Darkish matter has lengthy been some of the mysterious issues within the cosmos. It was first proposed within the 1930s as an concept to deal with the stellar motion in sure galaxies. The primary stable proof of darkish matter was collected by Vera Rubin, who studied the rotational motion of galaxies. The motion of those galaxies solely added up in the event that they contained a considerable amount of invisible mass. There have to be an unique and invisible materials, not like something that was recognized earlier than.
If darkish matter exists, it should have two important properties. First, it can’t work together strongly with gentle, in any other case we’d see it and it could not be "darkish". Second, it should work together gravitatively with different matter, to make seen matter transfer in a wierd method. We all know a number of issues that meet these situations, reminiscent of neutrinos or tiny black holes, however it may possibly't be darkish matter. We all know this partly as a result of we are able to now take his temperature.
Whereas some astronomers have proposed alternate options to darkish matter, reminiscent of sure fashions of "modified gravity". These don’t correspond nicely to the observations. Darkish matter doesn't simply have an effect on the rotation of galaxies. Like all mass, it deflects gentle by gravity. By observing the gravitational lens results of galaxies, we are able to map the distribution of their darkish matter. Darkish matter additionally impacts the best way galaxies group collectively. In different phrases, darkish matter directs the large-scale construction of the universe. From observations of clusters of galaxies, we discover that the darkish matter seems to be chilly.
Motion of atoms in a gasoline. Credit score: Greg L at English Wikipedia
For a super gasoline, its temperature is dependent upon the velocity at which the atoms bounce. The sooner they transfer, the extra kinetic vitality they’ve and the upper the gasoline temperature. The temperature of a gasoline additionally is dependent upon the mass of the atoms. The kinetic vitality of an atom is dependent upon mass, so a small atom of mass should transfer sooner than an atom of enormous mass to have the identical kinetic vitality. On the similar temperature, the atoms of a "heavier" gasoline transfer extra slowly than these of a "lighter" gasoline.
Darkish matter might not be a super gasoline, however we all know that it doesn’t work together strongly with itself, so the identical common rule applies. The temperature of darkish matter is dependent upon the motion and the mass of its particles.
Distribution map of galaxies. Credit score: M. Blanton and the SDSS.
On a cosmic scale, gravity is making an attempt to cut back darkish matter right into a mass and the motion of darkish matter is making an attempt to propagate on account of its motion. Which means that darkish matter will are likely to cluster in clumps. The size at which this agglomeration happens tells us the temperature of darkish matter, and subsequently the mass of its particles.
The usual mannequin of cosmology maintains that darkish matter is chilly. Which means that the darkish matter particles have a excessive mass. They’re typically known as huge weak interacting particles or WIMP. The chilly darkish matter mannequin works nicely on the size of galaxy clusters, however it doesn’t work as nicely on the size of particular person galaxies. One approach to take the temperature of darkish matter is to find out on what scale the agglomeration decomposes.
Utilizing the gravitational lens approach, a staff was in a position to look at how the sunshine of a distant quasar was affected by small tufts of darkish matter. Credit score: NASA / ESA / D. Participant (STScI)
Lately, a staff measured this scale by inspecting seven lens quasars. That is the place a distant quasar has a galaxy in entrance of it from our viewpoint. The quasar gentle is deflected across the galaxy like a lens. By measuring these lens quasars, the staff was in a position to cut back the size of darkish matter. From this, they decided the temperature of the darkish matter, and subsequently a minimal mass for the darkish matter particles.
Hubble photographs which reveal 4 distorted photographs of a background quasar and its host galaxy surrounding the central core of a large foreground galaxy. Credit score: NASA / ESA / A. Nierenberg (JPL) / T. Treu (UCLA)
They found that the mass of darkish matter particles have to be no less than 5.58 keV. It’s 3000 occasions extra huge than neutrinos, however it’s virtually 100 occasions smaller than the mass of an electron. This final level is attention-grabbing as a result of most WIMP fashions argue for a mass of particles 100 occasions heavier than a proton. If the darkish matter particles are gentle, they might certainly have already been detected in particle accelerator experiments.
Though this examine doesn’t dispute chilly darkish matter, it additionally permits scorching darkish matter. The authors say they would want to have a look at 4 dozen extra lens quasars to pin issues down. So you would say that the talk over chilly darkish matter is beginning to warmth up.
Reference: Hsueh, Jen-Wei, et al. "SHARP – VII. New constraints on the continual scattering properties of scorching darkish matter and the abundance of the substructure of quasars with irregular lens to move ratio. "