Why Cold Dark Matter?

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Why do we believe the Dark Matter in the Universe to be cold? What does it mean for Dark Matter to be cold? And how does this affect the evolution of structure formation in the Universe? This video answers these along with many other important questions regarding CDM in the standard model of Cosmology.
We will begin by examining the evidence for dark matter, which comes from the missing mass within galaxies. Observations of the velocity distribution of stars within galaxies show they have a flat rotation curve, instead of decreasing with radial distance from the galactic centre, as would be the case using Newtonian mechanics and the observed stellar mass distribution. The gravitational interaction between galaxies and clusters, as well as gravitational lensing of starlight provide a way to constrain the total gravitating mass of a galaxy. When compared to the mass from stars alone shows that a large majority of the mass is unseen. A spherical distribution of unseen mass around galaxies is capable of explaining the flat rotation curve and the total gravity from galaxies.
These dark matter halos form from the early Universe right after inflation occurs from the Quantum Foam left over from the Big Bang. The Heisenberg uncertainty principle leads to small density fluctuations across spacetime which manifest as slight over densities in dark matter. These are then capable of growing via self gravity, sowing the seeds of where galaxies are as we see them today. However, the temperature of dark matter in the universe has a massive effect on this structure formation.
Hot dark matter particles move with a higher average kinetic energy, which can cause the over densities to spread out and smooth out over time - an effect known as Free Streaming. Conversely, cold dark matter is not as affected by free streaming as smaller initial density perturbations can survive the smoothing effect and actually grow due to self gravity. In essence this means that the temperature of the dark matter in the Universe controls the minimum mass or size of structure which can form and we can observe today. With hotter dark matter erasing the smallest perturbations over time, leaving only the largest structures to survive the free streaming.
Observations, such as from the Sloan Digital Sky Survey (SDSS) catalogue galaxies in the night sky allowing us to deduce the clustering and size of structures in the Universe. Other observational evidence which originates from the initial density perturbations left after inflation such as the Cosmic Microwave Background (CMB) can also be used to justify this. With the conclusion that our Universe must contain cold dark matter for such structures to form. Cosmological hydrodynamical supercomputer simulations are used to further justify this by simulating universes with different dark matter temperatures, also predicting cold dark matter responsible compared to the sizes of galaxies in our universe.
This video was sponsored by Brilliant.
0:00 - LambdaCDM Model in Cosmology and Astrophysics
0:32 - Evidence for Dark Matter (Galactic Rotation Curves)
2:10 - Dark Matter Halos
2:28 - Quantum Foam, Inflation and Theoretical Physics in the Early Universe
3:16 - Growth of Cosmic Structures via Mergers and Self Gravity
4:04 - Effect of Temperature on Dark Matter and the Universe
4:47 - Free Streaming of Hot Dark Matter Particles
5:35 - Effect on Structure Size and Comparison to Observations

[@OVAstronomy]

To try everything Brilliant has to offer-free-for a full 30 days, visit brilliant.org/OVAstronomy . The first 200 of you will get 20% off Brilliant’s annual premium subscription.

[@xyhilwastaken]

Congrats on the sponsorship BTW! Really love seeing this channel grow. 😊

[@OVAstronomy]

@@xyhilwastaken Thanks for the support!

[@sebastiandierks7919]

Good video, thank you! Maybe to add some constructive criticism, I think it would have been helpful to verbalise that the chapter starting at 2:28 on quantum foam, inflation and bubbles of universes with different constants of nature is speculation in the realm of theoretical physics, while the rest of the video is experimentally tested physics. Inflation just provides a mechanism to blow up microscopic quantum fluctuations in the primordial plasma to create density perturbations of macroscopic size, which can then undergo gravitational collapse. But for dark matter and why it has to be cold, it is not necessary to explain/speculate where the density perturbations came from, be it inflation or just the initial conditions of our universe.

[@tybeedave]

inflation is like popcorn. photons are kernels, the puff is e/-e pairs. an electron is how much bigger than a photon? reminds me of frog eggs.

