Particle horizon

About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.

Dynamical evolution of the Universe in the quark-hadron phase transition and nugget formation

Abstract: We study the dynamics of first-order phase transition in the early Universe when it was $10-50 \mu s$ old with quarks and gluons condensing into hadrons. We look at how the Universe evolved through the phase transition in small as well as large super cooling scenario, specifically exploring the formation of quark nuggets and their possible survival. The nucleation of the hadron phase introduces new distance scales in the Universe, which we estimate along with the hadron fraction, temperature, nucleation time etc. It is of interest to explore whether there is a relic signature of this transition in the form of quark nuggets which might be identified with the recently observed dark objects in our galactic halo and account for the Dark Matter in the Universe at present.

Built-in inflation in f(G) gravity

Abstract: In this paper, we study the role of Gauss–Bonnet term for the early and late time accelerating phases of the universe with the help of two viable f(G) models in the background of flat FRW universe model. These models show inflationary behavior as well as the present accelerating expansion of the universe. The contribution of Gauss–Bonnet term in pressure and energy density is used to calculate equation of state (EoS) parameter for the modified fluid which behaves like cosmological constant with Ḣ = 0. We discuss early inflation and late accelerating expansion of the universe through scale factor evaluated from equation of continuity numerically.

Separate universe problem: 40 years on

Abstract: The claim that an overdense (positive-curvature) region in the early Universe cannot extend beyond some maximum scale and remain part of our Universe, first made 40 years ago, has recently been questioned by Kopp et al. Their analysis is elucidating and demonstrates that one cannot constrain the form of primordial density perturbations using this argument. However, the notion of a separate universe scale still applies and there is also an upper limit on the scale of a region collapsing to a black hole at any epoch, these scales being simply related. We calculate these scales for equations of state of the form $p=k\ensuremath{\rho}{c}^{2}$ with $\ensuremath{-}1lkl\ensuremath{\infty}$, refining earlier calculations on account of the Kopp et al. criticisms. For $\ensuremath{-}1/3lkl\ensuremath{\infty}$, the scale is always of the order of the cosmological particle horizon size, with a numerical factor depending on $k$. This confirms the earlier claim that a primordial black hole cannot be much larger than the particle horizon at formation. For $\ensuremath{-}1lkl\ensuremath{-}1/3$, as expected for some periods in the history of the Universe, the situation changes radically, in that a sufficiently large positive-curvature region produces a baby universe rather than a black hole. There is still a separate universe scale but the interpretation of these solutions requires care.

Anisotropy of the Cosmic Blackbody Radiation

Abstract: The universe is filled with thermal radiation having a current temperature of 2.75 K. Originating in the very early universe, this radiation furnishes strong evidence that the Big Bang cosmology best describes our expanding universe from an incredibly hot, compacted early stage until now. The model can be used to extrapolate our physics backward in time to predict events whose effects might be observable in the 2.75 K radiation today. The spectrum and isotropy are being studied with sophisticated microwave radiometers on the ground, in balloons, and in satellites. The results are as predicted by the simple theory: the spectrum is that of a blackbody (to a few percent) and the radiation is isotropic (to 0.01 percent) except for a local effect due to our motion through the radiation. However, a problem is emerging. Primordial fluctuations in the mass density, which later became the great clusters of galaxies that we see today, should have left an imprint on the 2.75 K radiation—bumpiness on the sky at angular scales of about 10 arc minutes. They have not yet been seen.

Islands in the Λ -sea: An alternative cosmological model

Abstract: We propose an alternative cosmological model in which our observable Universe is an island in a cosmological constant sea Initially the Universe is filled with cosmological constant of the currently observed value but is otherwise empty In this eternal or semieternal de Sitter spacetime, we show that local quantum fluctuations (upheavals) can violate the null energy condition and create islands of matter The perturbation spectra of quantum fields other than that responsible for the upheaval, are shown to be scale invariant With further cosmic evolution the island disappears and the local Universe returns to its initial cosmological constant dominated state