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Evidence for a sign change of the ISW effect in the very recent universe? hot voids and cold overdensities at $z<0.03$

This paper presents evidence for a significant, negative Integrated Sachs-Wolfe effect in the very recent universe ( $z<0.03$ ), where local underdensities exhibit a CMB temperature excess and overdensities show cooling, with an amplitude far exceeding standard predictions and potentially indicating dynamical dark energy.

Original authors: Frode K. Hansen, Diego Garcia Lambas, Andrés N. Ruiz, Facundo Toscano, Luis A. Pereyra

Published 2026-05-29

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CC BY 4.0

Original authors: Frode K. Hansen, Diego Garcia Lambas, Andrés N. Ruiz, Facundo Toscano, Luis A. Pereyra

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

The Big Picture: A Cosmic Temperature Surprise

Imagine the universe as a giant, invisible ocean of light called the Cosmic Microwave Background (CMB). This is the "afterglow" of the Big Bang, a faint temperature map that covers the entire sky. Usually, this map is very uniform, with only tiny, predictable ripples.

For a long time, scientists believed they understood how this light interacts with the "terrain" of the universe (galaxies and empty spaces). They had a rulebook called the Integrated Sachs-Wolfe (ISW) effect. Think of this rulebook like a physics textbook that says:

When light travels through a mountain (a dense cluster of galaxies), it should get slightly warmer.
When light travels through a valley (a giant empty void), it should get slightly cooler.

The Problem:
In previous studies, the authors found that near our own neighborhood of the universe, the "mountains" (galaxies) were actually making the light colder, not warmer. This was a massive surprise, like finding a campfire that freezes you instead of warming you.

The New Discovery:
In this paper, the authors asked: "If the mountains are freezing the light, what are the valleys doing?" They looked at the giant empty spaces (voids) right next to us (within about 400 million light-years).

The Result:
They found that the light passing through these empty voids is significantly hotter than it should be.

The Analogy: Imagine walking through a canyon. According to the old rulebook, the canyon should make you feel a breeze that cools you down. Instead, the authors found that the canyon is blowing a hot wind that heats you up.

This is the opposite of what the standard model of the universe predicts. It suggests that the "rulebook" (the physics of how gravity and dark energy work) might be broken or different in our very recent cosmic neighborhood.

How They Found It (The Detective Work)

To prove this wasn't just a fluke or a mistake in their telescope, they used a very careful method:

Mapping the Empty Spaces: They used a catalog of nearby galaxies (like a street map) to identify where the giant empty "voids" are. They didn't just guess; they used two different computer algorithms (named Sparkling and Revolver) to find these holes in the universe, ensuring they were looking at real empty spaces and not just random spots.
Taking the Temperature: They pointed the Planck satellite (a super-sensitive cosmic thermometer) at these empty spots.
The Simulation Test: They ran 10,000 computer simulations of what the universe should look like if the standard rules were true.
- The Finding: In 10,000 simulations, the voids were either cold or neutral. In the real data, the voids were hot. The chance of this happening by random luck is less than 0.2%.

Why Is This Happening? (The "Dark Energy" Mystery)

The paper suggests this might be caused by Dark Energy, the mysterious force pushing the universe apart.

The Standard Story: Dark energy pushes the universe apart, stretching the "gravity wells" (the mountains and valleys) so thin that light loses energy (cools down) in the valleys.
The New Story: The authors speculate that in our very recent cosmic neighborhood, Dark Energy might be behaving strangely. Instead of stretching the gravity wells, it might be making them deeper or stronger.
- The Analogy: Imagine a trampoline. Usually, if you put a heavy ball on it, the dip gets deeper as the fabric stretches. But here, it's as if the fabric is suddenly snapping back or the dip is getting deeper in a way that adds energy to anything rolling through it.

This "negative ISW effect" (heating in voids, cooling in galaxies) is about 10 times stronger than what standard physics predicts. It's like hearing a whisper when you expected a shout, but the whisper is actually a scream.

Did They Check Their Work?

Yes, they were very skeptical and ran many tests to make sure they weren't fooling themselves:

Different Maps: They checked the data using different ways of cleaning up the telescope's "static" (noise) and different frequency bands. The result was the same: the voids are hot.
Removing the "Quadrupole": They worried that a large, weird temperature bump in the sky (called a quadrupole) might be messing up their math. When they removed this bump, the result actually got stronger, not weaker.
Size Matters: They found that the bigger the void, the hotter the light. This matches the theory that a larger empty space would have a bigger effect if this new physics is real.

The Bottom Line

The authors have found strong evidence that in our local corner of the universe, the rules of gravity and light are behaving in a way that contradicts our current best theories.

Galaxies are cooling the cosmic background light.
Empty Voids are heating it up.

They don't claim to have solved the mystery of what exactly is causing this, but they suggest it points to a new, unknown behavior of Dark Energy that changes the sign of the gravitational effects we see. It's a "sign change" in the physics of our recent universe.

