How a lot do darkish matter particles weigh?

A Hubble House Telescope view of the galaxy NGC2985. The proof for darkish matter emerged within the Seventies when astronomers discovered an uncommon sample within the fee at which stars in a galaxy rotated the farther they have been from the centre.
| Picture Credit score: Judy Schmidt/NASA

Darkish matter is an enigmatic invisible substance supplying five-sixths of the matter of the universe. Not like photons, the particles of sunshine, the particles of darkish matter must have non-zero mass or else the dense and complex construction of matter on cosmic scales is not going to kind.

How gentle can a darkish particle then be? For many years scientists thought this minimal mass was about 10^-31 occasions the mass of a proton. However in Could this yr, theoretical physicists revised the restrict and pushed it up by an order of magnitude, to 2.3 × 10^-30 proton lots. This can be a vital replace on the earth of darkish matter.

Uniformly or in lumps?

To grasp these numbers and their significance, allow us to first construct a psychological image of darkish matter. Darkish matter is claimed to be all over the place within the universe. Does that imply it’s in your own home? In 1922, Dutch astronomer Jacobus Kapteyn studied the movement of stars neighbouring the Solar and concluded the density of “darkish matter” (utilizing that time period for one of many first occasions) should be 0.0003 photo voltaic lots per cubic gentle yr.

Since then, by a century of more and more refined measurements, the accuracy of Kapteyn’s conclusion has held up remarkably nicely. This density of darkish matter will be re-expressed because the heft of two protons per teaspoon, which implies your own home might comprise darkish matter with a mass equal of a trillion protons.

However this is able to even be naïve: Kapteyn’s and subsequent measurements are solely legitimate when concerning the million-cubic-lightyear quantity and doesn’t apply after we zoom in for a more in-depth look. It’s because stars, whose movement is used for the measurement, are themselves separated by a couple of gentle years. Whether or not or not darkish matter is current on smaller size scales would depend upon how it’s distributed: both uniformly or in lumps.

An occasional customer?

Let’s assume it’s unfold round like nice flour, which the usual theories of cosmology additionally predict. If it is available in lumps, the spacing between them could also be as massive as many gentle years and there’ll maybe be no darkish matter underneath your roof.

Now, since we all know the native density of darkish matter, the worth of the unknown mass of the darkish particle will decide the separation between two neighbouring particles. Whether it is 100 proton lots, the inter-particle separation will likely be 7 cm. Then darkish particles at any given second is not going to solely be in your own home but additionally in your head.

If darkish matter is fabricated from an elementary particle, the heaviest it may be is about 10¹⁹ occasions a proton’s mass. In that case the interparticle separation could be 30 km. So darkish matter received’t be a resident of your own home however will go to sometimes (for the reason that particles journey randomly at round 300 km/s).

Then once more, a ten²⁰ gram agglomerate of darkish particles could be aside by greater than the scale of the photo voltaic system, lowering our probability of discovering them.

Fluid reasonably than a flock

What about small lots? At 10^-11 proton lots, each pink blood cell in your physique will comprise a darkish matter particle. However now quantum physics turns into necessary. Each object can be a wave, with its wavelength given by the inverse of its momentum. Thus the lighter a darkish matter particle is, the bigger its wavelength will likely be. For 10^-11 proton lots, the wavelength will likely be about 2 cm, a lot bigger than its micrometre interparticle separation.

So for small lots, we should image a group of darkish particles as a fluid reasonably than as a flock of grains.

If we now dial the mass of a darkish particle all the way in which all the way down to 10^-31 proton lots, the wavelength is 200 gentle years, in regards to the measurement of a dwarf galaxy. The substance of a dwarf galaxy is mainly within the type of darkish matter, with solely about 1% contribution from stars. This straightforward truth interprets to a restriction on the darkish matter particle’s mass: it should be better than 10^-31 proton lots. If it have been decrease, its spatial extent would exceed the dwarf galaxy and we will’t kind a macroscopic object smaller than its microscopic constituents.

The time of computer systems

That is the place the paper from Could issues. Its authors have proven that this lore is just too simplistic and that researchers can do one thing sharper. First, utilizing information on how stars transfer in Leo II, a dwarf galaxy orbiting the Milky Manner, they inferred the darkish matter density in it as a operate of the gap from its centre. This density profile isn’t distinctive attributable to measurement uncertainties, in order that they generate a set in step with the stellar information. Subsequent, they numerically solved the Schrödinger equation after modifying it to account for gravity and obtained an ensemble of density profiles. Lastly, they carried out a statistical process to match the 2 units of density profiles — the empirical one from observing Leo II and the theoretical one from fixing the equation.

Their key discovering right here was that the inside areas of Leo II contained extra invisible mass and which darkish particles of 10^-31 proton mass couldn’t account for. Thus they surmised heavier particles are wanted to accommodate the inside crowding.

It’s not on a regular basis that particle physics will get to redraw a elementary objective submit by an order of magnitude. And it’s a signal of our occasions that this might solely have been performed with computer systems versus a blackboard.

Nirmal Raj is an assistant professor of theoretical physics on the Centre for Excessive Power Physics within the Indian Institute of Science, Bengaluru. nraj@iisc.ac.in