Galaxy Rewind

About this sky

You are looking out from the centre of the Milky Way. The view is fixed to the sky's directions, the way the stars are, so it doesn't turn with the Earth or the Sun, and only the galaxies move. From Earth it would look the same: we sit 26,000 light-years from the centre, too little to shift these galaxies visibly. Press Rewind to run the clock back about 11.7 billion years, to when the universe was 1.6 billion years old.

Each dot is where that galaxy's material was at the time on the clock. Its faint line is the path it took to get to where it is today, drawn in the colour of its type.

What moves them

The expansion of the universe moves nothing across the sky: it carries everything straight outward together. So all the drift you see here is gravity, as galaxies and groups pull on each other.

Positions are measured from the Milky Way, which moves too. The Milky Way's own small satellites, like the Magellanic Clouds, are left out. Distances in a galaxy's details are real distances at that time. The universe was smaller then, so most galaxies were closer to us. Our nearest neighbours are the exception: they are falling toward us now, so back then they were farther away.

Reading the screen

• z on the clock is the redshift: how much the universe has grown since then. At z = 4 it was 5 times smaller than today.
• The soft band is the disk of the Milky Way, which hides what lies behind it.
• The dashed line is the supergalactic plane, the flat sheet that most of our neighbouring galaxies lie in.
• Rings are groups and clusters of galaxies too far away to show one by one.

How it was made

The paths come from the Numerical Action method (Peebles 1989). It finds orbits that end exactly where galaxies are seen today and start out smooth and calm in the young universe. The masses of the Milky Way's neighbours and of the groups and clusters farther out come from Shaya, Tully, Hoffman and Pomarède (2017). The galaxies come from the Updated Nearby Galaxy Catalog (Karachentsev, Makarov and Kaisina 2013).

Gravity often allows more than one path that ends where a galaxy is today. When it does, the viewer shows the one whose speed today is closest to the galaxy's measured speed, as Shaya and colleagues did. Every path shown was also checked by running it forward again with finer time steps. Sideways motions are never fitted.

How good is it?

These paths are a reconstruction, not something anyone observed. Here is how they compare with what has been measured.

• Speed toward or away from us is measured for most of these galaxies. For galaxies with accurate distances within 26 million light-years, the model's speeds are typically off by 55 km/s, against 65 km/s for a plain expansion with no gravity. But many of those paths were chosen by their measured speed. Counting only the 102 galaxies with a single possible path, the model is off by 70 km/s against 55: no better than the plain expansion.
• Sideways motion, which is what you see moving here, has been measured for only 16 nearby galaxies. The model fits those worse than assuming they don't move sideways at all. So the drifts show what this model's gravity does, not what has been observed.
• M31 is the clearest miss. The model moves it sideways at about 200 km/s, while Gaia and Hubble measure under 100. The heavy groups nearby pull the Milky Way and M31 around, and the calmer solution Shaya and colleagues found wasn't reproduced. So M31 and its satellites drift farther than the real ones did.

Limits

• Some paths didn't fully solve, and their details say so.
• Light takes under 40 million years to reach us from these galaxies, and under 150 million years from the farthest groups. That is too small to show on this clock, so the viewer leaves it out.
• Big galaxies that lead a group move with their group's mass. Smaller galaxies follow their own paths through it.

Using it

• Drag to look around. Scroll or pinch to zoom. Arrow keys look around, + and − zoom, Space plays or pauses.
• Click or tap a galaxy to see its details. The search box finds one by name.
• The all-sky map shows the whole sky at once. Drag it sideways to turn it, and double-click a spot to look there.