrinos, including correlations with active galactic nuclei, gamma-ray bursts, and the Galactic plane. IceCube also conducts multi-messenger campaigns to alert other observatories of possible neutrino transients in real time. However, the source of these neutrinos remains elusive as no corresponding electromagnetic counterparts have been identified. This proceeding will give an overview of the detection principles of IceCube, the properties of the observed astrophysical neutrinos, the search for corresponding sources (including real-time searches), and plans for a next-generation neutrino detector, IceCube-Gen2.
Read more at https://arxiv.org/pdf/1705.00383.pdf

Fast Radio Bursts from Extragalactic Light Sails

Manasvi Lingam, Abraham Loeb
We examine the possibility that Fast Radio Bursts (FRBs) originate from the activity of extragalactic civilizations.
Our analysis shows that beams used for powering large light sails could yield parameters that are consistent with FRBs.
The characteristic diameter of the beam emitter is estimated through a combination of energetic and engineering constraints, and both approaches intriguingly yield a similar result which is on the scale of a large rocky planet.
Moreover, the optimal frequency for powering the light sail is shown to be similar to the detected FRB frequencies. These `coincidences’ lend some credence to the possibility that FRBs might be artificial in origin.
Other relevant quantities, such as the characteristic mass of the light sail, and the angular velocity of the beam, are also derived.
By using the FRB occurrence rate, we infer upper bounds on the rate of FRBs from extragalactic civilizations in a typical galaxy.
The possibility of detecting fainter signals is briefly discussed, and the wait time for an exceptionally bright FRB event in the Milky Way is estimated.
Read more at https://arxiv.org/pdf/1701.01109.pdf