arXiv subjects

Didam Duniya

Publications and source records attributed to Didam Duniya.

13 recordsLinked to original sources

arXiv2026

Hubble tension in k-essence: Evidence for robust tension alleviation

The Hubble tension has come to stay as a major problem in modern cosmology as it continues to plague the standard cosmological model ($Λ$CDM). As one of the viable, self-consistent dark energy theories, k-essence involves nontrivial self-interactions that can modify the background expansion beyond recombination; thereby impacting the sound horizon to last scattering, and hence, the inferred value of the Hubble constant. We examine this tension in two physically motivated k-essence models, dilaton and tachyon, using datasets from Planck and late-Universe probes including Pantheon+SH0ES, cosmic chronometer (CC), Supernova Cosmology Project Union compilation (Union3), Dark Energy Survey Year~5 (DESY5), and Dark Energy Spectroscopic Instrument (DESI) measurements. While $Λ$CDM exhibits inconsistent tension inferences, both k-essence models exhibit a substantial tension alleviation that is robust against the inclusion of the independent late-Universe cosmological datasets, giving consistent tension reduction irrespective of whether the observations are supernovae (Pantheon+SH0ES, Union3, DESY5) alone or in combination with cosmic chronometers (CC) and baryon acoustic oscillation measurements (DESI). The combined late-Universe dataset leads to only $0.14σ$ and $0.69σ$ offsets from the Planck prediction in the dilaton and tachyon models, respectively, compared to $5.89σ$ tension in $Λ$CDM. Both models demonstrate that the inferred tension alleviation is a stable, intrinsic consequence of the underlying k-essence dynamics rather than of model fine tuning: model parameters remain unchanged across datasets. The results establish that the apparent Hubble tension is not an unavoidable feature of late-Universe cosmology but depends critically on the description of dark energy.

astro-ph.CO↗

arXiv2026

Relativistic effects in k-essence

Relativistic effects are sensitive to subtle changes in dark energy. These effects grow on very large scales and at high redshifts, which will be the reach of upcoming surveys. We investigate these effects in both the linear and the angular galaxy power spectra in a late-time universe dominated by cold dark matter and k-essence, focusing on three core models (dilaton, tachyon, and DBI scalar fields) and contrasting their predictions with those of the concordance model. By enforcing identical present-day cosmological parameters, we isolate the imprints of k-essence dynamics and perturbations on very large scales. We found that relativistic corrections dominate on very large scales and grow with redshift, but are largely insensitive to k-essence microphysics in Fourier space, leading to strong degeneracies among the models. However, in the angular power spectrum, where line-of-sight integrals are naturally included, relativistic effects are significantly amplified, yielding better sensitivity to clustering k-essence. In particular, the tachyon exhibits clear deviations across multipoles and redshifts, with distinct imprints in the Doppler and the combined (velocity and gravitational) potentials contributions. Furthermore, our results show that neglecting relativistic corrections can lead to systematic misestimation of deviations of k-essence from the cosmological constant. Our results show the relativistic angular galaxy power spectrum as a more consistent and robust probe of ultra-large-scale physics. These findings underscore the need for full relativistic modelling in next-generation surveys that are targeting horizon-scale modes, where the imprint of non-standard dark energy is most pronounced.

astro-ph.CO↗

arXiv2025

Cosmic magnification in beyond-Horndeski gravity

Cosmic magnification is able to probe the geometry of large-scale structure on cosmological scales, thereby providing another window for probing theories of the late-time cosmic acceleration. It holds the potential to reveal new information on the nature of dark energy and modified gravity. By using the angular power spectrum, we investigated cosmic magnification beyond weak lensing (incorporating all known relativistic corrections) in beyond-Horndeski gravity$-$with both constant phenomenology and dynamic phenomenology, respectively. For both phenomenologies our results show that the total relativistic signal surpasses cosmic variance (considering an SKA2-like sky coverage) in the magnification angular power spectrum at low redshifts ($z\lesssim 0.5$), hence cosmic-variance reduction methods like multi-tracer analysis will not be needed for surveys at the given $z$. For the individual relativistic signals, we found that the Doppler magnification signal also surpasses cosmic variance and remains the dominant signal, at low $z$, for both phenomenologies. However, the integrated-Sachs-Wolfe, the time-delay, and the gravitational (potential) magnification signals, respectively, are subdominant to both the Doppler magnification signal and cosmic variance, at the same $z$; hence multi-tracer analysis will be needed to isolate these signals. At high redshifts ($z\gtrsim 3$), the integrated-Sachs-Wolfe, the time-delay, and the gravitational magnification signals, respectively, appear to surpass cosmic variance and dominate over the Doppler magnification signal for constant phenomenology; whereas for dynamic phenomenology, all these signals diminish significantly and are well below cosmic variance, at all $z$ (consistent with recent quintessence analysis). Suggesting that including time-variation in the parameters will be crucial in identifying the true signature of the beyond-Horndeski gravity.

