6.2.4 The speed and direction of the Earth's motion

The first significant claim to have detected the motion of the Earth relative to the ‘frame of isotropic 3 K radiation’ came in 1977 from a group at Berkeley, California. They concluded that the Earth is moving at a speed of (390 ± 60) km s⁻¹, in a direction towards the constellation Leo, relative to a frame in which the 3 K radiation is isotropic. Their conclusion resulted from observations of a variation of intensity with angle of the form predicted by Equation 14, which we have called a 24-hour variation.

Now this motion of the Earth through the 3 K radiation will be the resultant of several component motions:

1. The Earth's velocity about the Sun (of magnitude 30 km s⁻¹):

2. The velocity of the Sun itself about the Galactic centre (currently estimated to be of magnitude 230 km s⁻¹):

3. The velocity of the Galaxy relative to the Local Group;

4. Whatever velocity the Local Group has relative to the ‘frame of isotropic 3 K radiation’.

based on R.A. Muller (1978) in Scientific American, 238, 64–74 ©

based on R.A. Muller (1978) in Scientific American, 238, 64–74

Figure 28 The absolute motion of the Earth. The Earth travels in its orbit round the Sun at 30 km s⁻¹ and is being swept around the centre of the Galaxy at 230 km s⁻¹. Experiment shows that the Earth's net speed through the 3 K radiation is about 400 km s⁻¹. The Earth's net velocity lies in the same plane as its orbit round the Sun and at an angle tilted sharply upwards (northwards) from the plane of the Galaxy. In this diagram, the Earth's net velocity is depicted as a heavy arrow centred on the Sun (pointing upwards and to the right), since the two bodies travel together. Both are being carried by the Galaxy's own motion through the 3 K radiation. In order to account for the Earth's motion with respect to the 3 K radiation, the Galaxy must be travelling at about 600 km s⁻¹ in the direction shown by the coloured arrow centred on the disc of the Galaxy.

The Earth's speed of about 400 km s⁻¹, relative to this frame in which the radiation is isotropic, is comparable to its speed of about 230 km s⁻¹ relative to the Galactic centre of mass. Nevertheless, the Galaxy as a whole must be moving through the 3 K radiation even faster than the Earth, because the direction of the Solar System's orbital velocity round the Galaxy is almost opposite to the direction of maximum observed intensity of the 3 K radiation and hence opposite to the direction of the Earth's velocity through the 3 K radiation, as depicted in Figure 28. Adding the two velocity vectors gives the centre of the Galaxy a velocity whose magnitude is about 600 km s⁻¹ with respect to the 3 K radiation. Now if our Galaxy were isolated, this velocity could only be interpreted as a departure (and 600 km s⁻¹ would be an embarrassingly large departure) from the basic idea of cosmology that the expansion of the Universe is shared by all matter and radiation; an isolated galaxy, or the centre of mass of a cluster of galaxies, should not be moving with respect to the 3 K radiation. But our Galaxy is not isolated, it is a member of our Local Group of galaxies. It cannot be stationary with respect to the centre of mass of the Local Group but must, to avoid falling in, be travelling around in some quite complicated orbit. So the next step towards an understanding of the 600 km s⁻¹ is to subtract from it the velocity of our Galaxy in its movement about the Local Group. Unfortunately this velocity is not very well known, because the estimates of the masses of some of the galaxies in the Local Group are very rough. But current data give the speed of our Galaxy with respect to the Andromeda galaxy (which is the other major member of our Local Group) as 40 km s⁻¹. This might be an indication of the kind of velocity we have relative to the centre of mass of the Local Group. If so, it still leaves an unexplained speed of 500–600 km s⁻¹.

Our Local Group is thought to be a member of a cluster of clusters called the Local Supercluster. So the next step is to subtract the velocity of our Local Group with respect to the centre of mass of the Local Supercluster from the 600 km s⁻¹. If this is also the final step, the answer should be compatible with zero. Unfortunately, the uncertainties are, at this stage, too great to be able to decide whether the velocity of our Local Group can be entirely explained in terms of the effects of a Local Supercluster. If not, then the effects of other structures such as more distant superclusters and the voids between them, must also be taken into account.