Artículos con la etiqueta ‘Cosmology and Extragalactic Astrophysics (astro-ph.CO)’

Ya. B. Zeldovich (1914-1987): Chemist, Nuclear Physicist, Cosmologist

Por • 8 mar, 2014 • Category: Ambiente

Ya.B. Zeldovich was a pre-eminent Soviet physicist whose seminal contributions spanned many fields ranging from physical chemistry to nuclear and particle physics, and finally astrophysics and cosmology. March 8, 2014 marks Zeldovich’s birth centenary, and this article attempts to convey the zest with which Zeldovich did science, and the important role he played in fostering and mentoring a whole generation of talented Scientists.



A Thousand Problems in Cosmology: Horizons

Por • 27 oct, 2013 • Category: Opinion

This is one chapter of the collection of problems in cosmology, in which we assemble the problems that concern one of the most distinctive features of general relativity and cosmology—the horizons. The first part gives an elementary introduction into the concept in the cosmological context, then we move to more formal exposition of the subject and consider first simple, and then composite models, such as Λ CDM. The fourth section elevates the rigor one more step and explores the causal structure of different simple cosmological models in terms of conformal diagrams. The section on black holes relates the general scheme of constructing conformal diagrams for stationary black hole spacetimes. The consequent parts focus on more specific topics, such as the various problems regarding the Hubble sphere, inflation and holography.
This version contains only formulations of 97 problems. The full collection, with solutions included, is available in the form of a wiki-based resource at this http URL The cosmological community is welcome to contribute to its development.



Cosmological networks

Por • 25 oct, 2013 • Category: Leyes

Networks often represent systems that do not have a long history of studies in traditional fields of physics, albeit there are some notable exceptions such as energy landscapes and quantum gravity. Here we consider networks that naturally arise in cosmology. Nodes in these networks are stationary observers uniformly distributed in an expanding open FLRW universe with any scale factor, and two observers are connected if one can causally influence the other. We show that these networks are growing Lorentz-invariant graphs with power-law distributions of node degrees. New links in these networks not only connect new nodes to existing ones, but also appear at a certain rate between existing nodes, as they do in many complex networks.



Multiversality

Por • 30 jul, 2013 • Category: Leyes

Valid ideas that physical reality is vastly larger than human perception of it, and that the perceived part may not be representative of the whole, exist on many levels and have a long history. After a brief general inventory of those ideas and their implications, I consider the cosmological “multiverse” much discussed in recent scientific literature. I review its theoretical and (broadly) empirical motivations, and its disruptive implications for the traditional program of fundamental physics. I discuss the inflationary axion cosmology, which provides an example where firmly rooted, plausible ideas from microphysics lead to a well-characterized “mini-multiverse” scenario, with testable phenomenological consequences.



Is the CMB telling us that dark matter is weaker than weakly interacting?

Por • 9 jul, 2013 • Category: Leyes

If moduli, or other long-lived heavy states, decay in the early universe in part into light and feebly interacting particles (such as axions), these decay products could account for the additional energy density in radiation that is suggested by recent measurements of the CMB. These moduli decays will also, however, alter the expansion history of the early universe, potentially diluting the thermal relic abundance of dark matter. If this is the case, then dark matter particles must annihilate with an even lower cross section than required in the standard thermal scenario (sigma v < 3×10^-26 cm^3/s) if they are to make up the observed density of dark matter. This possibility has significant implications for direct and indirect searches for dark matter.



Why all these prejudices against a constant?

Por • 9 may, 2013 • Category: Educacion

The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the “great mystery” of the “nature of a dark energy”. We argue that these arguments are unconvincing, or ill-founded.



A Thousand Problems in Cosmology. Chapter 4: Black holes

Por • 6 may, 2013 • Category: Ciencia y tecnología

The fourth chapter of the collection of problems in cosmology, devoted to black holes. Consists of basic introduction, sections on Schwarzschild and Kerr black holes, a section on particles’ motion and collisions in general black hole space-times, and the astrophysical part. This version contains only formulations of 137 problems. The full collection, with solutions included, is available in the form of a wiki-based resource at www.universeinproblems.com. The cosmological community is welcome to contribute to its development.



What have we learned from observational cosmology ?

Por • 22 abr, 2013 • Category: Ciencia y tecnología

We review the observational foundations of the $\Lambda$CDM model, considered by most cosmologists as the standard model of cosmology. The Cosmological Principle, a key assumption of the model is shown to be verified with increasing accuracy. The fact that the Universe seems to have expanded from and hot and dense past is supported by many independent probes (galaxy redshifts, Cosmic Microwave Background, Big-Bang Nucleosynthesis and reionization). The explosion of detailed observations in the last few decades has allowed for precise measurements of the cosmological parameters within Friedman-Lema\^itre-Robertson-Walker cosmologies leading to the $\Lambda$CDM model: an apparently flat Universe, dominated by a cosmological constant, whose matter component is dominantly dark. We describe and discuss the various observational probes that led to this conclusion and conclude that the $\Lambda$CDM model, although leaving a number of open questions concerning the deep nature of the constituents of the Universe, provides the best theoretical framework to explain the observations.



Cosmology and Time

Por • 17 abr, 2013 • Category: Filosofía

Time has always played a crucial role in cosmology. I review some of the aspects of the present cosmological model which are more directly related to time, such as: the definition of a cosmic time; the existence of typical timescales and epochs in an expanding universe; the problem of the initial singularity and the origin of time; the cosmological arrow of time.



Fritz Hasenohrl and E = mc^2

Por • 4 abr, 2013 • Category: Ciencia y tecnología

In 1904, the year before Einstein’s seminal papers on special relativity, Austrian physicist Fritz Hasenohrl examined the properties of blackbody radiation in a moving cavity. He calculated the work necessary to keep the cavity moving at a constant velocity as it fills with radiation and concluded that the radiation energy has associated with it an apparent mass such that E = 3/8 mc^2. Also in 1904, Hasenohrl achieved the same result by computing the force necessary to accelerate a cavity already filled with radiation. In early 1905, he corrected the latter result to E = 3/4 mc^2. In this paper, Hasenohrl’s papers are examined from a modern, relativistic point of view in an attempt to understand where he went wrong. The primary mistake in his first paper was, ironically, that he didn’t account for the loss of mass of the blackbody end caps as they radiate energy into the cavity. However, even taking this into account one concludes that blackbody radiation has a mass equivalent of m = 4/3 E/c^2 or m = 5/3 E/c^2 depending on whether one equates the momentum or kinetic energy of radiation to the momentum or kinetic energy of an equivalent mass. In his second and third papers that deal with an accelerated cavity, Hasenohrl concluded that the mass associated with blackbody radiation is m = 4/3 E/c^2, a result which, within the restricted context of Hasenohrl’s gedanken experiment, is actually consistent with special relativity. Both of these problems are non-trivial and the surprising results, indeed, turn out to be relevant to the “4/3 problem” in classical models of the electron. An important lesson of these analyses is that E = mc^2, while extremely useful, is not a “law of physics” in the sense that it ought not be applied indiscriminately to any extended system and, in particular, to the subsystems from which they are comprised.