Tuning in to A.M.

No, we don’t mean A.M. on your radio- those parts of the dial that use amplitude modulation, and are the preserve of talkback, love song dedications, and late-night quizzes…

We mean annual modulation - the expected annual variation in the arrival rate of dark matter (DM) particles at the Earth, due to a combination of Earth’s orbital motion around our Sun, and our solar system’s orbital motion around the centre of our Milky Way galaxy.

Indirect evidence for the existence of DM tells us that galaxies contain much more mass than we can actually see, whether that’s by optical, radio, or other observations.

There has to be a distribution of non-visible mass throughout a galaxy, that has a gravitational effect on the visible mass within a galaxy- we know it’s there, we just can’t see it. Our thoughts regarding this dark matter are that it is in the form of particles, having some mass, but which do not interact with light or other forms of electromagnetic radiation.

Furthermore, this stuff exists throughout our galaxy- a “halo” in which the ordinary matter also resides, and through which- we think- the ordinary matter moves. The gas, dust, stars and planets in our galaxy, in a sense, swim through this halo of DM particles as the galaxy rotates.

The effect of this is that our solar system, as it orbits the centre of the Milky Way over a period of about 230 million years, faces a wind of DM particles. We’re moving relative to the DM halo, so we can think of this as a particle wind moving relative to us. The relative speed, assuming a static halo, is about 230 km/s.

But that relative motion is pretty well constant, so we might, at first, expect a constant stream of DM particles to pass the Earth.

What makes this interesting is that the Earth orbits our Sun once per year, and the plane in which the Earth orbits the Sun is inclined at an angle of about 60 degrees to the plane of the galaxy. Thus, the orbital velocity of the Earth around the Sun sometimes adds to, and sometimes subtracts from, our orbital velocity around the centre of the galaxy.

This leads to an annual variation in our velocity around the galaxy, and therefore also in the velocity of the expected DM wind.

For about half the year, we’re moving into the wind (think of cycling into a headwind); for the other half, we’re moving with the wind (think of cycling with a tailwind). For a DM wind at a constant relative speed, we would expect the Earth to face a constant number of particles per unit time (say, per day).

But, as we’ve seen, the wind speed is not constant, and this means a varying DM particle flux- when we’re moving into the wind, we would expect more particles per day; when moving with the wind, we would expect fewer particles per day.

When Earth moves against the DM wind, particle flux increases; when Earth moves with the wind, particle flux decreases.