# Can "dark matter" be the additional mass from the speed of the Galactic Cluster?

Considering that the Earth orbits the Sun at 100 000 km/h and that the Sun orbits the Milky Way at almost 1 000 000 km/h, could it be possible that the galactic cluster orbits some super cluster at speeds approaching 1 000 000 000 km/h?

If it did, how would that affect the mass of the cluster and how would we know or be able to measure it?

Can you show any calculations to prove that this additional mass is not in line with what's measured for "Dark Matter"?

• 1 080 000 000 km/h is the speed of light so it would be virtually impossible for a cluster to be orbiting a super cluster at that speed.
– Dean
Commented Feb 4, 2016 at 17:32

The invariant mass of a multi-particle system (in a galaxy the 'particles' being stars, etc.) can come (in part, or even in whole for systems of massless particles) from the kinetic energy of its constituent particles. The relativistic picture of gravity (i.e. general relativity) is a bit more complicated, but it is clear that in a multi-particle system (in its rest frame) the kinetic energy of individual particles contributes to the gravitational "force" in the form of the components representing pressure in the stress-energy tensor.

How the total kinetic energy of a system, which is not at rest, contributes to its gravitational attraction is a more complicated question, particularly as for a body's total kinetic energy to have a gravitational effect it must be travelling at relativistic speeds, which means we have to depart from Newtonian approximations (such as gravitational force). However it is not entirely wrong to say that the total kinetic energy of a body does contribute to its gravitational attraction.

Despite this, contributions to the gravitational attraction from the kinetic energy can be discounted as alternative explanation to dark matter for galactic rotation curves mainly because:

1) The centre of momentum frame of the galaxy is the correct frame to consider the contribution of kinetic energy, not its possible velocity relative to some other far away galaxies. Besides which the peculiar velocities of galaxies are very small compared to c.

2) Though the velocity of particles within the galaxy relative to the centre of momentum frame does contribute to the gravitational attraction, there just isn't enough mass moving at fast enough speed within the galaxy for the contribution to be more than negligible.

• How sure are we of the speeds and distances we estimate galactic clusters to be moving at? How fast are they moving and what assumptions are our estimates of those speeds based on? Commented Feb 6, 2016 at 1:13

The first part, "could the galactic cluster orbit another at 10000000km/h". Is no. For simple observational reasons. If the local group were in orbit, we would be able to see and measure the supercluster which we were orbiting. We don't see it so it isn't there.

I take it that you hypothesise that the 28% of the mass of the universe that is not visible, is in fact the moving mass of the galaxies. That is not the case. Dark matter is not the moving mass of visible matter.

First remember that velocity is a relative concept, and when specifying it, you must give a frame of reference. The relativistic mass of an object is dependent on the frame of reference. For clarity lets agree to use the frame of reference co-moving with our galaxy (which is very nearly inertial)

Our own Milky way has an excess of mass. Since we are in the Milky Way, we are not moving relative to it, there is low relative velocity, low moving mass.

The first observation of dark matter was in the rotation rates of spiral galaxies. There is a missing mass in the galaxy in its own frame of reference, and in its frame of reference. Thus the missing mass is not relativistic mass.

There is a "real" missing rest mass in the universe, probably supplied by a weakly interacting massive particle.

• Assume the frame of reference is all the other matter and clusters in the universe... or at least some supercluster... doesn't have to be an orbit necessarily... but seeing as that most planets orbits suns, and solar systems galactic cores... couldn't there be much much bigger stuff big enough to pull super clusters around them? Otherwise put, wouldn't the solar system, when travelling at 50% the speed of light, see the planets orbiting faster? Or does it all balance out?... Commented Feb 6, 2016 at 1:32

The Great Attractor at the heart of the Laniakea Supercluster meets the criteria.

Light does not make for a good sign post. If I continually accelerate I am always changing reference frames but my perspective on the speed of light will always be the same. How can you tell what ABSOLUTE frame of reference an observers is in when the absolute signpost of lightspeed looks the same from every relative perspective.

So if there was Dark Flow motion in common with the whole visible universe how would we detect it?

I would detect it by looking for the DARK MASS bloated by time dilation due to the dark flow motion.

Therefore finding extra DARK MASS in galactic spin stability implies additional time dilation due to dark flow.