If the Universe is expanding, then why do galaxies collide?

so something you hear from people all the time about the universe is that it is expanding and we've known this for nigh on a century now that when we look out into space all the other galaxies are moving away from us they're all red shifted and so people tend to ask well if all the galaxies are moving apart because the universe is expanding then how come galaxies collide so there's a constant push-pull going on in the universe all the time between gravity on one side and the expansion of the universe on the other so we know gravity exists because well we're not currently floating offices in the earth and the earth is still going around the Sun and the stars are all going around the center of the Milky Way and it's from observing those kind of things that we've been able to piece together a theory of gravity ie equations which describe what we're seeing our best theory is that of general relativity Einstein's theory and every place that we've tested that theory it's them right so the orbit of mercury or gas around the outskirts of a black hole so much so that now we can actually employ those equations in technology and so every time you pull out your phone and use your GPS to find out where you are in the world satellites around the earth are using Einstein's theory of general activity to help correct for gravitational influences and figure out where you are on planet earth on the other side you've got the expansion of the universe something we've accepted is happening since the 1920s when Hubble first looked out and dromeda nebula and found out that it wasn't a nebula in our own Milky Way you know only say a hundred thousand light years away but that it was a galaxy of stars in its own right that was actually millions of light years away and when he looks at other galaxies in the sky as well he found that the more distant they were the more their light had been red shifted which meant that they were moving away from us at a faster rate so if all the galaxies in the universe are moving away from us then if we sort of rewind time we obviously end up in that sort of Big Bang scenario where the universe starts from this at a impetus of lease mall incredibly dense point Hubble's discovery triggered so much debate about the universe and whether it was formed in a big bang and what its current state was as well but eventually this idea of an expanding universe in the Big Bang became accepted as more and more evidence was found say for example the discovery of the Cosmic Microwave Background in the 1960s by pin Dyson Wilson which was kind of like an echo of the radiation left over from the Big Bang now we don't really know what's causing this expansion of the universe we give it a name we call it dark energy well that doesn't necessarily mean that we know what it is we also know how much of the universe's energy budget is in dark energy we've been able to calculate that from studying early universe in the Cosmic Microwave Background and the expansion of the universe as well and found that it's about 75% of all of the universe's energy budget has gone into dark energy that's causing this expansion rather than being locked into matter so for example normal matter which is about 5% the universe's energy budget the rest of which is dark matter so it has a name and we know how much of the universe's energy budget it takes up but that's really all we know like we don't really know how it's causing the expansion that's something that people are still trying to answer like in research departments across the world right now so if you ask me in the comments I'm gonna know so whilst we think that this dark energy pervades all of the universe in all space because whichever direction we look into whatever distances we see this expansion happening gravity is a bit different though because first of all it depends on how close you are to something and then also how heavy the object is that you're close to so the equation that we always quote is G the gravitational constant times the mass of one object the mass of the second object divided by their separation squared this is why you know we can launch rockets off the earth that have managed to take the Voyager probes completely out of the solar system as long as we gave them enough energy to overcome that force of gravity then eventually they would escape the gravitational pull of the Sun when they got to a big enough distance so for small distances like in the solar system and for huge mass is like four galaxies then gravity dominates it wins out over dark energy it has a stronger pull than the push of the expansion of the universe outwards it's why the earth isn't moving away from the Sun because of the expansion of space we're at the same distance away from the Sun all the time because gravity holds us in that position against the push outwards of the expansion of space it's similar why stars in our own Milky Way are not moving away from the Sun because they're held in that position by the whole gravity of the whole Milky Way system but it's also why galaxies can also collide and merge because if you have galaxies of hundreds of billions of stars and then also containing dark matter that's like five ten times more than that as well then that force of gravity pulling those two massive objects together is going to be huge and so even if they are millions of light years away that force of gravity is still going to be strong enough to eventually pull them together and be the push of the expansion of the universe outwards so that they eventually collide and merge which is why it's not correct to say that all galaxies in the universe are moving away from us because the Milky Way is part of a little local group of galaxies which includes the large and small Magellanic Clouds which will be familiar sites in the