Late-time cosmology with phantom dark-energy in f(Q)f(Q) gravity

Motivated by the exciting features and a recent proposed general form of the function of non-metricity scalar Q, we investigate the cosmological implications in $f(Q)$ gravity, through the resulting effective dark energy sector, extracting analytical expressions for the dark energy density, equation-of-state parameter and the deceleration parameters. We show that even in the absence of a cosmological constant, the universe exhibits the usual thermal history, with the sequence of matter and dark energy eras, and the dark-energy equation-of-state parameter always lie in the phantom regime. Additionally, calculating the age of the universe, through the extracted analytical equations of the scenario at hand, we show that the result coincide with the value corresponding to $\Lambda$CDM scenario within 1$\sigma$. Moreover, we show the excellent agreement of the scenario at hand with Supernovae type Ia observational data. Lastly, comparing the cosmological behavior in the case of the absence of an explicit cosmological constant, with the one of the presence of a cosmological constant we show that $f(Q)$ gravity can mimic the cosmological constant in a very efficient way, providing very similar behavior, revealing the advantages and capabilitites of the scenario at hand.Comment: 17 pages, 5 figure

Sudden Future Singularities and their observational signatures in Modified Gravity

We verify the existence of Generalized Sudden Future Singularities (GSFS) in quintessence models with scalar field potential of the form $V(\phi)\sim \vert \phi\vert^n$ where $0<n<1$ and in the presence of a perfect fluid, both numerically and analytically, using a proper generalized expansion ansatz for the scale factor and the scalar field close to the singularity. This generalized ansatz includes linear and quadratic terms, which dominate close to the singularity and cannot be ignored when estimating the Hubble parameter and the scalar field energy density; as a result, they are important for analysing the observational signatures of such singularities. We derive analytical expressions for the power (strength) of the singularity in terms of the power $n$ of the scalar field potential. We then extend the analysis to the case of scalar tensor quintessence models with the same scalar field potential in the presence of a perfect fluid, and show that a Sudden Future Singularity (SFS) occurs in this case. We derive both analytically and numerically the strength of the singularity in terms of the power $n$ of the scalar field potential.Comment: 9 pages, 8 figures, contribution to the School and Workshops on Elementary Particle Physics and Gravity, 2-28 September 2017, Corfu, Greec

Inflation with non-canonical scalar fields revisited

We revisit inflation with non-canonical scalar fields by applying deformed-steepness exponential potentials. We show that the resulting scenario can lead to inflationary observables, and in particular to scalar spectral index and tensor-to-scalar ratio, in remarkable agreement with observations. Additionally, a significant advantage of the scenario is that the required parameter values, such as the non-canonicality exponent and scale, as well as the potential exponent and scale, do not need to acquire unnatural values and hence can accept a theoretical justification. Hence, we obtain a significant improvement with respect to alternative schemes, and we present distinct correlations between the model parameters that better fit the data, which can be tested in future probes. This combination of observational efficiency and theoretical justification makes the scenario at hand a good candidate for the description of inflation.Comment: 15 pages, 6 figures, 2 table

Primordial black holes and gravitational waves from non-canonical inflation

Primordial black holes (PBHs) can generically form in inflationary setups through the collapse of enhanced cosmological perturbations, providing us access to the early Universe through their associated observational signatures. In the current work we propose a new mechanism of PBH production within non-canonical inflation, using a class of steep-deformed inflationary potentials compatible with natural values for the non-canonical exponents. In particular, by requiring significant PBH production we extract constraints on the non-canonical exponents. Additionally, we find that our scenario can lead to the formation of asteroid-mass PBHs, which can account for the totality of the dark matter, as well as to production of solar-mass PBHs within the LIGO/VIRGO detection band. Finally, we find that the enhanced cosmological perturbations which collapse to form PBHs can produce a stochastic gravitational-wave (GW) background induced by second-order gravitational interactions. Very interestingly, we obtain a GW signal detectable by future GW experiments, in particular by SKA, LISA and BBO.Comment: 17 pages without appendices (23 in total), 7 figure

