#5 Kathryn Johnston - The Milky Way, Dark Matter vs MOND, Gaia

In this week's episode, David is joined by Kathryn Johnston. Prof Johnston is a Professor of Astronomy at Columbia University and the Dynamics Group Leader at the Flatiron Institute. Kathryn is a world renown expert for her work on Milky Way dynamics, especially tidal streams and the dark matter components of our galaxy.

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And of course right here on YouTube!

[paulpence]

You have one of the most soothing voices, , , listen to all your productions, to fall asleep, , , but it's way more than that, cant fall asleep because its all way to interesting... hahaha!!!

[victorburns]

This conversation answered a lot questions; some I didn't even know I had. Very insightful.

[Inugmi]

I think David definitely has his NPR voice developed now.

[celestromel]

Gee - Kathryn is such a great “simplified” of the hopeless complicated (to the rest of us). What a great colleague! Thanks to you both. Perhaps you could invite her back - as she relaxes, her enthusiasm becomes infectious.

[nicohocco]

great video that made me thoughtful and curious : so we have a spiral galaxy with a massive spining black hole in it's right center (as we think all spiral galaxy have), with a symetrical perfect bar of stars around it and symetric spiral arms around (certainly spining in the same direction the black hole does and the orientation of the galactic disc would certainly follow the orientation of the spinning black hole, as our planetary system is more or less aligned on the rotation of our sun, found no information about it, if anyone know about it please tell me) BUT as we noticed the Einstein equation were false at 80 %( missing mass) at the galactic scale to describe how this galaxy works, we then invented dark matter to fill the gap and we keep going on with Einstein/Schwarzschild equations and since this time we try to find justification to existence of dark matter. Then we say that the black hole in the middle of our galaxy is too "little" to be responsible of anything gravitational we see around it. OK but how are calculated the galactic black hole properties ? With the Einstein 80% false equation of course (more precisely the Schwarzschild solution from the Einstein equations). Heuuuu I m not an astrophysicist but that does not really makes any logical sense to me, so illogical that i dont see clearly how an astrophysicist can cope with that without questioning. Or im missing something due to my great lack of knowledge in astrophysics, please tell me. And also explain how dark matter would influence gravity to build this galactic shape an put a massive black hole right in its center but with no role ?

So, if im not wrong, what we think today about galactic massive black holes is by definition false by 80 %. Gravity ripples made by merging somehow modest black holes (dizains of solar masses) make their way to our detectors from more a billion lightyears through space(that we also dont know ANYTHING about), but a multi millon solar mass black hole would be too weak to have any influence on gravity at a light year galactic size level !?!? Personally as a logical result i think that dark matter has 80 % chances to end up as the string theory did, even if many great scientists spent their whole career working on it. What is true today was false yesterday and will be false tomorrow ... Lets hope im wrong which i prefer to be, regarding respect i have for generation of scientist working hardly to find dark matter. So even being totally wrong i will stay thoughtful and stay curious and keep whatching your videos that i really Love ;-)

[matthewbeyer]

What a flex of an intro! She was my boss now im hers!!! Now lets talk about cool shit!.... So gangster!

[fadedparadigm]

Wow, thank you so much for hosting and sharing this talk! I hadn't really taken the time to read much about dark matter other than hearing some quick quips about it accounting for a dominant percentage of mass.

It was really awesome to have you two talk in plain language about the evidence we see that's consistent with dark matter, and how it relates to our own galaxy. I had no idea our galaxy is apparently as large as it is, or that these dwarf galaxies are orbiting around in these nutty dynamical patterns based on some apparent mass distribution of dark matter that we're only just beginning to understand.

Great stuff! Thank you again!

[AndrewBlucher]

8:27 Ah! That's a great explanation @Katherine

This is a great episode David. I trained in Physics in the early 70's and have followed Astronomy and other sciences intently ever since. This discussion with Katherine is the first time I have grasped the relative size of the Milky Way's central black hole, and also the first time I have heard in simple terms how the competing theories of gravity might be distinguished.

Great work, both of you!

[iq]

The best podcast in the world. I listen to you at work and before I sleep.

[nias]

Serious question, suppose you had a device that made precision measurements of the Casimir pressure. If you put the device on a spacecraft and sent it away from Earth, taking periodic measurements when the thruster was off. What would be some possible implications for the vacuum or MOND (if any) if you saw the pressure increasing/decreasing.

Also, is out assumption that the quantum vacuum is the same everywhere to general?

[UrusaiJoe]

Discovered you lately, amazing podcast.

[GueranJones-xh]

THE WAY IMAGINE THE ARMS OF THE MILKY WAY ARE ENTERTWINED SIMILAR TO A BRAID OF HAIR? PEOPLE CORRECT ME....

[rolandsummers]

Your podcasts are great man! Thanks for
the great content!

[melissanoriega]

Wow! I love this! Subbed! I'm so excited to see more videos

[msxciejhello]

So glad you started a podcast. I've been following your channel from the beginning. Also, get John Michael Godier on the podcast😁

[grantgeorgebuffett]

I love all these deep dives into frontier astronomy. Great job!

[TheebayOffroader]

Another amazing interview. I still however find myself asking the age old question. Are space worms real?🤔

[lib]

I know it's more time consuming but some timestamps on these would be great.

[SnapDash]

Fascinating chat!

I've heard that there have been recent estimates for the number of starless planets; suggesting that there may be a great many more than previously acknowledged. Would that in turn impact the overall gravity (and particularly dark matter) calculations, or are planet-scale objects just too small to be significant? Has there been much work yet looking into that?

A starless/"rogue" planet Cool Worlds episode would be nifty-

On a rather different tangent... What's the chronology for the formation of the Solar System compared to its dynamics with the other stars that were around when it formed. How should we picture the nearby interstellar environment as the planets were forming, and how does that differ from what we see now?

[RoyBurnell-on]

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