New evidence AGAINST dark matter?!

there's one concept in physics that anytime i talk to people about it they're always really skeptical of it and that is dark matter matter that only interacts with gravity and not with light so we can't see it you know in any normal way but just like how we can't see the wind but we know it's there because the trees move we have our own ways of knowing that dark matter is there but they all rely on the fact that we have understood gravity right very recently there's been some research published that suggests that might not be the case so is this the end for dark matter now i've talked before on this channel about the decades worth of evidence and research that piled up and powered up in powder before astrophysicists begrudgingly agreed that dark matter must exist then it became accepted theory canon if you will and i've talked before on this channel as well about all of the legitimate alternate theories to gravity that there are that don't need dark matter that are sort of rivals to our current accepted theory of gravity which is einstein's theory of general relativity i'm going to summarize a little bit what those videos said i'm not going to go into as much detail so check those out as well if you're interested i'll link them up in little cards above and down in the video description below now one of these alternate theories of gravity is called monde modified newtonian dynamics and it essentially takes newton's equations of gravity and modifies them slightly with this extra term and those extra terms only matter and only start to dominate the equation when you have something really big that's accelerating very very slowly like a galaxy of stars that is rotating for example and the speed of rotation of a galaxy has always been in the top five pieces of evidence that we have for dark matter because they don't behave the way that we expect when we look at a galaxy we see that the brightest part of a galaxy is the very center so we assume there are more stars there and therefore that's where most of the mass is concentrated very similar to the solar system right where the majority of the mass more than 99 is actually concentrated in the center in the sun and if you look at the speed that the planets orbit around the sun you can see that it drops off as you get further away from the sun but in galaxies stars don't do that it actually rises as you get further out to the center until it sort of plateaus out towards the edge and this doesn't make sense not even just in einstein's theory of general relativity but newton's law of gravity as well that we all learn in high school it means that there must be more mass on the outskirts of the galaxy then there is in the middle of the galaxy but what's not where we see all the stars so therefore there must be a huge amount of matter that we can't see that's out there and black holes and gas and dust and brown dwarfs and all these other things that don't shine that's not enough to account for the amount of matter that would need to be there to explain those flat rotation curves at the edges of galaxies but if you use mond as your theory of gravity instead and you have this extra term in newton's equations then that problem goes away because the stars are orbiting at exactly the speed that you expect them to be on the outskirts even if there is no extra matter there but that's not everything solved monde also does have a lot of problems the main one being the fact that it predicted that gravitational waves wouldn't travel at the speed of light but then in 2017 we found that they did when we detected the simultaneous detection of gravitational waves and optical light and x-ray light and all sorts of different gamma rays and everything from the merger of two neutron stars so despite not having detected any dark matter or knowing what it's made of like come on particle physicists like what are you doing like we're waiting on you i gave you one job which to be there is very difficult because it doesn't interact with light and so therefore all of our usual ways of detecting things won't work with dark matter but despite the fact that we haven't detected any of it einstein's theory of general relativity combined with 90 of the universe's matter being dark matter still remains our best theory to explain what we have seen in the universe so all of the observations we've made and the evidence that we've collected is sort of piled up in its favor but this new research that has come out has been led by che and collaborators and they've added evidence to the pile in favor of mond and not dark matter so what did they actually find here and what were they actually testing to to understand that we really have to go back to the basics like physics 101. so say we have an object like a car of mass m newton's second law of motion says that the force needed to accelerate that car is proportional to the car's mass something we call the inertial mass of the car how much it resists motion at the same time newton's law of gravity says that if we were to drop that car from a big height above the ground say h the force felt by both the car and earth is proportional to the mass of the earth the mass of the car and how far apart the two are this mass is what's called the gravitational mass how much an object attracts other objects in the universe due to gravity now newton assumed that these two masses the inertial mass and the gravitational mass were one and the same and that's been confirmed