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Signatures of Galactic Expansion in Gaia DR3: Implications for the JWST Early-Galaxy Puzzle

This paper analyzes Gaia DR3 data to report evidence of statistically significant outward expansion in the Milky Way, suggesting that AGN-driven feedback may be a mechanism that accelerates galaxy growth and could help explain the unexpectedly rapid formation of massive galaxies observed by JWST.

Original authors: G. S. Karapetian, A. P. Mahtessian, L. E. Byzalov, M. A. Hovhannisyan, L. A. Mahtessian

Published 2026-03-25

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CC0 1.0

Original authors: G. S. Karapetian, A. P. Mahtessian, L. E. Byzalov, M. A. Hovhannisyan, L. A. Mahtessian

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ 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 Milky Way is "Breathing Out": A Simple Explanation

Imagine the Milky Way galaxy not as a giant, spinning pinwheel frozen in time, but more like a giant, slow-motion balloon that is currently inflating.

For decades, astronomers have believed that galaxies are like solid, stable wheels: they spin, they hold their shape, and everything inside them stays roughly where it belongs. But a new study using data from the European Space Agency's Gaia satellite suggests something surprising: our galaxy might be expanding from the inside out.

Here is the breakdown of what the researchers found, using everyday analogies.

1. The Mystery: Why are the Universe's "Teenagers" so big?

First, let's look at the problem that started this investigation. The James Webb Space Telescope (JWST) recently looked deep into the early universe and found massive, fully grown galaxies that existed when the universe was only a "baby" (less than 1 billion years old).

The Standard View: Think of building a house. Usually, you lay the foundation, then the walls, then the roof. It takes a long time. Standard physics says galaxies should take billions of years to grow big.
The Problem: JWST found "houses" that are already skyscrapers, built in what should have been just a few weeks of construction time.
The Question: How did they grow so fast?

2. The Hypothesis: The "Super-Engine" in the Center

The authors propose a radical idea: Galaxies might have a "turbocharger" in their center.

They suggest that the supermassive black hole at the center of a galaxy (the nucleus) isn't just a passive anchor. Instead, it might be an active engine, shooting out energy and matter like a garden hose turned on full blast. This "wind" pushes the stars and gas outward, causing the galaxy to expand rapidly.

If this happens in the early universe, it explains how those "teenager" galaxies grew so big so fast.

3. The Experiment: Checking the "Backyard"

To test if this "turbocharger" theory works, the scientists didn't look at the distant past; they looked right here, in our own neighborhood (the Milky Way). They used data from Gaia DR3, which is essentially a 3D map of over a billion stars, tracking their positions and speeds with incredible precision.

The Method: They divided the area around the center of our galaxy (within 5,000 light-years) into 27 different slices (like cutting a pizza into 27 pieces).
The Measurement: For every slice, they calculated: Are the stars moving away from the center (expanding) or toward it (contracting)?

4. The Results: The Pizza is Spreading

The results were striking:

21 out of 27 slices showed stars moving outward.
The speed of this expansion is between 3 and 50 km/s (about 6,700 to 112,000 mph). That sounds slow, but in the vast emptiness of space, that is a very fast "breath."
The Oddball: One slice (the 4th quadrant) showed stars moving inward. The authors think this is like a traffic jam caused by the galaxy's central "bar" (a long structure of stars) blocking the flow, or perhaps a specific zone of activity that behaves differently.

Even the "halo" (the fuzzy outer cloud of stars) and the "disk" (the flat part where we live) showed a small but clear outward push.

5. What Does This Mean?

If these findings hold up, it changes our understanding of how galaxies work:

The Center is Alive: The core of our galaxy isn't just sitting there; it's actively pushing matter outward.
The JWST Puzzle Solved: If our galaxy is expanding now, it's possible that in the early universe, this "push" was much stronger. This would explain how those ancient galaxies grew to massive sizes in record time. They didn't just slowly gather dust; they were blown up by their own central engines.

6. A Note of Caution

The authors are careful to say this is preliminary. It's like seeing a ripple in a pond and guessing it's a fish, but you need to wait to see if the ripple gets bigger.

The "What-ifs": Could this be an illusion caused by how we measure things? Could the galaxy's rotation be tricking the math?
The Future: They need more data and better tools to confirm that the galaxy is truly expanding and not just doing a weird dance.

The Bottom Line

This paper suggests that our galaxy might be inflating like a balloon driven by a powerful engine in its center. If true, it solves a major mystery about why the universe's first galaxies grew up so fast, and it suggests that the center of our own galaxy is far more active and dynamic than we previously thought.

In short: The Milky Way might be "breathing out," and that breath could be the key to understanding the rapid growth of the universe's oldest giants.

1. Problem Statement

The paper addresses two interconnected astrophysical challenges:

The JWST Early-Galaxy Puzzle: Recent observations by the James Webb Space Telescope (JWST) have revealed massive galaxies existing less than 1 Gyr after the Big Bang. This contradicts standard $\Lambda$ CDM cosmological models and galaxy formation theories, which predict significantly longer assembly timescales for such massive structures.
The Assumption of Gravitational Equilibrium: Standard galactic dynamics (based on Jeans equations) assume galaxies are in a steady state of gravitational equilibrium, with no large-scale radial expansion or contraction, particularly within the inner 10 kpc.
The Hypothesis: The authors propose that the standard equilibrium model may be incomplete. They hypothesize that the Milky Way (and potentially other galaxies) exhibits measurable central expansion driven by active galactic nuclei (AGN) feedback. If confirmed, this mechanism could explain the rapid assembly of massive galaxies in the early universe.

