🔭 astrophysics

Probing dark energy evolution with Quaia quasars through the integrated Sachs-Wolfe effect

This study utilizes a tomographic cross-correlation of the Quaia quasar catalogue and Planck CMB maps to detect the Integrated Sachs-Wolfe effect at 2.8σ significance, revealing an amplitude moderately higher than standard ΛCDM predictions that cannot be explained by current alternative dark energy models.

Original authors: M. Ghodsi Yengejeh, J. R. Bermejo-Climent, A. Kovács, I. Szapudi, I. Csabai

Published 2026-07-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: M. Ghodsi Yengejeh, J. R. Bermejo-Climent, A. Kovács, I. Szapudi, I. Csabai

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, expanding trampoline. In the middle of this trampoline, massive clusters of galaxies act like heavy bowling balls, creating deep dips in the fabric of space. If you were to roll a marble (a photon of light) across this trampoline, it would speed up as it falls into a dip and slow down as it climbs out. In a static universe, the speed it gains falling in would be exactly cancelled by the speed it loses climbing out, leaving the marble with no net change in energy.

But our universe isn't static; it's stretching. This is where things get tricky. As the marble rolls across the trampoline, the fabric itself is being pulled taut by an invisible, mysterious force called "dark energy." This force is causing the universe to expand faster and faster. Because the universe is stretching while the marble is rolling, the "dip" the marble falls into might get shallower before it even reaches the bottom, or the "hill" it has to climb might get lower before it gets there. This means the marble doesn't lose all the energy it gained, or it gains a little extra. This tiny, leftover energy change shows up as a slight warming or cooling of the light. This phenomenon is called the Integrated Sachs–Wolfe (ISW) effect. It's like a cosmic echo, a faint whisper from the universe's expansion history that tells us how dark energy is behaving. Scientists care deeply about this because while we know dark energy exists, we don't know what it is. Is it a constant force, or is it changing over time? The ISW effect is one of the few ways we can listen to that whisper to find out.

In this study, a team of astronomers decided to listen to that whisper using a massive, all-sky catalogue of quasars called "Quaia." Quasars are the brilliant, active hearts of distant galaxies, acting like cosmic lighthouses scattered across the universe. The researchers treated these quasars as a map of the universe's large-scale structure—the "bowling balls" creating the dips in our trampoline analogy. They then compared this map against a map of the Cosmic Microwave Background (CMB), which is the afterglow of the Big Bang, acting as the "backlight" that shines through the universe.

By cross-referencing the positions of these quasars with the temperature fluctuations in the CMB, the team looked for the specific signature of the ISW effect. They essentially asked: "Do the hot and cold spots in the ancient light line up with the super-clusters and giant voids of quasars in a way that suggests the universe's expansion is changing the light's energy?"

The team found a signal. They detected the ISW effect with a statistical significance of 2.8 σ (sigma). In the world of science, this is a "moderate" detection—it's more than a fluke, but not quite the gold standard of a definitive discovery (which usually requires 5 σ). They measured the strength of this signal to be an amplitude of AISW ≃ 1.69 ± 0.61 relative to the standard prediction. Think of this as a volume knob: the standard model of the universe (called ΛCDM) predicts a volume of 1.0. The team found the volume was turned up to about 1.69, but with a margin of error that means the true volume could be anywhere between roughly 1.08 and 2.30. So, while the signal is a bit louder than the standard model predicts, the uncertainty is large enough that it's still consistent with the standard model.

The researchers were very careful to check if this "louder" signal was real or just a glitch in their equipment or analysis. They tested their findings against different ways of cleaning up the CMB data, different ways of counting the quasars, and even split the sky into northern and southern halves. In every case, the result remained stable, suggesting the signal is robust and not an artifact of their methods.

However, when they tried to explain this slightly louder signal using new theories about dark energy that have been popular recently (specifically models where dark energy changes over time, known as w0waCDM), the results didn't quite fit. In fact, those alternative models predicted a signal that was weaker than the standard model, not stronger. The data they collected did not support these specific evolving dark energy theories; if anything, the data leaned slightly toward a stronger signal than even the standard model predicts, which the alternative models couldn't explain.

Ultimately, the paper concludes that while they have successfully measured the ISW effect using these quasars, the current data isn't precise enough to tell us if dark energy is changing or staying the same. The "volume" of the signal is a bit high, but the error bars are wide. The authors suggest that future surveys with even more quasars and better maps will be needed to turn this moderate detection into a clear, definitive answer about the nature of dark energy. For now, the cosmic whisper is heard, but its exact meaning remains a mystery waiting for a louder voice.

Technical Summary: Probing Dark Energy Evolution with Quaia Quasars through the Integrated Sachs–Wolfe Effect

Problem and Motivation
The physical nature of dark energy remains unknown despite its dominance in the cosmic energy budget. While the standard $\Lambda$ CDM model posits a cosmological constant ( $w = -1$ ), recent observations from DESI, DES, and other surveys suggest potential deviations, favoring dynamical dark energy models where the equation of state (EoS) evolves with time (e.g., $w_0w_a$ CDM). Distinguishing between these scenarios is challenging because they often yield similar expansion histories but predict different growth rates for large-scale structure and the evolution of gravitational potentials. The Integrated Sachs–Wolfe (ISW) effect offers a unique probe of this evolution: as Cosmic Microwave Background (CMB) photons traverse time-varying gravitational potentials in a dark-energy-dominated universe, they gain or lose energy, imprinting a secondary anisotropy correlated with large-scale structure tracers. However, detecting the ISW effect is difficult due to its small amplitude relative to primary CMB anisotropies and fundamental cosmic variance limits.

