Ethan Siegel | Demystifying Dark Matter | The Cartesian Cafe with Timothy Nguyen

Ethan Siegel is a theoretical astrophysicist and science communicator. He received his PhD from the University of Florida and held academic positions at the University of Arizona, University of Oregon, and Lewis & Clark College before moving on to become a full-time science writer. Ethan is the author of the book Beyond The Galaxy, which is the story of “How Humanity Looked Beyond Our Milky Way And Discovered The Entire Universe” and he has contributed numerous articles to ScienceBlogs, Forbes, and BigThink. Today, Ethan is the face and personality behind Starts With A Bang, both a website and podcast by the same name that is dedicated to explaining and exploring the deepest mysteries of the cosmos.

In this episode, Ethan and I discuss the mysterious nature of dark matter: the evidence for it and the proposals for what it might be.

#astronomy #physics #cosmos #darkmatter

Part I. Introduction
00:00:00 : Biography and path to science writing
00:07:26 : Keeping up with the field outside academia
00:11:42 : If you have a bone to pick with Ethan...
00:12:50 : On looking like a scientist and words of wisdom
00:18:24 : Understanding dark matter = one of the most important open problems
00:21:07 : Technical outline

Part II. Ordinary Matter
23:28 : Matter and radiation scaling relations
29:36 : Hubble constant
31:00 : Components of rho in Friedmann's equations
34:14 : Constituents of the universe
41:21 : Big Bang nucleosynthesis (BBN)
45:32 : eta: baryon to photon ratio and deuterium formation
53:15 : Mass ratios vs eta

Part III. Dark Matter
1:01:02 : rho = radiation + ordinary matter + dark matter + dark energy
1:05:25 : nature of peaks and valleys in cosmic microwave background (CMB): need dark matter
1:07:39: Fritz Zwicky and mass mismatch among galaxies of a cluster
1:10:40 : Kent Ford and Vera Rubin and and mass mismatch within a galaxy
1:11:56 : Recap: BBN tells us that only about 5% of matter is ordinary
1:15:55 : Concordance model (Lambda-CDM)
1:21:04 : Summary of how dark matter provides a common solution to many problems
1:23:29 : Brief remarks on modified gravity
1:24:39 : Bullet cluster as evidence for dark matter
1:31:40 : Candidates for dark matter (neutrinos, WIMPs, axions)
1:38:37 : Experiment vs theory. Giving up vs forging on
1:48:34 : Conclusion

Further learning:
E. Siegel. Beyond the Galaxy

More Ethan Siegel & Timothy Nguyen videos:
Brian Keating’s Losing the Nobel Prize Makes a Good Point but …
Testing Eric Weinstein's and Stephen Wolfram's Theories of Everything

Twitter:
@iamtimnguyen

Webpage:

Apple Podcasts:

Spotify:

[HansTube]

Would love to see more of Ethan Siegel.

[vishalchikkerur]

These interviews are excellent. Great encountering this caliber of discussion. Admiration from a fellow data scientist here Tim 👍🏽

[Joshua-byqv]

I subscribe to his column and podcast on Medium. While I enjoy Neil De Grasse Tyson, Dr. Siegal's writings are probably the best out there for the layperson because he forces you to think and know about basic concepts of physics, which are necessary to understand the topic matter. Great to see him here.

[Unique-Concepts]

This channel will be exploded in few months....Really such a high quality content I ever seen

[Woollzable]

Great talk. Ethan is a great scientific communicator. Thank you both.

[nerdcoke]

Love these videos thank you for curating!

[shawns]

I subscribed, the fundamental phenomenon of dilation perfectly explains galaxy rotation curves/dark matter. Mass that is dilated is smeared through spacetime relative to an outside observer. It's the phenomenon behind the phrase "mass becomes infinite at the speed of light". A graph illustrates its squared nature, dilation increases at an exponential rate the closer you get to the speed of light. A time dilation graph illustrates the same phenomenon, it's not just time that gets dilated.
Dilation will occur wherever there is an astronomical quantity of mass because high mass means high momentum. This includes the centers of very high mass stars and the overwhelming majority of galaxy centers.
The mass at the center of our own galaxy is dilated. This means that there is no valid XYZ coordinate we can attribute to it, you can't point your finger at something that is smeared through spacetime. More precisely, everywhere you point is equally valid. In other words that mass is all around us.
Dilation does not occur in galaxies with low mass centers because they do not have enough mass to achieve relativistic velocities. It has recently been confirmed in 6 very low mass galaxies including NGC 1052-DF2 and DF4 to have no dark matter. In other words they have normal rotation rates. All binary stars have normal rotation rates for the same reason.

