APOD: Dark Matter in a Simulated Universe (2024 Oct 20)

[APOD Robot]

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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[Ann]

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

Dark Matter in a Simulated Universe. Illustration Credit & Copyright: Tom Abel & Ralf Kaehler (KIPAC, SLAC), AMNH

---

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

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"!

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

The cosmic web is mostly occupied by vast voids: The bright regions are clusters and filaments of galaxies, dark regions are voids and blue dots are single galaxies inside the cosmic voids. Image credit: Either Ebrahim Yusofi or the cosmic web page in www.outerspacecentral.com.

---

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?

A schematic image of a void-dominated cosmic fluid that is occupied by vast voids: Negative pressure or repulsive force (like a concave lens) of vast voids on galaxies in the walls acts as the positive pressure or attractive force on it like a convex lens at a local scale (yellow circle).

---

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.

Our expanding and accelerating universe. Credit: NASA/Jet Propulsion Laboratory (JPL)

---

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.

Click to play embedded YouTube video.

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

A firefly in a void, with no pals around it. Credit: Ali Majdfar/Getty Images

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Ann

[JimB]

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?

[Ann]

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

[Christian G.]

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?

[smitty]

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.

[Chris Peterson]

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.

[johnnydeep]

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?

[Chris Peterson]

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.

[johnnydeep]

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?

[Chris Peterson]

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?

"Temperature" is a thermodynamic property that only makes sense across a statistically meaningful population of particles. A single atom doesn't have a "temperature". It reflects the average kinetic energy of a collection of particles.

Because our ordinary usage of "temperature" involves collisions between particles and the transfer of energy, and because particles of dark matter essentially don't interact, they don't really possess a temperature as such (although they may have very high velocities, so in some settings are treated as having a sort of temperature. But it's not something we measure; rather, it's a theoretical concept).

[VictorBorun]

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.

Let me say the say thing in more words.

Brown dwarfs' population and black holes' population have both the low viscosity as to pass the non-colliding criteria of the dark matter definition. What both of them do lack is the large enough mass.
In fact the map of the relic radiation is so smooth (denser spots having advantage of 1/30,000, that's almost zero) that we are led to suggest that the inflaton's heritage to the Big Bang was uniform in the same manner, with no clumps to create any black holes early on. Yet we know, by the quantity of the deuterium left from the first 20 minutes after the Big Bang, that the space was expanding fast enough to cut short the early nuclear syntheses and leave some deuterium fuel not turned to helium. And that shows that the non-baryon matter already was there.

So the black holes' population could be thought as today's dark matter, were the black holes surprisingly many and large, but it fails to fit the history.

[VictorBorun]

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.

there is a thing the relativity physicists call hot matter. They mean the particles that move at the speed of light or close. Mostly that is photons and neutrinos. A galaxy cluster with the escape velocity of 1,000 km/s would not keep such particles in its halo

Therefore the dark matter in a halo must be colder than that. Relativistic-ally cold.

[VictorBorun]

I wonder if baryon clumps (orange) are dots and dark matter clumps (grey) are filaments.
Because we arbitrarily set a threshold level to treat a more dense thing as a clump and the rest as an empty background.

We could have used the same density threshold for baryons and the dark matter. But then the picture would lose its appeal, because we'll be only left with galactic cores for clumps going baryon from dark matter, and so tiny things we would not pick in a picture frame of filaments and voids.

[Christian G.]

Thank you for your answers, Chris and Victor.

I'm sure they can capture dark matter under the right conditions

Coud the smaller and denser early universe be one such condition? I'm thinking specifically of the recent discovery of black holes just a few million years after the Big Bang that were more massive than expected; could one factor be that they have captured large amounts of dark matter, or would that be a negligeable part of their precocious mass?

How heavy is dark matter compared to baryonic matter, for that matter (no pun intended)? Galaxies have more dark matter mass than ordinary matter but that's because of the huge amount of dark matter, but say we compare a cubic meter of ordinary matter vs a cubic meter of dark matter, would the latter be heavier?

