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Archive for the ‘COSMOLOGY’ Category

Exploring evidence of interaction between dark energy and dark matter

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One of the most important problems of theoretical physics is to explain the fact that the universe is in a phase of accelerated expansion. Since 1998 the physical origin of cosmic acceleration remains a deep mystery. According to general relativity (GR), if the universe is filled with ordinary matter or radiation, the two known constituents of the universe, gravity should slow the expansion. Since the expansion is speeding up, we are faced with two possibilities, either of which would have profound implications for our understanding of the cosmos and of the laws of physics. The first is that 75% of the energy density of the universe exists in a new form with large negative pressure, called dark energy (DE). The other possibility is that GR breaks down on cosmological scales and must be replaced with a more complete theory of gravity. In this paper we consider the first option. The cosmological constant, the simplest explanation of accelerated expansion, has a checkered history having been invoked and subsequently withdrawn several times before. In quantum field theory, we estimate the value of the cosmological constant as the zero-point energy with a short-cut scale, for example the Planck scale, which results in an excessively greater value than the observational results….

read more at https://arxiv.org/pdf/1804.03296.pdf

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April 11, 2018 at 11:07 am

CPT symmetric universe

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Latham Boyle, Kieran Finn, Neil Turok
We propose that the state of the universe does it not spontaneously violate CPT. Instead, the universe before the Big Bang is the CPT reflection of the universe after the bang. Phrased another way, the universe before the bang and the universe after the bang may be re-interpreted as a universe/anti-universe pair, created from nothing. CPT selects a unique vacuum state for the QFT on such a spacetime, which leads to a new perspective on the cosmological baryon asymmetry, and a new explanation for the observed dark matter abundance. In particular, if we assume that the matter fields in the universe are described by the standard model of particle physics (including right-handed neutrinos), we predict that one of the heavy neutrinos is stable, and that its density automatically matches the observed dark matter density if its mass is 4.8×10^8 GeV. Among other predictions, we have: (i) that the three light neutrinos are majorana; (ii) that the lightest of these is exactly massless; and (iii) that there are no primordial long-wavelength gravitational waves. We mention connections to the strong CP problem and the arrow of time.
Read more at https://arxiv.org/pdf/1803.08928.pdf

Read also: “The Big Bang, CPT, and neutrino dark matter“, Latham Boyle, Kieran Finn, Neil Turok”

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March 29, 2018 at 7:12 am

Posted in COSMOLOGY

Cosmic Instability Could Have Created Dark Matter

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A proposed instability in the Higgs field could have seeded the Universe with primordial black holes that now serve as dark matter.

Our Universe may be sitting on the edge of destruction, as theory suggests that the Higgs field is in a metastable state. If this field tunneled to its “true” minimum energy state, the release of energy would be cataclysmic. The danger may be over-stated, as other physical mechanisms could have kept the Universe stable throughout its history. Nevertheless, the Higgs instability could have a major cosmological impact as the source of dark matter. According to this new scenario, dark matter consists of a large population of black holes that formed from fluctuations in the unstable Higgs field at the dawn of the Universe. These so-called primordial black holes have been proposed before, but this is the first hypothesis that doesn’t require physics beyond the standard model.

Physicists have long been aware that the Universe might rest in a “false vacuum.” This idea has recently taken on new urgency, as calculations based on the measured Higgs mass have shown that a lower energy state may exist for the Higgs field. Analyzing the implications of the Higgs instability, José Espinosa, from the Catalan Institution for Research and Advanced Studies (ICREA) in Spain, and David Racco and Antonio Riotto, both at the University of Geneva, have found that this menacing mechanism could—counterintuitively—be instrumental in creating the dark matter that makes galaxies and other life-accommodating structures possible. In their calculations, the team explored fluctuations in the Higgs field during an early expansion phase of the Universe, called inflation. Under certain assumptions, these fluctuations become seeds for microscopic black holes with masses around 1015 kg that could have a density consistent with cosmological predictions of dark matter.

Michael Schirber – https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.120.121301

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March 23, 2018 at 7:29 pm

Here’s What Stephen Hawking’s Final Paper Was Actually About

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March 21, 2018 at 8:29 pm

Posted in COSMOLOGY

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The Dawn of the Post-Naturalness Era

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Gian Francesco Giudice
In an imaginary conversation with Guido Altarelli, I express my views on the status of particle physics beyond the Standard Model and its future prospects.
read more at https://arxiv.org/pdf/1710.07663.pdf

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October 25, 2017 at 7:54 pm

Cosmology, Black Holes, and String Theory

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Eva Silverstein

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June 14, 2017 at 10:45 am

The Shape of Bouncing Universes

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John D. Barrow, Chandrima Ganguly
What happens to the most general closed oscillating universes in general relativity? We sketch the development of interest in cyclic universes from the early work of Friedmann and Tolman to modern variations introduced by the presence of a cosmological constant. Then we show what happens in the cyclic evolution of the most general closed anisotropic universes provided by the Mixmaster universe. We show that in the presence of entropy increase its cycles grow in size and age, increasingly approaching flatness. But these cycles also grow increasingly anisotropic at their expansion maxima. If there is a positive cosmological constant, or dark energy, present then these oscillations always end and the last cycle evolves from an anisotropic inflexion point towards a de Sitter future of everlasting expansion.

Read more at https://arxiv.org/pdf/1705.06647.pdf

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May 19, 2017 at 9:39 am

The cosmology of Poe

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Written by physicsgg

May 5, 2017 at 5:59 pm