[@OVAstronomy]

Thank you, and you make a good point! I have updated the chapter label.

[@ryanhegseth8720]

I be more interested in evidence that shows that dark matter actually exists. In reality dark matter is evidence that we got “gravity” wrong or galaxies, one of our assumptions has to be wrong.

[@jonathanbaincosmologyvideo3868]

Sorry, dark matter has nothing to do with rotation curves of galaxies, this is a common error. Rotation curves prove that galaxies have a binary pair at their core. This is a consequence of 3d-n-body Newtonian physics. Dark matter is just excess matter realized by statistical models based on light. This was observed as a consequence of gravitational lensing; first theorized by Laplace, and then observed in the 1920's by the Russians. But you are right. Dark-matter has to be colder than light-matter on average.

[@shawns0762]

Dark matter is dilated mass. In the 1939 journal "Annals of Mathematics" Einstein wrote - "The essential result of this investigation is a clear understanding as to why the Schwarzchild singularities (Schwarzchild was the first to raise the issue of General Relativity predicting singularities) do not exist in physical reality. Although the theory given here treats only clusters whose particles move along circular paths it does seem subject to reasonable doubt that more general cases will have analogous results. The Schwarzchild singularities do not appear for the reason that matter cannot be concentrated arbitrarily. And this is due to the fact that otherwise the constituting particles would reach the velocity of light." He was referring to the phenomenon of dilation (sometimes called gamma or y) mass that is dilated is smeared through spacetime relative to an outside observer. It's the phenomenon behind the phrase "mass becomes infinite at the speed of light". Time dilation is one aspect of dilation. General Relativity does not predict singularities when you factor in dilation. Dilation will occur wherever there is an astronomical quantity of mass because high mass means high momentum. Dilation is the original and correct explanation for why we cannot see light from the galactic center. It can be shown mathematically that the mass at the center of our own galaxy must be dilated. In other words that mass is all around us. This phenomenon does not occur in galaxies with very, very low mass because they do not have enough mass in their centers to achieve relativistic velocities. It has recently been confirmed in 5 very, very low mass galaxies to show no signs of dark matter. This is proof that dark matter is dilated mass. What we see in modern astronomy has been known since 1925. This is when the existence of galaxies was confirmed. It was clear that there should be an astronomical quantity of light emanating from our own galactic center. It wasn't until the 1960's that television and movies started to popularize black holes did they gradually become accepted. There was clarity in astronomy before idea of singularities took hold.

[@tybeedave]

it's not 'dark' matter, it's small matter. the largest dm particle is about the size of a neutrino. and represents an itty bitty black hole.

[@morningmadera]

here is your nobel prize

[@tybeedave]

@@morningmadera lol

[@enigmalfidelity]

The problem is that as we made scientific discoveries, we evolved with the ability to discover. We discovered fire cooks meat, making it easier to consume. We didn't know the actual benefits of cooked meat until we had forms of science investigate why. As we entered the age of the microscope, we started to discover microbiology. Viruses, disease. Suddenly, we were able to discern as to what made us sick. After that, we discovered ways to see how those single called organisms were formed. What they were made up of. This all happened very slowly. Many people had many theories as to what was going on in the world of micros. As we progressed, we've made discoveries of smaller and smaller particles, and lots based on theories of mathematics, i.e., Quarks and charms. Now, if someone had just given the scientists who discovered bacteria and viruses and neutron telescope, they wouldn't know what to look for. Why? They don't even know about the molecule yet. With dark matter, we see the "symptoms" of its presence, yet we can not measure any of its characteristics. We just don't have the tools to see what it is we are looking at. It's like "flatland" where as we can only perceive what's in and up to the 3rd dimension. Nothing beyond. With DM, we can't measure what we don't know to measure. Like bringing a measuring cup to measure flames in a fire.

[@unocoltrane2804]

That's just more speculation about it's nature. There's currently no way to confirm or debunk that.