[Chris Peterson]

Thank you for your answers, Chris and Victor.

I'm sure they can capture dark matter under the right conditions

Coud the smaller and denser early universe be one such condition? I'm thinking specifically of the recent discovery of black holes just a few million years after the Big Bang that were more massive than expected; could one factor be that they have captured large amounts of dark matter, or would that be a negligeable part of their precocious mass?

How heavy is dark matter compared to baryonic matter, for that matter (no pun intended)? Galaxies have more dark matter mass than ordinary matter but that's because of the huge amount of dark matter, but say we compare a cubic meter of ordinary matter vs a cubic meter of dark matter, would the latter be heavier?

Again, though, black holes don't easily capture material. It's not easy to fall into a black hole.

[johnnydeep]

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?

"Temperature" is a thermodynamic property that only makes sense across a statistically meaningful population of particles. A single atom doesn't have a "temperature". It reflects the average kinetic energy of a collection of particles.

But a collection of a million atoms could have the same temperature as a collection of 100 atoms due simply to the average velocity of the atoms being the same (however impeded by collisions those atoms may be). So, it makes sense to me to say that a single atom with that same average velocity could be said to have that same temperature.

Because our ordinary usage of "temperature" involves collisions between particles and the transfer of energy, and because particles of dark matter essentially don't interact, they don't really possess a temperature as such (although they may have very high velocities, so in some settings are treated as having a sort of temperature. But it's not something we measure; rather, it's a theoretical concept).

Why couldn't dark matter interact with itself via collisions? We already know dark matter can affect other dark matter via gravity.

[johnnydeep]

Thank you for your answers, Chris and Victor.

I'm sure they can capture dark matter under the right conditions

Coud the smaller and denser early universe be one such condition? I'm thinking specifically of the recent discovery of black holes just a few million years after the Big Bang that were more massive than expected; could one factor be that they have captured large amounts of dark matter, or would that be a negligeable part of their precocious mass?

How heavy is dark matter compared to baryonic matter, for that matter (no pun intended)? Galaxies have more dark matter mass than ordinary matter but that's because of the huge amount of dark matter, but say we compare a cubic meter of ordinary matter vs a cubic meter of dark matter, would the latter be heavier?

It would only make sense to compare the density (g/m³) of baryonic matter to that of dark matter. Unless you mean to compare, say, a proton, to a "comparable" particle of dark matter whatever that might be?

[Chris Peterson]

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?

"Temperature" is a thermodynamic property that only makes sense across a statistically meaningful population of particles. A single atom doesn't have a "temperature". It reflects the average kinetic energy of a collection of particles.

But a collection of a million atoms could have the same temperature as a collection of 100 atoms due simply to the average velocity of the atoms being the same (however impeded by collisions those atoms may be). So, it makes sense to me to say that a single atom with that same average velocity could be said to have that same temperature.

Except in actual usage, "temperature" simply isn't used that way. Absent a large enough population of particles for statistics to be meaningful, you would just consider the energy of a particle (which could be all over the place as it absorbs or emits photons and changes its energy state).

Because our ordinary usage of "temperature" involves collisions between particles and the transfer of energy, and because particles of dark matter essentially don't interact, they don't really possess a temperature as such (although they may have very high velocities, so in some settings are treated as having a sort of temperature. But it's not something we measure; rather, it's a theoretical concept).

Why couldn't dark matter interact with itself via collisions? We already know dark matter can affect other dark matter via gravity.

Particle collisions are extremely rare. At the particle scale, baryonic matter interacts with itself through the electromagnetic force. Not the very weak gravitational force. Very, very early in the Universe the particle density of dark matter may have been high enough that actual collisions occurred. No more.

[Christian G.]

Thank you for your answers, Chris and Victor.

I'm sure they can capture dark matter under the right conditions

Coud the smaller and denser early universe be one such condition? I'm thinking specifically of the recent discovery of black holes just a few million years after the Big Bang that were more massive than expected; could one factor be that they have captured large amounts of dark matter, or would that be a negligeable part of their precocious mass?

