Did the Big Bang happen?

did the universe come out of a black hole will the big bang repeat was the universe created from strings physicists have a lot of ideas about how the universe began and i'm constantly being asked to comment on them in this video i want to explain why you should not take these ideas seriously why not that's what we'll talk about today the first evidence that the universe expands was discovered by edwin hubble who saw that nearby galaxies all move away from us how this could happen was explained by none other than albert einstein yes that guy again his theory of general relativity says that space responds to the matter and energy in it by expanding and so as time passes matter and energy in the universe become more thinly diluted on average i say on average because inside of galaxies matter doesn't dilute but actually clumps and space doesn't expand but in this video we'll only look at the average over the entire universe so we know that the universe expands and on average matter in it dilutes but if the universe expands today this means if we look back in time the matter must have been squeezed together so the density was higher and a higher density of matter means a higher temperature this tells us that in the early universe mata was dense and hot really hot at some point mata must have been so hot that atoms couldn't keep electrons around them and even earlier there wouldn't even have been individual atomic nuclei just a plasma of elementary particles like quarks and gluons and photons and so on it's like the alphabet soup of physics and before that we don't know we don't know because we have never tested what matter does at energy densities higher than those which the large hadron collider can produce however we can just ignore this difficulty and continue using eisler's equations further back in time assuming that nothing changes what we find then is that the energy density of matter must once have been infinitely large this is a singularity and it's where our extrapolation into the past breaks down the moment at which this happens is approximately 13.7 billion years in the past and it's called the big bang the big bang didn't happen at any particular place in space it happened everywhere i explained this in more detail in this earlier video now most physicists me included think that the big bank singularity is a mathematical artifact and not what really happened it probably just means that einstein's theory stops working and we should be using a better one we think that that's what's going on because when singularities occur in other cases in physics that's the reason for example when a drop of water pinches off a tap then the surface curvature of the water has a singular point but this happens only if we describe the water as a smooth fluid if we take into account that it's actually made of atoms then the singularity would go away something like that is probably also why we get the big bang singularity we should be using a better theory one that includes the quantum properties of space unfortunately we don't have the theory for this calculation and so all that we can reliably say is if we extrapolate einstein's equations back in time we get the big bang singularity we think that this isn't physically correct so we don't know how the universe began and that's it then how come that you constantly read about all those other ideas for how the universe began because you were sitting around at the dentist and had nothing else to do okay but why do physicists put forward such ideas when the answer is we just don't know like you may have recently seen some videos about how a universe was allegedly born from a black hole the issue is that physicists can't accept the scientifically honest answer we don't know and leave it at that instead they change the extrapolation back in time by using a different set of equations and then you can do all kinds of other things really pretty much anything you want but wait this is science right you don't just get to make up equations unless possibly you're decorating a blackboard for a tv crew though actually i did this once and later they asked me what those equations were and i had to tell them they don't mean anything which was really embarrassing so even in this case my advice would be you shouldn't make up equations but in cosmology they do it anyway here's why suppose you're throwing a stone and you calculate where it falls using newton's laws if i give you the initial position and velocity you can calculate where the stone lands we call the initial position and velocity the initial state and the equation by which you calculate what happens the evolution law you can also use this equation the other way around if you know the final state that is the position at velocity at the moment the stone landed you can calculate where it's been at any time in between and where it came from it's kind of like when my kids have chocolate all over their fingers i can deduce where that came from okay but you didn't come here to hear me talk about stones this video was supposedly about the universe well in physics all theories we currently have work this way even that for the entire universe the equations are more complicated all right but we still have an initial state and an evolution law we put in some initial state calculate how it would look like today and compare that with our observations to see if it's correct but wait in this case we can only tell that the initial state and the equations together give the correct prediction for the final state how can we tell that the equations alone are correct let's look at the stone example again you could throw many stones from different places with different initial velocities and check that they always blend where the equations say you could also say take a video of the flight of the stone and check that the position at any moment agrees with the equations i'm sure that video would kill it on tiktok i threw a stone here's what happened next but in cosmology we can't do that we have only one universe so we can't test the equations by changing the initial conditions and we can't take any snapshots in between because we have to wait 13 billion years in cosmology we only have observations of the final state that is where the stone lands that's a problem because then you can take whatever equation you want and use it to calculate what happened earlier and for each possible equation there will be some earlier state that if you use the equation in the other direction we'll agree with the final position and velocity that you observed so it seems like in cosmology we can only test the combination of initial state and equation but not find out what either is separately and then we can't say anything about how the universe began that sounds bad but the situation isn't quite as bad for two reasons first the equations we use for the entire universe have been confirmed by other observations in which we can use the standard scientific methods there are many experiments which show that einstein's equations of general relativity are correct for example redshift in the gravitational field or the perihelion procession of mercury and so on we then take these well-confirmed equations and apply them to the entire universe this however doesn't entirely solve the problem that's because in cosmology we use further assumptions besides einstein's equations for example we use the cosmological principle about which i talked in an earlier video or we assume that the universe contains dark matter and dark energy and so on so saying that we trust the equations because they work in other cases doesn't entirely justify the current cosmological model but we have a second reason which does justify it it's like einstein's equations together with their initial values in the early universe provide a simple explanation for the observations we make today when i say simple i mean simple in a quantitative way you need few numbers to specify it if you used a different equation