Probing Physics Beyond the Standard Model through Combined Analyses of Next-Generation Type Ia Supernova, CMB, and BAO Surveys

Abstract

Observations of Type Ia supernovae (SNIa), baryon acoustic oscillations (BAO), and the cosmic microwave background (CMB), which probe the late-, intermediate-, and early-universe epochs, respectively, provide complementary constraints on the expansion history of the Universe. In this work, we forecast constraints on dark energy and other extensions to the standard cosmological model by combining the SNIa sample expected from the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), data from current and forthcoming CMB surveys, and BAO measurements from the Dark Energy Spectroscopic Instrument (DESI). For the CMB, we use temperature, polarization, and lensing power spectra ($TT/EE/TE/φφ$) from South Pole Telescope, the planned Advanced Simons Observatory, and a CMB-S4-like experiment. We derive constraints on $Λ{\rm CDM}$ and its extensions involving the dark energy equation of state parameters $(w_{0}, w_{a})$ and the sum of neutrino masses $\sum m_ν$, using a Markov Chain Monte Carlo (MCMC) sampling framework. We find that the LSST Year-3 SNIa sample can improve upon the DES Year-5 dark energy constraints by a factor of $\times2-\times2.5$, with the gains driven primarily by the significantly higher SNIa density in the LSST sample. Similarly, DESI-DR3 shows up to a $\times1.8$ improvement on dark energy parameters over DR2, driven largely by the substantial increase in low-redshift sample. Combining CMB with LSST-Y3-SNIa and DESI-DR3-BAO yields $σ(w_{0}) = 0.028$ and $σ(w_{a}) = 0.11$ for $w_{0} w_{a} {\rm CDM}$ cosmology with the results being largely independent of the CMB dataset. The constraints weaken by 10%-30% when freeing $\sum m_ν$ and spatial curvature. Moreover, the joint analysis of the three datasets can enable a $2-3σ$ detection of $\sum m_ν$.

Figures (13)

• Figure 1: Redshift distribution (left) and the distance modulus as a function of redshift (right) are presented for SNIa samples used in this work. Red distribution represents the LSST-Y3-SNIa mock dataset and we have also shown DES-Y5-SNIa in teal for comparison. The sample size of LSST-Y3-SNIa is roughly $\times3.3$ larger than DES-Y5-SNIa.
• Figure 2: Distance measurements $D_{M}/r_{d}$ from DESI DR2 release (teal circles, abdulkarim25) along with the expectations from the next DR3 release. For DR3, we show the low- (high-redshift) measurements from the clustering of galaxies and quasars (Ly-$\alpha$ forest) at $z \lesssim 2$ ($z>2$) in yellow squares (red hexagons). The DR2 values are actual measurements while the DR3 values are expectations from $\Lambda {\rm CDM}$ (black curve) along with the forecasted errors.
• Figure 3: Constraints on dark energy EoS parameters and the matter density: Teal color shows the constraints from DES-Y5-SNIa sample: dash-dotted corresponds to the actual constraints similar to the original works des24_sne_cosmologyvincenzi24steinhardt25 while the solid contours correspond to a mock dataset with DES-Y5-SNIa-like errors, but simulated assuming the fiducial cosmology as specified in Table \ref{['tab_parameters']}, that is different from the one derived using DES-Y5-SNIa. The expected constraints from LSST-Y3-SNIa are shown in red. We find that the uncertainties on dark energy parameters can be reduced by $\times 2-\times2.5$ with LSST-Y3-SNIa relative to DES. The majority of this constraining power comes from the increased number density in the LSST sample compared to DES, with roughly $\times 1.3 - \times 1.5$ contributed by objects at $z \ge 1$.
• Figure 4: Comparing the constraints in the $r_{d}H_{0} - \Omega_{m}$ plane for $\Lambda {\rm CDM}$ cosmology between DESI DR2 (teal) abdulkarim25 and the upcoming DR3 dataset (orange). For DR2, we present constraints derived from the actual DESI DR2 dataset abdulkarim25, shown with dash‑dotted curves, and from the mock data vectors, drawn with a different cosmology given in Table \ref{['tab_parameters']}, shown with solid curves. For DR3, we show the constraints for the sample containing BAO measurements from galaxies/quasars at $z \lesssim 2$ in red and the ones from Ly-$\alpha$ forest at $z > 2$ in yellow. We see $\times1.5$ improvement in the uncertainties on both parameters when switching from DR2 to DR3. While not shown in this figure, the uncertainties in $w_{0}, w_{a}$ improve by $\times 1.8$ with DR3 relative to the DR2 measurements. Such improvements will be crucial in addressing the growing tension between CMB and BAO.
• Figure 5: Comparison of the constraints derived using the Fisher formalism in open gray contours and MCMC in red for the LSST-Y3-SNIa. For MCMC, we present the results for both the unbinned (filled contours) and binned (open contours). The filled red contours are the same as in Fig. \ref{['fig_w0wa_lsst_des']}. The posteriors are significantly non-Gaussian which are not captured by the Fisher formalism. We find that binning the SNIa sample reduces the constraining power by $\sim 30\%$ for the dark energy parameters.