Starship Asterisk*

Dark Matter in a Simulated Universe

Explanation: Is our universe haunted? It might look that way on this dark matter map. The gravity of unseen dark matter is the leading explanation for why galaxies rotate so fast, why galaxies orbit clusters so fast, why gravitational lenses so strongly deflect light, and why visible matter is distributed as it is both in the local universe and on the cosmic microwave background. The featured image from the American Museum of Natural History's Hayden Planetarium Space Show Dark Universe highlights one example of how pervasive dark matter might haunt our universe. In this frame from a detailed computer simulation, complex filaments of dark matter, shown in black, are strewn about the universe like spider webs, while the relatively rare clumps of familiar baryonic matter are colored orange. These simulations are good statistical matches to astronomical observations. In what is perhaps a scarier turn of events, dark matter -- although quite strange and in an unknown form -- is no longer thought to be the strangest source of gravity in the universe. That honor now falls to dark energy, a more uniform source of repulsive gravity that seems to now dominate the expansion of the entire universe.

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Well, that's a nice surprise! A non-comet, non-aurora APOD! We haven't had many of them lately.


My first thought, when I just glimpsed the APOD, was, "Wow, that's a lot of fireflies in that thicket of branches!".


Note that even the real fireflies follow the "filaments" and avoid the "voids"!

The cosmic web itself is made up of filaments and voids:


The picture above was used in a ResearchGate paper and I was going to quote from it, but I'll refrain, because these guys did not grow up speaking English, and their attempt at using it as a second language in a scientific paper was just moderately successful. However, the point they are trying to make is interesting, because they claim that the pressure of cosmic voids may be a source of both dark matter and dark energy.

Like this, perhaps?


So the authors said that the voids are like bubbles with surface tension that keep growing in size, thus "pushing" on the Universe, making it grow larger. They also said that the universe is in fact dominated by its large voids, and that the effects are felt locally more than globally. I couldn't follow their reasoning, however, both because of their less than perfect grasp of English and my own very far from perfect grasp of the underlying scientific concepts. Note that I stole the longer image caption above from the authors. I tried to improve their grammar, but I don't know if I succeeded. I think I know what they are trying to say, however.

The publication source is here, but you have to download the PDF yourself if you want to read it.

But clearly, when the universe was young, it was much denser than it is today and the voids must have been much smaller. Not only are the voids now larger, but as the universe expands, the voids are dominating larger and larger parts of the universe.


For some reason I am reminded of the murmurations of starlings, when starlings form huge clouds of hundreds of thousands or even millions of individuals that fly and dance together in amazing configurations. They even create filaments and voids sometimes.


Now we just have to figure out what makes fireflies imitate that structure of the universe.


Ann

Ann wrote: ↑Sun Oct 20, 2024 6:07 am

Fireflies Event at Norton Creek 2021. Image credit: I don't know, but the picture can be found here: https://dlia.org/event/fireflies-2021/

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Note that even the real fireflies follow the "filaments" and avoid the "voids"!


Now we just have to figure out what makes fireflies imitate that structure of the universe.


Ann

Could it be that the photographer was using a timed exposure and the "filaments" are actually single fireflies flashing their morse code messages as they fly around to attract a mate?

JimB wrote: ↑Sun Oct 20, 2024 8:44 am

Ann wrote: ↑Sun Oct 20, 2024 6:07 am

Fireflies Event at Norton Creek 2021. Image credit: I don't know, but the picture can be found here: https://dlia.org/event/fireflies-2021/

---

Note that even the real fireflies follow the "filaments" and avoid the "voids"!


Now we just have to figure out what makes fireflies imitate that structure of the universe.


Ann

Could it be that the photographer was using a timed exposure and the "filaments" are actually single fireflies flashing their morse code messages as they fly around to attract a mate?

Sounds like a plausible explanation, Jim!

Ann

What about black holes in relation to dark matter? Is it fair to presume that they swallow it like everything else? Might the billions of solar masses of the largest ones include copious amounts of dark matter pulled in early on in the history of the universe?

Question: when a theory is founded on a circular definition, why should we be surprised when that theory yields surprising or puzzling results? Relativity is founded on a circular definition: the operational definition of time, i.e., "time is that which is measured by clocks." When we ask for the definition of a clock, however, we're told: "A clock is a device that measures time." Proponents of the operational definition will go on to say that "a clock is a device that measures and counts regularly recurring phenomena," but how would we know that phenomena are regularly recurring unless they had already been measured by another clock, etc., ad infinitum? As physicist Julian Barbour correctly pointed out in his book The End of Time, "Relativity is not about an abstract concept of time at all: it is about physical devices called clocks. Once we grasp that, many difficulties fall away."

