scholarly journals Decay of baryon inhomogeneities in an expanding universe

2021 ◽  
Vol 81 (9) ◽  
Author(s):  
Pratik K. Das ◽  
Sovan Sau ◽  
Abhisek Saha ◽  
Soma Sanyal
Keyword(s):  
Phase Transition ◽  
Diffusion Coefficients ◽  
Interaction Cross Section ◽  
Temperature Dependent ◽  
Transition Dynamics ◽  
Background Density ◽  
Walker Metric ◽  
Expansion Of The Universe ◽  
Subsequent Phase ◽  
The Universe

AbstractBaryon inhomogeneities are generated early in the universe. These inhomogeneities affect the phase transition dynamics of subsequent phase transitions, they also affect the nucleosynthesis calculations. We study the decay of the inhomogeneities in the early universe using the diffusion equation in the Friedmann–Lemaître–Robertson–Walker metric. We calculate the interaction cross section of the quarks with the neutrinos, the electrons and the muons and obtain the diffusion coefficients. The diffusion coefficients are temperature dependent. We find that the expansion of the universe causes the inhomogeneities to decay at a faster rate. We find that the baryon inhomogeneities generated at the electroweak epoch have low amplitudes at the time of the quark hadron transition and hence will not affect the phase transition dynamics unless they are generated with a amplitude greater than $$10^{5}$$ 10 5 times the background density. After the quark hadron transition, we include the interaction of the muons with the hadrons till 100 MeV. We find that large density inhomogeneities generated during the quark hadron transition with sizes of the order of 1 km must have amplitudes greater than $$10^{5} $$ 10 5 times the background density to survive upto the nucleosynthesis epoch. This puts constraints on any models that generate these inhomogeneities

scholarly journals BARYOGENESIS VIA DENSITY FLUCTUATIONS WITH A SECOND ORDER ELECTROWEAK PHASE TRANSITION

2003 ◽  
Vol 18 (26) ◽  
pp. 4851-4868 ◽  
Author(s):  
BISWANATH LAYEK ◽  
SOMA SANYAL ◽  
AJIT M. SRIVASTAVA
Keyword(s):  
Phase Transition ◽  
Baryon Asymmetry ◽  
Density Fluctuations ◽  
Second Order ◽  
Baryon Production ◽  
Electroweak Phase Transition ◽  
Expansion Of The Universe ◽  
Second Order Transition ◽  
Background Temperature ◽  
The Universe

We consider the presence of cosmic string induced density fluctuations in the universe at temperatures below the electroweak phase transition temperature. Resulting temperature fluctuations can restore the electroweak symmetry locally, depending on the amplitude of fluctuations and the background temperature. The symmetry will be spontaneously broken again in a given fluctuation region as the temperature drops there (for fluctuations with length scales smaller than the horizon), resulting in the production of baryon asymmetry. The time scale of the transition will be governed by the wavelength of fluctuation and, hence, can be much smaller than the Hubble time. This leads to strong enhancement in the production of baryon asymmetry for a second order electroweak phase transition as compared to the case when transition happens due to the cooling of the universe via expansion. For a two-Higgs extension of the Standard Model (with appropriate CP violation), we show that one can get the required baryon to entropy ratio if fluctuations propagate without getting significantly damped. If fluctuations are damped rapidly, then a volume factor suppresses the baryon production. Still, the short scale of the fluctuation leads to enhancement of the baryon to entropy ratio by at least 3–4 orders of magnitude compared to the conventional case of second order transition where the cooling happens due to expansion of the universe.

scholarly journals Observational constraints and stability in viscous gravity

2020 ◽  
Vol 98 (12) ◽  
pp. 1119-1124
Author(s):  
T. Mirzaei Rezaei ◽  
Alireza Amani ◽  
E. Yusofi ◽  
S. Rouhani ◽  
M.A. Ramzanpour
Keyword(s):  
Dark Energy ◽  
Cosmological Parameters ◽  
Teleparallel Gravity ◽  
System Stability ◽  
Accelerated Expansion ◽  
Tetrad Field ◽  
Dark Energy Component ◽  
Walker Metric ◽  
Expansion Of The Universe ◽  
The Universe

In this paper, we study the [Formula: see text] gravity model in the presence of bulk viscosity by the flat Friedmann–Robertson–Walker metric. The field equation is obtained by teleparallel gravity with a tetrad field. The universe components are considered matter and dark energy, with the dark energy component associated with viscous [Formula: see text] gravity. After calculating the Friedmann equations, we obtain the energy density, pressure, and equation of state of dark energy in terms of the redshift parameter. Afterward, we plot the corresponding cosmological parameters versus the redshift parameter and examine the accelerated expansion of the universe. In the end, we explore the system stability using a function called the speed sound parameter.

