# What is the fastest a galaxy or other body has been measured to (or can) travel?

Our solar system travels at an average speed of 515,000 mph and our galaxy at 1.3 million mph through space. What is the fastest moving body in space recorded and the fastest can a black hole or galaxy travel through the universe?

• Now the question emphasizes that it should be a celestial body, in which case my discussion about particles doesn't really fit… – pela Jun 30 '18 at 0:09
• @pela feel free to revise it to fit your answer as well. – Muze the good Troll. Jun 30 '18 at 2:48

### Hubble velocities

The Universe expands and carries galaxies away from each other with a relative velocity proportional to the distance between them. This is Hubble's law, and if the Universe is infinitely large, there is no limit to how fast two galaxies may recede from each other. In our observable Universe, the most distant galaxy observed (GN-z11; Oesch et al. 2016) recedes from the Milky Way at more than twice the speed of light ($c\simeq300\,000\,\mathrm{km}\,\mathrm{s}^{-1}$).

### Peculiar velocities

However, this is not a motion through space, which seems to be what you are asking about. Nothing is able to travel through space faster than the speed of light.

Galaxies generally move through space with velocities of the order of a few $100\,\mathrm{km}\,\mathrm{s}^{-1}$; in massive clusters, they may acquire velocities of a few $1000\,\mathrm{km}\,\mathrm{s}^{-1}$ (e.g. Karachentsev et al. 2006). These so-called peculiar velocities are wrt. to the "global" frame, i.e. the frame in which an observer that follows the cosmic expansion lies still. This is also the frame in which the cosmic microwave background is (statistically) the same in all directions.$^\dagger$

Black holes, stars, planets, and gas cloud move around in the reference frame of their galaxies with characteristic velocities of the order of $100\,\mathrm{km}\,\mathrm{s}^{-1}$, so this will typically be smaller than that of their host galaxy, if the galaxy lies in a large cluster.

When massive celestial bodies such as neutron stars or black holes merge, they reach velocities of the order of half the speed of light, as discussed further in Rob Jeffries' answer. This is as measured in the frame of the center of mass.

### Velocities of particles

EDIT: The following was written before the question was changed to emphasize celestial bodies: Much higher velocities are found for small particles such as cosmic rays; these are massive particles accelerated to extreme energies, e.g. by supernova explosions. The record holder of such particles is still (I think) the so-called Oh-My-God particle which traveled at $0.9999999999999999999999951c$ (in Earth's frame), much faster than the velocities reached in the Large Hadron Collider.

$^\dagger$Of course you can always define a frame that moves with almost the speed of light, such that all object acquire that speed "artificially", but then the Universe isn't isotropic anymore.

• How are these speeds determined? I thought we could only measure relative velocities and these are stated like they are absolute values. – jmh Jun 29 '18 at 22:20
• @jmh Yes, you're right. I though it'd be too much to go into that, but you're right that it shouldn't go unmentioned, so I'll edit. – pela Jun 29 '18 at 23:22
• Trying to understand, we don't actually see GN-z11 receding from as at twice the speed of light right ? We see its past image recede from us at a given speed and from that infer how far it is receding from us "now" (as far as this actually means something) , which would mean that light emitted from it "now" will never each us ? – Guiroux Jul 4 '18 at 12:09
• @Guiroux We don't measure its velocity as in "measure its distance now and measure its distance 1 second later, and take the difference" (I know you know this). We see its image from the past, and we measure its redshift. From the redshift (and assuming that our model of cosmology is correct), we can calculate that right now, GN-z11 increases its distance from MW at twice the speed of light. We can also calculate how fast it receded from us when it emitted the light we see now. You might be surprised to hear that at that time, it receded at four times the speed of light :) – pela Jul 4 '18 at 21:27
• @pela well I actually allowed myself to ask such a questions because I had notion of how we can measure that kind of velocity using redshift, this kind of ultra basic stuff. For this part "You might be surprised to hear that at that time, it receded at four times the speed of light", I just don't see how it is possible, but I'll do my research to understand it. Thx for your time. – Guiroux Jul 5 '18 at 7:23

If we are talking about any significant celestial bodies, then it will be the relative velocities of a pair of merging black holes.

The typical relative velocity due to the orbital speed of the black hole components just before merger is greater than half the speed of light.

Nothing comes close to that on a galactic scale. Typical peculiar velocities for galaxies in a cluster are a few hundred to a few thousand km/s.

The plot below shows the inferred relative velocities (bottom) of merging black holes from the first LIGO gravitational wave detection. 