⚛️ general relativity

Reconstructing the kinematics of Laniakea using Type Ia Supernovae

This paper proposes a kinematic framework that utilizes Type Ia supernovae within the Laniakea supercluster to directly reconstruct its ellipsoidal peculiar velocity field by analyzing the monopole, dipole, and quadrupole of luminosity distance anisotropies.

Original authors: Francesco Sorrenti, Erick Pastén, Leonardo Giani

Published 2026-07-14

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CC BY 4.0

Original authors: Francesco Sorrenti, Erick Pastén, Leonardo Giani

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, invisible ocean. For a long time, scientists thought this ocean was perfectly calm and flat, with water flowing at the same speed everywhere. But if you look closely at the "waves" of light coming from exploding stars (Type Ia supernovae), you can see that the water isn't flat at all. It's sloshing around, swirling, and stretching in weird ways.

In this new study, a team of researchers decided to map the specific currents of a massive cosmic whirlpool called Laniakea. Think of Laniakea not as a solid island, but as a giant, invisible bubble of space about 110 Mpc/h (roughly 360 million light-years) across, where thousands of galaxies, including our own Milky Way, are all drifting together.

The Cosmic "Stretchy" Bubble

The main discovery here is that the authors built a new mathematical "lens" to look at these exploding stars. Instead of just measuring how far away they are, they looked at how the light from these stars is stretched or squished depending on the direction you look.

They found that inside the Laniakea bubble, space isn't just expanding; it's behaving like a piece of dough being pulled in different directions.

The Squeeze: The whole bubble is actually shrinking slightly as it flows toward a central point. The team measured this "shrinking" (or negative expansion) at a rate of −5.7 km/s/Mpc (with a margin of error of +2.4 and −2.2).
The Stretch: While the whole thing is shrinking, it's also getting squashed in one direction and stretched in another, like a rubber ball being squeezed by a giant hand. This "squishing" is called shear. They found three different shear values: −4.9 ± 1.5 km/s/Mpc, 0.54 (+0.93/−0.90) km/s/Mpc, and 4.3 (+1.3/−1.2) km/s/Mpc.

This means the universe around us isn't just a boring, uniform expansion. It's a complex, lopsided flow, and the light from supernovae carries the fingerprints of this motion.

The "Bulk Flow" Drift

The researchers also tracked the "bulk motion" of this entire bubble. Imagine you are on a raft in a river; even if you aren't paddling, the current carries you. The team found that our Solar System is being carried by a current moving at about 299 km/s (with a range of +56/−58) toward a specific spot in the sky. This direction lines up with a massive cluster of galaxies called the Shapley Supercluster, acting like a cosmic magnet pulling everything toward it.

What They Ruled Out (and What They Didn't)

The paper is very careful about what it doesn't claim.

It's not a perfect sphere: The authors explicitly argue against the idea that Laniakea is a simple, round ball. If you tried to model it as a sphere, you would get the wrong answer (specifically, you might think it's expanding when it's actually shrinking). The "ellipsoidal" (egg-shaped) model is the one that fits the data.
It's not a solved mystery: The paper does not say they have "solved" the Hubble Tension (the disagreement about how fast the universe is expanding) or proven that local structures are the only reason for it. They suggest that these local flows could be a piece of the puzzle, but they don't claim to have fixed the whole problem yet.
It's not just a guess: They didn't just theorize; they tested their method using "mock" data (computer simulations). In these simulations, when they picked the right shape for the bubble, their method worked perfectly. But when they picked the wrong shape (like making the bubble too big or shifting it), the results got messy and biased. This proves that how you choose which stars to look at matters a huge amount.

How Sure Are They?

The authors are confident in their method but cautious about the exact numbers.

The Shape: They are quite sure the direction of the "stretch" (the eigenvectors) matches the egg-shape of Laniakea found in previous studies. The directions they found line up well with the "principal axes" of the bubble.
The Speed: The numbers for how fast it's shrinking or stretching are measured from the supernova data, but they come with "error bars" (uncertainty ranges). For instance, the expansion is −5.7 (+2.4/−2.2) km/s/Mpc. This means the real number is likely somewhere between −3.3 and −8.1.
The Future: The paper suggests that with future telescopes like LSST and ZTF, which will find thousands more supernovae, we will be able to map these cosmic currents even better. Right now, they have shown it's possible to do this, but the current data still has some wiggle room.

The Bottom Line

This paper is like finding a new way to read the wind. By looking at how the light from exploding stars is distorted, the authors showed that our local neighborhood of the universe is a dynamic, egg-shaped flow that is shrinking and stretching in specific ways. It's a fresh, independent way to map the invisible currents of the cosmos, confirming that Laniakea is a real, physical structure with its own unique dance, distinct from the smooth, uniform expansion of the rest of the universe.

Technical Summary: Reconstructing the kinematics of Laniakea using Type Ia Supernovae

Problem Statement
The Cosmological Principle posits a homogeneous and isotropic Universe on large scales; however, low-redshift observables often exhibit anisotropies attributed to local large-scale structure (LSS) and peculiar velocities. While supernova surveys have been used to quantify these anisotropies via dipole or higher multipole corrections, the mapping between these fitted multipoles and the underlying kinematics of the local velocity field (expansion, shear, and observer displacement) remains nontrivial. Standard perturbed FLRW machinery struggles to disentangle global cosmological parameters from environmental contributions without strong modeling assumptions. Recent analyses have identified anomalously large Bulk Flows and suggested that local inhomogeneities, such as the Laniakea supercluster, may impact cosmological inference, including the Hubble tension. Existing reconstructions of the local velocity field often rely on density surveys and Newtonian relations, potentially neglecting vorticity or suffering from biases due to incomplete sky coverage. There is a need for a physically motivated framework that directly links luminosity distance anisotropies to the specific kinematic dynamics of known superstructures like Laniakea.