Methodology
This study performs a tomographic cross-correlation analysis between the Quaia quasar catalogue (Storey-Fisher et al. 2024) and Planck CMB temperature maps.

Data: The analysis utilizes the Planck PR3 (2018) full-mission data, primarily using the SMICA component-separation map, with NILC and SEVEM maps used for robustness checks. The Quaia catalogue provides a full-sky sample of $\sim$ 1.3 million quasars (limiting magnitude $G < 20.5$ ) with photometric redshifts extending to $z \sim 4$ .
Tomography: The Quaia sample is split into two redshift bins ( $\bar{z} \approx 0.97$ and $\bar{z} \approx 2.10$ ) and analyzed as a total sample ( $\bar{z} \approx 1.54$ ), covering a comoving volume of approximately $(10 h^{-1} \text{ Gpc})^3$ .
Modeling: Theoretical angular power spectra ( $C_\ell^{Tg}$ ) are computed using the pyCCL library under the Limber approximation. The baseline model is flat $\Lambda$ CDM (Planck 2018 parameters). Alternative models include the Chevallier–Polarski–Linder (CPL) parameterization ( $w(a) = w_0 + w_a(1-a)$ ) using best-fit parameters from DES Year 6 and DESI BAO constraints.
Analysis: The cross-power spectra are estimated using NaMaster with a pseudo- $C_\ell$ formalism to correct for mask-induced mode coupling. The ISW amplitude ( $A_{\text{ISW}}$ ) is defined as the ratio of the measured signal to the theoretical $\Lambda$ CDM prediction.
Robustness: Covariance matrices are estimated from 2,000 correlated Gaussian realizations. The analysis tests stability against variations in CMB maps, quasar bias models (including an alternative Laurent et al. 2017 model), sky coverage (North/South splits), and multipole ranges ( $\ell_{\text{max}}$ ).

Key Results

Detection Significance: The study detects the ISW effect at a significance of $2.8\sigma$ ( $2.78\sigma$ ) for the total sample.
Measured Amplitude: The best-fit amplitude is $A_{\text{ISW}} = 1.69 \pm 0.61$ relative to the Planck $\Lambda$ $Λ$ CDM prediction.
- Low-redshift bin ( $\bar{z}=0.97$ ): $A_{\text{ISW}} = 1.19 \pm 0.56$ ( $2.13\sigma$ ).
- High-redshift bin ( $\bar{z}=2.10$ ): $A_{\text{ISW}} = 2.86 \pm 1.62$ ( $1.77\sigma$ ).
Model Comparison:
- The measured amplitude is moderately higher than the fiducial $\Lambda$ CDM prediction ( $A_{\text{ISW}}=1$ ), though consistent within $1.1\sigma$ .
- The alternative $w_0w_a$ CDM models (favored by DES/DESI) predict a lower ISW amplitude than $\Lambda$ CDM (suppressed by up to 20%). Consequently, these models do not resolve the observed excess; in fact, they worsen the fit ( $\Delta\chi^2 > 0$ ) compared to the rescaled $\Lambda$ CDM model.
- An exploratory test with a constant $w = -0.65$ model yielded a fit comparable to the rescaled $\Lambda$ CDM, highlighting the signal's sensitivity to the EoS, though such a value is constrained by other data.
Robustness: The result is stable across different CMB component separation maps (SMICA, NILC, SEVEM), bias models, hemispheric splits, and multipole binning choices. The signal remains consistent even when restricted to the linear regime ( $\ell_{\text{max}} = 100$ ), suggesting the measurement is not dominated by non-linear Rees-Sciama effects or small-scale systematics.

Significance and Conclusions
The paper establishes a robust detection of the ISW effect using the Quaia quasar catalogue, extending previous measurements to a broad redshift range. The primary finding is that the measured ISW amplitude is slightly elevated compared to standard $\Lambda$ CDM expectations, a trend observed in other recent studies (e.g., Stölzner et al. 2018). Crucially, the alternative dynamical dark energy models currently favored by DESI and DES constraints do not account for this discrepancy, as they predict a signal weaker than $\Lambda$ CDM.

The authors conclude that while the current data cannot statistically distinguish between $\Lambda$ CDM and the tested CPL models due to large uncertainties (driven by cosmic variance and the sparse nature of quasar tracers), the result serves as a valuable consistency check. The modest excess signal motivates future work. The paper posits that improved tomographic ISW measurements will require higher source densities and deeper redshift coverage, which will be provided by upcoming data from Gaia DR4, Euclid, and DESI. These future surveys are expected to clarify the origin of the amplitude discrepancy and better constrain the time evolution of dark energy.

More like this