Black Holes, Dark Matter & Quantum Gravity, what's new in Loop Quantum Gravity

loop quantum gravity is an attempt to unite general relativity with quantum mechanics in one of our previous films we told the story of this theory and how it might resolve the singularities inside black holes and replace the Big Bang with a Big Bounce but since then there have been new developments from theory and observations that may have dramatic consequences for how we see the universe so we traveled to Penn State with a biannual loop quantum gravity conference was taking place in this episode we'll focus on the quest to uncover the secrets of what lies at the center of black holes and meet some of the scientists who are focusing their attention on this most challenging of puzzles starting with Rodolfo Gambini who in 2013 was one of the first to apply new quantum gravity to the heart of a black hole quantized completely the the black hole and we unexpectedly discover our discreet structure typically of typical of the loop approaches but that in this very simple case was not expected and we also observed that the central singularity is completely resolved there is no singularity at all you have a point of maximum curvature with large fluctuations but in principle one can go further on the other side of the blue hole whatever the means so what happened to a black hole it forms and then what is this black hole exploding back in our universe or is it possible that something has happened namely is it possible that these black holes divide itself from our universe and creates a new region namely a new universe according to most loop researchers matter that falls into a black hole will eventually bounce back out forming its time reverse a white hole just as nothing can escape a black hole nothing can get into a white hole black holes bouncing into white holes could be linked to fast radio bursts energetic signals astronomers have recently discovered that have been dubbed the biggest mystery in astronomy but an alternative is that white holes quantum tunnel into another distant region of space-time or maybe even into a new universe with different values for the constants of nature so there are two scenarios which are inequivalent and they are competing and now we have to understand better I mean one thing is observations so so either we observe the first scenario which is observable or if we don't observe it then we always have the second one which is not but but we also have other theoretical considerations tests we can we can check how those scenarios are consistent with other areas of physics with other phenomena black hole evolves by evaporating by getting smaller and smaller by letting off these particles that are known as Hawking particles or Hawking quanta and and there was a question when we first came up with the idea that black holes might transform into white holes there's a question of whether this might happen early in their lives or quite late and and our most recent thinking is that it actually happens deep into the lifetime of the black hole so the black hole mostly evaporates via these Hawking quanta and and then it reaches a very very small size what we call the Planck mass which is about the mass of a hair human hair and it's only once it's reached this size that it transforms so this leads to this incredible timescale that I've never seen anything like elsewhere in physics much much longer than the age of the universe let's see 55 orders of magnitude larger than the age of the universe it's quite quite an extraordinary timescale I don't think he's a result of the yet but the calculation seems to favor a longer time technically M cube or M is mass of the of the initial black hole rather than M square so which is a much longer time M square would mean that the black hole can explode when it's still big in microscopic M cube would mean that the black hole has to become very small evaporators to be small before tunneling into a white hole there are calculations to be done the problem is that calculation have been done but in very crude approximations so the questions are this reliable or by doing a better approximation we go we're the same better I I don't know I I haven't I'm not sure yet but the weight seems to go toward a long life long life means less plausible the idea that they're connected to faster oliver's still there but it's less plausible and more plausible the connection to dark matter physicists don't know what makes up the huge percentage of the matter of the universe so this is the famous problem of dark matter and and there are two indications that there's something interesting with black holes and dark matter so if if many black holes were created in the very early universe and they evaporated down to these very small black holes then you could have these small black holes making up a bunch of matter and there they wouldn't interact with anything and we wouldn't know that they were present necessarily and so this could be one candidate dark matter and so that would be another experimental Avenue to try to look for signatures of these old black holes LIGO the gravitational wave Observatory and Virgo and the other observatories are starting to measure gravitational waves and they're finding black holes that are like ten solar masses and 30 solar masses in this range and so there are many many more of these kinds of these this type of black holes than we expect and they also might make up some portion of dark matter so it may be that the the picture that's emerging of dark matter is very much more complicated that it's made up of multiple different kinds of things and and that those things are just becoming visible to us now one of the beautiful novelty in this conference it's a prediction by Gina Bianchini how are Goethe which is very simple if the black holes were formed in some thermal ensemble in primordial fluctuations they are likely to be formed according to a distribution which has to do with a vacancy volton entropy since the 70s it's been recognized that black holes have this remarkable entropy a very very high entropy so this is called the bekenstein Hawking entropy and and this entropy has a really beautiful property which is that it's closely related to the surface area of the black hole the way we understand black holes is that they are not simple objects they are not simple solutions of general relativity they are really complex objects have a temperature they ought they have an entropy and they consist of a immense amoonguss number of states and one can ask what are the statistical properties and what can we predict for their typical behavior a very interesting aspect of it is that when you spin a black hole the entropy actually decreases there are fewer allowed configurations for the microstructure of the black hole what would happen what would be the case if a black hole was formed in thermal equilibrium so if you had a bath of temperature all around what you would expect is that every possible configuration of the black hole would be equally likely and so you should form the the black hole's that have the most possible configurations and it turns out that the the black hole's that have the most possible configurations would be black holes that have zero spin so then the question the non-trivial question is that when you see that these black holes are in equilibrium in a equilibrium that would allow you to find the most likely configuration and one one paucity is that black holes produced in the early universe primordia of the course these might be in equilibrium especially the ones that are formed in now one of the phases were the pressure drops the probability of producing them is enhanced and the matter that produced them was already in thermal equilibrium because producing that phase would have a mass that is accessible to present experiments