When we discover puzzling things about the nature of gravity, it might behoove us to go back and think more closely about the circular definition upon which relativity is founded. Just a thought.

Christian G. wrote: ↑Sun Oct 20, 2024 12:54 pm What about black holes in relation to dark matter? Is it fair to presume that they swallow it like everything else? Might the billions of solar masses of the largest ones include copious amounts of dark matter pulled in early on in the history of the universe?

Well, for the most part black holes don't swallow very much. And billion mass black holes represent only a small fraction of all energy in the Universe, only a small fraction of the mass of ordinary matter, even. I'm sure they can capture dark matter under the right conditions, but part of the capture process for ordinary matter involves electromagnetic interactions, which of course don't exist in dark matter.

So many stupid questions:

- Dark matter is affected by gravity and has gravity of its own, so presumably it could create clumps of greater density due to random concentration fluctuations, correct?

- Dark matter is unaffected by the electromagnetic field and so doesn't generate any photons. Does that imply that dark matter has a temperature of absolute zero? That is, don't all things that vibrate (i.e., have a non-zero temperature) emit photons a la "black body radiation"? If so, dark matter must have a zero temperature.

- Dark energy has "repulsive gravity"? That seems like a poor metaphor. Gravity is gravity. Dark energy works in opposition to gravity, but is not a type of gravity that is repulsive, right?

johnnydeep wrote: ↑Sun Oct 20, 2024 7:10 pm So many stupid questions:

- Dark matter is affected by gravity and has gravity of its own, so presumably it could create clumps of greater density due to random concentration fluctuations, correct?

- Dark matter is unaffected by the electromagnetic field and so doesn't generate any photons. Does that imply that dark matter has a temperature of absolute zero? That is, don't all things that vibrate (i.e., have a non-zero temperature) emit photons a la "black body radiation"? If so, dark matter must have a zero temperature.

- Dark energy has "repulsive gravity"? That seems like a poor metaphor. Gravity is gravity. Dark energy works in opposition to gravity, but is not a type of gravity that is repulsive, right?

Not stupid at all!

Dark matter does form "clumps". We see that in the halos it forms around galaxies.

Temperature isn't about photons, it's about the energy of a particle, generally given by its speed. Dark matter particles (assuming the particle theory is correct) certainly move, so they have kinetic energy and they can transfer that energy to other particles, which means they have a temperature. They don't have to emit blackbody photons for that to be true.

I agree that "repulsive gravity" is a very poor term for dark energy... which as far as we know is unrelated to gravity.

Chris Peterson wrote: ↑Sun Oct 20, 2024 7:21 pm

johnnydeep wrote: ↑Sun Oct 20, 2024 7:10 pm So many stupid questions:

- Dark matter is affected by gravity and has gravity of its own, so presumably it could create clumps of greater density due to random concentration fluctuations, correct?

- Dark matter is unaffected by the electromagnetic field and so doesn't generate any photons. Does that imply that dark matter has a temperature of absolute zero? That is, don't all things that vibrate (i.e., have a non-zero temperature) emit photons a la "black body radiation"? If so, dark matter must have a zero temperature.

- Dark energy has "repulsive gravity"? That seems like a poor metaphor. Gravity is gravity. Dark energy works in opposition to gravity, but is not a type of gravity that is repulsive, right?

Not stupid at all!

Dark matter does form "clumps". We see that in the halos it forms around galaxies.

Temperature isn't about photons, it's about the energy of a particle, generally given by its speed. Dark matter particles (assuming the particle theory is correct) certainly move, so they have kinetic energy and they can transfer that energy to other particles, which means they have a temperature. They don't have to emit blackbody photons for that to be true.

I agree that "repulsive gravity" is a very poor term for dark energy... which as far as we know is unrelated to gravity.

Thanks. A few more...

Can we then measure the temperature of dark matter? I'd guess not since the temperature of other stuff in the universe is measurable by the frequency of light they emit, and dark matter emits no light, and we also don't know how fast dark matter "particles" are moving?

Does a single hydrogen atom or a single proton have a temperature that's directly proportional to its velocity?