A single star can reshape an entire galaxy. New research reveals that the chaotic nature of galaxies means their simulations are inherently unpredictable over long timescales, even from identical starting points.
Summary
Researchers from Leiden University have demonstrated that simulations of galaxies like the Milky Way do not always produce identical outcomes, even when starting from the same initial conditions. This is not a flaw but a reflection of the chaotic nature of galaxies and the limitations of simulation methods.
Key Findings
- Tetsuro Asano and Simon Portegies Zwart (Leiden Observatory) created hundreds of models of Milky Way-like galaxies, each consisting of a flat disk of stars embedded in dark matter.
- In each experiment, they ran two nearly identical simulations that differed by a tiny detail, such as a slight shift in the position of a single star. Over time, these small differences grew into visible structural changes: spiral arms developed differently, and the central bar rotated differently.
- This behavior resembles the butterfly effect, where small causes lead to large consequences. It challenges the traditional view that a galaxy, containing hundreds of billions of stars, behaves smoothly with small disturbances averaging out.
Resolution of Paradox
- Previous research had conflicting results: some simulations suggested galaxies become more chaotic with more stars, while others found the opposite.
- The new study explains that this discrepancy arises from the “softening” of gravity at very small distances in simulations. Softening treats stars as small clouds rather than point masses, which smooths out the strong, close interactions that drive chaos.
- By systematically varying the degree of softening, the researchers showed when a simulation accurately reflects a real galaxy and when it does not.
Implications
- The Milky Way becomes unpredictable after about one million years, which is extremely short compared to its age of ~10 billion years (equivalent to a second in a human lifetime).
- Some features, such as the formation of a central bar of stars, emerge consistently in all simulations. Other properties, like the exact shape of the bar or the structure of spiral arms, depend strongly on tiny differences.
- Small differences do not grow indefinitely; the butterfly effect has limits. Simulations may end up looking different in details but still produce recognisable spiral galaxies.
Statement
Simon Portegies Zwart said: "We have now quantified how choices in a simulation determine how much of that chaos you see. That not only explains how a single star can reshape an entire galaxy, but also how we can model this reliably."