like like when we're gone mass of 2025 solar masses and if they are in equilibrium they would have a small spin now when a star collapses star spins so become smaller spins faster faster faster so it produces a black hole which spins very fast so what coal produces by stars are spinning faster but promoting black holes are probably not spinning this prediction wasn't recognized before and we've started to build it out and try to understand what comes out of it and there's a compelling a connection with the gravitational wave measurements that are happening right now so at LIGO and Virgo they're detecting pairs of black holes that are rotating around one another and crashing into each other and they can't met yet measure the individual spins of the black holes very effectively but they can measure a kind of combination of the two spins and so far the measurements are consistent with each of the individual black holes having zero spin and so this connection between black hole entropy predicting that you would expect black holes that were formed in the early universe in thermal equilibrium to have zero spin and the fact that the LIGO is measuring black holes that are consistent with zero spin is really compelling to us there's this exciting moment now where they are starting to make measurements of black holes on a weekly basis so you can even check the the grace database and they show you every every day what they've measured at LIGO they don't release the the numbers yet but they're coming and and so we're starting to have a picture of a whole population of black holes and since this this prediction of zero spinning for the buckles is a statistical prediction it's about a population of black holes there's the possibility really test it through these like no measurements if you'd asked me five years ago that we were this close to being able to check a prediction of quantum gravity I would have been surprised and and I'm really excited about the possibility that that this would be happening in the next few years so this is just a mechanism if if black holes can be formed in thermal equilibrium it's a mechanism to form them with zero spin and you might think of another mechanism you might think that there was a star that had some spin it formed a black hole that had some spin but then it was sucking on matter in such a way that it's pinned it down it got to very low spin so the one might think of other mechanisms so zero spin black holes on their own is not enough to say we've we've tested quantum gravity but if we can start to connect different sets of ideas so if we could start to show that these black holes were formed in the early universe and that they have zero spin you start to make connections between the the various parts of the ideas then I think it would be a much more a test of quantum gravity if these results are confirmed would they tell us which theory of quantum gravity is correct one of the remarkable things about black hole entropy is that so far it seems to be independent of your favorite theory of quantum gravity the bekenstein Hawking entropy has been derived in loop quantum gravity and string theory and in numerous approaches it seems to be a very robust feature of of quantum gravity but it would tell you that black holes have a statistical nature they have a microstructure so so even if it wouldn't select one theory for you it would be be a big progress black holes seem to destroy information that falls into them but quantum mechanics says that information can never be destroyed how to resolve this paradox may give us clues that will help to distinguish between competing theories string theory the the belief is that information goes very very slowly through through very tiny Corrections out of the horizon while we believe that for answering this question it is essential to explain what happens with information that we thought we think in classical physics semi classical physics shows falls directly into the singularity another great idea which is tantalizing is related but dark matter again it could be not just black holes produced in the early universe but black holes from a previous universe that went through the balance something Francesca VDOT has been pushing and it's it's it's plausible of course it demands the the one has to believe the bounds won't have to believe this transition so there are many things which are not confirmed but these are all things which are more and more plausible I would say today when scientists examined the oldest light in the universe the Cosmic Microwave Background they find tiny differences in temperature which are plotted in what is called the power spectrum the most accepted way to explain this power spectrum is assuming there was a strange form of matter known as the inflaton field which drives a hyper rapid period of expansion called inflation if there is a a pre Big Bang face in a bouncing cosmology in which the universe would have undergone a contraction before the current expansion well in this contracting phase a lot of black holes could have formed and they could have been in the remnant phase and according to what also other scientists are studying for instance Barnard car or Robert Brandenburger so it's possible for those pretty big bank black holes to pass through the bounds and be present today what I found very interesting is that in this case this black hole in the contracting phase provides exactly the kind the right kind of matter for the universe to give the right power spectrum later at a later time so this happens without the need for inflation so there is a current scenario in which promote a black holes are there and not only they are there in order to provide that matter but they also provide provide the right source for getting rid of inflation for the moment the effort is to see whether the scenario is consistent with standard cosmology where the all the energy balances go through through cosmology direct observation of these small white holes they're very little they interact only gravitationally they're dark they're dark matter that's exactly what they are so how do we see them so for the moment the weight is this is the only explanation of dark matter that actually does not require new physics just generativity in quantum mechanics so that would make the plausibility a direct observation people are squeezing the head but I haven't seen anything credible yet at the moment I think all of us want to to see quantum gravity before we're too worried about distinguishing theories it's not that we have a menu of a thousand theories that we have to distinguish it's actually it's we have very few consistent theories and it would be just wonderful to see that gravity is quantum mechanical in our next episode we'll see how new developments in loop quantum gravity may be able to explain what something is the biggest mystery in physics dark energy I mean if if correct it would solve that issue there is no fine-tuning and we would have an explanation of well what is it that it comes from and we'll get an update on the experimental search for evidence of the Big Bounce very exciting idea interesting is that this kind of correlations can have observable consequence

Channel: skydivephil

Views: 11,146

Rating: 4.9026546 out of 5

Keywords: black hole, black holes, cosmology, quantum gravity, loop quantum gravity, physics, entropy, stephen hakwing, holographic principle, space, space-time, nasa, big bang, inflation, quantum physics, multiverse, black hole entropy, string theory, what happened before the big bang?, relativity, quantum mechanics, big bounce, science, cosmic, carlo rovelli, francesca viadotto, dark matter, dark energy, white hole, fast radio bursts, astronomy, einstein, LIGO, VIRGO, GRAVITATIONAL WAVES, cosmos

Id: HFJtDLR4bTo

Length: 17min 45sec (1065 seconds)

Published: Sat Mar 14 2020