That is possible. However:

The early-universe evidence for dark matter is so strong that even the (non-mainstream) researchers who favor modifications to gravity to explain dark matter in galaxies still have to include dark matter in the early universe. Then they have to get rid of it by the time galaxies start to form to make way for their gravity theory.

Another perspective is that dark matter is a much simpler hypothesis than a modification to gravity. We already know of one dark matter species -- neutrinos -- and it's really not a stretch to suppose that there is another. The only special feature that we know it has to have is that it has to reach the observed abundance.

Yeah, but in context it's the hypothesis that sterile neutrinos explain the anomaly. I edited the post above after checking the research article.

They also say, > we reject with high CL the hypothesis of a sterile neutrino of mass around 1 eV.

Viable sterile neutrino dark matter models are generally at least 1000 times heavier than that, in the keV range. That's because if the dark matter particle were too light, its thermal motion would eliminate variations in the density of the universe at the scales of dwarf galaxies, preventing those galaxies (which we observe and hence know to exist) from forming.

That reads like they are ruling out the possibility that sterile neutrinos are the explanation for a particular anomaly seen in past experiments, and not that they rule out the possibility of sterile neutrinos in general.

Edit: see for example this figure from the article. They rule out parameters to the right of the red/blue curves, in particular ruling out the parameters that would have explained the "reactor antineutrino anomaly (RAA)".

That works for gas but not for stars, black holes, asteroids, planets, etc. Their collision rate is negligible, similarly to dark matter. The key point about those galactic collisions is that the lens mass lies ahead of the gas, not the stars.

> These things tend to clump together and do things like fall into stars where we can detect them by their emission lines.

I think that requires some additional assumptions. Dark matter indeed couldn't be gas, because we can detect gas on its own, and also gas would lose energy and fall into things (whereas dark matter does not). However, high-density low-brightness objects made of ordinary matter (like free-floating asteroids or planets) would behave just like dark matter and would be incredibly difficult to detect. Granted, it would not be easy to explain how they all formed.

But that's why I went to the very early-universe evidence for dark matter. Because irrespective of what form ordinary matter took in the late universe, it was all the same in the hot early universe.

For example, we cannot rule out that the dark matter might be asteroid-mass black holes (e.g. figure 10 of this article). Why couldn't it just be asteroids?

The main lines of evidence against such a possibility are related to the early universe. This is a time when the the universe was very hot. Asteroids could not exist in such an environment; they would dissociate into diffuse plasma like all the rest of the ordinary matter. In this context, all ordinary matter is equally detectable, in the sense that it has an equal impact on what we observe. But what do we observe?

1. The relative abundances of light elements throughout the universe. We understand nuclear physics and can predict the ratios of hydrogen, deuterium, helium, etc. that should have emerged from the Big Bang. What we find is largely consistent with ordinary matter comprising only 5% of the total energy density today. If the density of ordinary matter were higher, we should find less deuterium and more helium than we do. The first figure of this paper (page 9) illustrates nicely how the primordial element abundances depend on how much ordinary matter there is.

2. Temperature variations in the cosmic microwave background. In the early universe, the ordinary matter and photons were tightly coupled, which led to such effects as pressure oscillations and sound waves. Dark matter, on the other hand, only interacted via gravity. This causes them to have very different effects on the evolution of temperature and density variations in the early universe, which manifest themselves to us in the cosmic microwave background. Here's an animation of how changing the density of ordinary matter ("baryons") would alter the "power spectrum" of the cosmic microwave background temperature, which is something we have measured extremely precisely, e.g. the top panel of this figure.

If a cluster of galaxies is virialized (its constituents are orbiting stably), we call it a cluster, not a supercluster. Superclusters are expanding with the Hubble flow by definition. A supercluster could certainly have a virialized cluster at its center though!

We can see the initial density fluctuations as temperature fluctuations in the cosmic microwave background (CMB). Almost all of the CMB was causally disconnected at its emission time, as the horizon scale at the time is around 1 degree on the sky. We see temperature variations larger than that, and since they are not causally connected, we know that they must have been frozen in time since whatever process created them in the much earlier universe. (Likely inflation, as I noted in another comment.)

