5 research outputs found
Scalar induced gravitational waves in modified teleparallel gravity theories
Primordial black holes (PBHs) forming out of the collapse of enhanced
cosmological perturbations provide access to the early Universe through their
associated observational signatures. In particular, enhanced cosmological
perturbations collapsing to form PBHs are responsible for the generation of a
stochastic gravitational-wave background (SGWB) induced by second-order
gravitational interactions, usually called scalar induced gravitational waves
(SIGWs). This SGWB is sensitive to the underlying gravitational theory; hence
it can be used as a novel tool to test the standard paradigm of gravity and
constrain possible deviations from general relativity. In this work, we study
the aforementioned GW signal within modified teleparallel gravity theories,
developing a formalism for the derivation of the GW spectral abundance within
any form of gravitational action. At the end, working within viable
models without matter-gravity couplings, and accounting for the effect of
mono-parametric gravity at the level of the source and the propagation
of the tensor perturbations, we show that the respective GW signal is
indistinguishable from that within GR. Interestingly, we find that in order to
break the degeneracy between different theories through the portal of
SIGWs one should necessarily consider non-minimal matter-gravity couplings at
the level of the gravitational action.Comment: 16 pages without appendices (24 in total), 2 figure
Signatures of Superstring theory in NANOGrav
In this Letter, we extract for the first time signatures of Superstring
theory in the recently released NANOGrav data. We concentrate on the primordial
gravitational wave (GW) spectrum induced by the gravitational potential of a
population of primordial black holes (PBHs) generated in the framework of
no-scale Supergravity. In particular, working within Wess-Zumino type no-scale
Supergravity we find naturally-realised inflection-point inflationary
potentials, which can give rise to the formation of microscopic PBHs triggering
an early matter-dominated era (eMD) and evaporating before Big Bang
Nucleosythesis (BBN). Remarkably, we obtain an abundant production of
gravitational waves, whose profile is quite distinctive, characterized by a
strong oscillatory pattern and being in strong agreement with NANOGrav data.
Hence, such a signal can act as a potential signature of no-scale Supergravity
and Superstring theory at the current and near-future GW observations.Comment: Minor changes: new references and small discussion adde
Induced gravitational waves from flipped SU(5) superstring theory at
The no-scale flipped SU(5) superstring framework constitutes a very promising
paradigm for physics below the Planck scale providing us with a very rich
cosmological phenomenology in accordance with observations. In particular, it
can accommodate Starobinsky-like inflation, followed by a reheating phase,
which is driven by a light "flaton" field, and during which the GUT phase
transition occurs. In this Letter, we extract for the first time a
gravitational-wave (GW) signal which naturally arises in the context of the
flipped SU(5) cosmological phenomenology and is related to the existence of an
early matter era (eMD) driven by the flaton field. Specifically, we study GWs
non-linearly induced by inflationary perturbations and which are abundantly
produced during a sudden transition from the flaton-driven eMD era to the
late-time radiation-dominated era. Remarkably, we find a GW signal with a
characteristic peak frequency depending only on the string
slope and reading as , where is the
fiducial string slope being related directly to the reduced Planck scale
as . Interestingly enough,
lies within the frequency range; hence
rendering this primordial GW signal potentially detectable by SKA, NANOGrav and
PTA probes at their very low frequency region of their detection bands.Comment: Accepted in Physics Letters B. arXiv admin note: text overlap with
arXiv:2307.0860
No constraints for f(T) gravity from gravitational waves induced from primordial black hole fluctuations
Abstract Primordial black hole (PBH) fluctuations can induce a stochastic gravitational wave background at second order, and since this procedure is sensitive to the underlying gravitational theory it can be used as a novel tool to test general relativity and extract constraints on possible modified gravity deviations. We apply this formalism in the framework of f(T) gravity, considering three viable mono-parametric models. In particular, we investigate the induced modifications at the level of the gravitational-wave source, which is encoded in terms of the power spectrum of the PBH gravitational potential, as well as at the level of their propagation, described in terms of the Green function which quantifies the propagator of the tensor perturbations. We find that, within the observationally allowed range of the f(T) model-parameters, the obtained deviations from general relativity, both at the levels of source and propagation, are practically negligible. Hence, we conclude that realistic and viable f(T) theories can safely pass the primordial black hole constraints, which may offer an additional argument in their favor
Gravitational wave signatures of no-scale supergravity in NANOGrav and beyond
In this Letter, we derive for the first time a characteristic three-peaked GW signal within the framework of no-scale Supergravity, being the low-energy limit of Superstring theory. We concentrate on the primordial gravitational wave (GW) spectrum induced due to second-order gravitational interactions by inflationary curvature perturbations as well as by isocurvature energy density perturbations of primordial black holes (PBHs) both amplified due to the presence of an early matter-dominated era (eMD) era before Big Bang Nucleosythesis (BBN). In particular, we work with inflection-point inflationary potentials naturally-realised within Wess-Zumino type no-scale Supergravity and giving rise to the formation of microscopic PBHs triggering an eMD era and evaporating before BBN. Remarkably, we obtain an abundant production of gravitational waves at the frequency ranges of nHz, Hz and kHz and in strong agreement with Pulsar Time Array (PTA) GW data. Interestingly enough, a simultaneous detection of all three nHz, Hz and kHz GW peaks can constitute a potential observational signature for no-scale Supergravity