2. Methodology

The study utilizes Gaia Data Release 3 (DR3) to test for radial expansion in the Milky Way.

Data Selection:
- Source: Gaia DR3 stellar data.
- Region: Stars within a galactocentric distance ( $r_{gc}$ ) of 5 kpc.
- Sample Size: The analysis covers 27 distinct spatial sectors. Due to data completeness variations, sample sizes per sector range from 21 to 31,777 stars (totaling over 26,000 stars for global averages).
- Quality Filters: Strict criteria were applied to ensure kinematic reliability:
  - Parallax $> 0$ and Parallax Error Ratio $> 5$ .
  - Radial Velocity and Proper Motion (RA/Dec) must have signal-to-noise ratios $> 5$ (i.e., $|v/\sigma_v| > 5$ ).
Coordinate Transformation:
- Stellar positions and velocities were transformed from the heliocentric frame to the Galactocentric frame using the Astropy library.
- The Galactocentric Radial Velocity ( $v_{radial\_gc}$ ) was calculated for each star using the dot product of the position vector ( $\vec{r}$ ) and velocity vector ( $\vec{v}$ ):
  $v_{radial\_gc} = \frac{\vec{r} \cdot \vec{v}}{|\vec{r}|}$
Analysis Strategy:
- Sectorization: The Galaxy was divided into 27 sectors covering all quadrants, disk ( $|z| < 0.5$ kpc), and halo regions.
- Binning: Data was binned in radial intervals of 0.25 kpc.
- Statistical Testing: For each bin and sector, the mean velocity and standard error ( $\pm \sigma/\sqrt{N}$ ) were computed. A t-test was performed to determine if the mean velocity significantly differed from zero.

3. Key Results

The analysis reveals a statistically significant, asymmetric pattern of outward motion across the Galaxy.

Global Expansion:
- Disk: Mean outward velocity of +2.9 ± 0.6 km/s ( $p < 0.001$ ).
- Halo: Mean outward velocity of +1.7 ± 0.7 km/s ( $p \approx 0.02$ ).
- Whole Galaxy: Mean outward velocity of +2.3 ± 0.6 km/s ( $p < 0.001$ ).
Sectoral Asymmetry (Quadrant Analysis):
- Quadrants 1, 2, and 3: Exhibit consistently positive (outward) mean velocities.
  - Quadrant 1 ( $x>0, y>0$ ): Strongest signal, mean +17.2 ± 0.7 km/s. In the southern disk subsample, this reaches +49.9 ± 7.3 km/s.
  - Quadrants 2 & 3: Positive means ranging from +2.5 to +5.1 km/s.
- Quadrant 4 ( $x>0, y<0$ ): Exhibits negative (inward) velocities, with a mean of −21.0 ± 0.7 km/s.
  - Note: The authors suggest this negative gradient in Q4 may be an artifact of the Galactic bar's configuration, absorption effects, or a localized activity zone, as velocities transition to positive beyond $\sim6$ kpc.
Consistency with Previous Work: These findings align with the authors' prior studies on globular clusters (17–31 km/s outward) and stars along the south-north axis (~19 km/s outward).

4. Key Contributions

Empirical Evidence of Expansion: Provides the first large-scale, sector-resolved evidence suggesting the Milky Way is undergoing a net radial expansion of 3–50 km/s within 5 kpc of the center.
Methodological Framework: Demonstrates a robust pipeline for analyzing Gaia DR3 data to detect subtle kinematic trends by dividing the Galaxy into spatial sectors and applying rigorous error filtering.
Re-evaluation of Equilibrium: Challenges the standard assumption that galactic disks and halos are in strict hydrostatic equilibrium, suggesting dynamic activity in the Galactic nucleus.
Cosmological Implication: Links local Galactic kinematics to the "JWST Early-Galaxy Puzzle," proposing that AGN-driven feedback could accelerate galaxy assembly timescales.

5. Significance and Limitations

Significance:
- If confirmed, the detection of Galactic expansion implies that AGN-driven outflows are a dominant mechanism for galaxy evolution, capable of injecting energy and matter to drive rapid growth.
- This offers a potential solution to the JWST paradox: if nuclei drive expansion, massive galaxies could reach sizes of 10–20 kpc in less than 1 Gyr, consistent with early-universe observations.
- It revives the hypothesis (originally proposed by V. Ambartsumian and D. Lynden-Bell) that galaxies originate from active central nuclei.
Limitations and Caveats:
- Preliminary Nature: The authors explicitly state these results are preliminary.
- Systematics: Potential residual systematics in Gaia DR3 data and projection effects from Galactic rotation could contribute to the observed signals.
- Asymmetry: The strong negative signal in the 4th quadrant requires further investigation to rule out sampling biases or specific local dynamical effects (e.g., the Galactic bar).
- Future Work: Verification requires future Gaia data releases and complementary spectroscopic surveys to confirm the expansion is a global, physical phenomenon rather than an observational artifact.

Conclusion: The paper presents compelling, albeit preliminary, evidence that the Milky Way is expanding. If validated, this finding would necessitate a paradigm shift in galaxy formation theory, suggesting that active nuclei play a more critical role in rapid galaxy assembly than currently modeled.