Dark Matter
ΛCDM, MoND and the Second Big Bang.
If you think of the universe as a cup of coffee, as far as we know, coffee will remind us of dark energy, which is almost 70% of the universe, and the milk we add will remind us of dark matter. The sugar we put in it will remind us of the matter we see, which is 5% of the universe.
For those who don’t know the subject, we say “dark matter and dark energy” because we have no idea about them and cannot see them. Naturally, you might say, how can you compare it to milk? But that’s a pointless question, as it doesn’t mean that dark matter will be dark.
First of all, all we know about dark matter is that it has a very strong gravitational effects and does not interact with the objects we see in the universe except by gravity. If you ask why we say this, there are two reasons:
The first reason is the mass inconsistency in galaxy clusters. Galaxy clusters are formed by the coexistence of galaxies in groups. Just as the planets in our solar system are held together by a force, there is a force that holds galaxies together. This indicates that there are packets of energy in the spaces between galaxies that need to be taken seriously and are connected by invisible filaments. But, mass calculations of galaxy clusters “apparently” show that these clusters shouldn’t exist, that they shouldn’t stay together, or even move away from each other very quickly.
Because of this problem in the early 20th century, astronomers wanted to focus on a single galaxy to better understand the situation: the Andromeda galaxy. It is both a galaxy known for its proximity to us and a spiral galaxy. Spiral is important because almost most galaxies in the universe are spirals. Naturally, any conclusion to be drawn from this would constitute an induction that could be applied to the entire universe.
And second reason,
The observations were surprising. Because, according to calculations, the rotation speed of the galaxy should be like the one on the right. However, the observations were that the rotation of the galaxy was much faster. That’s why it was said that there is something we don’t see, and this thing is not only gravitationally affecting visible objects but also allowing them to defy physics as we know it.
Therefore, the discrepancy between the expected and observed rotation curves is explained by adding a halo of dark matter surrounding the galaxy.
Despite numerous searches, no dark matter particles have yet been found. Quite a few candidates for dark matter have been proposed, from axions to sterile neutrinos to even more bizarre and mysterious candidates like weakly interacting massive particles (WIMPs), which had been a favorite of string theorists. What’s more, a recent study said dark photons1 could also be good candidates.
But the problem is that even though we had concrete observations, we couldn’t say, “Yes, dark matter, we’re sure,” for a single particle. Don’t be fooled by what I call concrete observation. Yes, we are observing something that is not within our formulas, but that doesn’t mean we need an external phenomenon.
Despite this, those who look definitively at such a phenomenon, especially those who have published their articles in respected journals of the scientific community, have always continued their research by looking at dark matter as “something that is definitely”.
I guess they don’t like black coffee.
We have many models, but two of them stand out, one is the ΛCDM model and the other is Modified Newtonian dynamics.
The ΛCDM model is referred to as the standard model of Big Bang cosmology as it combines the cosmological constant Λ, which Einstein called “my biggest blunder”, and the dark matter-related CDM
ΛCDM is a very imaginary model in the sense of trying to find the unknown by taking the unknown into account when there are so many unknowns. In short, while extraordinary claims require extraordinary evidence, we have added these claims to the models.
However, there are also those who drink coffee without sugar and milk.
Those who looked at it from a more realistic point of view said, “Why do we add imaginary things to our models that we know nothing about?” and formulas were developed that challenged this model. The most notable is MoND.
The idea of MoND (Modified Newtonian Dynamics) was inspired by the rotation of galaxies. First published in 1983 by Israeli physicist Mordehai Milgrom, the hypothesis’ original motivation was to explain why the velocities of stars in galaxies were observed to be larger than expected based on Newtonian mechanics.
MOND proposes to alter the acceleration of gravity depending on how strong the acceleration is. For large accelerations, which apply to anything within the solar system, the theory is the same as Newton’s, which means that the force on an object is proportional to its acceleration, F = ma. For very tiny accelerations, however, the force is proportional to more like the square of the acceleration. What this means is that for objects that are very distant from a galactic center, the force on them is independent of the distance, and thus their rotational curve is smooth.
Of course, just adding a small term to Newton’s gravity means that you also have to modify Einstein’s equations as well. So MoND has been generalized in various ways, such as AQUAL (AQUAL is a theory of gravity based on Modified Newtonian Dynamics ‘MOND’, but using a Lagrangia), which stands for a quadradic lagrangian.
