APOD: Dark Matter in a Simulated Universe (2021 Oct 31)

[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]

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

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That's a striking illustration of dark matter and baryonic matter in the Universe. I like it!

A fun thing about the map is that dark matter is very appropriately shown as dark▮, but the parts where there is not a lot of dark matter shown as light gray▮. But we all know that the Universe is pretty dark, because the night sky is dark. Do you remember why it is dark? What is the correct answer?

1) It's dark because the Universe is young, and the speed of light is slow compared with the size of the Universe, which means that much of the light that the Universe has produced has not had time to reach us.

2) It's dark because the Universe is expanding, and the wavelengths of light are stretched along with it, so that much of the light of the Universe has been stretched far beyond the visible part of the spectrum.

3) It's dark because the Universe is not just expanding but accelerating, so that the ever-growing size of the Universe is really outrunning the speed of light, which means that much of the light of the Universe will never reach us!

4) It's dark because there is not an infinite number of stars in the Universe. Come on, people. What makes anyone think that there are so many stars in the Universe that there is not a single point along any arbitrary line of sight in the Universe that wouldn't eventually "hit a star"?

Hubble Ultra Deep Field. Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team

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Surely there isn't a star along every line of sight in the Hubble Deep Field? Of course there are more stars in the parts of the Universe that are not observable. But, no, we can't see them.

Small Big Bad Wolf.png (21.06 KiB) Viewed 6028 times

Anyway, take a look at the APOD again and note how the light gray stuff behind the dark filaments of dark matter almost looks like expanding bubbles, huffing and puffing like the Big Bad Wolf. Maybe that's where the dark energy is??

Ann

[orin stepanek]

I skimmed through today's APOD! Very interesting; and took about
an hour! Wow; it seems there are more things that we can''t see and
feel than we can see and feel! If all that dark matter is
making the universe grow; it'll get a lot larger than it already is!

[jmhaas@bellsouth.net]

Looks like the neural pathways in the human brain.

[Chris Peterson]

4) It's dark because there is not an infinite number of stars in the Universe. Come on, people. What makes anyone think that there are so many stars in the Universe that there is not a single point along any arbitrary line of sight in the Universe that wouldn't eventually "hit a star"?

That one is arguable (not necessarily wrong, just not certain). It is possible that the extent of the Universe is infinite, and if so, contains an infinite number of stars. Of course, this poses no problem for a dark sky, given that we only need consider the observable universe for that (related to your #3), and there is certainly only a finite star count in any finite volume.

[ta152h0]

Does that mean the Universe will never collapse? pass the ice cold one

[Chris Peterson]

Does that mean the Universe will never collapse? pass the ice cold one

The current evidence suggests that dark energy now dominates gravity, and therefore for the last few billion years (and for the infinite future) its expansion rate has been increasing. So it will head towards some variation of heat death, not a collapse or bounce.

[neufer]

<<In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. Predictions of the cold dark matter paradigm are in general agreement with observations of cosmological large-scale structure.

In the hot dark matter paradigm, popular in the early 1980s and less so now, structure does not form hierarchically (bottom-up), but forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way.

Since the late 1980s or 1990s, most cosmologists favor the cold dark matter theory (specifically the modern Lambda-CDM model) as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation) to the lumpy distribution of galaxies and their clusters we see today—the large-scale structure of the universe. Dwarf galaxies are crucial to this theory, having been created by small-scale density fluctuations in the early universe; they have now become natural building blocks that form larger structures.
..................................................
Several discrepancies between the predictions of the particle cold dark matter paradigm and observations of galaxies and their clustering have arisen:

• The cuspy halo problem: The density distributions of dark matter halos in cold dark matter simulations (at least those that do not include the impact of baryonic feedback) are much more peaked than what is observed in galaxies by investigating their rotation curves.
  
  The missing satellites problem: Cold dark matter simulations predict large numbers of small dark matter halos, more numerous than the number of small dwarf galaxies that are observed around galaxies like the Milky Way.
  
  The disk of satellites problem: Dwarf galaxies around the Milky Way and Andromeda galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly about their parent galaxies.
  
  Galaxy morphology problem: If galaxies grew hierarchically, then massive galaxies required many mergers. Major mergers inevitably create a classical bulge. On the contrary, about 80% of observed galaxies give evidence of no such bulges, and giant pure-disc galaxies are commonplace. That bulgeless fraction was nearly constant for 8 billion years.

Some of these problems have proposed solutions, but it remains unclear whether they can be solved without abandoning the CDM paradigm.>>

[zendae1]

I tried viewing it 3D in case there was a teapot but it was turtles.

[johnnydeep]

I tried viewing it 3D in case there was a teapot but it was turtles.

"Turtles all the way down!" From https://en.wikipedia.org/wiki/Turtles_all_the_way_down

"Turtles all the way down" is an expression of the problem of infinite regress. The saying alludes to the mythological idea of a World Turtle that supports a flat Earth on its back. It suggests that this turtle rests on the back of an even larger turtle, which itself is part of a column of increasingly large turtles that continues indefinitely.

The exact origin of the phrase is uncertain. In the form "rocks all the way down", the saying appears as early as 1838.[1] References to the saying's mythological antecedents, the World Turtle and its counterpart the World Elephant, were made by a number of authors in the 17th and 18th centuries.[2][3]

[Nukeit]

Resembles a neurolink or even possibly mycorrhiza. "to be belittled" at best.

[neufer]

Click to play embedded YouTube video.

The Bullet Cluster: X-ray image (pink) superimposed over a visible light image (galaxies), with matter distribution calculated from gravitational lensing (blue)

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Resembles a neurolink or even possibly mycorrhiza.

"to be belittled" at best.