The f(Q) theories of modified gravity arise from the consideration of non-metricity as the basic geometric quantity, and have been proven to be very efficient in describing the late-time Universe. We use the Big Bang Nucleosynthesis (BBN) formalism and observations in order to extract constraints on various classes of f(Q) models. In particular, we calculate the deviations that f(Q) terms bring on the freeze-out temperature  $$T_f$$  <math> <msub> <mi>T</mi> <mi>f</mi> </msub> </math>   in comparison to that of the standard  $$\Lambda $$  <math> <mi>Λ</mi> </math>  CDM evolution, and then we impose the observational bound on  $$ \left| \frac{\delta {T}_f}{{T}_f}\right| $$  <math> <mfenced> <mfrac> <mrow> <mi>δ</mi> <msub> <mi>T</mi> <mi>f</mi> </msub> </mrow> <msub> <mi>T</mi> <mi>f</mi> </msub> </mfrac> </mfenced> </math>   to extract constraints on the involved parameters of the considered models. Concerning the polynomial model, we show that the exponent parameter should be negative, while for the power-exponential model and the new hyperbolic tangent-power model we find that they pass the BBN constraints trivially. Finally, we examine two DGP-like f(Q) models, and we extract the bounds on their model parameters. Since many gravitational modifications, although able to describe the late-time evolution of the Universe, produce too-much modification at early times and thus fall to pass the BBN confrontation, the fact that f(Q) gravity can safely pass the BBN constraints is an important advantage of this modified gravity class.

Greece

Fotios Anagnostopoulos

Viktor Gakis

China

Emmanuel Saridakis

Cyprus

Spyros Basilakos

New models and big bang nucleosynthesis constraints in f(Q) gravity

We use observational data from Supernovae (SNIa) Pantheon sample, as well as from direct measurements of the Hubble parameter from the cosmic chronometers (CC) sample, in order to extract constraints on the scenario of Barrow holographic dark energy. The latter is a holographic dark energy model based on the recently proposed Barrow entropy, which arises from the modification of the black-hole surface due to quantum-gravitational effects. We first consider the case where the new deformation exponent  $$\Delta $$  <math> <mi>Δ</mi> </math>   is the sole model parameter, and we show that although the standard value  $$\Delta =0$$  <math> <mrow> <mi>Δ</mi> <mo>=</mo> <mn>0</mn> </mrow> </math>  , which corresponds to zero deformation, lies within the  $$1\sigma $$  <math> <mrow> <mn>1</mn> <mi>σ</mi> </mrow> </math>   region, a deviation is favored. In the case where we let both  $$\Delta $$  <math> <mi>Δ</mi> </math>   and the second model parameter to be free we find that a deviation from standard holographic dark energy is preferred. Additionally, applying the Akaike, Bayesian and Deviance Information Criteria, we conclude that the one-parameter model is statistically compatible with  $$\Lambda \hbox {CDM}$$  <math> <mrow> <mi>Λ</mi> <mtext>CDM</mtext> </mrow> </math>   paradigm, and preferred comparing to the two-parameter one. Finally, concerning the present value of the Hubble parameter we find that it is close to the Planck value.

Observational constraints on Barrow holographic dark energy

We study how the cosmological constraints from growth data are improved by including the measurements of bias from Dark Energy Survey (DES). In particular, we utilize the biasing properties of the DES Luminous Red Galaxies (LRGs) and the growth data provided by the various galaxy surveys in order to constrain the growth index ($$\gamma $$ <math><mi>γ</mi></math> ) of the linear matter perturbations. Considering a constant growth index we can put tight constraints, up to $$\sim 10\%$$ <math><mrow><mo>∼</mo><mn>10</mn><mo>%</mo></mrow></math>  accuracy, on $$\gamma $$ <math><mi>γ</mi></math> . Specifically, using the priors of the Dark Energy Survey and implementing a joint likelihood procedure between theoretical expectations and data we find that the best fit value is in between $$\gamma =0.64\pm 0.075$$ <math><mrow><mi>γ</mi><mo>=</mo><mn>0.64</mn><mo>±</mo><mn>0.075</mn></mrow></math>  and $$0.65\pm 0.063$$ <math><mrow><mn>0.65</mn><mo>±</mo><mn>0.063</mn></mrow></math> . On the other hand utilizing the Planck priors we obtain $$\gamma =0.680\pm 0.089$$ <math><mrow><mi>γ</mi><mo>=</mo><mn>0.680</mn><mo>±</mo><mn>0.089</mn></mrow></math>  and $$0.690\pm 0.071$$ <math><mrow><mn>0.690</mn><mo>±</mo><mn>0.071</mn></mrow></math> . This shows a small but non-zero deviation from General Relativity ($$\gamma _{\mathrm{GR}}\approx 6/11$$ <math><mrow><msub><mi>γ</mi><mi>GR</mi></msub><mo>≈</mo><mn>6</mn><mo>/</mo><mn>11</mn></mrow></math> ), nevertheless the confidence level is in the range $$\sim 1.3-2\sigma $$ <math><mrow><mo>∼</mo><mn>1.3</mn><mo>-</mo><mn>2</mn><mi>σ</mi></mrow></math> . Moreover, we find that the estimated mass of the dark-matter halo in which LRGs survive lies in the interval $$\sim 6.2 \times 10^{12} h^{-1} M_{\odot }$$ <math><mrow><mo>∼</mo><mn>6.2</mn><mo>×</mo><msup><mn>10</mn><mn>12</mn></msup><msup><mi>h</mi><mrow><mo>-</mo><mn>1</mn></mrow></msup><msub><mi>M</mi><mo>⊙</mo></msub></mrow></math>  and $$1.2 \times 10^{13} h^{-1} M_{\odot }$$ <math><mrow><mn>1.2</mn><mo>×</mo><msup><mn>10</mn><mn>13</mn></msup><msup><mi>h</mi><mrow><mo>-</mo><mn>1</mn></mrow></msup><msub><mi>M</mi><mo>⊙</mo></msub></mrow></math> , for the different bias models. Finally, allowing $$\gamma $$ <math><mi>γ</mi></math>  to evolve with redshift [Taylor expansion: $$\gamma (z)=\gamma _{0}+\gamma _{1}z/(1+z)$$ <math><mrow><mi>γ</mi><mrow><mo>(</mo><mi>z</mi><mo>)</mo></mrow><mo>=</mo><msub><mi>γ</mi><mn>0</mn></msub><mo>+</mo><msub><mi>γ</mi><mn>1</mn></msub><mi>z</mi><mo>/</mo><mrow><mo>(</mo><mn>1</mn><mo>+</mo><mi>z</mi><mo>)</mo></mrow></mrow></math> ] we find that the $$(\gamma _{0},\gamma _{1})$$ <math><mrow><mo>(</mo><msub><mi>γ</mi><mn>0</mn></msub><mo>,</mo><msub><mi>γ</mi><mn>1</mn></msub><mo>)</mo></mrow></math>  parameter solution space accommodates the GR prediction at $$\sim 1.7-2.9\sigma $$ <math><mrow><mo>∼</mo><mn>1.7</mn><mo>-</mo><mn>2.9</mn><mi>σ</mi></mrow></math>  levels.