southern sky to those of you in the southern hemisphere and then also the Andromeda galaxy are very familiar sight to people in the Northern Hemisphere and because of the force of gravity on all these galaxies in this group they are all destined to one day merge and become a single galaxy so Andromeda is not red shifted its light hasn't been stretched by the expansion of space instead its light is blue shifted because Andromeda is actually moving towards us so the light has been squashed as it's given off from Andromeda on its way to us so because that blue shift we tend to say that it means that I'm dromeda it's coming towards us but that's not quite the right language because on dromeda is much bigger than Milkyway sort of like three to four times bigger Andromeda has a trillion stars in it whereas the Milky Way is something like 200 to 400 billion last count and so whilst yes they will both be exerting a force of gravity on each other Andromeda will be exerting the bigger Pole on the Milky Way and so technically we're the ones moving towards Andromeda and when we get there we are gonna collide with it emerge probability of two stars colliding though is very very low check out my previous video that I've done on that and what happened is that beautiful spiral structure that we think the Milky Way has and the beautiful spiral structure that we've seen that Andromeda hounds is gonna get torn apart by all those forces of gravity the stars will get flung out everywhere and eventually the stars will settle down on these random chaotic orbits giving us this kind of giant blob of a revenant galaxy that people like to call milk comida oh and by the way this is happening like four billion years in the future I don't wanna panic anybody I don't want anyone to have to think they have to go running out and buying like galactic merger insurance today because it's so far in the future but after we've merged with Andromeda the question is you know will we merge with anything else again but it's quite unlikely because our local Group of galaxies is in what we call a super void and a super void is essentially a very low density part of the universe they've got incredibly low lumbers of galaxies in that region of space than you would expect from like the normal density and it means that gravity's got absolutely no chance of winning in those regions and the expansion of space is going to dominate so those super boys just get bigger and bigger and bigger leaving the galaxies that were originally in them extremely isolated in the vastness of space but it all depends on whether gravity or dark energy this expansion of space will win the overall fight in the end this is what cosmologists are currently trying to measure they're trying to determine how much mass is there in total in the universe and is that enough to counteract against the expansion of space and so with the equations that we have to describe gravity and the expansion of the universe you can then work out what the critical density of Massah would be in the universe for those two things to perfectly balance that's one scenario that we could end up with the expansion slows down and we end up with this nice happy equilibrium where the universe reaches a specific size and we don't grow any further but it could be that once we've measured the amount of mass in the universe it's actually less than that critical density in which case there's nothing to stop the expansion of the universe it would continue accelerating for eternity moving everything in the universe away from each other isolating galaxies in the vastness of space so that when you look up you see a dark night sky or you could have so much mass in the universe that its density is above that critical density in which case you'd completely halt the expansion and you would draw everything back in again under the force of gravity ruling everything back down to that impetus null point where the Big Bang was in what we like to call a Big Crunch or again AB Gib as some people like to call it because it's Big Bang backwards so this is what cosmologists are trying to do right now is measure that density of matter in the universe and there's lots of different teams doing this worldwide using lots of different methods and what's really interesting is that as the decades wear on and they drill down how precise they're able to get that value as technology improves and the different data techniques improve as well we get closer and closer to thinking that we live in a universe which has the exact critical density to balance gravity and dark energy perfectly always don't mmm whoo oh my universe my nails are shocking I really need to paint them again just know one look at my nails okay for the whole video I'm just gonna like claw a nail like this the entire time on the other side so let's just sort of think about what each of those two things I wait for you then mister motorbike mrs. motorbike I don't know shouldn't that's general relativity Einstein's theory that he came up with to describe how gravity oh where is the Milky Way is say like 200 to 400 stars just to just two hundred stars that's all there is or Kanab Gib organ a big no we're gonna add Gabe I'm so sick around as fast as I can I'm so sick of expanded as fast as I wonder if I could get there quicker if it wasn't for gravity that does not fit I'm so sorry

Channel: Dr. Becky

Views: 84,026

Keywords: space, astronomy, big questions, astrophysics, science, scicomm, womeninscience, womeninSTEM, dr becky, becky smethurst, rebecca smethurst, big bang, dark energy, dark matter, gravity, galaxy, galaxy mergers, galaxy collisions, hubble, telescopes, physics, andromeda, M31, milky way, milkomeia, universe, expansion, big crunch, gnab gib, flat universe, cosmology, end of the universe

Id: 2OjNokItg3E

Length: 12min 6sec (726 seconds)

Published: Wed Sep 11 2019