Is our universe one of many?

BY KER THAN
As physicists have delved deeper and deeper into nature’s mysteries, they have been forced to accept the unsettling fact that our universe is suspiciously fine-tuned to support life. The amount of matter in the universe, the mass of the electron, the strength of gravity – if the value of any of these deviated only a tiny bit from what they actually are, then galaxies and stars could not form and biological life could not exist. The best theory that physicists have come up with to explain this cosmic coincidence is called the String Theory Landscape.

The String Theory Landscape combines elements from two of the strangest and most enduring ideas in modern physics – string theory and cosmic inflation – to argue that we live in a multiverse made up of infinitely many “pocket universes,” of which our perfectly calibrated universe is just one. This five-part series tells the story of how theoretical physicists at Stanford helped develop the String Theory Landscape – and in the process sparked a fierce and still ongoing debate about what science is and what it should be…

Read more at https://news.stanford.edu/2018/09/10/landscape-theory/

Introduction to neutrino astronomy

neutrino flux

Energy dependence of the neutrino fluxes produced by the different nuclear processes in the Sun

Andrea Gallo Rosso, Carlo Mascaretti, Andrea Palladino, Francesco Vissani
This writeup is an introduction to neutrino astronomy, addressed to astronomers and written by astroparticle physicists. While the focus is on achievements and goals in neutrino astronomy, rather than on the aspects connected to particle physics, we will introduce the particle physics concepts needed to appreciate those aspects that depend on the peculiarity of the neutrinos. The detailed layout is as follows: In Sect.~1, we introduce the neutrinos, examine their interactions, and present neutrino detectors and telescopes. In Sect.~2, we discuss solar neutrinos, that have been detected and are matter of intense (theoretical and experimental) studies. In Sect.~3, we focus on supernova neutrinos, that inform us on a very dramatic astrophysical event and can tell us a lot on the phenomenon of gravitational collapse. In Sect.~4, we discuss the highest energy neutrinos, a very recent and lively research field. In Sect.~5, we review the phenomenon of neutrino oscillations and assess its relevance for neutrino astronomy. Finally, we offer a brief overall assessment and a summary in Sect.~6. The material is selected – i.e., not all achievements are reviewed – and furthermore it is kept to an introductory level, but efforts are made to highlight current research issues. In order to help the beginner, we prefer to limit the list of references, opting whenever possible for review works and books.

Read more at https://arxiv.org/pdf/1806.06339.pdf

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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

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Lets Talk About Black Hole Singularities

Abraham Loeb
Does the collision of black hole singularities imprint an observable quantum signature on the resulting gravitational wave signal?

The singularities at the centers of astrophysical black holes mark the breakdown of Einstein’s theory of gravity, General Relativity. They represent the only breakdown sites accessible to experimentalists, since the other known singularity, the Big Bang,is believed to be invisible due to the vast expansion that occurred afterwards during cosmic inflation…
Read more https://arxiv.org/ftp/arxiv/papers/1805/1805.05865.pdf

Is Dark Matter Made of Primordial Black Holes?

Astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize in the last century that most of the matter in the universe was not visible. About 84% of the matter in the cosmos is dark matter, much of it located in halos around galaxies. It was dubbed dark matter because it does not emit light, but it is also mysterious: it is not composed of atoms or their usual constituents like electrons and protons.

Meanwhile, astronomers have observed the effects of black holes and recently even detected gravitational waves from a pair of merging black holes. Black holes usually are formed in the explosive death of massive stars, a process that can take many hundreds of millions of years as a star coalesces from ambient gas, evolves and finally dies. Some black holes are inferred to exist in the early universe, but there is probably is not enough time in the early universe for the normal formation process to occur. Some alternative methods have been proposed, like the direct collapse of primordial gas or processes associated with cosmic inflation, and many of these primordial black holes could have been made.

CfA astronomer Qirong Zhu led a group of four scientists investigating the possibility that today’s dark matter is composed of primordial black holes, following up on previously published suggestions. If galaxy halos are made of black holes, they should have a different density distribution than halos made of exotic particles. There are some other differences as well – black hole halos are expected to form earlier in a galaxy’s evolution than do some other kinds of halos. The scientists suggest that looking at the stars in the halos of faint dwarf galaxies can probe these effects because dwarf galaxies are small and faint (they shine with a mere few thousand solar luminosities) where slight effects can be more easily spotted. The team ran a set of computer simulations to test whether dwarf galaxy halos might reveal the presence of primordial black holes, and they find that they could: interactions between stars and primordial halo black holes should slightly alter the sizes of the stellar distributions. The astronomers also conclude that such black holes would need to have masses between about two and fourteen solar masses, right in the expected range for these exotic objects (although smaller than the black holes recently spotted by gravitational wave detectors) and comparable to the conclusions of other studies. The team emphasizes, however, that all the models are still inconclusive and the nature of dark matter remains elusive.

Reference(s):
“Primordial Black Holes as Dark Matter: Constraints from Compact Ultra-faint Dwarfs,” Qirong Zhu, Eugene Vasiliev, Yuexing Li, and Yipeng Jing, MNRAS 476, 2, 2018 (https://arxiv.org/abs/1710.05032)

https://www.cfa.harvard.edu/news/su201816