Dark Matter: The Elusive Fibers of the Universe - Xuejian Shen - 06/23/2023

Despite being effectively invisible, dark matter is believed to compose 85% of the material of the universe--but what is it? Join us for a 30-minute lecture investigating the true nature of dark matter, followed by a panel Q&A consisting of several astrophysicists to answer your questions about astronomy, astrophysics, and space science. Timestamps below:
00:00 Announcements
04:52 Intro to Dark Matter Presentation
05:49 Dark Matter Presentation
37:08 Q&A for Dark Matter Presentation
37:19 "What assumptions go into dark matter simulations?"
40:09 "Does dark matter evolve over time?"
42:21 "Is dark matter connected to dark energy?"
44:39 "Is there evidence for dark matter vibration or spin?"
46:58 Intermission
52:43 Q&A Panel Introductions
55:02 "What is the mass of an eV (electron volt)?"
57:20 "How are dark matter halos observed for dwarf galaxies?"
58:38 "Do JWST observations change our view of bottom-up galaxy formation?"
59:28 "How are the distributions of dark matter and ordinary matter different?"
1:01:13 "Does dark matter not make disks?"
1:03:21 "Can particle collider experiments rule out certain dark matter candidates?"
1:05:13 "Can we gain insight about dark matter from gravitational wave observations?"
1:07:08 "What is the limiter in understanding dark matter: theory or observations?"
1:10:08 "Are there any galaxies without dark matter?"
1:13:34 "Is there a topological relation between dark matter and normal matter?"
1:15:15 "Tell us more about Pluto and friends!"
1:17:35 "Is dark matter represented as a particle or a field in simulations?"
1:19:27 "Can the observations be explained without dark matter?"
1:27:24 "Does dark matter have different phases: solid, liquid, gas?"
1:29:16 "Are there dark forces which only apply to dark matter?"
1:31:28 "Why do we simulate astrophysical systems?"
1:35:38 "Are there many types of dark matter?"
1:36:11 "Why do stars oscillate and are they observable?"
1:39:16 "Do scientists still hold to MOND as an alternative to dark matter?"
1:41:53 "How do we believe the Kuiper Belt formed?"
1:45:58 "What is the consensus on phosphene gas in Venus' atmosphere?"
1:51:55 "What does dark matter do?"
1:54:15 "Has dark matter changed its overall amount over time?"
1:56:08 "How can one succeed at learning math (or anything)?"
2:00:39 Concluding Remarks
Title: Dark Matter: The Elusive Fibers of the Universe
Speaker: Xuejian Shen
Abstract:
Scientists believe that most of the mass of the universe is composed of dark matter, a hypothetical material with no (or extremely weak) interactions with the normal matter we encounter in our daily lives. Despite being effectively invisible, dark matter is essential for the formation of large structures, like galaxies, in the universe. The fundamental nature of dark matter is one of the biggest question in modern physics. Could it be black holes? Could it be some new particles that have so far eluded detection in collider experiments? In this talk, I will discuss the compelling evidence for dark matter from astrophysical observations and introduce several hypotheses about the true nature of dark matter.

[@johngrundowski3632]

Thanks- great program🌌

[@iamjimgroth]

Well now I understand why I was dreaming I was at a cosmology conference. 😂

[@fredfonebone5108]

This was a very interesting talk. And congratulations DOCTOR Shen.

[@TheJerryRN]

Hello Caltech from the corn fields of central Ohio.. Wright State University. I greatly appreciate everything you are doing for outreach.. Bring on the science! Jerry

[@CaltechAstro]

Thank you so much for the kind words, Jerry! Bring on the science!

[@dianemacdonell8811]

So glad I found this!

[@CaltechAstro]

Hope you found it useful! Lots of good content from our other lectures too, but this was the first formal treatment of dark matter. It's definitely a challenging subject.

[@michaelccopelandsr7120]

I need help with my new years resolution. I've figured out how to change the stars. My idea for changing the stars includes Orion and Pleiades (Subaru). I figure it's time to put something up there that's relevant to us, don't you think? Take Orion's belt and Betelgeuse becomes the head with a baseball hat. Below the belt are two legs bending at the knee. The feet aligning perfectly under the bent knees. The 3 stars of Orion's belt align perfectly as the 3 fat belt loops on a baseball uniform. The spear pointing at "Subaru" is the bat being swung and "Pleiades" is the baseball flying away after being hit. Put it all together and you get, "THE ALL-STAR." In my case, I see a left-handed batter and I imagine a "7" on the jersey. Which makes him, "Mickey." (As it should be ;-) But you can put any number you want, making, "THE ALL-STAR," any player you want. It'd be wrong of me to not, at least, try. This is me, trying. Pass it on, please and thank you. Don't worry, where I come from, crazy is a compliment. ;-P

[@mikemorgan8294]

My theory is dark matter is Moscovium. While this element doesn't exist stably here on earth, we know it exists somewhere in the universe. Since Moscovium is utilized to create gravity, this would explain the extra mass that holds the universe together.

[@CaltechAstro]

I am unaware of any evidence for moscovium or any other super-heavy unstable elements being found in other parts of our universe.

[@toonmoene8757]

The dark matter halo of galaxy is often pictured as spherical. Is that borne out by observation (gravitational lensing) ? Does that also mean the dark matter halo doesn't rotate. If so, why not ?

