Reconstruction of dark energy using DESI DR2

Robot-controlled optical fibers will help create 3D map of the cosmos.

We present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Lyman-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI), to be released in the DESI Data Release 1. DESI BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1 < z < 4.2. To mitigate confirmation bias, a blind analysis was implemented to measure the BAO scales. DESI BAO data alone are consistent with the standard flat ΛCDM cosmological model with a matter density Ωm=0.295±0.015. Paired with a baryon density prior from Big Bang Nucleosynthesis and the robustly measured acoustic angular scale from the cosmic microwave background (CMB), DESI requires H 0=(68.52±0.62) km s-1 Mpc-1. In conjunction with CMB anisotropies from Planck and CMB lensing data from Planck and ACT, we find Ωm=0.307± 0.005 and H 0=(67.97±0.38) km s-1 Mpc-1. Extending the baseline model with a constant dark energy equation of state parameter w, DESI BAO alone require w=-0.99+0.15 -0.13. In models with a time-varying dark energy equation of state parametrised by w 0 and wa , combinations of DESI with CMB or with type Ia supernovae (SN Ia) individually prefer w 0 > -1 and wa < 0. This preference is 2.6σ for the DESI+CMB combination, and persists or grows when SN Ia are added in, giving results discrepant with the ΛCDM model at the 2.5σ, 3.5σ or 3.9σ levels for the addition of the Pantheon+, Union3, or DES-SN5YR supernova datasets respectively. For the flat ΛCDM model with the sum of neutrino mass ∑ mν free, combining the DESI and CMB data yields an upper limit ∑ mν < 0.072 (0.113) eV at 95% confidence for a ∑ mν > 0 (∑ mν > 0.059) eV prior. These neutrino-mass constraints are substantially relaxed if the background dynamics are allowed to deviate from flat ΛCDM.

The Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2) galaxy and quasar clustering data represents a significant expansion of data from Data Release 1 (DR1), providing improved statistical precision in baryon acoustic oscillation (BAO) constraints across multiple tracers, including bright galaxies, luminous red galaxies, emission line galaxies, and quasars. In this paper, we validate the BAO analysis of DR2. We present the results of robustness tests on the blinded DR2 data and, after unblinding, consistency checks on the unblinded DR2 data. All results are compared with those obtained from a suite of mock catalogs that replicate the selection and clustering properties of the DR2 sample. We confirm the consistency of DR2 BAO measurements with DR1 while achieving a reduction in statistical uncertainties due to the increased survey volume and completeness. The combined BAO precision, including both statistical and systematic errors, improves from ∼0.52% in DR1 to 0.30% in DR2—a factor of 1.7 gain. We assess the impact of analysis choices, including different data vectors (correlation function vs power spectrum), modeling approaches and systematics treatments, and an assumption of the Gaussian likelihood, finding that our BAO constraints are stable across these variations and assumptions with a few minor refinements to the baseline setup of the DR1 BAO analysis. We summarize a series of pre-unblinding tests that confirmed the readiness of our analysis pipeline, the final systematic errors, and the DR2 BAO analysis baseline. The successful completion of these tests led to the unblinding of the DR2 BAO measurements, ultimately leading to the DESI DR2 cosmological analysis, with their implications for the expansion history of the Universe and the nature of dark energy presented in the DESI key paper (companion paper).

The Hubble tension and attempts to resolve it by modifying the physics of (or at) recombination motivate finding ways to determine H 0 and the sound horizon at the epoch of baryon decoupling r d in ways that rely neither on a recombination model nor on late-time Hubble data. In this work, we investigate what one can learn from the current and future BAO data when treating r d and H 0 as independent free parameters. It is well known that baryon acoustic oscillations (BAOs) give exquisite constraints on the product r d H 0. We show here that imposing a moderate prior on Ωm h 2 breaks the degeneracy between r d and H 0. Using the latest BAO data, including the recently released the extended Baryon Oscillation Spectroscopic Survey Data Release 16, along with a Ωm h 2 prior based on the Planck best-fit Λ cold dark matter (ΛCDM) model, we find r d = 143.7 ± 2.7 Mpc and H 0 = 69.6 ± 1.8 km s−1 Mpc−1. BAO data prefers somewhat lower r d and higher H 0 than those inferred from Planck data in a ΛCDM model. We find similar values when combing BAO with the Pantheon supernovae, the Dark Energy Survey Year 1 galaxy weak lensing, Planck or SPTPol cosmic microwave background lensing, and the cosmic chronometer data. We perform a forecast for the Dark Energy Spectroscopic Instrument (DESI) and find that, when aided with a moderate prior on Ωm h 2, DESI will measure r d and H 0 without assuming a recombination model with an accuracy surpassing the current best estimates from Planck.