Also, gravity can only amplify already existing density variations. Thus the smaller-scale (causally connected) CMB temperature variations, and the density variations in the universe today (responsible for galaxies and larger-scale structure), must have originated from similar initial density variations. In fact we understand quite well (mathematically) how density variations gravitationally amplify over time, and a wide range of observations generally all point to initial density variations having essentially the same average amplitude at every scale (the one part in 10-100 thousand that I mentioned).

We don't know, but the most popular hypothesis is that the density variations originated as quantum fluctuations during inflation (the hypothesized early period of accelerated expansion). They would begin around the Planck scale but rapidly expand due to inflation. This process creates fluctuations over a huge range of scales, as fluctuations created earlier grow larger than later ones, and that matches what we observe.

Superclusters are still expanding, they are just overdense regions that would eventually collapse if there were no dark energy. Assuming dark energy persists, only our Local Group will remain nearby.

I'm not sure actually! They look like they could be indicating the "virial radius" of each dark matter halo, which is a common way of approximating the system's size. As context, the virial radius of the Milky Way's halo is something like 700000 light years in radius, over ten times larger than its galactic disk. So these spheres would be much larger than galaxies, but they would generally contain galaxies at their centers.

The precise definition of the virial radius varies, but a typical definition is that it's the radius inside which the average density is 200 times the cosmological mean. That would mean that each sphere is exactly 200 times denser than the cosmological mean.

The basic idea of the virial radius is that the material inside this radius should be orbiting stably. There's a theoretical reason for the factor of 200 (technically the theory suggests 178, but it's approximate enough that people usually round it), and its derivation uses the idea that stably orbiting material should obey the virial theorem. That's where the name comes from.

There's a very nice recent review article about the origins of massive black holes.

Supermassive black holes form because of galaxies, not the reverse.

It has been suggested that supermassive black holes might form from "seed" primordial black holes, which would have existed before galaxies. But even then, it's the galaxy-scale initial density variations that allow galaxies to form around these seeds and grow them to supermassive scales.

What happened is that the universe cooled enough that all of the free protons and electrons condensed into neutral hydrogen. Neutral atoms interact with light much more weakly than do free electric charges, so this process made the universe transparent to light.

No, the time light took to reach us only correlates with the amount that the light's frequency is shifted and not with the light's absolute frequency. For example, the CMB is actually a whole spectrum of frequencies. Those frequencies don't take (significantly) different amounts of time to reach us; if they did, the CMB spectrum wouldn't be such a perfect blackbody spectrum.

Galaxies exist in the first place because the early universe was not completely uniform. Some regions were slightly denser than others. These density variations were initially at the level of one part in 10-100 thousand (10^(-5) to 10^(-4)), but gravity amplified them over time. Denser regions tended to pull in surrounding matter, becoming still denser. Eventually, the densest patches formed galaxies.

However, initial variations in the density of the universe also existed at scales much larger than galaxies. Due to this large-scale structure, galaxies are now moving toward regions of higher density and away from regions of lower density.

Here's an example movie. "z" in the corner is the redshift, essentially inverse time (smaller is later). The key point is that galaxies form (the yellowish color) but continue moving as they coalesce into still larger systems.

See this comment! About 56000 years.

I just lazily took pi/l for l=2500, the largest l that Planck papers plot. Indeed, the ground-based telescopes push to somewhat higher l.

Hmm, around 1 degree (where the fluctuation power peaks), the time scale for the CMB to change would be of order a billion years, or one part in ~10^(9) per year. I wonder how far off that kind of sensitivity is.

Sorry, I typed "reionization" but meant "recombination"... I've fixed that.

To clear things up:

• Recombination is a process that occurred at a time of around 370000 years. At this time, the universe cooled enough that all of the free protons and electrons condensed into neutral hydrogen. Without all of the free electric charges, the universe became transparent. (The "re" in "recombination" is a complete misnomer.)

• Reionization is a process that occurred at a time of around 200 million to 1 billion years. This is what those videos are showing. When the first galaxies formed, the light emitted by their stars and black hole accretion disks ionized essentially all of the neutral hydrogen in the universe. (The universe didn't become opaque again, though, just because the hydrogen was far too sparse by this time.)

