Dark Matter: The Situation has Changed

hi everybody we haven't talked about dark matter for some time which is why today i want to tell you how my opinion about dark matter has changed over the past 20 years or so in particular i want to discuss whether dark matter is made of particles or if not what else it could be let's get started first things first dark matter is the hypothetical stuff that astrophysicists think makes up 80 percent of the matter in the universe or 24 of the combined matter energy dark matter should not be confused with dark energy these are two entirely different things dark energy is what makes the universe expand faster dark matter is what makes galaxies rotate faster though that's not the only thing dark matter does as we will see in a moment but what is dark matter 20 years ago i thought dark matter is most likely made of some kind of particle that we haven't measured so far because well i'm particle physicist by training and if a particle can explain an observation why look any further also at the time there were quite a few proposals for new particles that could fit the data like some supersymmetric particles or axions so the idea that dark matter is stuff made of particles is implausible to me and like the obvious explanation that's why just among us i always thought dark matter is not a particularly interesting problem sooner or later they'll find the particle give it a name and someone would get a nobel prize and that's that but well that hasn't happened physicists have tried to measure dark matter particles since the mid-1980s but no one's ever seen one there have been a few anomalies in the data but these have all gone away upon closer inspection instead what's happened is that some astrophysical observations have become increasingly difficult to explain with the particle hypothesis before i get to the observations that particle dark matter doesn't explain i'll first quickly summarize what it does explain which are the reasons astrophysicists thought it exists in the first place historically the first evidence for dark matter came from galaxy clusters galaxy clusters are made of a few hundred up to a thousand or so galaxies that are held together by their gravitational pull they move around each other and how fast they move depends on the total mass of the cluster the more mass the faster the galaxies move turns out that galaxies in galaxy clusters move way too fast to explain this with the mass that we can attribute to the visible matter so fritz zwicky conjectured in the 1930s that there must be more matter in galaxy clusters just that we can't see it he called it donclemateria dark matter it's a similar story for galaxies the velocity of a star which orbits around the center of a galaxy depends on the total mass within this orbit but the stars in the outer parts of galaxies just orbit too fast around the center the velocity should drop with distance to the center of the galaxy but it doesn't instead the velocity of the stars becomes approximately constant at a far distance to the galactic center this gives rise to the so-called flat rotation curves again you can explain that by saying there's dark matter in the galaxies then there is gravitational lensing these are galaxies or galaxy clusters which bend light that comes from an object behind them this object behind them then appears distorted and from the amount of distortion you can infer the mass of the lens again the visible matter just isn't enough to explain the observations then there are the temperature fluctuations in the cosmic microwave background these fluctuations are what you see in this sky map all these spots here are deviations from the average temperature which is about 2.7 kelvin the red spots are a little warmer the blue spots a little colder than the average astrophysicists analyzed the microwave background using its power spectrum where the vertical axis is roughly the number of spots and the horizontal axis is their size with the larger sizes on the left and increasingly smaller spots to the right to explain this power spectrum again you need dark matter finally there's the large scale distribution of galaxies and galaxy clusters and interstellar gas and so on as you see in the image from this computer simulation normal matter alone just does not produce enough structure on short scales to fit the observations and again adding dark matter will fix the problem so you see dark matter was a simple idea that fit to a lot of observations which is why it was such a good scientific explanation but that was the status 20 years ago and what's happened since then is that observations have piled up that dark matter cannot explain for example particle dark matter predicts a density in the course of small galaxies that peaks whereas the observations say the density distribution should be flat dark matter also predicts too many small satellite galaxies these are small galaxies that fly around a larger host the milky way for example should have many hundreds but actually only has a few dozen also these small satellite galaxies are often aligned in planes dark matter does not explain why we also know from observations that the mass of a galaxy is correlated to the fourth power of the rotation velocity of the outermost stars this is called the baryonic tully fischer relation and it's just an