Gravity Without Mass: UAH Study Proposes Alternative to Dark Matter

Very much the latter:

https://en.wikipedia.org/wiki/Negative_mass

The thing about dark matter is that it seems to be highly non-interactive.

If it could interact it would be able to shed its momentum through collisions like normal matter, and thus fall towards the center of the galaxy and clump like normal matter.

Instead the gravitational evidence, like galaxy rotation curves, indicates that the dark matter is spread out in a huge diffuse halo around the galaxies, several times the radius of the visible galaxy.

Thus in the same way, dark matter would be unable to form a "dark accretion disk" around a black hole, where it could shed it's momentum in order to lose enough orbital velocity to fall into the black hole.

Thus the only dark matter that would enter would be that captured during formation and that which hits it directly like a meteor, so the ratio would be significantly lower.

This of course assumes dark matter is a particle (or several), which is the current leading hypothesis but it's by no means settled yet.

At least that's my understanding.

Isn't redshifting decrease in wavelength? The way even a mesh can reflect radio waves I always accounted to "the wave is so big it can't fit trough the holes of the mesh", so in that sense the radiation redshift equals to it spreading (even tho on interaction it manifests in a single point.

Could you elaborate?

It’s a set of observations about “something” that behaves like matter[1], and interacts only gravitationally. There are many theories proposed but none has any real answers or solutions. The following is taken from Cern[2]:

> Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum “missing” after a collision. Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. One theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having very little in common with matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together.

And thus at least whoever wrote this and the people who approved that publication agree with the notion that it is observations for which no concrete theory exists that explains them.

[1] i used “behaves like matter” in the sense that follows this definition:

> In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a−3 . This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor ρ ∝ a−4 , and a cosmological constant, which does not change with respect to a (ρ ∝ a0). The different scaling factors for matter and radiation are a consequence of radiation redshift: For example, after gradually doubling the diameter of the observable Universe via cosmic expansion of General Relativity, the scale, a, has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled);[51] the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.[d] The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.[52][e] In principle, "dark matter" means all components of the universe which are not visible but still obey ρ ∝ a−3 . In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons". Context will usually indicate which meaning is intended.

https://en.m.wikipedia.org/wiki/Dark_matter

[2] https://home.cern/science/physics/dark-matter

Redshift is an increase in wavelengths and a decrease in frequency. As an object moves further away from a reference point, all waves emitted from that object have an increased wavelength.

Imagine a basketball player bouncing a ball up and down at a fix frequency. The wavelength is inversely proportional to the frequency.

The wave here is the ball bouncing up and down.

Now imagine the moment the ball hits the ground they start moving away from you at a fixed speed.

Because they have accelerated, light now needs to travel some additional distance since the ball at the top is further away from you than the ball at the bottom was.

But their new frequency remains the same because the displacement is now accounted for in the wavelength.

If they stop moving, opposite happens and it blueshifts.

If they keep accelerating, the distance increases and so does the redshifting.