arXiv subjects

Subir Sarkar

Publications and source records attributed to Subir Sarkar.

At least 127 records · Page 7Linked to original sources

arXiv1999

Scale of SU(2)_R symmetry breaking and leptogenesis

Models of leptogenesis often invoke the out-of-equilibrium decays of heavy right-handed neutrinos in order to create a baryon asymmetry of the universe through the electroweak phase transition. Their presumed existence argues strongly for the presence of an $SU(2)_R$ gauge symmetry. We study the equilibrating effects of the resulting additional right-handed interactions and find that successful leptogenesis requires that $m_N \gtrsim 10^{16}$ GeV if $m_N > m_{W_R}$, and $m_{W_R} \gtrsim 3 \times 10^6 GeV (m_N /10^2 GeV)^{2/3}$ if $m_{N} m_{W_R}$ option is excluded in a supersymmetric theory with gravitinos.

hep-ph↗

arXiv1999

Big bang nucleosynthesis limit on N_nu

Recently we presented a simple method for determining the correlated uncertainties of the light element abundances expected from big bang nucleosynthesis, which avoids the need for lengthy Monte Carlo simulations. We now extend this approach to consider departures from the Standard Model, in particular to constrain any new light degrees of freedom present in the thermal plasma during nucleosynthesis. Since the observational situation regarding the inferred primordial abundances has not yet stabilized, we present illustrative bounds on the equivalent number of neutrino species N_nu for various combinations of individual abundance determinations. Our 95% C.L. bounds on N_nu range between 2 and 4, and can easily be reevaluated using the technique provided when the abundances are known more accurately.

hep-ph↗

arXiv1999

Big Bang Nucleosynthesis: Reprise

Recent observational and theoretical developments concerning the primordial synthesis of the light elements are reviewed, and the implications for dark matter mentioned.

astro-ph↗

arXiv1998

Sensitivity of Astrophysical Observations to Gravity-Induced Wave Dispersion in Vacuo

We discuss possible signatures of quantum gravity for the propagation of light, including an energy-dependent velocity (refractive index), dispersion in velocity at a given energy, and birefringence. We also compare the sensitivities of different astrophysical observations, including BATSE data on GRB 920229, BeppoSAX data on GRB 980425, the possible HEGRA observation of GRB 920925c, and Whipple observations of the active galaxy Mrk 421. Finally, we discuss the prospective sensitivities of AMS and GLAST.

astro-ph↗

arXiv1998

CMB Anisotropy in the Decaying Neutrino Cosmology

It is attractive to suppose for several astrophysical reasons that the universe has close to the critical density in light (~30 eV) neutrinos which decay radiatively with a lifetime of ~10^{23} sec. In such a cosmology the universe is reionized early and the last scattering surface of the cosmic microwave background significantly broadened. We calculate the resulting angular power spectrum of temperature fluctuations in the cosmic microwave background. As expected the acoustic peaks are significantly damped relative to the standard case. This would allow a definitive test of the decaying neutrino cosmology with the forthcoming MAP and PLANCK surveyor missions.

astro-ph↗

arXiv1998

Quantifying uncertainties in primordial nucleosynthesis without Monte Carlo simulations

We present a simple method for determining the (correlated) uncertainties of the light element abundances expected from big bang nucleosynthesis, which avoids the need for lengthy Monte Carlo simulations. Our approach helps to clarify the role of the different nuclear reactions contributing to a particular elemental abundance and makes it easy to implement energy-independent changes in the measured reaction rates. As an application, we demonstrate how this method simplifies the statistical estimation of the nucleon-to-photon ratio through comparison of the standard BBN predictions with the observationally inferred abundances.

astro-ph↗

arXiv1998

Extremely high energy cosmic rays from relic particle decays

The expected proton and neutrino fluxes from decays of massive metastable relic particles is calculated using the HERWIG QCD event generator. The predicted proton spectrum can account for the observed flux of extremely high energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff, for a decaying particle mass of O(10^{12}) GeV. The lifetime required is of O(10^{20}) yr if such particles constitute all of the dark matter (with a proportionally shorter lifetime for a smaller contribution). Such values are plausible if the metastable particles are hadron-like bound states from the hidden sector of supersymmetry breaking which decay through non-renormalizable interactions. The expected ratio of the proton to neutrino flux is given as a diagonistic of the decaying particle model for the forthcoming Pierre Auger project.

hep-ph↗

arXiv1998

Potential Sensitivity of Gamma-Ray Burster Observations to Wave Dispersion in Vacuo

The recent confirmation that at least some gamma-ray bursters (GRBs) are indeed at cosmological distances raises the possibility that observations of these could provide interesting constraints on the fundamental laws of physics. Here we demonstrate that the fine-scale time structure and hard spectra of GRB emissions are very sensitive to the possible dispersion of electromagnetic waves in vacuo with velocity differences $δv \sim E/E_{\QG}$, as suggested in some approaches to quantum gravity. A simple estimate shows that GRB measurements might be sensitive to a dispersion scale $E_{QG}$ comparable to the Planck energy scale $E_{P} \sim 10^{19}$ GeV, sufficient to test some of these theories, and we outline aspects of an observational programme that could address this goal.

astro-ph↗

arXiv1997

Cosmological implications of neutrinos

Massive neutrinos were the first proposed, and remain the most natural, particle candidate for the dark matter. In the absence of firm laboratory evidence for neutrino mass, considerations of the formation of large scale structure in the universe provide a sensitive, albeit indirect, probe of this possibility. Observations of galaxy clustering and large angle anisotropy in the cosmic microwave background have been interpreted as requiring that neutrinos provide about 20% of the critical density. However the need for such `hot' dark matter is removed if the primordial spectrum of density fluctuations is tilted below scale-invariance, as is often the case in physically realistic inflationary models. This question will be resolved by forthcoming precision measurements of microwave background anisotropy on small angular scales. This data will also improve the nucleosynthesis bound on the number of neutrino species and test whether decays of relic neutrinos could have ionized the intergalactic medium.

