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Post Info TOPIC: Space-time
Blobrana


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RE: Space-time
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Title: Foam-like structure of the Universe
Authors: A.A.Kirillov, D.Turaev

On the quantum stage spacetime had the foam-like structure. When the Universe cools, the foam structure tempers and does not disappear. We show that effects caused by the foamed structure mimic very well the observed Dark Matter phenomena. Moreover, we show that in a foamed space photons undergo a chaotic scattering and together with every discrete source of radiation we should observe a diffuse halo. We show that the distribution of the diffuse halo of radiation around a point-like source repeats exactly the distribution of dark matter around the same source, i.e. the DM halos are sources of the diffuse radiation.

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Blobrana


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Title: Constraints on the size of the extra dimension from Kaluza-Klein gravitino decay
Authors: David Gherson (IPNL)
(Version v2)

We study the consequences of the gravitino decay into dark matter. We suppose that the lightest neutralino is the main component of dark matter. In our framework gravitino is heavy enough to decay before Big Bang Nucleosynthesis starts. We consider a model coming from a five dimensional supergravity compactified on S^1/Z_2 with gravity in the bulk and matter localized on tensionless branes at the orbifold fixed points. We require that the dark matter, which is produced thermally and in the decay of Kaluza-Klein modes of gravitino, has an abundance compatible with observation. We deduce from our model that there are curves of constraints between the size of the extra-dimension and the reheating temperature of the universe after inflation.
 
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Blobrana


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General Relativity
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Astronomy/Physics Lecture - General Relativity
The origin of Einstein's general theory of relativity from the equivalence principle. Spacetime and Black Holes.



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Blobrana


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Posts: 131433
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RE: Space-time
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Title: Towards observable signatures of other bubble universes
Authors: Anthony Aguirre, Matthew C Johnson, Assaf Shomer
(Version v2)

We evaluate the possibility of observable effects arising from collisions between vacuum bubbles in a universe undergoing false-vacuum eternal inflation. Contrary to conventional wisdom, we find that under certain assumptions most positions inside a bubble should have access to a large number of collision events. We calculate the expected number and angular size distribution of such collisions on an observer's "sky," finding that for typical observers the distribution is anisotropic and includes many bubbles, each of which will affect the majority of the observer's sky. After a qualitative discussion of the physics involved in collisions between arbitrary bubbles, we evaluate the implications of our results, and outline possible detectable effects. In an optimistic sense, then, the present paper constitutes a first step in an assessment of the possible effects of other bubble universes on the cosmic microwave background and other observables.

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Blobrana


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Posts: 131433
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Other bubble universes
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It has long been accepted, at least in theory, that other universes might exist and might even collide with ours. Yet the idea that we would ever be able to see the aftermath of such collisions, and so find evidence of other universes, has seemed beyond the scope of science. That is set to change.
Anthony Aguirre of the University of California, Santa Cruz, thinks the proof of cosmic collisions could be all around us, as imprints in the cosmic microwave background (CMB) left over from the big bang.
According to the standard model of cosmology, our universe underwent a phase of exponential expansion, known as inflation, just after the big bang. In theory, inflation could still be happening, with bubbles of space-time suddenly blowing up to create new pocket universes. The cosmological parameters, such as the rate of expansion, and the laws of physics could be different in each new universe, potentially giving rise to new types of matter.

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Title: Towards observable signatures of other bubble universes
Authors: Anthony Aguirre, Matthew C Johnson, Assaf Shomer

We evaluate the possibility of observable effects arising from collisions between vacuum bubbles in a universe undergoing false-vacuum eternal inflation. Contrary to conventional wisdom, "typical" observers inside a bubble should have access to a large number of collision events. We calculate the expected number and angular size distribution of such collisions on an observer's "sky", finding that for typical observers the distribution is anisotropic and includes many collisions, each of which will affect the majority of the observer's sky. After a qualitative discussion of the physics involved in collisions between arbitrary bubbles, we evaluate the implications of our results, and outline possible observable effects. In an optimistic sense, then, the present paper constitutes a first step in an assessment of the effects of other bubble universes on the cosmic microwave background and other observables.

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Blobrana


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General Relativity
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Title: General Relativity Today
Authors: Thibault Damour

After recalling the conceptual foundations and the basic structure of general relativity, we review some of its main modern developments (apart from cosmology) : (i) the post-Newtonian limit and weak-field tests in the solar system, (ii) strong gravitational fields and black holes, (iii) strong-field and radiative tests in binary pulsar observations, (iv) gravitational waves, (v) general relativity and quantum theory.

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Blobrana


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The standard model
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Scientists question our understanding of the universe
Cosmologists from around the world will meet at Imperial College London next week to challenge the theories behind the 'standard model' used to understand the universe. Speakers at the four-day conference, jointly organised by Imperial and the University of Alabama in Huntsville, and sponsored by the U.S. National Science Foundation, will cover a wide range of unanswered questions on how the universe was formed and what has been happening to it after formation.

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Blobrana


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Space-time
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The universe is a string-net liquid
In 1998, just after he won a share of the Nobel prize for physics, Robert Laughlin of Stanford University in California was asked how his discovery of "particles" with fractional charge, now called quasi-particles, would affect the lives of ordinary people.

"It probably won't, unless people are concerned about how the universe works" - Robert Laughlin.

Well, people were. Xiao-Gang Wen at the Massachusetts Institute of Technology and Michael Levin at Harvard University ran with Laughlin's ideas and have come up with a prediction for a new state of matter, and even a tantalising picture of the nature of space-time itself. Levin presented their work at the Topological Quantum Computing conference at the University of California, Los Angeles, early this month.

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Blobrana


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Is the universe a fractal?
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Written across the sky is a secret, a hidden blueprint detailing the original design of the universe itself. The spread of matter throughout space follows a pattern laid out at the beginning of time and scaled up to incredible proportions by nearly 14 billion years of cosmic expansion. Today that pattern is gradually being decoded by analysing maps of the distribution of the stars, and what has been uncovered could shake modern cosmology to its foundations.
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Blobrana


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Posts: 131433
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RE: Space-time
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Title: The Universe as a topological defect
Authors: Andres Anabalon, Steven Willison, Jorge Zanelli

Four-dimensional Einstein's General Relativity is shown to arise from a gauge theory for the conformal group, SO(4,2). The theory is constructed from a topological dimensional reduction of the six-dimensional Euler density integrated over a manifold with a four-dimensional topological defect. The resulting action is a four-dimensional theory defined by a gauged Wess-Zumino-Witten term. There is a pure gravitational sector in which the Einstein's field equations hold and the unique coupling constant in the action is shown to be restricted to take integer values.

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