Are Galaxies Stationary, or Do they Move Through Space?

Newton’s first law of motion says that everything will remain at rest or move at a constant speed in a straight line unless a force acts on it.

We’ve already discovered that objects as massive as galaxies can be in orbit, but are they moving through space?

And, if so, how do we know that and what do we measure their movement against?

We’re going to answer all of these questions, and a few more, in this article.

How We Measure Speed

Measuring speed is always done relative to something else. When you run along a track, your speed is measured relative to the non-moving track.

Except, the track is moving… relative to the sun. Earth is both rotating on its axis, producing the day and night, and orbiting the sun, creating the year.

The sun is not stationary either though. It is orbiting the center of the Milky Way galaxy every 225 million years or so and traveling at a speed in excess of half a million miles per hour compared to the bright galactic center.

Depending on what you measure your run against, you might be moving at 8 mph or half a million!

But what about the Milky Way moving, what do we measure that against?

How Do We Measure Our Galaxy’s Speed Through Space?

You might know that virtually all galaxies are moving away from us due to the expansion of space. It appears that we (or at least our local group) are static in the middle of an expanding universe. We know that is an illusion because if we were in another galaxy, the Milky Way would appear to be racing away from it.

To measure whether and how fast our galaxy is moving through space, we need another ‘static’ measurement to compare against. This is where the CMB Dipole comes in.

The Cosmic Microwave Background Dipole Anisotropy

Okay, I’ll hold my hands up and admit that this subsection title is intimidating.

Just what the heck is a Dipole Anisotropy?

Before I answer that, let’s take one step back to the cosmic microwave background (CMB). This is the cold remains of the big bang, think of it like an echo which we read as a uniform static wherever we look in the universe.

ESA map of the cosmic microwave background
The Cosmic Microwave Background (source)

The CMB has a temperature of below 3 Kelvin, which means it is only three degrees above absolute freezing – the coldest possible temperature. More importantly, this background radiation is incredibly uniform. No matter where scientists measure it, it varies by less than one part in ten thousand.

Except, we see a greater degree of temperature variation in two distinct areas. Near the constellation of Aquarius, the temperature is 0.0035 K below average, and it is 0.0035 K above average in Leo.

These two poles of temperature, cold near Aquarius and warm in Leo, are what give us our dipole in the ‘dipole anisotropy’. Anisotropy means that the CMB has different temperatures in different directions.

These are the easiest galaxies to see for a new astronomer

While this might be interesting, what relevance does it have for this discussion?

Well, cosmologists quickly realized that we see the temperature dipole only because we are moving relative to the uniform CMB. If we were static, the temperature would be completely uniform and there would be no dipole.

The fluctuation – the dipole – we measure is caused by light stretching ‘behind’ us and compressing ‘in front’ of us, leading to minute temperature variations.

All of which means we know how fast the local group of galaxies is moving, and in which direction.

How Fast is the Local Group of Galaxies Traveling Through Space?

Using the CMB dipole as the absolute arbiter of our speed through space has our Local Group of galaxies, which includes Andromeda Galaxy and the Large and Small Magellanic Clouds, moving at 627±22 km/s through space. That’s about 1.3 million miles per hour.

To our minds that is incredibly fast. Especially when you realize that the fastest any ‘vehicle’ made by people has traveled is the Parker Solar Probe, which hit a top speed of 163 km/s in November 2021.

Yet, on a universal scale, it is sedentary, being just 0.2% of the speed of light.

Now we know that our galaxy isn’t stationary, the last question to answer is: where is it heading?

Laniakea and The Great Attractor – Why The Milky Way is Not Stationary

The Great Attractor sounds like something from a sci-fi movie, but it is a genuine scientific phenomenon.

Our galaxy and the Local Group to which we are gravitationally bound, are part of a much (much) larger structure called Laniakea (say: lan-ee-a-KAY-a). See the video below for more details but, in summary, it is our home supercluster, one of the largest structures in the universe.

The Great Attractor is the most gravitationally dense area of Laniakea (which means ‘immeasurable heaven’ in Hawaiian) and our local group, along with hundreds of others, are slowly sliding towards it. At its center is the Norma Cluster, a rich galaxy cluster 222 million light-years away in Centaurus, obscured by the center of our galaxy.

We also appear to be heading towards the Shapley Supercluster, which is the largest gravitationally bound structure in our neighborhood of the universe. Gravitationally bound means the large concentration of galaxies within it pull together instead of being spread apart by cosmic expansion.

The opposite of the Shapley Supercluster and Great Attractor is the Dipole Repeller, which appears to push galaxies away from it. Whether this is actually so is of dubious merit. Popular scientific opinion is that this region is simply devoid of galaxies because they are being pulled away from it. See more on the Dipole Repeller in the video below.

Galactic Speed and the Expansion of Space

As we mentioned earlier in this article, outside of our Local Group, all galaxies appear to be moving away from us.

Since we are not connected to them gravitationally, they – and we – are influenced by the expansion of space. In simple terms, more space is being created between us, pushing us further and further apart.

One unexpected result of this is galaxies traveling away from us faster than the speed of light. Nothing can travel faster than light in space, but these galaxies are being pushed away by the creation of space.

That is why we can never visit over 97% of the galaxies we can see, even if we could travel at light speed.

Summary

We measure speed relative to something else. Walking to the park, you are moving at 3 mph relative to the ground, but 500,000 mph relative to the center of our galaxy.

Our galaxy’s speed is measured relative to the dipole anisotropy of the cosmic microwave background, which tells us we are moving at 1.3 million mph.

The Milky Way, along with the rest of the Local Group, is steadily heading for a gravitationally dense local part of the universe called the Great Attractor.

When will we get there?

The latest thinking is that it will take so long to arrive that it will have been destroyed by dark energy long before it pulls us in.


Written by Adam Kirk