arXiv subjects

Anastasios Avgoustidis

Publications and source records attributed to Anastasios Avgoustidis.

25 records · Page 2Linked to original sources

arXiv2012

Gravitational shocks as a key ingredient of Gamma-Ray Bursts

We identify a novel physical mechanism that may be responsible for energy release in $γ$-ray bursts. Radial perturbations in the neutron core, induced by its collision with collapsing outer layers during the early stages of supernova explosions, can trigger a gravitational shock, which can readily eject a small but significant fraction of the collapsing material at ultra-relativistic speeds. The development of such shocks is a strong-field effect arising in near-critical collapse in General Relativity and has been observed in numerical simulations in various contexts, including in particular radially perturbed neutron star collapse, albeit for a tiny range of initial conditions. Therefore, this effect can be easily missed in numerical simulations if the relevant parameter space is not exhaustively investigated. In the proposed picture, the observed rarity of $γ$-ray bursts would be explained if the relevant conditions for this mechanism appear in only about one in every $10^4-10^5$ core collapse supernovae. We also mention the possibility that near-critical collapse could play a role in powering the central engines of Active Galactic Nuclei.

astro-ph.HE↗

arXiv2012

Cosmic superstring trajectories in warped compactifications

We explore the generic motion of cosmic (super)strings when the internal compact dimensions are warped, using the Klebanov-Strassler solution as a prototypical throat geometry. We find that there is no dynamical mechanism which localises the string at the tip of the throat, but rather that the motion seems to explore both internal and external degrees of freedom democratically. This indicates that cosmic (super)strings formed by inflationary brane-antibrane annihilation will have sufficient internal motion for the gravitational wave signals from the string network to be suppressed relative to the signal from a `standard' cosmic string network.

hep-th↗

arXiv2012

Decoupling Survives Inflation: A Critical Look at Effective Field Theory Violations During Inflation

We investigate the validity of effective field theory methods and the decoupling of heavy fields during inflation. Considering models of inflation in which the inflaton is coupled to a heavy (super-Hubble) degree of freedom initially in its vacuum state, we find that violations of decoupling are absent unless there is a breakdown of the slow-roll conditions. Next we allow for a temporary departure from inflation resulting in a period of non-adiabaticity during which effective field theory methods are known to fail. We find that the locality of the event and energy conservation lead to a tight bound on the size of the effects of the heavy field. We discuss the implications for the power spectrum and non-gaussianity, and comment on the connection with recent studies of the dynamics of multi-field inflation models. Our results further motivate the use of effective field theory methods to characterize cosmic inflation, and focus the question of observability of additional degrees of freedom during inflation to near the Hubble scale or below - as anticipated from the Wilsonian notions of decoupling and naturalness.

hep-th↗

arXiv2012

The Importance of Slow-roll Corrections During Multi-field Inflation

We re-examine the importance of slow-roll corrections during the evolution of cosmological perturbations in models of multi-field inflation. We find that in many instances the presence of light degrees of freedom leads to situations in which next to leading order slow-roll corrections become significant. Examples where we expect such corrections to be crucial include models in which modes exit the Hubble radius while the inflationary trajectory undergoes an abrupt turn in field space, or during a phase transition. We illustrate this with two examples -- hybrid inflation and double quadratic inflation. Utilizing both analytic estimates and full numerical results, we find that corrections can be as large as 20%. Our results have implications for many existing models in the literature, as these corrections must be included to obtain accurate observational predictions -- particularly given the level of accuracy expected from CMB experiments such as Planck

astro-ph.CO↗

arXiv2011

Constraints on the fundamental string coupling from B-mode experiments

We study signatures of cosmic superstring networks containing strings of multiple tensions and Y-junctions, on the cosmic microwave background (CMB) temperature and polarisation spectra. Focusing on the crucial role of the string coupling constant $g_s$, we show that the number density and energy density of the scaling network are dominated by different types of string in the $g_s \sim 1$ and $g_s \ll 1$ limits. This can lead to an observable shift in the position of the B-mode peak --- a distinct signal leading to a direct constraint on $g_s$. We forecast the joint bounds on $g_s$ and the fundamental string tension $μ_F$ from upcoming and future CMB polarisation experiments, as well as the signal to noise in detecting the difference between B-mode signals in the limiting cases of large and small $g_s$. We show that such a detectable shift is within reach of planned experiments.

astro-ph.CO↗

arXiv2011

Scaling configurations of cosmic superstring networks and their cosmological implications

We study the cosmic microwave background temperature and polarisation spectra sourced by multi-tension cosmic superstring networks. First we obtain solutions for the characteristic length scales and velocities associated with the evolution of a network of F-D strings, allowing for the formation of junctions between strings of different tensions. We find two distinct regimes describing the resulting scaling distributions for the relative densities of the different types of strings, depending on the magnitude of the fundamental string coupling g_s. In one of them, corresponding to the value of the coupling being of order unity, the network's stress-energy power spectrum is dominated by populous light F and D strings, while the other regime, at smaller values of g_s, has the spectrum dominated by rare heavy D strings. These regimes are seen in the CMB anisotropies associated with the network. We focus on the dependence of the shape of the B-mode polarisation spectrum on g_s and show that measuring the peak position of the B-mode spectrum can point to a particular value of the string coupling. Finally, we assess how this result, along with pulsar bounds on the production of gravitational waves from strings, can be used to constrain a combination of g_s and the fundamental string tension mu_F. Since CMB and pulsar bounds constrain different combinations of the string tensions and densities, they result in distinct shapes of bounding contours in the (mu_F, g_s) parameter plane, thus providing complementary constraints on the properties of cosmic superstrings.

astro-ph.CO↗

arXiv2010

Constraints on cosmic opacity and beyond the standard model physics from cosmological distance measurements

We update constraints on cosmic opacity by combining recent SN Type Ia data compilation with the latest measurements of the Hubble expansion at redshifts between 0 and 2. The new constraint on the parameter $ε$ parametrising deviations from the luminosity-angular diameter distance relation ($d_L=d_A(1+z)^{2+ε}$), is $ε=-0.04_{-0.07}^{+0.08}$ (2-$σ$). For the redshift range between 0.2 and 0.35 this corresponds to an opacity $Δτ<0.012$ (95% C.L.), a factor of 2 stronger than the previous constraint. Various models of beyond the standard model physics that predict violation of photon number conservation contribute to the opacity and can be equally constrained. In this paper we put new limits on axion-like particles, including chameleons, and mini-charged particles.

astro-ph.CO↗