hep-ph↗

arXiv1997

Ruling out a critical density baryonic universe

It has been suggested by Bartlett and Hall that our universe may have the critical density in baryons by virtue of specific interactions with a `shadow' world. We show that this possibility is severely constrained by primordial nucleosynthesis, stellar evolution and the thermalization of the cosmic microwave background. In particular, recent observations of small angular-scale anisotropy in the cosmic microwave background conclusively rule out all such baryon-dominated cosmologies.

hep-ph↗

arXiv1997

Multiple inflation

Attempts at building an unified description of the strong, weak and electromagnetic interactions usually involve several stages of spontaneous symmetry breaking. We consider the effects of such symmetry breaking during an era of primordial inflation in supergravity models. In cases that these occur along flat directions at intermediate scales there will be a succession of short bursts of inflation which leave a distinctive signature in the spectrum of the generated scalar density perturbation. Thus measurements of the spectral index can directly probe the structure of unified theories at very high energy scales. An observed feature in the power spectrum of galaxy clustering from the APM survey may well be associated with such structure. If so, this implies a characteristic suppression of the secondary Doppler peaks in the angular power spectrum of temperature fluctuations in the cosmic microwave background.

hep-ph↗

arXiv1997

Nucleosynthesis bounds on a time-varying cosmological 'constant'

We constrain proposed phenomenological models for a vacuum energy which decays with the expansion of the universe from considerations of standard big bang nucleosynthesis. Several such models which attempt to solve the cosmological age problem are disfavored or even ruled out.

astro-ph↗

arXiv1997

Evading the cosmological domain wall problem

Discrete symmetries are commonplace in field theoretical models but pose a severe problem for cosmology since they lead to the formation of domain walls during spontaneous symmetry breaking in the early universe. However if one of the vacuua is favoured over the others, either energetically, or because of initial conditions, it will eventually come to dominate the universe. Using numerical methods, we study the evolution of the domain wall network for a variety of field configurations in two and three dimensions and quantify the rate at which the walls disappear. Good agreement is found with a recent analytic estimate of the termination of the scaling regime of the wall network.

hep-ph↗

arXiv1996

Primordial Nucleosynthesis and Dark Matter

The cosmological abundance of nucleons determined from considerations of Big Bang nucleosynthesis allegedly provides compelling evidence for non-nucleonic dark matter. Recent developments in measurements of primordial light element abundances, in particular deuterium and helium, require reexamination of this important issue. The present situation is uncertain but exciting.

astro-ph↗

arXiv1996

Natural Supergravity inflation

We identify a new mechanism in supergravity theories which leads to successful inflation without any need for fine tuning. The simplest model yields a spectrum of density fluctuations tilted away from scale-invariance and negligible gravitational waves. We demonstrate that this is consistent with the observed large-scale structure for a cold dark matter dominated, critical density universe. The model can be tested through measurements of microwave background anisotropy on small angular scales.

hep-ph↗

arXiv1996

Supersymmetric inflation and large-scale structure

In effective supergravity theories following from the superstring, a modulus field can quite naturally set the neccessary initial conditions for successful cosmological inflation to be driven by a hidden sector scalar field. The leading term in the scalar potential is {\em cubic} hence the spectrum of scalar density perturbations neccessarily deviates from scale-invariance, while the generation of gravitational waves is negligible. The growth of large-scale structure is then consistent with observational data assuming a critical density cold dark matter universe, with no need for a component of hot dark matter. The model can be tested thorough measurements of cosmic microwave background anisotropy on small angular scales.

hep-ph↗

arXiv1996

Big Bang nucleosynthesis and physics beyond the Standard Model

The Hubble expansion of galaxies, the $2.73\dK$ blackbody radiation background and the cosmic abundances of the light elements argue for a hot, dense origin of the universe --- the standard Big Bang cosmology --- and enable its evolution to be traced back fairly reliably to the nucleosynthesis era when the temperature was of $\Or(1)$ MeV corresponding to an expansion age of $\Or(1)$ sec. All particles, known and hypothetical, would have been created at higher temperatures in the early universe and analyses of their possible effects on the abundances of the synthesized elements enable many interesting constraints to be obtained on particle properties. These arguments have usefully complemented laboratory experiments in guiding attempts to extend physics beyond the Standard $SU(3)_{\c}{\otimes}SU(2)_Ł{\otimes}U(1)_{Y}$ Model, incorporating ideas such as supersymmetry, compositeness and unification. We first present a pedagogical account of relativistic cosmology and primordial nucleosynthesis, discussing both theoretical and observational aspects, and then proceed to examine such constraints in detail, in particular those pertaining to new massless particles and massive unstable particles. Finally, in a section aimed at particle physicists, we illustrate applications of such constraints to models of new physics.

hep-ph↗

arXiv1996

No Crisis for Big Bang Nucleosynthesis

Contrary to a recent claim, the inferred primordial abundances of the light elements are quite consistent with the expectations from standard big bang nucleosynthesis when attention is restricted to direct observations rather than results from chemical evolution models. The number of light neutrino (or equivalent particle) species ($N_ν$) can be as high as 4.53 if the nucleon-to-photon ratio ($η$) is at its lower limit of $1.65 \times 10^{-10}$, as constrained by the upper bound on the deuterium abundance in high redshift quasar absorption systems. Alternatively, with $N_ν= 3$, $η$ can be as high as $8.90 \times 10^{-10}$ if the deuterium abundance is bounded from below by its interstellar value.

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