[Ubu]

Dark matter and energy sound like cosmological phlogiston. Can any testable predictions be derived from the idea?

[booJay]

Super hyped that Ethan will be making videos regularly. I've been following him for years, and if Sabine Hossenfelder has taught us anything, there's a huge appetite for this kind of stuff!

[Raytheon]

Ethan @29:18 asks a mathematician (Timothy) if he wants to see the math It's like asking a dental hygienist if you should quit drinking soda or coffee.

[jabowery]

Core-cusp tension sets a limit on cross-section under some assumptions about dark matter.

[Thinking_Fast_And_Slow]

If cost were not a factor, then you would want to do all experiments possible. But some of the experiments take a large portion of available funding at the expense of other possible experiments. You need to have some way to decide how to allocate scarce resources.

[marcussandzik]

I wish Eric would do this with you. That would be huge. Can’t we all just get along. ? Maybe DM each other.

[johnmancini]

Hi Timothy, I really enjoy your youtube channel. Would you be willing to make a video about ChatGPT, or any of the LLM that have been coming out lately, from a somewhat math / programming angle? Perhaps a similar format you use with guests but you doing a solo lecture?

[StephenPaulKing]

Unfortunately Planck mass black holes have an effectively zero scattering cross-section!

[hu]

Loved this discussion, learned a lot! But to counter Ethan’s point near end, we can’t afford to do every possible experiment covering every possibility. The country is already bankrupt, this would throw in the towel. Therefore, we need to be very judicious with the selection of our experiments looking for justifications as to which have higher probability of providing a most useful result. And towards that end, the theorists still need to be out in front because on average theory is much cheaper than experiment.

[hacknisty]

This is truly amazing ! Thank you very much !
Mr Siegel is doing an awesome job at explaining science to non scientific people

[anamariasanchez]

Sería genial que estubieran subtitulados en español

[timjohnson]

Since Ethan didn’t give a proper steel-manning of MOND and the bullet cluster problem, I would like to give my understanding of Milgrom’s MOND response. After all, the question I submitted to Brian Keating about the bullet cluster was asked by Brian in his interview of Milgrom.

Milgrom said that non-colliding galaxy clusters have always and admittedly been a problem for MOND. MOND works for galaxy rotation curves and when scaled up to clusters, MOND eliminates the need for much of the DM. However, there is still missing mass in the clusters (a factor of ~2, whereas lambda CDM requires a factor of ~5 DM to normal matter). Milgrom supposes that there is some unexplained/unknown baryonic matter in galaxy clusters.

Milgrom says this unknown baryonic matter in clusters (that MOND has always assumed is missing) continues its travel with the galaxies in the cluster (i.e. it is not slowed by some kind of self-interaction the way the gas is slowed as shown by the x-rays). So in Milgrom’s view, it is this unknown baryonic matter that explains why we see that most of the mass/gravity in the bullet clusters is with the galaxies and not the gas. Milgrom’s response seems reasonable to me, and I think Ethan should respond to this explanation instead of saying the Modified Gravity community “say things [about the bullet cluster problem], but those things don’t address this point” because Milgrom clearly understands and addresses the point.

As an aside, Sabine Hossenfelder repeatedly says that the bullet cluster is a huge problem for both MOND and lambda CDM (possibly an equivalent problem though I don’t want to speak for her). Since Tim Nguyen has connections to Sabine and Ethan, it would be awesome if you could have them both on for a short discussion. I think a lot of people would be really interested. Curt Jaimungal does a good job of hosting these types of discussions and frames them as theolocutions instead of debates, which I think is helpful.

[markphc]

So long as he's getting enough vitamin D, it's all good