اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدMinimal Theory of Bigravity: construction and cosmology
84
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Following the path of minimalism in alternative theories of gravity, we construct the Minimal Theory of Bigravity (MTBG), a theory of two interacting spin-2 fields that propagates only four local degrees of freedom instead of the usual seven ones and that allows for the same homogeneous and isotropic cosmological solutions as in Hassan-Rosen bigravity (HRBG). Starting from a precursor theory that propagates six local degrees of freedom, we carefully choose additional constraints to eliminate two of them to construct the theory. Investigating the cosmology of MTBG, we find that it accommodates two different branches of homogeneous and isotropic background solutions, equivalent on-shell to the two branches that are present in HRBG. Those branches in MTBG differ however from the HRBG ones at the perturbative level, are both perfectly healthy and do not exhibit strong coupling issues nor ghost instabilities. In the so-called self-accelerating branch, characterized by the presence of an effective cosmological constant, the scalar and vector sectors are the same as in General Relativity (GR). In the so-called normal branch, the scalar sector exhibits non-trivial phenomenology, while its vector sector remains the same as in GR. In both branches, the tensor sector exhibits the usual HRBG features: an effective mass term and oscillations of the gravitons. Therefore MTBG provides a stable nonlinear completion of the cosmology in HRBG.
قيم البحث
اقرأ أيضاً
A modified gravitational theory explains early universe and late time cosmology, galaxy and galaxy cluster dynamics. The modified gravity (MOG) theory extends general relativity (GR) by three extra degrees of freedom: a scalar field $G$, enhancing th
e strength of the Newtonian gravitational constant $G_N$, a gravitational, spin 1 vector graviton field $phi_mu$, and the effective mass $mu$ of the ultralight spin 1 graviton. For $t < t_{rm rec}$, where $t_{rm rec}$ denotes the time of recombination and re-ionization, the density of the vector graviton $rho_phi > rho_b$, where $rho_b$ is the density of baryons, while for $t > t_{rm rec}$ we have $rho_b > rho_phi$. The matter density is parameterized by $Omega_M=Omega_b+Omega_phi+Omega_r$ where $Omega_r=Omega_gamma+Omega_ u$. For the cosmological parameter values obtained by the Planck Collaboration, the CMB acoustical oscillation power spectrum, polarization and lensing data can be fitted as in the $Lambda$CDM model. When the baryon density $rho_b$ dominates the late time universe, MOG explains galaxy rotation curves, the dynamics of galaxy clusters, galaxy lensing and the galaxy clusters matter power spectrum without dominant dark matter.
We revisit spatially flat FLRW cosmology in light of recent advances in standard model relativistic fluid dynamics. Modern fluid dynamics requires the presence of curvature-matter terms in the energy-momentum tensor for consistency. These terms are l
inear in the Ricci scalar and tensor, such that the corresponding cosmological model is referred to as ``Ricci cosmology. No cosmological constant is included, there are no inflaton fields, bulk viscosity is assumed to be zero and we only employ standard Einstein gravity. Analytic solutions to Ricci cosmology are discussed, and we find that it is possible to support an early-time inflationary universe using only well-known ingredients from the Standard Model of physics and geometric properties of space-time.
We present a short review of possible applications of the Wheeler-De Witt equation to cosmological models based on the low-energy string effective action, and characterised by an initial regime of asymptotically flat, low energy, weak coupling evolut
ion. Considering in particular a class of duality-related (but classically disconnected) background solutions, we shall discuss the possibility of quantum transitions between the phases of pre-big bang and post-big bang evolution. We will show that it is possible, in such a context, to represent the birth of our Universe as a quantum process of tunneling or anti-tunneling from an initial state asymptotically approaching the string perturbative vacuum.
Recent measurements at the LHC suggest that the current Higgs vacuum could be metastable with a modest barrier (height 10^{10-12}{GeV})^{4}) separating it from a ground state with negative vacuum density of order the Planck scale. We note that metast
ability is problematic for big bang to end one cycle, bounce, and begin the next. In this paper, motivated by the approximate scaling symmetry of the standard model of particle physics and the primordial large-scale structure of the universe, we use our recent formulation of the Weyl-invariant version of the standard model coupled to gravity to track the evolution of the Higgs in a regularly bouncing cosmology. We find a band of solutions in which the Higgs field escapes from the metastable phase during each big crunch, passes through the bang into an expanding phase, and returns to the metastable vacuum, cycle after cycle after cycle. We show that, due to the effect of the Higgs, the infinitely cycling universe is geodesically complete, in contrast to inflation.
We study for the first time the dynamical properties and the growth index of linear matter perturbations of the Finsler-Randers (FR) cosmological model, for which we consider that the cosmic fluid contains matter, radiation and a scalar field. Initia
lly, for various FR scenarios we implement a critical point analysis and we find solutions which provide cosmic acceleration and under certain circumstances we can have de-Sitter points as stable late-time attractors. Then we derive the growth index of matter fluctuations in various Finsler-Randers cosmologies. Considering cold dark matter and neglecting the scalar field component from the perturbation analysis we find that the asymptotic value of the growth index is $gamma_{infty}^{(FR)}approxfrac {9}{16}$, which is close to that of the concordance $Lambda$ cosmology, $gamma^{(Lambda)} approxfrac{6}{11}$. In this context, we show that the current FR model provides the same Hubble expansion with that of Dvali, Gabadadze and Porrati (DGP) gravity model. However, the two models can be distinguished at the perturbation level since the growth index of FR model is $sim18.2%$ lower than that of the DPG gravity $gamma^{(DGP)} approx frac{11}{16}$. If we allow pressure in the matter fluid then we obtain $gamma_{infty}^{(FR)}approxfrac{9(1+w_{m})(1+2w_{m})}{2[8+3w_{m}% (5+3w_{m})]}$, where $w_{m}$ is the matter equation of state parameter. Finally, we extend the growth index analysis by using the scalar field and we find that the evolution of the growth index in FR cosmologies is affected by the presence of scalar field.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