Alleviating both H0H_0 and σ8\sigma_8 tensions in Tsallis cosmology

We present how Tsallis cosmology can alleviate both $H_0$ and $\sigma_8$ tensions simultaneously. Such a modified cosmological scenario is obtained by the application of the gravity-thermodynamics conjecture, but using the non-additive Tsallis entropy, instead of the standard Bekenstein-Hawking one. Hence, one obtains modified Friedmann equations, with extra terms that depend on the new Tsallis exponent $\delta$ that quantifies the departure from standard entropy. We show that for particular $\delta$ choices we can obtain a phantom effective dark energy, which is known to be one of the sufficient mechanisms that can alleviate $H_0$ tension. Additionally, for the same parameter choice we obtain an increased friction term and an effective Newton's constant smaller than the usual one, and thus the $\sigma_8$ tension is also solved. These features act as a significant advantage of Tsallis modified cosmology.Comment: 14 pages, 4 figure

Optimal extent of initial parathyroid resection in patients with multiple endocrine neoplasia syndrome type 1: A meta-analysis

BACKGROUND: Hyperparathyroidism is an almost universal feature of multiple endocrine neoplasia type 1 syndrome. We present a systematic review and meta-analysis of the postoperative outcomes of patients undergoing initial operative treatment of primary hyperparathyroidism complicating multiple endocrine neoplasia 1. METHODS: A comprehensive literature search was performed with a priori defined exclusion criteria for studies comparing total parathyroidectomy, subtotal parathyroidectomy, and less than subtotal parathyroidectomy. RESULTS: Twenty-one studies incorporating 1,131 patients (272 undergoing total parathyroidectomy, 510 subtotal parathyroidectomy, and 349 less than subtotal parathyroidectomy) were identified. Pooled results revealed increased risk for long-term hypoparathyroidism in total parathyroidectomy patients (relative risk 1.61; 95% confidence interval, 1.12-2.31; P = .009) versus those undergoing subtotal parathyroidectomy. In the less than subtotal parathyroidectomy or subtotal parathyroidectomy comparison group, a greater risk for recurrence of hyperparathyroidism (relative risk 1.37; 95% confidence interval, 1.05-1.79; P = .02), persistence of hyperparathyroidism (relative risk 2.26; 95% confidence interval, 1.49-3.41; P = .0001), and reoperation for hyperparathyroidism (relative risk 2.48; 95% confidence interval, 1.65-3.73; P < .0001) was noted for less than subtotal parathyroidectomy patients, albeit with lesser risk for long-term for hypoparathyroidism (relative risk 0.47; 95% confidence interval, 0.29-0.75; P = .002). CONCLUSION: Subtotal parathyroidectomy compares favorably to total parathyroidectomy, exhibiting similar recurrence and persistence rates with a decreased propensity for long-term postoperative hypoparathyroidism. The benefit of the decreased risk of hypoparathyroidism in less than subtotal parathyroidectomy is negated by the increase in the risk for recurrence, persistence, and reoperation. Future studies evaluating the performance of less than subtotal parathyroidectomy in specific multiple endocrine neoplasia 1 phenotypes should be pursued in an effort to delineate a patient-tailored, operative approach that optimizes long-term outcomes

Κοσμολογικές επιπτώσεις θεωριών ενοποίησης

In this thesis, we study cosmological aspects of some well-known dark energy models, and extending our investigation into Unified Theories. We explore the existence of geodesically complete singularities in quintessence and scalar-tensor quintessence models using analytical expressions for the scale factor and the scalar field. Next, using the Tsallis entropy and applying the first law of thermodynamics we construct several cosmological scenarios. The sequence of dark energy span and, depending on the value of the parameter of dark energy equation of state, during the evolution, experience the phantom-divide crossing and can be quintessence or phantom-like. Finally, we revisit inflation with non-canonical scalar fields by applying deformed steepness exponential potentials. A significant advantage of this approach is that free parameters, such as the non-canonicality exponent do not need to acquire unnatural values. The resulting scenario can lead to inflationary observables in remarkable agreement with observations.Στη παρούσα διατριβή, μελετάμε κοσμολογικές πτυχές ορισμένων από τα γνωστά μοντέλα σκοτεινής ενέργειας, επεκτείνοντας την έρευνά μας στις Ενοποιημένες Θεωρίες. Εξετάζουμε την ύπαρξη γεωδαισιακά πλήρων ανωμαλιών στα quintessence και βαθμοτανυστικά quintessence μοντέλα χρησιμοποιώντας αναλυτικές εκφράσεις για τον συντελεστή κλίμακας και το βαθμωτό πεδίο. Στη συνέχεια, χρησιμοποιώντας την εντροπία Tsallis και εφαρμόζοντας τον πρώτο νόμο της θερμοδυναμικής κατασκευάζουμε νέα κοσμολογικά σενάρια σκοτεινής ενέργειας. Το Σύμπαν εμφανίζει τη συνηθισμένη θερμική ιστορία, με την ακολουθία της εποχής της ύλης και σκοτεινής ενέργειας, και σύμφωνα με την τιμή της μη εκτατής παραμέτρου δ η καταστατική εξίσωση σκοτεινής ενέργειας μπορεί να είναι quintessence, phantom-like, ή να βιώνει το phantom-divide crossing κατά τη διάρκεια της εξέλιξης. Στην περίπτωση όπου έχουμε απουσία της κοσμολογικής σταθεράς, το σενάριο μπορεί να μιμηθεί πολύ καλά την ΛCDM Κοσμολογία, σε εξαιρετική συμφωνία με τα παρατηρησιακά δεδομένα Supernovae Ia. Τέλος, επανεξετάζουμε τον πληθωρισμό με μη κανονικά βαθμωτά πεδία. Ένα σημαντικό πλεονέκτημα αυτής της προσέγγισης είναι ότι οι ελεύθερες παράμετροι, όπως ο εκθέτης μη κανονικότητας δεν χρειάζεται να λαμβάνει αφύσικες τιμές. Το σενάριο που προκύπτει μπορεί να οδηγήσει σε πληθωριστικές παρατηρήσιμες ποσότητες οι οποίες βρίσκονται σε αξιοσημείωτη συμφωνία με τις παρατηρήσεις

