Fifty years of cosmological particle creation [CL]

In the early sixties Leonard Parker discovered that the expansion of the universe can create particles out of the vacuum, opening a new and fruitfull field in physics. We give a historical review in the form of an interview that took place during the Conference ERE2014 (Valencia 1-5, September, 2014).

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L. Parker and J. Navarro-Salas
Fri, 24 Feb 17

Comments: 8 pages

Why Boltzmann Brains Are Bad [CL]

Some modern cosmological models predict the appearance of Boltzmann Brains: observers who randomly fluctuate out of a thermal bath rather than naturally evolving from a low-entropy Big Bang. A theory in which most observers are of the Boltzmann Brain type is generally thought to be unacceptable, although opinions differ. I argue that such theories are indeed unacceptable: the real problem is with fluctuations into observers who are locally identical to ordinary observers, and their existence cannot be swept under the rug by a choice of probability distributions over observers. The issue is not that the existence of such observers is ruled out by data, but that the theories that predict them are cognitively unstable: they cannot simultaneously be true and justifiably believed.

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S. Carroll
Mon, 6 Feb 17

Comments: 27 pages. Invited submission to a volume on Current Controversies in Philosophy of Science, eds. Shamik Dasgupta and Brad Weslake

Einstein's 1917 Static Model of the Universe: A Centennial Review [CL]

We present a historical review of Einstein’s 1917 paper ‘Cosmological Considerations in the General Theory of Relativity’ to mark the centenary of a key work that set the foundations of modern cosmology. We find that the paper followed as a natural next step after Einstein’s development of the general theory of relativity and that the work offers many insights into his thoughts on relativity, astronomy and cosmology. Our review includes a description of the observational and theoretical background to the paper; a paragraph-by-paragraph guided tour of the work; a discussion of Einstein’s views of issues such as the relativity of inertia, the curvature of space and the cosmological constant. Particular attention is paid to little-known aspects of the paper such as Einstein’s failure to test his model against observation, his failure to consider the stability of the model and a mathematical oversight in his interpretation of the role of the cosmological constant. We discuss the insights provided by Einstein’s reaction to alternate models of the universe proposed by Willem de Sitter, Alexander Friedman and Georges Lema\^itre. Finally, we consider the relevance of Einstein’s static model of the universe for today’s ’emergent’ cosmologies.

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C. ORaifeartaigh, M. OKeeffe, W. Nahm, et. al.
Thu, 26 Jan 17

Comments: Submitted to the European Physical Journal (H).70-page review, 4 figures

Evidence for a White-light Flare on 10 September 1886 [SSA]

We present evidence for the occurrence of a white-light flare on 10 September 1886. It represents the third of such rare events reported in the history of astronomy. The flare was mentioned by Valderrama (1886, L’Astronomie 5, 388). In this article we have used the original logbook of the observer, J. Valderrama y Aguilar, an amateur astronomer that lived in Madrid and Santa Cruz de Tenerife at that time.

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J. Vaquero, M. Vazquez and J. Almeida
Tue, 24 Jan 17

Comments: 11 pages, 3 figures, accepted for publication in Solar Physics

A maximum magnetic moment to angular momentum conjecture [CL]

Conjectures play a central role in theoretical physics, especially those that assert an upper bound to some dimensionless ratio of physical quantities. In this paper we introduce a new such conjecture bounding the ratio of the magnetic moment to angular momentum in nature. We also discuss the current status of some old bounds on dimensionless and dimensional quantities in arbitrary spatial dimension. Our new conjecture is that the dimensionless Schuster-Wilson-Blackett number, c{\mu}/JG^{(1/2)}, where {\mu} is the magnetic moment and J is the angular momentum, is bounded above by a number of order unity. We verify that such a bound holds for charged rotating black holes in those theories for which exact solutions are available, including the Einstein-Maxwell theory, Kaluza-Klein theory, the Kerr-Sen black hole, and the so-called STU family of charged rotating supergravity black holes. We also discuss the current status of the Maximum Tension Conjecture, the Dyson Luminosity Bound, and Thorne’s Hoop Conjecture.

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J. Barrow and G. Gibbons
Tue, 24 Jan 17

Comments: 24 pages, no figures

The role of cosmology in modern physics [CL]

Subject of this article is the relationship between modern cosmology and fundamental physics, in particular general relativity as a theory of gravity on one side, together with its unique application in cosmology, and the formation of structures and their statistics on the other. It summarises arguments for the formulation for a metric theory of gravity and the uniqueness of the construction of general relativity. It discusses symmetry arguments in the construction of Friedmann-Lema\^itre cosmologies as well as assumptions in relation to the presence of dark matter, when adopting general relativity as the gravitational theory. A large section is dedicated to $\Lambda$CDM as the standard model for structure formation and the arguments that led to its construction, and to the of role statistics and to the problem of scientific inference in cosmology as an empirical science. The article concludes with an outlook on current and future developments in cosmology.

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B. Schaefer
Wed, 18 Jan 17

Comments: 9 pages, invited contribution to the workshop “Why trust a theory?”, Dec.2015 in Munich

Records of sunspot and aurora activity during 581-959 CE in Chinese official histories in the periods of Suí, Táng, and the Five Dynasties and Ten Kingdoms [CL]

Recent studies of radioisotopes in tree rings or ice cores suggest that extreme space weather events occurred in the pre-telescope age. Observational records of naked-eye sunspots and low-latitude auroras in historical documents in pre-telescopic age can provide useful information on past solar activity. In this paper, we present the results of a comprehensive survey of records of sunspots and auroras in Chinese official histories from the 6th century to the 10th century, in the period of Su\’i, T\’ang, the Five Dynasties and Ten Kingdoms. These official histories contain records of continuous observations with well-formatted reports conducted under the policy of the government. A brief comparison of the frequency of observations of sunspots and auroras with the observations of radioisotopes as an indicator of solar activity during the corresponding periods is provided. Based on our data, we survey and compile the records of sunspots and auroras in historical documents from variouslocations and in several languages, and ultimately provide these as open data to the scientific community.

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H. Tamazawa, A. Kawamura, H. Hayakawa, et. al.
Tue, 13 Dec 16

Comments: 2016/12/10 accepted for publication in PASJ