Context. Cosmological parameters and dark energy (DE) behavior are generally constrained assuming a priori models. Aims. We work out a model-independent reconstruction to bind the key cosmological quantities and the DE evolution. Methods. Through the model-independent Bézier interpolation method, we reconstructed the Hubble rate from the observational Hubble data and derived analytic expressions for the distances of galaxy clusters, type Ia supernovae, and uncorrelated baryonic acoustic oscillation (BAO) data. In view of the discrepancy between Sloan Digital Sky Survey (SDSS) and Dark Energy Spectroscopic Instrument (DESI) BAO data, they were kept separate in two distinct analyses. Correlated BAO data were employed to break the baryonic–dark matter degeneracy. All these interpolations enable us to single out and reconstruct the DE behavior with the redshift z in a totally model-independent way. Results. In both analyses, with SDSS-BAO or DESI-BAO datasets, the constraints agree at a 1–σ confidence level (CL) with the flat ΛCDM model. The Hubble constant tension appears solved in favor of the Planck satellite value. The reconstructed DE behavior exhibits deviations at small z (&gt; 1–σ CL), but agrees (&lt; 1–σ CL) with the cosmological constant paradigm at larger z. Conclusions. Our method hints at a slowly evolving DE, consistent with a cosmological constant at early times.

This paper presents an alternative way of analysing baryon acoustic oscillation (BAO) distance measurements via rotations to define new quantities ${\cal D}^{\rm perp}$ and ${\cal D}^{\rm par}$. These quantities allow simple tests of consistency with the Planck $\Lambda$cold dark matter (Λ CDM) cosmology.1 The parameter ${\cal D}^{\rm perp}$ is determined with negligible uncertainty from Planck under the assumption of $\Lambda$CDM. Comparing with measurements from the Dark Energy Spectroscopic Instrument (DESI), we find that the measurements of ${\cal D}^{\rm perp}$ from Data Release 2 (DR2) move into significantly better agreement with the Planck $\Lambda$CDM cosmology compared to DESI Data Release 1 (DR1). The quantity in the orthogonal direction ${\cal D}^{\rm par}$ provides a measure of the physical matter density in the $\Lambda$CDM cosmology. The DR2 measurements of ${\cal D}^{\rm par}$ remain consistent with Planck $\Lambda$CDM despite the substantial improvement in their accuracy compared to the earlier DR1 results. Comparing Planck and DESI BAO measurements, we find no significant evidence in support of evolving dark energy. We also investigate a rotation in the theory space of the $w_0$ and $w_a$ parametrization of the dark energy equation-of-state $w(z)$. We show that the combination of DESI BAO measurements and the CMB constrain $w(z=0.5) = -0.996 \pm 0.046$, very close to the value expected for a cosmological constant. We present a critique of the statistical methodology employed by the DESI collaboration and argue that it gives a misleading impression of the evidence in favour of evolving dark energy. An appendix shows that the cosmological parameters determined from the Dark Energy Survey 5 Year supernova sample are in tension with those from DESI DR2 and parameters determined by Planck.

When measuring the Baryon Acoustic Oscillations (BAO) scale from galaxy surveys, one typically assumes a fiducial cosmology when converting redshift measurements into comoving distances and also when defining input parameters for the reconstruction algorithm. A parameterised template for the model to be fitted is also created based on a (possibly different) fiducial cosmology. This model reliance can be considered a form of data compression, and the data is then analysed allowing that the true answer is different from the fiducial cosmology assumed. In this study, we evaluate the impact of the fiducial cosmology assumed in the BAO analysis of the Dark Energy Spectroscopic Instrument (DESI) survey Data Release 1 (DR1) on the final measurements in DESI 2024 III. We utilise a suite of mock galaxy catalogues with survey realism that mirrors the DESI DR1 tracers: the bright galaxy sample (BGS), the luminous red galaxies (LRG), the emission line galaxies (ELG) and the quasars (QSO), spanning a redshift range from 0.1 to 2.1. We compare the four secondary AbacusSummit cosmologies against DESI's fiducial cosmology (Planck 2018). The secondary cosmologies explored include a lower cold dark matter density, a thawing dark energy universe, a higher number of effective species, and a lower amplitude of matter clustering. The mocks are processed through the BAO pipeline by consistently iterating the grid, template, and reconstruction reference cosmologies. We determine a conservative systematic contribution to the error of 0.1% for both the isotropic and anisotropic dilation parameters α iso and α AP. We then directly test the impact of the fiducial cosmology on DESI DR1 data.

