Dark matter
Hot dark matter is another class of dark matter, for example neutrinos, but it is unlikely to be responsible for galaxy formation and rotation rates. Due to its ultra-relativistic speeds, hot matter could not create the small-scale structure of the early universe, as it would not clump in dense enough regions from which galaxies could form in the early universe. As a result, although neutrinos have been observed, I do not believe that they can be the full story behind dark matter. The other candidate is warm dark matter, for example sterile neutrinos. 16 With properties in between cold and hot, it would interact weakly with particles travelling close to or at the speed of light which, according to simulations, will cause the universe to turn out in a similar way if dark matter is cold. The two can be distinguished as warm dark matter’s unique coupling and inter action with radiation forms an early universe distinct to that formed by cold darkmatter. 17 Unfortunately, current telescopes do not have the resolution required to measure the structure so far back in time or the distribution of matter throughout the galaxy.
Some physicists have proposed that there is no missing matter in the universe, rather our understanding of gravity and its equations are wrong. CalledMONDs 18 they propose altering the equations of gravity so that they act differently at extremely low accelerations, far below any observed in the solar system. One facet of MONDs is that the centre of baryonic mass is the centre of mass for a galaxy. However, studies have shown that this is not true. For example, with the Bullet cluster (figure 5), where two galaxy clusters collided, the baryonic
Figure 9: The Bullet Cluster
matter (pink) has mixed while the majority of mass (blue), deduced by gravitational lensing, has just passed straight through, with little interaction as expected from dark matter. 19 Even though a few MONDs claim to explain this phenomenon, I think NGC 1052-DF2 provides much more compelling evidence that it is not the equations that are wrong.
NGC 1052-DF2 (figure 6) is a galaxy, which appears to have very little dark matter. According to MONDs its behaviour should be the same as other galaxies, but this is not the case, such are its uniquely low velocities. So, ironically, the lack of dark matter is the final nail in the coffin for MONDs and definitive proof that dark matter exists.
Figure 10: NGC 1052-DF2
The search for dark matter is being carried out at a blistering pace. Every year new experiments are published, and new hypotheses proposed. I believe that the most likely answer to the dark matter question is the axion particle. WIMP predictions have time and time again been wrong. There are only a certain number of times you can alter a theory and its predictions, lowering the interaction cross- section to smaller and smaller values. I would add that there may be no one single answer. Dark matter may not consist of one single particle. Axions, MACHOs and WIMPs or even a mixture of cold, warm and hot dark matter could all contribute to the incredible universe that we see today.
16 Neutrinos which only interact through gravity. 17 Williams (2019). 18 Modified Newtonian dynamics. 19 Seigel (2017).
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