with a series of experiments called the earthverse experiments that show that's true at least on the scales that we can test in the lab and that's something that's now known as the equivalence principle but in mond the equivalence principle doesn't hold the inertial mass doesn't equal the gravitational mass instead the inertial mass this how much an object resists motion is also dependent on the gravitational pull of every other single object in the entire universe in other words if you try to accelerate your car you're doing it fighting against the pull of the andromeda galaxy as well in the theory of mon this is called the external field effect or efe and it would mean that there would be a relationship between the speed a galaxy rotates and how dense of an environment that's found in i.e how many galaxy neighbors does it have does it have a lot of galaxy neighbors or is it found relatively alone in the universe and to the scientists disbelief in this research study that is what they found they found this relationship so in 153 galaxies that they studied they found that in very dense environments where there was lots of galaxies grouped together all those galaxies were pulling on the stars on the outskirts of the other galaxies meaning that the rotation curve sort of dipped on the outskirts where the stars were being slowed down by the pull of the galaxies nearby whereas galaxies that are in a much more isolated environment that are left fairly alone in the universe there's nothing pulling on the stars of the edges of those galaxies and so the rotation curve is once again quite flat because there's nothing to slow down the stars on the edges now apparently the authors spent months checking their work just to make sure that it was robust and would stand up to very intense scrutiny because they knew that it was going to get that they checked it for measurement error and systematic error and statistical errors but by the end of it they just couldn't deny what they found you know even the most die-hard of dark matter fans could not deny what had been found now having said that that does not mean that we are going to know just tear up the theory of dark matter and throw it out the window because this is just one piece of evidence it does not outweigh the giant pile of decades worth of research and evidence we already have in favor of dark matter there's more work still to be done here including doing this again but with a much larger sample of galaxies you know i will be a lot happier if this is found with 153 000 galaxies rather than just 153 because that could just turn out to be you know random statistical fluctuation or something like that and there are also still the other observations of the universe that monde fails to explain including the mass in galaxy clusters where galaxies are all grouped together where it still needs a small amount of dark matter to explain the observations so as usual more work does need to be done but i'm really curious and interested to see what will come out of this you know whether it'll be a full shake off of the theory of dark matter i don't know i i doubt that personally but i can imagine a future where perhaps you know the best bits of each theory are sort of merged together into a sort of newer shinier better theory that we have and maybe it turns out that mond or dark matter and general relativity will each of them maybe got it half right before we get to the bloopers i just want to thank this week's video sponsor brilliant brilliant is a website with learning courses across a huge range of science and math topics the courses are interactive and this is what i love about them because not only does it engage you with the topic that you're learning about but it also really helps you to visualize what's going on and actually remember what you've learned as well now as part of their ridiculously detailed astronomy course they have a section on cosmology where you can learn more about dark matter why we think it exists and the math that underpins this best theory we have to explain our observations of the universe so if that sounds like something that you've always wanted to wrap your head around or learn about the maths behind dark matter and you want to support me and my channel then head to brilliant.org forward slash dr becky that's d-r-b-e-c-k-y and sign up completely for free plus the first 200 people that go to that link that it's in the video description will get 20 off an annual premium subscription which i think is pretty incredible so thank you so much to brilliant which to be fair is very difficult because they're difficult the rotation curve stayed fairly flat because the stars on the outskirts weren't being pulled on anything from motorbike i'd sooner try modifying gravity and to the authors of this papers like disbelief that's what this dish believe i believe in dishes

Channel: Dr. Becky

Views: 820,017

Keywords: astronomy, astrophysics, space, physics, science, dr becky, becky smethurst, big bang, dark matter, rotation curves, galaxy clusters, scientist, astrophysicist, cosmology, drbecky, Becky Smethurst, Rebecca Smethurst, gravity, general relativity, einstein, newton, modified gravity, scientific process, testing theories, galaxies, womeninSTEM, womeninscience, female scientist, MOND, hypothesis, academic life, research, research life, phd life, evidence, statistics, equivalence principle, EFE, mach

Id: mvmwqx5vjps

Length: 11min 23sec (683 seconds)

Published: Thu Jan 28 2021