The search for dark matter – an unknown and invisible matter that is thought to make up the vast majority of matter in the universe – is at a crossroads. Although it was proposed almost 70 years ago and has been searched intensely – with large particle collisions, detectors deep underground and even instruments in space – it is still nowhere to be found.
But astronomers have promised to leave “no rocks” and have begun throwing their nets wider into the galaxy. The idea is to extract information from astrophysical objects that may have witnessed lumps of it as they passed by. We have just proposed a new method of doing this by tracking galactic gas ̵
Physicists believe that dark matter has a tendency to structure itself in a hierarchy of halos and subhalos via gravity. The masses of these lumps fall on a spectrum, with lower mass volumes expected to be more. Is there a limit to how easy they could be? It depends on the nature of the dark matter particles.
Hot versus cold
Dark matter cannot be seen directly. We know it exists because we can see the gravitational effects it has on the surrounding matter. There are various theories as to what dark matter may actually be. The standard model suggests that it is cold, which means that it moves very slowly and only interacts with other matter through gravity. This would be consistent with the fact that it consists of particles known as actions or WIMPS. Another theory, however, suggests that it is hot, meaning that it moves at higher speeds. One such particle candidate is the sterile neutrino.
If dark matter is cold, a Milky Way galaxy may contain one or two subhalos weighing as much as 1010 Suns, and probably hundreds with masses of about 108 The sun. If dark matter is hot, halo is lighter than about 108 The sun cannot be formed easily. So tuning dark halos with mass of light can tell us something about dark matter.
We believe that the existence of halos with lower mass can be revealed by carefully planned observations. Astronomers have already become pretty good at this game of hide and seek with dark matter halos and have devised observations to capture the damage they leave behind.
So far, observations have mainly targeted changes in the distribution of stars in the Milky Way. For example, the large Magellanic cloud, a smaller galaxy orbiting ours, appears to have a dark matter halo massive enough to trigger a huge wake – driving stars from across large regions to move in harmony.
A few of the smaller halos of dark matter that are believed to whiz inside the Milky Way can occasionally penetrate large star features, such as spherical clusters (spherical collection of stars) and leave narrative holes in them. Halos of dark matter can also affect how light bends around astrophysical objects in a process called gravity lens.
But the signals left in the star distributions are weak and prone to confusion with the stars’ own motions. Another way to study the effect of halos is by looking at the galactic gas it affects. Galaxies have lots of hot gas (with a temperature of about 106 degrees Kelvin) that extend to their edge and provide a wide net to capture these dark fabric halos.
Using a combination of analytical calculations and computer simulations, we have shown that dark halos are heavier than 108 Solar masses can compress the hot gas through which they move. These will create local spikes in the density of the gas, which can be picked up by X-ray telescopes. These are predicted to be small in the order of a few percent, but they will be within range of the upcoming Lynx and Athena telescopes.
Our models also predict spikes in the density of the cooler galactic gas (with a temperature of about 105 K) will be even more significant. This means that the cooler gas can detect the passage of dark matter halos even more sensitively than the hot gas.
Another promising way to observe the dark matter-induced fluctuations in the gas is via photons (light particles) from the cosmic microwave background – the light left over from the Big Bang. This light disperses the very energetic electrons in the hot gas in a way that we can detect, which provides a complementary approach to the other studies.
Over the next few years, this new method can be used to test models of dark matter. Whether dark fabric halos under 108 solar masses are found in the number predicted or not, we will learn something useful. If the numbers agree, the common cosmological model would have passed an important test. If they are missing or far fewer than expected, the standard model would be ruled out and we will have to find a more viable alternative.
Dark matter remains a mystery, but there is an enormous amount of work to be done to solve it. Whether the answer comes from instruments on earth or astrophysical probes, it will undoubtedly be one of the most important discoveries of the century.