Universe Today

For almost a century, astronomers and cosmologists have postulated that space is filled with aninvisible mass known as “dark matter”. Accounting for 27% of the mass and energy in the observable universe, the existence of this matter was intended to explain all the “missing” baryonic matter in cosmological models. Unfortunately, the concept of dark matter has solved one cosmological problem, only to create another.

If this matter does exist, what is it made of? So far, theories have ranged from saying that it is made up of cold, warm or hot matter, with the most widely-accepted theory being the Lambda Cold Dark Matter (Lambda-CDM) model. However, a new study produced by a team of European astronomer suggests that the Warm Dark Matter (WDM) model may be able to explain the latest observations made of the early Universe.

But first, some explanations are in order. The different theories on dark matter (cold, warm, hot) refer not to the temperatures of the matter itself, but the size of the particles themselves with respect to the size of a protogalaxy – an early Universe formation, from which dwarf galaxies would later form.

Diagram showing the Lambda-CBR universe, from the Big Bang to the the current era. Credit: Alex Mittelmann/Coldcreation

The size of these particles determines how fast they can travel, which determines their thermodynamic properties, and indicates how far they could have traveled – aka. their “free streaming length” (FSL) – before being slowed by cosmic expansion. Whereas hot dark matter would be made up of very light particles with high FSLs, cold dark matter is believed to be made up of massive particles bigger that a protogalaxy (hence, a low FSL).

Cold dark matter has been speculated to take the form of Massive Compact Halo Objects(MACHOs) like black holes; Robust Associations of Massive Baryonic Objects (RAMBOs) like clusters of brown dwarfs; or a class of undiscovered heavy particles – i.e. Weakly-Interacting Massive Particles (WIMPs), and axions.

The widely-accepted Lambda-CDM model is based in part of the theory that dark matter is “cold”. As cosmological explanations go, it is the most simple and can account for the formation of galaxies or galaxy cluster formations. However, there remains some holes in this theory, the biggest of which is that it predicts that there should be many more small, dwarf galaxies in the early Universe than we can account for.

In short, the existence of dark matter as massive particles that have low FSL would result in small fluctuations in the density of matter in the early Universe – which would lead to large amounts of low-mass galaxies to be found as satellites of galactic halos, and with large concentrations of dark matter in their centers.

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Post completo en: Universe Today

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