Just like the ΛCDM model, AQUAL could explain these inconsistencies in the universe and does not need phenomena such as dark matter or dark energy to do so. What makes things even more interesting is that a recent study that has already been done supports this theory. Indeed, according to the study, the MOND theory was almost perfectly compatible with the observed and simulated galactic spin curves. The research made us all ask the following question: could dark matter be refuted?2
While all these discussions were going on, a mind-blowing theory emerged.3
Freese and Winkler, talk about a second “dark big bang” that occurred within a month of the first big bang in their still-unpeer-reviewed work. These two physicists even go further, suggesting that their models could produce a range of “exciting experimental signatures” detectable by current and future devices, implying that “direct testing of the dark big bang origin of dark matter may be possible.”
The origins of matter and radiation lie in the Hot Big Bang which terminates inflation and releases the vacuum energy into a hot plasma of particles. The latter contains the photons, leptons and quarks of our visible Universe, and, in the standard picture, also the dark matter.
So they say: Despite numerous studies to date, no non-gravitational link between the visible and the invisible (dark matter) has been detected, so we propose a big bang model. As the days rolled by in the universe’s infancy, regular matter cooled into atoms, a process known as Big Bang nucleosynthesis (BBN). At some point around the point of BBN, the dark quantum field did finally decay and transform its state, sparking the Dark Big Bang that created dark matter.
The reason this theory is important to them is because it can be proven. For example, Pulsars are dead stars that regularly shine through space, like a kind of lighthouse. So pulsars could enable us to detect powerful gravitational waves in spacetime that were created during the Dark Big Bang phase. It is even suggested that a project known as the International Pulsar Timing Array (IPTA) may have already identified possible evidence for a dark big bang.
The Dark Big Bang phase transition generates strong gravitational radiation,” the team said in their study. “[W]e investigated the sensitivity of ongoing and upcoming pulsar timing array experiments to the gravitational wave signal from the Dark Big Bang. We found that already the ongoing IPTA run (which combines several individual PTA experiments) has an exciting discovery potential for Dark Big Bangs which occur around or after BBN.
Another reason why the study is mind-blowing is the questions that accompany the proposal.
If we apply the example of a balloon to this, we inflate the two nested balloons so that the outer one (the big bang we know) is inflated first, and then when we apply an excessive blowing force to the second inner balloon (where phenomena such as dark matter occur), instead of bursting the outer balloon, it continues to inflate with it. Then, do we owe the overinflated universe, which continues to inflate at an excessive rate, to the second explosion? In other words, was the second explosion actually the real trigger of the inflating universe? So where do we place dark energy?
I think we need a second cup of coffee.
As crazy as those who divide the universe into dimensions like string theory and look for phenomena in these dimensions, those who search for these phenomena in mirror worlds, and even those who search for these phenomena in parallel universes, this study is just as crazy, but there is a difference: it is provable. That is why I am citing this work.
This is what makes some studies valuable. Being open to experimentation and observation as well as thought-provoking. Otherwise, there are universes that we all dream of; however, only the provable ones in these universes are worth seeing and telling.
Dark photons are hypothetical new particles that are the force carriers for a new force in the dark sector, much like how the photon is the force carrier for electromagnetism. Unlike photons, however, they can have mass. In particular, the ultralight dark photon — with a mass as small as twenty orders of magnitude less than the electron — is a good candidate for dark matter. James S. Bolton, Andrea Caputo, Hongwan Liu and Matteo Viel. Comparison of Low-Redshift Lyman-α Forest Observations to Hydrodynamical Simulations with Dark Photon Dark Matter. Physical Review Letters, 129, 211102 – Published 18 November 2022 DOI: 10.1103/PhysRevLett.129.211102
When the SPARC database was analyzed by Chae et al. (2020, 2021) with an analytic spherical MOND model from Famaey & McGaugh (2012), it was a surprise that external fields inferred from internal dynamics based on such a simple model agreed with those expected from cosmic environments. Kyu-Hyun Chae, Distinguishing Dark Matter, Modified Gravity, and Modified Inertia with the Inner and Outer Parts of Galactic Rotation Curves. 2022 December 12, Published by the American Astronomical Society. The Astrophysical Journal. DOI: 10.3847/1538–4357/ac93fc
Katherine Freese, Martin Wolfgang Winkler. Dark Matter and Gravity Waves from a Dark Big Bang. 22 Feb 2023. Cornell University. arXiv:2302.11579 [astro-ph.CO]. DOI: 10.48550/arXiv.2302.11579.
so cool, love that you mentioned MOND