Methodology
The authors develop a kinematic framework relating the monopole, dipole, and quadrupole of the luminosity distance to an ellipsoidal peculiar velocity field describing the Laniakea supercluster. The methodology proceeds as follows:

Reference Frame Transformation: The analysis explicitly connects three reference frames: the Laniakea comoving frame (centered on the dynamical center), the Solar System frame (observer location), and the CMB rest frame. The velocity field in the Laniakea frame is modeled as an anisotropic ellipsoidal flow characterized by a diagonal tensor $\Delta H$ . This is transformed into the Solar System frame via a rotation matrix $R$ and a displacement vector $p$ , and subsequently to the CMB frame.
Kinematic Decomposition: The velocity field in the CMB frame is decomposed into a scalar expansion ( $\theta$ ), a traceless shear tensor ( $\sigma_{ij}$ ), and a dipolar bulk motion ( $d$ ). The authors derive expressions linking these kinematic quantities to the multipolar corrections in the luminosity distance ( $d_L$ ) and the corrected redshift ( $z_{cos}$ ).
Data Selection: The study utilizes the Pantheon+SH0ES compilation of Type Ia supernovae (SNe Ia). The sample is partitioned into two sub-samples based on whether the supernovae lie inside or outside the effective ellipsoidal volume of Laniakea, as defined by previous work using CosmicFlows-4 data. Positions are converted to Cartesian coordinates in the supergalactic frame using a fiducial $\Lambda$ CDM distance.
Statistical Analysis: A Markov Chain Monte Carlo (MCMC) routine is employed to constrain cosmological parameters ( $H_0, \Omega_{m,0}$ ) globally, while kinematic parameters (expansion $\theta$ , shear eigenvalues $\sigma_i$ , eigenvectors $e_i$ , and dipole amplitude/direction) are constrained separately for the "inside" and "outside" Laniakea regions. The external region is modeled with a pure dipole, while the internal region includes the full ellipsoidal expansion and shear.
Validation: The robustness of the method is tested using mock SNe Ia datasets where an ellipsoidal velocity field is injected into a specific volume. The recovery of kinematic parameters is evaluated under different geometric selections (enlarged, rotated, and shifted volumes) to assess sensitivity to volume definition errors.

Key Contributions

Theoretical Framework: The paper establishes a direct physical link between luminosity distance multipoles and the expansion/shear of a local peculiar velocity field, moving beyond phenomenological dipole fits to a kinematic interpretation.
Independent Reconstruction: It demonstrates that SNe Ia can serve as direct kinematic tracers of large-scale structures, offering a method independent of galaxy density reconstructions or distance indicators used in traditional peculiar velocity surveys.
Laniakea Kinematics: The authors apply this framework to the Laniakea supercluster, recovering its expansion scalar, shear eigenvalues, and principal axis directions directly from supernova data.

Results

Kinematic Parameters: The analysis of Pantheon+ data yields a negative expansion scalar ( $\theta \approx -5.7^{+2.4}_{-2.2}$ km/s/Mpc) inside Laniakea, indicating a convergent flow toward the dynamical center. The shear eigenvalues display a clear anisotropic pattern (one negative, one near zero, one positive), qualitatively consistent with the ellipsoidal model proposed in [45] but with magnitudes approximately twice as large.
Dipole and Bulk Flow: The inferred dipole amplitude and direction are consistent within $2\sigma$ with the bulk motion found in CosmicFlows-4, pointing toward the Shapley supercluster.
Eigenvector Alignment: The recovered eigenvector directions for the shear tensor show very good agreement with the principal axes of the Laniakea ellipsoid defined in [45].
Cosmological Parameters: The global constraints on $H_0$ ( $73.6^{+1.1}_{-1.0}$ km/s/Mpc) and $\Omega_{m,0}$ ( $0.332^{+0.019}_{-0.018}$ ) are consistent with the main Pantheon+ analysis.
Mock Validation: The mock analysis confirms that the method accurately recovers injected kinematic parameters when the supernova selection geometry matches the true velocity field region. However, incorrect volume selection (e.g., including too many external supernovae) leads to significant dilution of the anisotropic signal and biases in both kinematic and cosmological parameters. Notably, the eigenvector directions remain robust even when amplitudes are biased, provided the selection does not include a large number of external supernovae.

Significance and Claims
The paper claims that luminosity distance anisotropies within the volume dynamically dominated by Laniakea encode signatures of its large-scale kinematics, specifically its expansion and shear. By properly accounting for reference frame transformations and selecting supernovae within the superstructure's volume, the authors show that SNe Ia can independently reconstruct the ellipsoidal kinematics of Laniakea.

The significance of this work lies in providing a physically motivated interpretation of luminosity distance anisotropies as direct probes of local flow dynamics, rather than just statistical corrections. The authors suggest that this framework opens the possibility of using future supernova observations (specifically from LSST and ZTF) to reconstruct the expansion and shear of multiple nearby superstructures independently of galaxy density fields. The paper maintains a modest tone, noting that the current model assumes a sharp boundary between the internal ellipsoidal flow and the external dipole, and that future extensions could incorporate more realistic continuous transitions and external velocity contributions. The results are presented as a proof-of-concept that the methodology is viable and robust, particularly for determining the orientation of underlying velocity fields.