Technical Summary: Evidence for a Sign Change of the ISW Effect in the Very Recent Universe

Problem Statement
Previous studies (Luparello et al. 2023; Hansen et al. 2023; Garcia Lambas et al. 2024) have established a significant cooling of Cosmic Microwave Background (CMB) photons passing through galactic halos in nearby massive filaments ( $z < 0.04$ ), with temperatures approximately $27\mu\text{K}$ lower than expected in standard $\Lambda$ CDM simulations. This cooling exceeds the predicted amplitude of the Rees-Sciama (RS) effect by an order of magnitude and contradicts the standard Integrated Sachs-Wolfe (ISW) effect, which predicts heating in overdensities and cooling in underdensities at these redshifts. Given recent hints of dynamical dark energy from the Dark Energy Spectroscopic Instrument (DESI), the authors investigate whether a non-standard evolution of gravitational potentials in the very recent Universe ( $z \lesssim 0.1$ ) could cause a sign reversal of the ISW effect. Specifically, if gravitational potentials are growing rather than decaying, CMB photons traversing local voids (underdensities) should experience heating rather than cooling.

Methodology
The study utilizes CMB intensity maps from the Planck satellite (Public Release 3 and 4) and WMAP data, alongside galaxy catalogues from the 2MASS Redshift Survey (2MRS) to identify voids.

Voids Identification: Two independent void-finding algorithms, Sparkling and Revolver, were applied to the 2MRS catalogue within the redshift range $z < 0.03$ . To account for algorithmic and stochastic uncertainties, the authors generated 200 realizations of void maps (combining volume-limited and full galaxy samples) and constructed "void weight maps." These maps assign a weight to each pixel based on the frequency with which it is identified as part of a void across different realizations.
Temperature Measurement: The mean CMB temperature within these voids was calculated using a weighted average over the void weight maps. To isolate the signal from noise and large-scale fluctuations, the authors applied Spherical Mexican Hat Wavelets (SMHW) with a scale of $4^\circ$ , corresponding to the median angular size of the voids ( $\sim 10^\circ$ ).
Control and Consistency:
- Quadrupole Removal: To prevent artificial anticorrelations between cold galaxy regions and hot void regions caused by large-scale CMB modes, the quadrupole was removed from both the data and simulations.
- Simulations: Results were compared against 10,000 simulated SMICA CMB maps and 1,000 simulations for other component separation methods (SEVEM, NILC, Commander).
- Mock Catalogues: 100 mock galaxy catalogues were generated from the Multi-Dark Planck 2 (MDPL2) simulation to test the robustness of the void identification and temperature measurement against random fluctuations in the density field.
- Frequency Independence: Tests were performed using different Planck frequency channels (100 GHz, 217 GHz) and WMAP Q-band (41 GHz) to ensure the signal is achromatic.

Key Results

Significant Temperature Excess: The authors found a significant positive temperature excess in the direction of local voids. Depending on the void sample (all open voids, median size, or largest quartile) and the fraction of the void radius used, the significance ranges from $2.0\sigma$ to $3.5\sigma$ above the mean of simulations.
Line-of-Sight Dependence: The signal is strongest for voids with an extended underdense region along the line of sight ( $\Delta_{LOS} < 0$ ). For these specific voids, the significance increases to $3.7\sigma$ – $4.0\sigma$ . This aligns with the hypothesis that a longer path through an underdense potential well enhances the effect.
Size Scaling: The temperature excess increases with void size, reaching $3.5\sigma$ for the largest voids (largest quartile). This scaling is consistent with an ISW/RS-like mechanism where larger structures yield stronger signals.
Temperature Difference: When comparing the mean temperature in the central parts of the largest voids ( $\sim +30\text{--}40\mu\text{K}$ ) with the previously measured cooling around galactic halos ( $\sim -27\mu\text{K}$ ), the observed difference is approximately $69\mu\text{K}$ . This difference is $6.5\sigma$ larger than the maximum difference found in 10,000 simulations.
Robustness: The results remain consistent across different component separation methods (SMICA, SEVEM, NILC, Commander), different data releases (PR3, PR4), and different frequency bands. The signal persists when splitting the sky into hemispheres or analyzing sub-redshift ranges.
Anticorrelation Check: The study confirms that removing the quadrupole eliminates the strong anticorrelation between galaxy and void temperatures seen in simulations, ensuring the observed hot voids are not merely a statistical compensation for cold galaxy regions.

Significance and Claims
The paper claims to provide evidence for a negative ISW effect in the very recent Universe ( $z < 0.03$ ). The observed phenomenon—hot voids and cold overdensities—represents a sign change relative to the standard $\Lambda$ CDM prediction, where dark energy causes potentials to decay, leading to heating in overdensities and cooling in voids.

The authors argue that the amplitude of the observed effect (an order of magnitude larger than predicted RS/ISW effects) and its sign reversal suggest an unknown physical process. They speculate this could be related to dynamical dark energy or non-standard models where gravitational potentials grow rather than decay at low redshifts. The findings are consistent with recent DESI results hinting at dynamical dark energy and theories involving ultra-light axions or Galilean gravity that predict such sign changes.

The authors remain cautious, noting that a detailed cross-correlation with the non-linear density field is required for a precise theoretical comparison, which is deferred to future work. However, the consistency of the signal across multiple algorithms, frequency bands, and the specific dependence on void size and line-of-sight geometry strongly supports the hypothesis of an anomalous ISW effect in the local Universe.

The Big Picture: A Cosmic Temperature Surprise

How They Found It (The Detective Work)

Why Is This Happening? (The "Dark Energy" Mystery)

Did They Check Their Work?

The Bottom Line

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