How heavy is dark matter compared to baryonic matter, for that matter (no pun intended)? Galaxies have more dark matter mass than ordinary matter but that's because of the huge amount of dark matter, but say we compare a cubic meter of ordinary matter vs a cubic meter of dark matter, would the latter be heavier?

It would only make sense to compare the density (g/m³) of baryonic matter to that of dark matter. Unless you mean to compare, say, a proton, to a "comparable" particle of dark matter whatever that might be?

Thanks, indeed I should have been more precise, I meant comparing them at similar densities as well, not just volume.

[Christian G.]

And speaking of density, another enigma for me is why doesn't dark matter contract on and on, collapse, how does it put the brakes on gravity, in the same way for example that electron degeneracy pressure puts the brakes in white dwarfs (unless there might exist super compact dark matter bodies but that's getting a little speculative)

[Chris Peterson]

And speaking of density, another enigma for me is why doesn't dark matter contract on and on, collapse, how does it put the brakes on gravity, in the same way for example that electron degeneracy pressure puts the brakes in white dwarfs (unless there might exist super compact dark matter bodies but that's getting a little speculative)

Gravity does not cause collapse unless there is some mechanism for removing energy from the material. Gravity causes all the particles or bodies in a system to be in orbit around each other. Solar systems don't collapse. Globular clusters don't collapse. The only time we see material collapse (as in star formation) is where the densities are high enough that friction and fluid dynamics become significant, causing orbital decay. But those things are mediated by the electromagnetic force, which does not affect dark matter.

[Christian G.]

And speaking of density, another enigma for me is why doesn't dark matter contract on and on, collapse, how does it put the brakes on gravity, in the same way for example that electron degeneracy pressure puts the brakes in white dwarfs (unless there might exist super compact dark matter bodies but that's getting a little speculative)

Gravity does not cause collapse unless there is some mechanism for removing energy from the material. Gravity causes all the particles or bodies in a system to be in orbit around each other. Solar systems don't collapse. Globular clusters don't collapse. The only time we see material collapse (as in star formation) is where the densities are high enough that friction and fluid dynamics become significant, causing orbital decay. But those things are mediated by the electromagnetic force, which does not affect dark matter.

Enigma solved, thank you!

[johnnydeep]

And speaking of density, another enigma for me is why doesn't dark matter contract on and on, collapse, how does it put the brakes on gravity, in the same way for example that electron degeneracy pressure puts the brakes in white dwarfs (unless there might exist super compact dark matter bodies but that's getting a little speculative)

Gravity does not cause collapse unless there is some mechanism for removing energy from the material. Gravity causes all the particles or bodies in a system to be in orbit around each other. Solar systems don't collapse. Globular clusters don't collapse. The only time we see material collapse (as in star formation) is where the densities are high enough that friction and fluid dynamics become significant, causing orbital decay. But those things are mediated by the electromagnetic force, which does not affect dark matter.

Enigma solved, thank you!

Yes, Chris can be quite persuasive.

[johnnydeep]

Thank you for your answers, Chris and Victor.



Coud the smaller and denser early universe be one such condition? I'm thinking specifically of the recent discovery of black holes just a few million years after the Big Bang that were more massive than expected; could one factor be that they have captured large amounts of dark matter, or would that be a negligeable part of their precocious mass?

How heavy is dark matter compared to baryonic matter, for that matter (no pun intended)? Galaxies have more dark matter mass than ordinary matter but that's because of the huge amount of dark matter, but say we compare a cubic meter of ordinary matter vs a cubic meter of dark matter, would the latter be heavier?

It would only make sense to compare the density (g/m³) of baryonic matter to that of dark matter. Unless you mean to compare, say, a proton, to a "comparable" particle of dark matter whatever that might be?

Thanks, indeed I should have been more precise, I meant comparing them at similar densities as well, not just volume.

Well, if the density is the same then the (gravitational) effect would be too! As for the average grams of baryonic matter versus dark matter in a given inter-galactic or intra-galactic m³, I believe the dark matter density is much smaller. Chris?