astro-ph.CO↗

arXiv2025

Investigating the imprint of quintessence in cosmic magnification

We study cosmic magnification beyond lensing in a late-time universe dominated by quintessence and cold dark matter. The cosmic magnification angular power spectrum, especially going beyond the well-known lensing effect, provides an independent avenue for investigating the properties of quintessence, and hence, dark energy. By analysing the magnification power spectrum at different redshifts, it is possible to extract new information about the large-scale imprint of dark energy, including whether we are able to disentangle different models from one another. Using three well-known quintessence models, we analyse the cosmic magnification angular power spectrum while taking relativistic corrections into account. We found that it will be difficult to distinguish between quintessence models, and quintessence from the cosmological constant, in lensing magnification angular power spectrum on large scales, at redshifts $z \,{\leq}\, 1$; whereas, when relativistic corrections are incorporated, the total magnification angular power spectrum holds the potential to distinguish between the models, at the given $z$. At $z \,{\geq}\, 3$, the lensing magnification angular power spectrum can be a reasonable approximation of the total magnification angular power spectrum. We also found that both the total relativistic and the Doppler magnification signals, respectively, surpass cosmic variance at $z \,{\leq}\, 0.5$: hence the effect may be detectable at the given $z$. On the other hand, the ISW and the time-delay magnification signals, respectively, are surpassed by cosmic variance on all scales, at epochs up to $z \,{=}\, 4.5$, with the gravitational-potential magnification signal being zero.

astro-ph.CO↗

arXiv2024

Qualitative probe of interacting dark energy with redshift-space distortions

The imprint of interacting dark energy (IDE) needs to be correctly identified in order to avoid bias in constraints on IDE. This paper investigates the large-scale imprint of IDE in redshift space distortions, using $Euclid$-like photometric prescriptions. A first attempt at incorporating the IDE dynamics in the galaxy (clustering and evolution) biases is made. Without IDE dynamics taken into account in the galaxy biases, as is conventionally done, the results suggest that for a constant dark energy equation of state parameter, an IDE model where the dark energy transfer rate is proportional to the dark energy density exhibits an alternating, positive-negative effect in the redshift space distortions angular power spectrum. However, when the IDE dynamics is incorporated in the galaxy biases, it is found that the apparent positive-negative alternating effect vanishes: implying that neglecting IDE dynamics in the galaxy biases can result in ''artefacts'' that can lead to incorrect identification of the IDE imprint. In general, the results show that multi-tracer analysis will be needed to beat down cosmic variance in order for the redshift space distortions angular power spectrum as a statistic to be a viable diagnostic of IDE. Moreover, it is found that redshift space distortions hold the potential to constrain IDE on large scales, at redshifts $z \,{\leq}\, 1$; with the scenario having IDE dynamics incorporated in the biases showing better potential.

astro-ph.CO↗

arXiv2023

Which is a better cosmological probe: Number counts or cosmic magnification?

