The Current Acceleration of the Universe: Comparing SDSS and DESI Data
Joint analysis of DESI DR2 data (over 14 million galaxies and quasars), cosmic microwave background (Planck), and Type Ia supernovae points to a preference for dynamic dark energy. Comparison with SDSS reveals three key findings: 1) DESI+Planck combination yields w0 = -0.41^{+0.21}_{-0.22} versus w0 = -0.71^{+0.19}_{-0.18} from SDSS+Planck; 2) the deceleration parameter q0 from DESI+Planck (0.10^{+0.21}_{-0.23}) is consistent with no acceleration, while SDSS+Planck gives q0 = -0.22^{+0.20}_{-0.21}, indicating accelerated expansion; 3) the discrepancy is explained by the different minimum effective redshift: z_eff ≈ 0.295 (DESI) and z_eff ≈ 0.15 (SDSS). Since w0 and q0 characterize the current epoch, data near z=0 constrain them directly; lacking such data (as in DESI), the parameters are determined by extrapolation of the CPL parametrization, which shifts q0 to positive values and w0 significantly above -1. Adding Pantheon+ supernovae restores low-redshift information, returning q0 to negative values and reducing the tension with ΛCDM.
Context
The question of whether the Universe is accelerating right now is fundamental to understanding dark energy. Back then, Edwin Hubble showed that the Universe expands, and Georges Lemaître tied this to a unified theory described by Friedmann equations. Recent data from the DESI instrument combined with cosmic microwave background (Planck) show a preference for evolving dark energy, yet leave the sign of the deceleration parameter in the present epoch undetermined. Since the expansion of the Universe is sensitive to the lowest redshifts, having data as close to z=0 as possible is critically important.
Methods
The analysis used publicly available cosmological MCMC chains for SDSS DR16 and DESI DR2 within the CPL (Chevallier–Polarski–Linder) parameterization of dark energy, along with Hubble's law to link distances and redshifts. From these chains, the parameters w0 and q0 — the present-day deceleration parameter — were calculated. Additionally, Type Ia supernova data from the Pantheon+ catalog were incorporated. To test the hypothesis about the influence of low redshifts, the Main Galaxy Sample (MGS) from SDSS, the one nearest to us, was removed. Extensions with massive neutrinos were also considered.
Results
The joint analysis of DESI+Planck yields w0 = −0.41+0.21−0.22 and q0 = +0.10+0.21−0.23, allowing for both acceleration and deceleration. Meanwhile, SDSS+Planck confidently indicates acceleration: w0 = −0.71+0.19−0.18, q0 = −0.22+0.20−0.21, consistent with predictions by Adam Riess and other pioneers of the acceleration discovery. The key difference is the minimum effective redshift: zeff≈0.15 for SDSS versus ≈0.295 for DESI. Removing the low-redshift 'anchor' MGS from SDSS weakens the constraint and shifts it toward q0 = −0.10+0.33−0.35, approaching DESI. Adding Pantheon+ to DESI restores acceleration: q0 = −0.37±0.06, in agreement with ΛCDM.
Implications
The results show that the apparent lack of acceleration in DESI data does not necessarily imply new physics, but rather reflects an extrapolation of dark energy into the z→0 range without direct measurements. To reliably determine the sign of q0 and the dynamics of w0, data at z<0.2 are needed. This is critical for interpreting discrepancies between surveys and for testing models beyond ΛCDM.
Future development
Future DESI analyses could use a finer tomographic binning of the Bright Galaxy Survey (BGS) sample, enabling an effective redshift around z≈0.18 — similar to SDSS. This will directly test whether the tension with ΛCDM is alleviated with improved low-z coverage.
Impact
The work impacts fundamental cosmology and methods for analyzing dark energy, as well as planning for future surveys like Euclid and the Roman Space Telescope.
Next steps
Conduct detailed sensitivity forecasts for low-redshift bins of BGS and cross-check the results with identical CMB priors to eliminate systematic effects.
Key open problems
Directly connected to the unsolved problem of the nature of dark energy and its time dynamics. It also resonates with the 'H0 tension' issue, as local measurements of the Hubble constant also rely on low-redshift data.
🎯 If we trusted only DESI and CMB, we might conclude that the Universe had already transitioned to deceleration about a billion years ago, and then started accelerating again — but most likely this is an illusion caused by a lack of data near us.
Key numbers
- w0 (DESI+Planck): −0.41+0.21−0.22
- q0 (DESI+Planck): 0.10+0.21−0.23
- q0 (SDSS+Planck): −0.22+0.20−0.21
- Minimum zeff (SDSS): 0.15
- Minimum zeff (DESI): 0.295