We study eight different gamma-ray burst (GRB) data sets to examine whether current GRB measurements — that probe a largely unexplored part of cosmological redshift (z) space — can be used to reliably constrain cosmological model parameters.We use three Amati-correlation samples and five Combo-correlation samples to simultaneously derive correlation and cosmological model parameter constraints. The intrinsic dispersion of each GRB data set is taken as a goodness measurement. We examine the consistency between the cosmological bounds from GRBs with those determined from better-established cosmological probes, such as baryonic acoustic oscillation (BAO) and Hubble parameter H(z) measurements.We use the Markov chain Monte Carlo method implemented in MontePython to find best-fit correlation and cosmological parameters, in six different cosmological models, for the eight GRB samples, alone or in conjunction with BAO and H(z) data.For the Amati correlation case, we compile a data set of 118 bursts, the A118 sample, which is the largest — about half of the total Amati-correlation GRBs — current collection of GRBs suitable for constraining cosmological parameters. This updated GRB compilation has the smallest intrinsic dispersion of the three Amati-correlation GRB data sets we examined. We are unable to define a collection of reliable bursts for current Combo-correlation GRB data.Cosmological constraints determined from the A118 sample are consistent with — but significantly weaker than — those from BAO and H(z) data. They also are consistent with the spatially-flat ΛCDM model, in which dark energy is the cosmological constant Λ, as well as with dynamical dark energy models and non-spatially-flat models. Since GRBs probe a largely unexplored region of z, it is well worth acquiring more and better-quality burst data which will give a more definitive answer to the question of the title.

Recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) show hints of tension with data from the cosmic microwave background (CMB) when interpreted within the standard model of cosmology. In this short note we discuss the consequences of one solution to this tension, a small but negative spatial curvature with R k = 21 H 0 -1, which DESI measures at 2σ when combined with CMB data. We describe the physical role of curvature in cosmological distance measures tied to recombination, i.e. the CMB and BAO, and the relation to neutrino mass constraints which are relaxed to ∑ mν < 0.10 eV at 95% confidence when curvature is allowed to deviate from zero. A robust detection of negative curvature would have significant implications for inflationary models: improved BAO measurements, particularly from future high-redshift spectroscopic surveys, will be able to distinguish curvature from other solutions to the DESI-CMB tension like phantom dark energy at high significance.

The cosmic distance duality relation (CDDR) is a fundamental and practical condition in observational cosmology that connects the luminosity distance and angular diameter distance. Testing its validity offers a powerful tool to probe new physics beyond the standard cosmological model. In this work, for the first time, we present a novel consistency test of CDDR by combining HII galaxy data with a comprehensive set of Baryon Acoustic Oscillations (BAO) measurements. The BAO measurements include two-dimensional (2D) BAO and three-dimensional (3D) BAO from the Sloan Digital Sky Survey (SDSS), as well as the latest 3D BAO data from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2). We adopt four different parameterizations of the distance duality relation parameter, η(z), to investigate possible deviations and their evolution with cosmic time. To ensure accurate redshift matching across datasets, we reconstruct the distance measures through a model-independent Artificial Neural Network (ANN) approach. We find no significant deviation from the CDDR (less than 68% confidence level) among four parameterizations. Furthermore, our results show that the constraints on η(z) obtained separately from 2D and 3D BAO measurements are consistent at the 68% confidence level. This indicates that there is no significant tension between the two datasets under the four parameterizations considered. Our ANN reconstruction of HII galaxies could provide constraints on the CDDR at redshifts beyond the reach of Type Ia supernovae. Finally, the consistency of our results supports the standard CDDR and demonstrates the robustness of our analytical approach.

Recent cosmological observations have provided numerous new observations with an increasing level of precision, ushering in an era of precision cosmology. The exquisite quality of these observations opens up new possibilities in terms of measuring fundamental constants with good precision on scales that are complementary to laboratory references. In particular, the cosmic microwave background (CMB) temperature and polarisation spectra contain a wealth of information that goes well beyond the basic cosmological parameters. In this paper, we update the precision on a cosmological determination of the gravitational constant, G, by using the latest Planck data release (PR4) in combination with the latest baryon acoustic oscillation (BAO) from the Dark Energy Spectroscopic Instrument (DESI) data release 1 and the BBN prior on the primordial helium fraction. We demonstrate a precision of 1.8%, corresponding to a ∼40% improvement with regard to previous results in the literature. This is comparable to the level achieved by Cavendish in 1873 using a torsion balance. However, it is a complementary measurement because it has been obtained under wildly different physical environments compared to laboratory results or even studies of the very nearby Universe. Our analysis takes into account the modification of the primordial helium fraction predicted by Big Bang nucleosynthesis (BBN), induced by a variation in G. We also point out the importance of the polarisation data in attaining the ultimate level of precision. In particular, we discuss the constraints that can be obtained by considering either the low-ℓ or the high-ℓ part of the spectra. Within the ΛCDM model, we find G = (6.75 ± 0.12)×10−11 m3 kg−1 s−2. This measurement is compatible with laboratory results within one standard deviation. Finally, we show that this cosmological measurement of G is robust against several assumptions made on the cosmological model, particularly when considering a non-standard dark energy fluid or non-flat models.

