Astounding Interconnectedness of the Universe: Dark Matter Halos

Updated: Jul 17

The dark matter forms the skeleton on which galaxies form, evolve, and merge. 5/6th of the mass in the universe is made up of dark matter according to modern cosmological models (Planck Collaboration et al. 2016) The fluctuations due to Gravitational instability grew over time. The effects of gravity on scales comparable to the size of galaxies or larger, appear to be anomalously strong. Initially, Gas and dark matter were well mixed; and as the universe evolved, gas dissipated and fell to the centers of the dark matter halos. For large enough dark matter halos, gas-cooled, which resulted in the formation of stars and protogalaxy. The earliest observations were done in 1933 by Zwicky on the Coma cluster of galaxies. Zwicky calculated velocity dispersion of 1600 km/s for the galaxies in Coma, indicating that the cluster had an enormous amount of internal kinetic energy Using redshifts derived from optical spectroscopy. For a dynamically stable structure, the required gravitational potential energy was 20 times what would have been deduced from the distribution of visible stars and gas alone. For this deficit, Zwicky proposed that some sort of “dark matter”, cold enough to be effectively invisible in his optical observations, might be responsible.


The term "dark matter" was first coined by Zwicky and, Rubin and Ford rediscovered the idea. Rubin and ford Started with a study of the Andromeda galaxy, M31, over time they observed that rotation speeds in the outskirts of spiral galaxies were far higher than possible for any stable systems bound together by their visible gas and stars alone. The concept of a ‘dark halo’ around every galaxy was eventually introduced as the mass needed to explain the observations was enormous. At the time it was confusing whether this matter was normal, baryonic matter, some novel form of non-baryonic matter, or in the form of cold gas or compact objects. The evidence that we have nowadays for dark matter is much more broadly based than it was in the 1930s or the 1970s. In, the cosmic microwave background (CMB), the fluctuation spectrum strongly constrains the composition of the universe when it was less than a million years old. Interactions with photons have a substantial effect in smoothing out the density variation of normal matter as a result of the high radiation density at the time. The weakly interacting matter is free to go about its business, however, the fluctuations grow when the corresponding particle cools down sufficiently to become non-relativistic.


The baryonic matter coupling to photons fraction is only Ωbar = 0.045 ± 0.003, while the total matter density in the universe, as a fraction of the critical density, is Ωm = 0.267 ± 0.029, which constrains the ratio of two components from comparative heights of the second and third acoustic peaks in the CMB angular power spectrum. This difference must come from a component with interactions of the weak scale or less. according to this dark components could have decayed since that time, but measurements of large-scale structure and cosmic expansion find a consistent value for the total matter density at low redshift.


An independent estimate of the baryon density is provided by the abundance of light elements which is roughly consistent with CMB measurements and much lower than the density needed to explain large-scale structure or cosmic expansion. Hence, the best evidence of cold dark matter—or more specifically for a non-baryonic, pressure-less component that dominates the matter density and is non-relativistic comes from the largest scales in the universe. In proposed extensions to the Standard Model of particle physics, the idea of a weakly interacting particle massive enough to be “cold” (or nonrelativistic) at the time of the CMB was not unwelcomed. According to the standard cosmological model of structure formation the universe not only contains known particles— Photons, baryons, and a small contribution from hot or warm neutrinos— but also two dominant dark components, weakly interacting cold dark matter and a cosmological constant or some similar form of “dark energy”.


On smaller scales, the CDM model suggests where the galaxies may form, but it does not specify how galaxies form or evolve. How the fluctuations grow into virialized halos and predict the abundance and clustering of dark matter halos as a function of mass and redshift is shown by Analytic theory and numerical simulations of structure formation. The galaxies can be placed in these halos with the help of Empirical models in a way that is consistent with the galaxy abundances and clustering measured in surveys. To test this incredible prediction of 10 decades or more of invisible structure filling our universe, we need to study dark matter in the highly nonlinear regime, deep in the heart of halos. the smallest and oldest dark matter structures end up here, and it is also where dark matter reaches its highest density.


New physics—scattering, annihilation, or decay into other particles—should be most evident here. The Milky Way is embedded in a dark matter halo, and we reside relatively close to its center (within the central 3% in radius), hence, any local study of dark matter must come to grips with the highly nonlinear regime of CDM structure formation. The small objects should form first, and halos should grow and merge over time according to the power spectrum of matter. Because of the process of merging of dark matter halos, the galaxies within these halos form stars (in situ) as well as grow through merging (ex-situ).


The surroundings of the galaxies get affected after they are formed due to the energetic processes within, which results in various kinds of feedback influencing future gas accretion and star formation. Hence, clearly, the growth, internal properties, and spatial distribution of galaxies are likely to be closely connected to the growth, internal properties, and spatial distribution of dark matter halos.


To put it simply, the dark matter in the Universe is arranged in dark matter halos and the luminous matter in the Universe is arranged in galaxies, and in a cold dark matter model.

Cold dark matter could comprise new and however undiscovered huge interacting particles (WIMPs), which happen for the model in super-symmetric augmentations of the Standard Model of molecule physics. "Cold" implies that these particles have rather little thermal speeds, which permits the formation of small structures, ordinarily down to far under one solar mass. CDM together with the significantly mysterious dark energy (normally indicated "Λ") are the prevailing components of the ΛCDM model, in which all the ordinary matter accounts for only 4.6% of the total. ΛCDM has at this point become the "standard cosmological model.”


References:


· https://arxiv.org/pdf/1711.01693.pdf

· https://www.hindawi.com/journals/aa/2011/604898/

· http://arxiv.org/abs/1001.4635.

· http://adsabs.harvard.edu/pdf/1972ApJ...176....1G

· https://authors.library.caltech.edu/37466/1/1984ApJ___281____1F.pdf

· http://adsabs.harvard.edu/pdf/1985ApJS...58...39B

· https://iopscience.iop.org/article/10.1086/304888/meta

· https://iopscience.iop.org/article/10.1086/383219/meta

· https://academic.oup.com/mnras/article/398/4/1858/982583?login=true

· https://academic.oup.com/mnras/article/331/1/98/1035377?login=true

· https://iopscience.iop.org/article/10.1086/338765/meta




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