[@CaltechAstro]

Insightful question! The dark matter halos of galaxies are roughly spherical based both on the results of computational models as well as observations from gravitational lensing in the real universe. At a fine level, they appear to be "tri-axial" which is just a fancy way of saying that they aren't perfect spheres and deviate by a factor of

[@michaelccopelandsr7120]

My idea so I get to name it! What I mean is, no one has claimed it so I'm officially calling, "Dibs." Voyager 1 is now in the, "Milky Way's interstellar time" or "Mikey's Time." "V-ger's" message has sped up now that it's outside our Sun's, "Time Bubble," or, "Terran Time." It will be faster, still, when "V-ger" sends a message from beyond the Milky Way's time bubble. Then there's Outside the Local Group time bubble. So on and so on until we get outside any influence and into the, "True Interstellar Time Standard." Or, "T.I..." ;-P Now that "V-ger" is outside our Sun's reach, in interstellar space, it's now in the Milky Way's faster moving, Interstellar Time or "Mikey's Time." This can be proven by turning off everything except its clock and transmitter. Have "V-ger" read time for as long as possible. They WILL show the flow of time speeds up the further away you get from any celestial bodies. Until you reach the Milky Way's time standard or "Mikey's Time." •Our sun's time bubble: "Terran Time" we know and have measured. In a lifetime, our head is one second younger than our feet. •Milky Way's time bubble or "Mikey's Time." The rate/flow of TIME outside any influence but within the Milky Way: We just got there and are still figuring what the difference is. Wild guess I'd say time will increase in speed, now and until V-ger is outside the Ort cloud. •Local Group's time bubble or the rate/flow of time outside of any influence but within the Local Group: Name still open and unknown. Wild guess .08% to a couple seconds faster, maybe. Used just for reference. •Outside any influence in the, "True Interstellar Time Standard." (or T.I...) ;-P This name is NOT up for grabs. The rate/flow of time is fastest here. (Time flows fastest here so it's best to use a motor boat and hold tight. Always applies when you're in T.I....) ;-P A minute is a minute in all. It's the rate/flow I'm talking about. Heck, rivers of time flowing differently might explain dark energy and dark matter. The Milky Way's Interstellar Time Standard will be known as, "Mikey's Time." Pass it on, please and thank you.

[@mirceapintelie361]

Is it possible to perform an experiment to see if the "mass" of the dark matter fluctuate or not?

[@morganisles4222]

In the images at 16:40 we can see that the coloured fibres become more pronounced the closer to the centre we look. Does this imply that the amount of dark matter in the universe is increasing with time?

[@CaltechAstro]

Nice observation, but there is a slightly different explanation. Dark matter isn't increasing with time, but it does get more concentrated in structures over time. Gravity is an attractive force on matter, so over time, regions of high density will draw more mass into them and become even denser. So this graphic is just showing that the filaments of dark matter are increasing in their density, even as the voids between those filaments are decreasing their density as dark matter preferentially moves from underdense regions to overdense regions of the universe.

[@morganisles4222]

Thank you for the response. To check that I’ve understood: you’re saying that the total quantity of dark matter in the universe stays constant, but it works its way closer and closer to regions of concentrated ordinary matter, like galaxies? If this is the case, how do we know this? I imagine that we can’t use gravitational lensing to detect dark matter that is outside of galaxies or globular clusters.@@CaltechAstro

[@CaltechAstro]

Yes, your understanding is roughly correct, but that the dark matter works its way closer to regions of concentrated matter, both ordinary matter as well as other dark matter. For gravity, mass is mass, regardless of whether its dark or baryonic (ordinary). We know this is the case from a bunch of different observations: gravitational lensing, the power spectrum of the CMB, the rotation curves of galaxies and clusters of galaxies, etc. Gravitational lensing reveals the distribution of mass at the location of the lensing object (foreground) not the lensed object (background), so you can use it to identify regions of high mass density, and it basically only occurs near galaxies and clusters of galaxies.

[@morganisles4222]

@@CaltechAstro So in summary, you would say that the total amount of dark matter appears to be generally the same when comparing nearby areas to distant ones? And these survey images appear to show dark matter decreasing with radius from the earth merely because dark matter flows via gravity from low to high density regions, not because of an increase in the dark matter ‘substance’ over time?

[@shawns0762]

Most people don't know that Einstein said that singularities are not possible. In the 1939 journal "Annals of Mathematics" he wrote "The essential result of this investigation is a clear understanding as to why the Schwarzchild singularities (Schwarzchild was the first to raise the issue of General relativity predicting singularities) do not exist in physical reality. Although the theory given here treats only clusters whose particles move along circular paths it does seem to be subject to reasonable doubt that more general cases will have analogous results. The Schwarzchild singularities do not appear for the reason that matter cannot be concentrated arbitrarily. And this is due to the fact that otherwise the constituting particles would reach the velocity of light." He was referring to the phenomenon of dilation (sometimes called gamma or y) mass that is dilated is smeared through spacetime relative to an outside observer. This is illustrated in a common 2 axis dilation graph with velocity on the horizontal line and dilation on the vertical. The graph shows the squared nature of the phenomenon, dilation increases at an exponential rate the closer you get to the speed of light. General relativity does not predict singularities when you factor in dilation. Einstein is known to have repeatedly spoken about this. Nobody believed in black holes when he was alive for this reason. Wherever you have an astronomical quantity of mass, dilation will occur because high mass means high momentum. There is no place in the universe where mass is more concentrated than at the center of a galaxy. According to Einstein's math, the mass at the center of our own galaxy must be dilated. In other words that mass is all around us. This is the explanation for the abnormally high rotation rates of stars in spiral galaxies, the missing mass is dilated mass. According to Einstein's math, galaxies with very, very low mass would not have dilated mass because they do not have enough mass at the center to achieve relativistic velocities. To date, this has been confirmed with 5 very low mass galaxies, all showing no signs of dark matter, in other words they have predictable star rotation rates. This is virtual proof that dilation is the governing phenomenon in galactic centers, there can be no other realistic explanation for this fact.

[@wisdon]

define boring