observational fact dark matter does not explain it it's a similar issue with renzo's rule that says if you look at the rotation curve of a galaxy then for every feature in the curve for the visible emission like a wiggle or bump there's also a feature in the rotation curve again that's an observational fact but it makes absolutely no sense if you think that most of the matter in the galaxies is dark matter the dark matter should remove any correlation between the luminosity and the rotation curves then there are collisions of galaxy clusters at high velocity like the bullet cluster or the elgato cluster these are difficult to explain with particle dark matter because dark matter creates friction and that makes such high relative velocities incredibly unlikely yes you heard that correctly the bullet cluster is a problem for dark matter not evidence for it and yes you can fumble with the computer simulations for dark matter and add more and more parameters to try to get it all right but that's no longer a simple explanation and it's no longer predictive so if it's not dark matter then what else could it be the alternative explanation to particle dark matter is modified gravity the idea of modified gravity is that we are not missing a source for gravity but that we have the law of gravity wrong modified gravity solves all the riddles that i just told you about there's no friction so high relative velocities are not a problem it predicted the tully fischer relation it explains ransom's rule and satellite alignments it removes the issue with density peaks and galactic cores and it solves the missing satellites problem but modified gravity does not do well with the cosmic microwave background and the early universe and it has some issues with galaxy clusters so that looks like a battle between competing hypothesis and that's certainly how it's been portrayed and how most physicists think about it but here's the thing purely from the perspective of data the simplest explanation is that particle dark matter works better in some cases and modified gravity better in others a lot of astrophysicists reply to this well if you have dark matter anyway why also have modified gravity to which the answer is because dark matter has difficulties explaining a lot of observations on its own it's no longer parametrically the simplest explanation but wait you may want to say you can't just use dark matter for observations a b c and modified gravity for observations x y z well actually you can totally do that nothing in the scientific method forbids that but more importantly if you look at the mathematics modified gravity and particle dark matter are actually very similar dark matter adds new particles and modified gravity adds new fields but because of quantum mechanics fields are particles and particles are fields so it's the same thing really the difference is the behavior of these fields or particles it's the behavior that changes from the scales of galaxies to galaxy clusters and filaments and to the early universe so what we need is a kind of phase transition that explains why and under which circumstances the behavior of these additional fields or particles changes so that we need two different sets of equations and once you look at it this way it's obvious why we have not made progress on the question of what dark matter is for such a long time they're just the wrong people working on it it's not a problem you can solve with particle physics and general relativity it's a problem for condensed metaphysics that's the physics of gases fluids and solids and so on so the conclusion that i have arrived at is that the distinction between dark matter and modified gravity is a false dichotomy the answer isn't either or it's both the question is just how to combine them this video was sponsored by not vpn not vpn is an app that keeps you safe as you browse the internet you install it on your phone or laptop use it to connect to one of their servers and then you browse the web from there this keeps your data safe even on a public wireless using this app has the added benefit that you can choose your virtual location from any one of their more than 5000 servers all over the world so if you ever encounter a video website that is blocked where you are you can just connect to a server in a different country and access the website from there viewers of this channel can now benefit from a special offer with a huge discount if you use the link notvpn.com sabine and the coupon code nordvpn works on pretty much all platforms android windows ios what have you once again that's not vpn.com sabine and the cooper code zabine thanks for watching see you next week

Channel: Sabine Hossenfelder

Views: 306,436

Rating: 4.9355388 out of 5

Keywords: physics, space, astronomy, dark matter, modified gravity, what is dark matter, dark energy, science, astrophysics, galaxies, rotation curves, cosmic microwave background, dark matter vs modified gravity, dark matter problems, modified gravity problems, is dark matter science, is dark matter real, hossenfelder, science without the gobbledygook

Id: 4_qJptwikRc

Length: 11min 49sec (709 seconds)

Published: Sat May 01 2021

Reddit Comments

[u/asdaaaaaaaa]

That's a bomb ass shirt.

[u/You_are_Retards]

good video.
I now dont understand just as much about dark matter as i did before