DYNAMICAL THEORY OF PHASE TRANSITIONS AND COSMOLOGICAL EW AND QCD PHASE TRANSITIONS

2008 ◽  
Vol 23 (17n20) ◽  
pp. 1325-1335
Author(s):  
SANG PYO KIM
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Real Time ◽  
Dynamical Theory ◽  
Quench Rate ◽  
Dynamical Phase ◽  
Time Formalism ◽  
Dynamical Phase Transitions ◽  
Expansion Of The Universe ◽  
The Universe

We critically review the cosmological EW and QCD phase transitions. The EW and QCD phase transitions would have proceeded dynamically since the expansion of the universe determines the quench rate and critical behaviors at the onset of phase transition slow down the phase transition. We introduce a real-time quench model for dynamical phase transitions and describe the evolution using a canonical real-time formalism. We find the correlation function, the correlation length and time and then discuss the cosmological implications of dynamical phase transitions on EW and QCD phase transitions in the early universe.

scholarly journals Causal gravitational waves as a probe of free streaming particles and the expansion of the Universe

2021 ◽  
Vol 2021 (2) ◽  
Author(s):  
Anson Hook ◽  
Gustavo Marques-Tavares ◽  
Davide Racco
Keyword(s):  
Phase Transition ◽  
Gravitational Waves ◽  
Gravity Waves ◽  
Wave Spectrum ◽  
Spectral Feature ◽  
Low Frequency ◽  
Low Frequencies ◽  
Expansion Of The Universe ◽  
The Difference ◽  
The Universe

Abstract The low frequency part of the gravitational wave spectrum generated by local physics, such as a phase transition or parametric resonance, is largely fixed by causality, offering a clean window into the early Universe. In this work, this low frequency end of the spectrum is analyzed with an emphasis on a physical understanding, such as the suppressed production of gravitational waves due to the excitation of an over-damped harmonic oscillator and their enhancement due to being frozen out while outside the horizon. Due to the difference between sub-horizon and super-horizon physics, it is inevitable that there will be a distinct spectral feature that could allow for the direct measurement of the conformal Hubble rate at which the phase transition occurred. As an example, free-streaming particles (such as the gravity waves themselves) present during the phase transition affect the production of super-horizon modes. This leads to a steeper decrease in the spectrum at low frequencies as compared to the well-known causal k3 super-horizon scaling of stochastic gravity waves. If a sizable fraction of the energy density is in free-streaming particles, they even lead to the appearance of oscillatory features in the spectrum. If the universe was not radiation dominated when the waves were generated, a similar feature also occurs at the transition between sub-horizon to super-horizon causality. These features are used to show surprising consequences, such as the fact that a period of matter domination following the production of gravity waves actually increases their power spectrum at low frequencies.

scholarly journals TWO NONCOMMUTATIVE PARAMETERS AND REGULAR COSMOLOGICAL PHASE TRANSITION IN THE SEMICLASSICAL DILATON COSMOLOGY

2008 ◽  
Vol 23 (15) ◽  
pp. 1079-1091 ◽  
Author(s):  
WONTAE KIM ◽  
EDWIN J. SON
Keyword(s):  
Phase Transition ◽  
Equations Of Motion ◽  
Accelerated Expansion ◽  
Dilaton Gravity ◽  
Initial State ◽  
Future Singularity ◽  
Flat Spacetime ◽  
Modified Equations ◽  
Expansion Of The Universe ◽  
The Universe

We study cosmological phase transitions from modified equations of motion by introducing two noncommutative parameters in the Poisson brackets, which describes the initial- and future-singularity-free phase transition in the soluble semiclassical dilaton gravity with a nonvanishing cosmological constant. Accelerated expansion and decelerated expansion appear alternatively, where the model contains the second accelerated expansion. The final stage of the universe approaches the flat spacetime independent of the initial state of the curvature scalar as long as the product of the two noncommutative parameters is less than one. Finally, we show that the initial-singularity-free condition is related to the second accelerated expansion of the universe.

Black Hole Universe: a grand cosmic recycler and Big Bang generator?

2019 ◽  
Author(s):  
Matheus Pereira Lobo
Keyword(s):  
Black Hole ◽  
Big Bang ◽  
Subsequent Increase ◽  
Expansion Of The Universe ◽  
The Universe

We propose the discussion of a highly speculative idea for the scenario where black hole collisions and their subsequent increase in sizes exceed the expansion of the universe.

Cosmological constant

2018 ◽  
Author(s):  
Michael Kachelriess
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Accelerated Expansion ◽  
Vacuum Fluctuations ◽  
Present Epoch ◽  
Modify Gravity ◽  
Expansion Of The Universe ◽  
The Universe

The contribution of vacuum fluctuations to the cosmological constant is reconsidered studying the dependence on the used regularisation scheme. Then alternative explanations for the observed accelerated expansion of the universe in the present epoch are introduced which either modify gravity or add a new component of matter, dubbed dark energy. The chapter closes with some comments on attempts to quantise gravity.

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