Derive Lovelock gravity from string theory in cosmological background

It was proved more than three decades ago, that the first order α′ correction of string effective theory could be written as the Gauss-Bonnet term, which is the quadratic term of Lovelock gravity. In cosmological background, with an appropriate field redefinition, we reorganize the infinite α′ corrections of string effective action into a finite term expression for any specific dimension. This finite term expression matches Lovelock gravity exactly and thus fix the couplings of Lovelock gravity by the coefficients of string effective action. This result thus provides a strong support to string theory.

Superluminality in DHOST theory with extra scalar

We consider DHOST Ia theory interacting gravitationally with an additional conventional scalar field minimally coupled to gravity. At the linearized level of perturbations about cosmological background, we find that in the presence of a slowly rolling extra scalar field, one of the modes generically propagates at superluminal speed. This result is valid for any stable cosmological background. We identify a subclass of DHOST Ia theories in which this superluminality property is absent, and all modes may propagate (sub)luminally. We discuss possible implications for the interacting DHOST Ia theories.

Non-comoving cold dark matter in a $$\varLambda $$CDM background

We examine the evolution of peculiar velocities of cold dark matter (CDM) in localized arrays of inhomogeneous cosmic structures in a $$\varLambda $$ Λ CDM background that can be identified as a frame comoving with the Cosmic Microwave (CMB). These arrays are constructed by smoothly matching to this cosmological background regions of Szekeres-II models whose source is an imperfect fluid reinterpreted as non-comoving dust, keeping only first order terms in v/c. Considering a single Szekeres-II region matched along two comoving interfaces to a $$\varLambda $$ Λ CDM background, the magnitudes of peculiar velocities within this region are compatible with values reported in the literature, while the present day Hubble expansion scalar differs from that of the $$\varLambda $$ Λ CDM background value by a 10% factor, a result that might provide useful information to the ongoing debate on the $$H_0$$ H 0 tension. While the models cannot describe the virialization process, we show through a representative example that structures of galactic cluster mass reach the onset of this process at redshifts around $$z\sim 3$$ z ∼ 3 .

Impact of relativistic effects on the primordial non-Gaussianity signature in the large-scale clustering of quasars

ABSTRACT Relativistic effects in clustering observations have been shown to introduce scale-dependent corrections to the galaxy overdensity field on large scales, which may hamper the detection of primordial non-Gaussianity fNL through the scale-dependent halo bias. The amplitude of relativistic corrections depends not only on the cosmological background expansion, but also on the redshift evolution and sensitivity to the luminosity threshold of the tracer population being examined, as parametrized by the evolution bias be and magnification bias s. In this work, we propagate luminosity function measurements from the extended Baryon Oscillation Spectroscopic Survey (eBOSS) to be and s for the quasar (QSO) sample, and thereby derive constraints on relativistic corrections to its power spectrum multipoles. Although one could mitigate the impact on the fNL signature by adjusting the redshift range or the luminosity threshold of the tracer sample being considered, we suggest that, for future surveys probing large cosmic volumes, relativistic corrections should be forward modelled from the tracer luminosity function including its uncertainties. This will be important to quasar clustering measurements on scales $k \sim 10^{-3}\, h\, {\rm Mpc}^{-1}$ in upcoming surveys such as the Dark Energy Spectroscopic Instrument (DESI), where relativistic corrections can overwhelm the expected fNL signature at low redshifts z ≲ 1 and become comparable to fNL ≃ 1 in the power spectrum quadrupole at redshifts z ≳ 2.5.

Gravitational waves in modified Gauss–Bonnet gravity

We study the gravitational waves (GWs) in modified Gauss–Bonnet gravity. Applying the metric perturbation around a cosmological background, we obtain explicit expressions for the wave equations. It is shown that the speed of the traceless mode is equal to the speed of light. An additional massive scalar mode appears in the propagation of the GWs. To find phenomena beyond the general relativity, the scalar mode mass is calculated as a function of the background curvature in some typical models.

On the Relationship between the Cosmological Background Field and the Higgs Field

It is shown that gravity and quantum physics can be unified upon the basis of a quark description in terms of a recently discovered third type Dirac particle. It requires the awareness of a polarisable second elementary dipole moment next to the angular moment (spin) and the awareness of an (unbroken) omnipresent energetic cosmological background field. The unification has been made explicit by relating the two major gravitational constants of nature (the gravitational constant and Milgrom’s acceleration constant) with the two major nuclear constants of nature (the weak interaction boson and the Higgs boson).

On the Relationship between the Cosmological Background Field and the Higgs Field

It is shown that gravity and quantum physics can be unified upon the basis of a quark description in terms of a recently discovered third type Dirac particle. It requires the awareness of a polarisable second elementary dipole moment next to the angular moment (spin) and the awareness of an (unbroken) omnipresent energetic cosmological background field. The unification has been made explicit by relating the two major gravitational constants of nature (the gravitational constant and Milgrom’s acceleration constant) with the two major nuclear constants of nature (the weak interaction boson and the Higgs boson).

Induced gravitational waves in a general cosmological background

Gravitational waves (GWs) are inevitably produced by second-order terms in cosmological perturbation theory. Most notably, the so-called induced (GWs) are a window to the small scales part of the primordial spectrum of fluctuations and a key counterpart to the primordial black hole (PBH) scenario. However, semi-analytical solutions are only known for matter and radiation domination eras. In this paper, we present new analytic integral formulas for the induced GWs on subhorizon scales in a general cosmological background with a constant equation-of-state. We also discuss applications to a peaked primordial scalar power spectrum and the PBH scenario.