The next generation of cosmological surveys will have unprecedented measurement precision, hence they hold the power to put theoretical ideas to the most stringent tests yet. However, in order to realise the full potential of these measurements, we need to ensure that we apply the most effective analytical tools. We need to identify which cosmological observables are the best cosmological probes. Two commonly used cosmological observables are galaxy redshift number counts and cosmic magnification. Both of these observables have been investigated extensively in cosmological analyses, but only separately. In the light of interacting dark energy (IDE) emerging as a plausible means of alleviating current cosmological tensions, we investigate both observables on large scales in a universe with IDE, using the angular power spectrum: taking into account all known terms, including relativistic corrections, in the observed overdensity. Our results suggest that (given multi-tracer analysis) measuring relativistic effects with cosmic magnification will be relatively better than with galaxy redshift number counts, at all redshifts z. Conversely, without relativistic effects, galaxy redshift number counts will be relatively better in probing the imprint of IDE, at all z. At low z (up to around z = 0.1), relativistic effects enable cosmic magnification to be a relatively better probe of the IDE imprint; while at higher z (up to z < 3), galaxy redshift number counts become the better probe of IDE imprint. However, at z = 3 and higher, our results suggest that either of the observables will suffice.

astro-ph.CO↗

arXiv2022

Imprint of f(R) gravity in the cosmic magnification

f(R) gravity is one of the simplest viable modifications to General Relativity: it passes local astrophysical tests, predicts both the early-time cosmic inflation and the late-time cosmic acceleration, and also describes dark matter. In this paper, we probe cosmic magnification on large scales in f(R) gravity, using the well-known Hu-Sawicki model as an example. Our results indicate that at redshifts z < 3, values of the model exponent n > 1 lead to inconsistent behaviour in the evolution of scalar perturbations. Moreover, when relativistic effects are taken into account in the large scale analysis, our results show that as z increases, large-scale changes in the cosmic magnification angular power spectrum owing to integral values of n tend to share a similar pattern, while those of decimal values tend to share another. This feature could be searched for in the experimental data, as a potential "smoking gun" for the given class of gravity models. Furthermore, we found that at z = 1 and lower, relativistic effects lead to a suppression of the cosmic magnification on large scales in f(R) gravity, relative to the concordance model; whereas, at z > 1, relativistic effects lead to a relative boost of the cosmic magnification. In general, relativistic effects enhance the potential of the cosmic magnification as a cosmological probe.

astro-ph.CO↗

arXiv2022

Understanding the relativistic overdensity of galaxy surveys

The main goal of galaxy surveys is to map the distribution of the galaxies, for the purpose of understanding the properties of this distribution and its implications for the content and the evolution of the universe. However, in order to realise the full potential of these surveys, we need to ensure that we are using the correct analysis: a relativistic analysis, which has been widely studied recently. In this work, the known relativistic overdensity of galaxy surveys is re-examined. Unlike in previous works, a consistent approach for incorporating both the relativistic number-count overdensity and the relativistic cosmic magnification overdensity in the total observed overdensity of a generic survey, is presented. Since in practice, analyses are often done for specific sample types (flux-limited or volume-limited) the approach in this work allows for the total observed overdensity to be easily reduced to either of the individual overdensities by applying the same limiting conditions as for extracting the corresponding data samples. This is not obvious, or in some cases not possible, with the results of previous works. Thus, the calculations in this work serve to generalise the expression for the total observed overdensity. However, care must be taken to apply it appropriately: the type of the data sample in consideration needs to be taken into account.

astro-ph.CO↗

arXiv2019

Probing beyond-Horndeski gravity on ultra-large scales

The beyond-Horndeski gravity has recently been reformulated in the dark energy paradigm - which has been dubbed, Unified Dark Energy (UDE). The evolution equations for the given UDE appear to correspond to a non-conservative dark energy scenario, in which the total energy-momentum tensor is not conserved. We investigate both the background cosmology and, the large-scale imprint of the UDE by probing the angular power spectrum of galaxy number counts, on ultra-large scales; taking care to include the full relativistic corrections in the observed overdensity. The background evolution shows that only an effective mass smaller than the Planck mass is needed in the early universe in order for predictions in the given theory to match current observational constraints. We found that the effective mass-evolution-rate parameter, which drives the evolution of the UDE, acts to enhance the observed power spectrum and, hence, relativistic effects (on ultra-large scales) by enlarging the UDE sound horizon. Conversely, both the (beyond) Horndeski parameter and the kineticity act to diminish the observed power spectrum, by decreasing the UDE sound horizon. Our results show that, in a universe with UDE, a multi-tracer analysis will be needed to detect the relativistic effects in the large-scale structure. In the light of a multi-tracer analysis, the various relativistic effects hold the potential to distinguish different gravity models. Moreover, while the Doppler effect will remain significant at all epochs and, thus can not be ignored, the integrated Sachs-Wolfe, the time-delay and the potential (difference) effects, respectively, will only become significant at epochs near z=3 and beyond, and may be neglected at late epochs. In the same vein, the Doppler effect alone can serve as an effective cosmological probe for the large-scale structure or gravity models, in the angular power spectrum - at all z.

