Unravelling the Universe one particle at a time

Dark Matter - II 

Hello readers, this article is in continuation with my previous article on dark matter. So, if you have not read it before I’d recommend you to do check it out. Without further ado, let’s begin.

We left the article with a series of unanswered questions. So, let’s pick the first one from the list. What can and cannot be dark matter? What are the possible candidates of dark matter? 

Dark Matter - A scientific bewilderment

What dark matter cannot be?

The abundant amount of light elements created during the big bang nucleosynthesis (production of nuclei other than those of the lightest isotope of hydrogen after the big bang during the earlier phases of the universe) can rule out the possibility that dark matter particles are baryonic (Baryons contain an odd number of quarks, minimum 3. Protons and neutrons are the most common example of baryonic particles). The nucleosynthesis depends strongly on the baryon-photon ratio. This is also supported by the observations of cosmic microwave background radiation.

1. The main baryonic candidates are the massive astrophysical compact halo object (MACHO) class of candidates. These mainly include brown dwarf stars, Jupiter-like planets, and 100 solar mass (1 solar mass = 1.989 * 10³⁰ Kg) black holes. Searches such as MACHO collaboration and EROS-2 have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy.

   

   How MACHOS can focus light via gravitational lensing

   
   
2. Next in line with the particles that can be ruled out from the equation is relativistic neutrinos. Neutrinos are expected to have been produced profusely in the very initial stages of the universe. Similar to the microwave background radiation which are the fossil remnants of the hot radiation which characterised the dense phase of the early universe, we also expect fossil remnants of neutrinos which now form a background. With an estimated density of about 150/cm³, per species and this summed up over all the six species, we expect a fossil neutrino background with a number density of 1000/cm³. Even with this density, and mass as low as 0.1eV to 0.01 eV, it would account for less than a per cent of the missing dark matter.

   

   Cosmic neutrino background

   
   
3. There are a few other proposals that can be easily ruled out from basic astrophysical considerations. Highly relativistic protons trapped in halos of galaxies is one of them. Other rejected baryonic candidates are brown dwarfs, old white dwarfs, neutron stars, stellar-mass black holes, solid H₂, dense cold molecular clouds in galaxies, etc.

   

   Brown dwarf

   

   Ancient white Dwarf Star

   

   Neutron star

   

   A black hole weighing 70 solar masses

   
   

   

   Giant interstellar hydrogen iceberg

   
   

   

   Molecular clouds in the neighbouring WLM galaxy

   
   

What are some possible candidates for dark matter?

The range of ideas when it comes to what dark matter can be has no shortage. Serious candidates with masses ranging from 10^-5 eV to 10⁴ solar masses have been proposed. That’s a whooping range of masses of over 75 orders of magnitude!
As we have already discarded the notion of baryonic matter to be the dark matter, we shift our focus to non-baryonic matter. The non-baryonic candidates are basically elementary particles that are either not yet discovered or have non-standard properties. There are many propositions for the possible candidates. Many of them, like axions, neutralinos, gravitinos or composites these have been theorised. So, let’s look into them one by one.

1. A fraction of a second after the Big Bang, the universe was so hot that new particles and anti-particles were created and destroyed all the time. Calculations show that a stable particle of mass near 100 GeV and interacting with weak forces will leave just about the right amount of “leftovers” to account for the observed dark matter density. In particle physics, the Standard Model says that each particle has a heavier partner of different spin but similar interactions. The lightest of these particles is stable in many cases, which is an excellent dark matter candidate. Many theories which talk about higher dimensions, talk of different dimensions altogether in which these particles are curled up. The lightest of these particles, e.g., Kaluza-Klein particle, make for excellent dark matter candidates.

   

   Kaluza-Klein Theory

   
   
2. There are other possible dark matter candidates which do not fit into the above framework. One of these particles is the axions. An explanation for why axion is a good candidate goes beyond the scope of this article. Its explanation requires a pre-requisite knowledge of various types of symmetries which we would talk about in later articles.

   

   Newly found quasiparticles mimic hypothetical dark matter axions

   
   
3. Primordial black holes have also been suggested as a possible candidate for dark matter. Primordial black holes are hypothetical black holes that were formed after the big bang. It is found that black holes in the intermediate-mass range of one solar mass to a thousand solar mass and sub-lunar black holes in the range of 10¹⁷ – 10²¹ Kg can still produce all the dark matter. There are many constraints to that and the mathematics to it is still blurry, but we can be optimistic.

   

   Primordial black holes just after big bang

   
   
4. We have a few exotic candidates that have been suggested – WIMPzillas, gravitinos, gluinos, Q-balls, Q-nuggets, SIMPS, etc. There is a range of possible dark matter models. One other model is that baryons can be ‘packaged’ in non-luminous forms. There is also evidence that much of the dark matter may be made up of as yet undiscovered particles with several experiments all over the world trying to detect these. Many of these particles are in the preferred range of 100 GeV to a TeV. There could be dark matter objects or clumps made up of these particles bound by their mutual self-gravity and limits have already been placed on the abundance of these objects.

   

   Gravitino - Warm dark matter

   
   
5. There are several new classes of dark matter objects. One of the favoured dark matter candidates called the WIMPs (weakly interacting massive particles) has masses from about 10 GeV to 1 TeV. It can gravitate to form a new class of objects in dark matter halos or around the galactic centre. The role of dark matter in planetary formation and evolution has been considered by several authors.

   

   Theories of dark matter

   
   
   
6. Another alternate candidate to standard dark matter is the mirror matter-type dark matter. They have the right properties to be identified with the non-baryonic dark matter in the universe and make for an excellent candidate. I hate to say it but we are not ready for an in-depth discussion on mirror particles just yet.

Keeping in mind that I do not have to make my articles very lengthy, I would finish this topic for today. Needless, to say this will open new horizons to our discussions and give us a lot more fundamental words to use for the future. I hope the article kept you engaged, made you curious and fascinated you. Because these are the seeds for an inquiring mind that is ready for facing the challenges of Physics head-on. And even if today was a boring lecture, well, there would always be parts that you just do not like so much. Don't be disheartened and consider it one necessary evil. With that note, I would take your leave. Hope to see you soon.

 Auf Wiedersehen!

Footnote: I have written mass in terms of energy, more specifically electron volts(eV) in the whole article. This way of representing masses with electron-volts has its roots in the famous mass-energy equivalence equation, E = mc². Dividing 1 eV energy with c² gives the mass in kg.

1eV = 1.6 x 10-19 joules (J)

The various prefixes like K, M, G, and T. stands for kilo, mega, giga, and terra respectively, with the orders of magnitude being 10³, 10⁶, 10⁹, and 10¹², respectively.