The Dark Energy Spectroscopic Instrument (DESI) collaboration, combining their baryon acoustic oscillation (BAO) data with cosmic microwave background (CMB) anisotropy and supernovae data, have found significant indication against the Lambda cold dark matter ($\Lambda$CDM) cosmology. This can also be interpreted as the significance of the detection of the $w_a$ parameter that measures variation of the dark energy equation of state. DESI’s DR2 article quotes exclusion of $\Lambda$CDM for combinations of BAO and CMB data with each of three different and overlapping supernovae compilations (at 2.8σ for Pantheon+ , 3.8σ for Union3, and 4.2σ for DESY5). We show that one can neither choose amongst nor average over these three different significances. We demonstrate how a principled statistical combination yields a combined exclusion significance of 3.1σ. Further we argue that, faced with these competing significances, the most secure inference from the DESI DR2 results is the 3.1σ level exclusion of $\Lambda$CDM obtained from combining DESI + CMB alone, omitting supernovae.

The recent results from the first-year baryon acoustic oscillations (BAO) data released by the Dark Energy Spectroscopic Instrument (DESI), combined with cosmic microwave background (CMB) and Type Ia supernova (SN) data, have shown a detection of significant deviation from a cosmological constant for dark energy. In this work, we utilize the latest DESI BAO data in combination with the SN data from the full 5 yr observations of the Dark Energy Survey and the CMB data from the Planck satellite to explore potential interactions between dark energy and dark matter. We consider four typical forms of the interaction term Q. Our findings suggest that interacting dark energy (IDE) models with Q ∝ ρ de support the presence of an interaction where dark energy decays into dark matter. Specifically, the deviation from ΛCDM for the IDE model with Q = β H 0 ρ de reaches the 3σ level. These models yield a lower value of Akaike information criterion than the ΛCDM model, indicating a preference for these IDE models based on the current observational data. For IDE models with Q ∝ ρ c, the existence of interaction depends on the form of the proportionality coefficient Γ. The IDE model with Q = β H ρ c yields β = 0.0003 ± 0.0011, which essentially does not support the presence of the interaction. In general, whether the observational data support the existence of interaction is closely related to the model. Our analysis helps to elucidate which type of IDE model can better explain the current observational data.

The Lambda cold dark matter (ΛCDM) cosmological model provides a good description of a wide range of astrophysical and cosmological observations. However, severe challenges to the phenomenological ΛCDM model have emerged recently, including the Hubble constant tension and the significant deviation from the ΛCDM model reported by the Dark Energy Spectroscopic Instrument (DESI) collaboration. Despite many explanations for the two challenges having been proposed, their origins are still intriguing mysteries. Here, we investigate the DESI baryon acoustic oscillation (BAO) measurements to interpret the Hubble constant tension. Employing a nonparametric method, we find that the dark energy equation of state w ( z ) evolves with redshift from DESI BAO data and Type Ia supernovae. From the Friedmann equations, the Hubble constant ( H 0 ) is derived from w ( z ) model-independently. We find that the values of H 0 show a descending trend as a function of redshift, and can effectively resolve the Hubble constant tension. Our study finds that the two unexpected challenges to the ΛCDM model can be understood in one physical framework, e.g., dynamical dark energy.

We use the new gamma-ray bursts (GRBs) data, combined with the baryon acoustic oscillation (BAO) observation from the spectroscopic Sloan digital sky survey (SDSS) data release, the newly obtained A parameter at z=0.6 from the WiggleZ dark energy survey, the cosmic microwave background (CMB) observations from the 7-year Wilkinson microwave anisotropy probe (WMAP7) results, and the type Ia supernovae (SNeIa) from Union2 set, to constrain a phenomenological model describing possible interactions between dark energy and dark matter, which was proposed to alleviate the coincidence problem of the standard ΛCDM model. By using the Markov chain Monte Carlo (MCMC) method, we obtain the marginalized 1σ constraints Ωm=0.2886±0.0135, rm=−0.0047±0.0046, and wX=−1.0658±0.0564. We also consider other combinations of these data for comparison. These results show that: (1) the energy of dark matter is slightly transferring to that of dark energy; (2) even though the GRBs+BAO+CMB data present less stringent constraints than SNe+BAO+CMB data do, the GRBs can help eliminate the degeneracies among parameters.