Remember the CMB light originated everywhere. So there will always be a distance such that light originating from that distance is just reaching us now. Cosmic expansion doesn't come into play here.

> The Andromeda galaxy is expected to collide with the Milky Way in approximately 4.5 billion years. Does this time take into account the expansion of space in between the two galaxies?

Space expanding doesn't physically do anything. It's just a convention that's useful in some contexts. (It represents a choice of coordinates on spacetime.)

Since the misconceived reification of expanding space is pretty deeply ingrained in the public consciousness, here are some articles discussing the point further.

(1) A diatribe on expanding space. This is pretty technical, but it's the most direct attack on the idea of expanding space. One key quote is that

> there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost.

For example, the Milky Way-Andromeda system is no longer expanding, so cosmic expansion is simply no longer relevant to it.

(2) The kinematic origin of the cosmological redshift. Very well written and less technical, although there are mathematical arguments. The main point of this article is that the cosmological redshift -- often framed as a consequence of space expanding -- is more precisely viewed as just a Doppler shift.

(3) On The Relativity of Redshifts: Does Space Really "Expand"? The least technical of the batch. This article is also focused on the interpretation of the cosmological redshift. It includes the choice paragraph:

> While it may seem that railing against the concept of expanding space is somewhat petty, it is actually important to set the scene straight, especially for novices in cosmology. One of the important aspects in growing as a physicist is to develop an intuition, an intuition that can guide you on what to expect from the complex equation under your fingers. But if you assuming that expanding space is something physical, something like a river carrying distant observers along as the universe expands, the consequence of this when considering the motions of objects in the universe will lead to radically incorrect results.

Missing the redshift factor (should be ~55 million years), so I guess that would be ~40 km diameter?

According to this paper, the last scattering surface has a comoving thickness of about 19 Mpc, which corresponds to a physical thickness of (19 Mpc)/1100 ~ 17 kpc or a duration of (17 kpc)/c ~ 56000 years.

Edit: The above concerns how thick the last scattering surface is at any given point on the sky (which is connected to how long recombination -- the process by which the universe became transparent -- took, as well as how opaque the universe was before recombination). I just realized that you are instead asking how the recombination time varied between different patches of the sky. Temperature variations in the CMB are around the 10^-4 level (one part in ten thousand), which implies that the recombination time varied to a similar degree. 10^(-4) of 370000 years is 37 years, so the spatial variation in the recombination time is of order tens of years.

That's right, the scales are precisely proportional in that way.

With respect to whether such resolution is possible, I can't say much about the instrumentation side, but I can point out a major physical challenge. According to our calculations, there simply wasn't much structure on very small scales in the early universe, due to diffusion damping. Photons were able to gradually diffuse between hot and cold regions, allowing their temperatures to equalize. This effectively smoothed out the early universe; due to photon travel times, it affected small scales more than large scales.

While movement is expected in principle, the cosmic microwave background (CMB) is static over human time scales.

The light comprising the CMB last scattered at the same time everywhere, when the universe was about 370000 years old. The CMB that we see consists of the light that is just now reaching us. As time goes on, light from more and more distant regions is able to reach us. In this way, the CMB depicts a spherical slice of the 370000-year-old universe (the "last scattering surface") at an ever increasing distance as time goes on.

Over what time scale should we expect to see the CMB change, then? The smallest scales we can resolve, currently, are about 0.07 degrees on the sky, which corresponds to about 50000 light years (15 kpc) at the distance of the CMB. (This is actually remarkably small due to the angular diameter turnover!)

For a 50000-light-year structure, light from the far end takes 50000 years longer to reach us than light from the near end. Does this mean that we should expect CMB temperature fluctuations on those scales to change in about 50000 years? Well not quite. The CMB is redshifted by a factor of 1100, which means it's time dilated by the same factor. So we expect fluctuations on the (currently) smallest resolved scales to change over a (1100 times longer) time scale of about 55 million years.

It will become continuously dimmer and redder, eventually moving into radio frequencies. If dark energy persists, the CMB frequency will halve (wavelength will double) every ~12 billion years.