astro-ph.CO↗

arXiv2016

Large-scale imprint of relativistic effects in the cosmic magnification

Apart from the known weak gravitational lensing effect, the cosmic magnification acquires relativistic corrections owing to Doppler, integrated Sachs-Wolfe, time-delay and other (local) gravitational potential effects, respectively. These corrections grow on very large scales and high redshifts z, which will be the reach of forthcoming surveys. In this work, these relativistic corrections are investigated in the magnification angular power spectrum, using both (standard) noninteracting dark energy (DE), and interacting DE (IDE). It is found that for noninteracting DE, the relativistic corrections can boost the magnification large-scale power by ~ 40% at z = 3, and increases at lower z. It is also found that the IDE effect is sensitive to the relativistic corrections in the magnification power spectrum, particularly at low z---which will be crucial for constraints on IDE. Moreover, the results show that if relativistic corrections are not taken into account, this may lead to an incorrect estimate of the large-scale imprint of IDE in the cosmic magnification; including the relativistic corrections can enhance the true potential of the cosmic magnification as a cosmological probe.

astro-ph.CO↗

arXiv2016

Dark energy homogeneity in general relativity: Are we applying it correctly?

Thus far, there does not appear to be an agreed (or adequate) definition of homogeneous dark energy (DE). This paper seeks to define a valid, adequate homogeneity condition for DE. Firstly, it is shown that as long as $w_x \neq -1$, DE must have perturbations. It is then argued, independent of $w_x$, that a correct definition of homogeneous DE is one whose density perturbation vanishes in comoving gauge: and hence, in the DE rest frame. Using phenomenological DE, the consequence of this approach is then investigated in the observed galaxy power spectrum - with the power spectrum being normalized on small scales, at the present epoch z = 0. It is found that for high magnification bias, relativistic corrections in the galaxy power spectrum are able to distinguish the concordance model from both a homogeneous DE and a clustering DE - on super-horizon scales.

gr-qc↗

arXiv2015

Probing the imprint of interacting dark energy on very large scales

The observed galaxy power spectrum acquires relativistic corrections from lightcone effects, and these corrections grow on very large scales. Future galaxy surveys in optical, infrared and radio bands will probe increasingly large wavelength modes and reach higher redshifts. In order to exploit the new data on large scales, an accurate analysis requires inclusion of the relativistic effects. This is especially the case for primordial non-Gaussianity and for extending tests of dark energy models to horizon scales. Here we investigate the latter, focusing on models where the dark energy interacts non-gravitationally with dark matter. Interaction in the dark sector can also lead to large-scale deviations in the power spectrum. If the relativistic effects are ignored, the imprint of interacting dark energy will be incorrectly identified and thus lead to a bias in constraints on interacting dark energy on very large scales.

astro-ph.CO↗

arXiv2014

Clustering of quintessence on horizon scales and its imprint on HI intensity mapping

Quintessence can cluster only on horizon scales. What is the effect on the observed matter distribution? To answer this, we need a relativistic approach that goes beyond the standard Newtonian calculation and deals properly with large scales. Such an approach has recently been developed for the case when dark energy is vacuum energy, which does not cluster at all. We extend this relativistic analysis to deal with dynamical dark energy. Using three quintessence potentials as examples, we compute the angular power spectrum for the case of an HI intensity map survey. Compared to the concordance model with the same small-scale power at z=0, quintessence boosts the angular power by up to ~15% at high redshifts, while power in the two models converges at low redshifts. The difference is mainly due to the background evolution, driven mostly by the normalization of the power spectrum today. The dark energy perturbations make only a small contribution on the largest scales, and a negligible contribution on smaller scales. Ironically, the dark energy perturbations remove the false boost of large-scale power that arises if we impose the (unphysical) assumption that the dark energy is smooth.

astro-ph.CO↗