It's all dark up there!

Dark Matter

 

Hello readers, it has not been a long time since we last talked about dark matter. However, one blog was not enough to cover what I wanted to convey. So, here is another one. But, before moving ahead, I would strongly recommend reading the articles I had posted on particles and dark matter and dark energy. Many topics are in continuation with the mentioned articles. So, without further ado, let’s dive deeper into the realm of mystery and the intriguing universe we live in.

One of the most intriguing things about the universe is, it is not dominated by ordinary baryonic matter (hadrons with spin number of the form n/2), but by a form of non-luminous matter that is as much as five times the amount of baryonic matter.

The universe is very nearly spatially flat, as is indicated by the measurement of cosmic background radiation, yet 70% of its mass is unaccounted for. Detailed measurements of the universe revealed a value that is 30% that of the critical mass density. The unaccounted matter called the dark matter has eluded us for decades. Several experiments to detect it has been running for several years without yielding a positive result.

What observational evidence do we have of the existence of dark matter?

Zwicky, in 1973 was trying to estimate the masses of large clusters of galaxies. The dynamic mass of the cluster, deduced from the motion of the galaxies, were at least a hundred times their luminous mass. This led Zwicky to conclude that most of the matter in such clusters is not made up of luminous objects like stars, or clusters of stars, but consists of matter that does not radiate.

Dark Matter Halos and Galaxy Mass Distributions

One more phenomenon that we discussed in one of my previous articles was how at a very large scale, the stars or other orbiting bodies, orbiting the galaxy at larger distances from the galactic centre, move around it at more or less the same velocity as objects much closer to the centre, contrary to what is expected. The rotational velocity is supposed to be proportional to r^-2, but here it’s almost constant as if the mass is somewhat related to the distance from the centre. This tells us mass is growing even after light dies out. As much as 90% of the galaxy mass is due to dark matter. Mass increases as we move farther and farther away from the central region. The relation between increasing mass and radius may seem strange to us but it would become clear as we move further. So, let’s mark it as one of our questions.

Difference between measured and calculated rotational velocities

To better emphasise our previous point, we have what we call ultra-diffusive galaxies, these have sizes of giants but the luminosity of dwarf galaxies. They have a large amount of dark matter but very few stars, making them difficult to detect due to their inherent faintness. An example could be the coma cluster.

Coma cluster

Apart from the velocity distribution of galaxies and galaxy clusters, there is other evidence pointing to the existence of dark matter. Extended emission in X-ray observations of clusters of galaxies indicates the presence of hot gas distributed throughout the cluster volume. From the observed emitted rays, luminosity can be measured. It depends on the density, temperature, and volume of the cluster. The mass required to hold hot gas in the cluster estimated requires a vast amount of dark matter.

The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch of the locations of the X-rays and the inferred mass is undeniable.

The very presence of complex structures is also proof of the existence of dark matter. Cosmic Microwave Background anisotropy measurements indicate a model with predominant dark matter. The gravity from dark matter increases compaction, allowing the formation of structures.

The large scale structure of the Universe

How do we classify dark matter?

An important classification for dark matter particles is the ‘hot’ vs ‘cold’ classification.

Hot vs Cold Dark Matter

Hot dark matter particles are those that are described by a relativistic equation of state at the time when galaxies could just start to form. As far as I think, the immensely hot universe might have led to the creation of particles with low mass and high kinetic energy, making the velocities > 0.1 c (c = speed of light).

Cold dark matter is those that are described by a non-relativistic equation of state at the time when galaxies could just start to form.

Hot matter cannot cluster on galaxy scales until it has cooled to non-relativistic speeds. Warm dark matter is another hypothesised form of dark matter that has properties intermediate between those of hot dark matter and cold dark matter. The most common warm dark matter particles candidates are sterile neutrinos and gravitinos.

From observations from our current favourite model for the universe, the matter is mostly cold dark matter (with a large cosmological constant).

Do dark matter decay? What might be their half-life?

Dark matter has survived until the present day, accounting for almost 26% of the present energy density of the universe. It is still unknown whether they are stable or have a finite but very long lifetime. This could be a possibility because there is no theoretical basis predicting their stability.

Emission line-like spectral features at energy E equivalent to 3.5keV in the long exposure X-ray observations of several dark matter-dominated objects, such as the stack of 73 galaxy clusters and Andromeda and Perseus galaxy cluster has recently been observed. The spectral lines may indicate a decay of some sort. The possibility that this spectral feature may be the signal from decaying dark matter has piqued a lot of interest, and many dark matter models explaining this signal have been proposed.

 

Dark matter (X) annihilates into dark photons (A) which couple to Standard Model particles that eventually decay into photons

What can and can not be dark matter? What might be some of the possible candidates for dark matter? What is this new thing called mirror dark matter? What is our plan to detect dark matter particles? These are few questions that have yet not been answered. To keep this article short and brief, I would talk about these topics in my next article. The topic might be overwhelming at this point. But, I hope you liked the article.

Auf Wiedersehen!

Image Credits: Google Images