Modified cosmology through nonextensive horizon thermodynamics

Abstract We construct modified cosmological scenarios through the application of the first law of thermodynamics on the universe horizon, but using the generalized, nonextensive Tsallis entropy instead of the usual Bekenstein–Hawking one. We result to modified cosmological equations that possess the usual ones as a particular limit, but which in the general case contain extra terms that appear for the first time, that constitute an effective dark energy sector quantified by the nonextensive parameter $$\delta$$ δ . When the matter sector is dust, we extract analytical expressions for the dark energy density and equation-of-state parameters, and we extend these solutions to the case where radiation is present too. We show that the universe exhibits the usual thermal history, with the sequence of matter and dark-energy eras, and according to the value of $$\delta$$ δ the dark-energy equation-of-state parameter can be quintessence-like, phantom-like, or experience the phantom-divide crossing during the evolution. Even in the case where the explicit cosmological constant is absent, the scenario at hand can very efficiently mimic $$\Lambda \hbox {CDM}$$ ΛCDM cosmology, and is in excellent agreement with Supernovae type Ia observational data

Modified cosmology through Kaniadakis horizon entropy

We apply the gravity-thermodynamics conjecture, namely the first law of thermodynamics on the Universe horizon, but using the generalized Kaniadakis entropy instead of the standard Bekenstein–Hawking one. The former is a one-parameter generalization of the classical Boltzmann–Gibbs–Shannon entropy, arising from a coherent and self-consistent relativistic statistical theory. We obtain new modified cosmological scenarios, namely modified Friedmann equations, which contain new extra terms that constitute an effective dark energy sector depending on the single model Kaniadakis parameter K. We investigate the cosmological evolution, by extracting analytical expressions for the dark energy density and equation-of-state parameters and we show that the Universe exhibits the usual thermal history, with a transition redshift from deceleration to acceleration at around 0.6. Furthermore, depending on the value of K, the dark energy equation-of-state parameter deviates from $$\Lambda$$CDM cosmology at small redshifts, while lying always in the phantom regime, and at asymptotically large times the Universe always results in a dark-energy dominated, de Sitter phase. Finally, even in the case where we do not consider an explicit cosmological constant the resulting cosmology is very interesting and in agreement with the observed behavior

Kaniadakis holographic dark energy and cosmology

We construct a holographic dark energy scenario based on Kaniadakis entropy, which is a generalization of Boltzmann-Gibbs entropy that arises from relativistic statistical theory and is characterized by a single parameter K which quantifies the deviations from standard expressions, and we use the future event horizon as the Infrared cutoff. We extract the differential equation that determines the evolution of the effective dark energy density parameter, and we provide analytical expressions for the corresponding equation-of-state and deceleration parameters. We show that the universe exhibits the standard thermal history, with the sequence of matter and dark-energy eras, while the transition to acceleration takes place at $$z\approx 0.6$$. Concerning the dark-energy equation-of-state parameter we show that it can have a rich behavior, being quintessence-like, phantom-like, or experience the phantom-divide crossing in the past or in the future. Finally, in the far future dark energy dominates completely, and the asymptotic value of its equation of state depends on the values of the two model parameters