then the initial state would be more difficult you'd need to put in more numbers and the theory wouldn't explain as much just think of the equations as a kind of machine you put in some assumptions about how the universe began do the math and you get out a prediction for how it looks like today this is a good explanation if the prediction agrees with observations and the initial state was simple the simpler the better and for this you only need the observations from today you don't need to wait some billion years unless of course you would like to you know what let's just wait together okay how about you wait and we talk again in 10 billion years while you wait the cosmologists who are patient enough justify using one particular equation at one particular initial state by showing that this combination is a simple explanation in the sense that we can calculate a lot of data from it the simplest explanation that we have found is the standard model for cosmology which is also called lambda cdm and it's based on einstein's equations this model explains for example how our observations of the cosmic microwave background fits together with our observations of galactic filaments they came out of the same initial distribution of matter the alphabet soup of the early universe if we used a different equation that still be some initial state but it wouldn't be a simple one anymore the requirement that an explanation is simple is super important and it's not just because otherwise people fall asleep before you're done explaining it's because without it we can't do science at all take the idea that the earth was created 6000 years ago with all dinosaur bones in place because god made it so this isn't wrong but it isn't simple so it's not a scientific explanation evolution and geology in contrast are simple explanations for how those dinosaur bones ended up where they are i explained this in more detail in my new book existential physics which has just appeared that said let us then look at what physicists do when they talk about different ideas for how the universe began for this they changed the equations as we go back in time typically the equations are very similar to einstein's equations at the present time but they differ early in the universe and then they also need a different initial state so you might no longer find a big bang as i said earlier you can always do this because for any evolutionary there will be some initial state that will give you the right prediction for today the problem is that this makes a simple explanation more complicated so these theories are not scientifically justifiable they don't improve the explanatory power of the standard cosmological model another way to put it is that all those complicated ideas for how the universe began are unnecessary to explain what we observe it's actually worse because you might think we just have to wait for better observations and then maybe we'll see that the current cosmological model is no longer the simplest explanation but if there was an earlier phase of the universe that was indeed more complicated than the simple initial state that we use today we couldn't use the scientific method to decide whether it's correct or not the scientific method as we know it just doesn't cover this case science fails sure making better observations can help us improve the current models a little more but eventually we'll run into this problem that more complicated explanations are always possible and never scientifically justified so what's with all those ideas about the early universe here's one that's been kind of popular recently an idea that was put forward by nicodem poblovsky for this you changed general relativity by adding new terms to the equations called torsion this removes the big bang singularity and replaces it with a bounce our universe then came out of a bottleneck that's quite similar to a black hole just without the singularity can you do this you can certainly do this in the sense that there's maths for it but on that count you can do many other things like broccoli that's maths for broccoli so why not make the universe out of broccoli i know this sounds crazy but there are lots of examples for this like penrose's cyclic cosmology that we talked about some months ago or the equiotic universe which starts with a collision of higher dimensional membranes or the idea that we came out of a five-dimensional black hole which made headlines a few years ago or the idea that the universe began with a guess of strings which seems to never have been particularly popular or the no boundary proposal which has that the universe began with only space and no time an idea put forward by jim hartle and stephen hawking or geometrogenesis which is the idea that the universe began as a highly connected network that then lost most of its connections and condensed into something that is indistinguishable from the space we inhabit and so on have you ever wondered how come there are so many different ideas for the early universe it's because by the method that physicists currently use there are infinitely many stories you can invent for the early universe the physicists who work on this always come up with some predictions for observables but since these hypotheses are already unnecessarily complicated anyway you can make them fit to any possible observation and even if you'd rule out some of them there are infinitely many others you could make up this doesn't mean that these ideas are wrong it just means that we can't tell if they're right or wrong my friend tim palmer suggested to call them a scientific when it comes to the question how the universe began we are facing the limits of signs itself it's a question i think we'll never be able to answer just like we'll never be able to answer the question why women pluck off their eyebrows and then paint them back on some questions defy answers so if you read yet another headline about some physicists who thinks that our universe could have begun this way or that way you should really read this as a creation myth written in the language of mathematics it's not wrong but it isn't scientific either the big bang is the simplest explanation we know and that is probably wrong and that's it that's all that science can tell us if you want to know more about how we use equations in physics and how these equations work check out our sponsor brilliant brilliant is an amazing tool for learning with courses on a large variety of topics in science and mathematics all their courses are interactive and will challenge you with questions so you can check your understanding right away you can take them conveniently on your laptop or phone so you can learn what you want when you want and at your own pace to make sense of the stuff with the evolution laws and initial conditions that i talked about today have a look for example at their courses on differential equations and on gravitational physics i now also have a course on quantum mechanics at brilliant that accompanies my videos on the topic our new course will give you an introduction to interference superpositions and entanglement the uncertainty principle and belt theorem we just added two new lessons to the course and more may follow i had a lot of fun working with brilliant's team on this and i hope you'll enjoy too to support this channel and learn more about brilliant go to brilliant.org sabine and sign up for free the first 200 subscribers using this link will get 20 off the annual premium subscription thanks for watching see you next week

Channel: Sabine Hossenfelder

Views: 393,279

Keywords: big bang, cosmology, universe, big bang theory, physics, astrophysics, how did the universe begin, theories for beginning of universe, black hole, universe black hole, do we live in a black hole, we don't know how the universe began, is the big bang true, is the big bang religion, hossenfelder, science without the gobbledygook

Id: CAVUvq6BE1E

Length: 16min 58sec (1018 seconds)

Published: Sat Aug 27 2022