Charalambos Pittordis, "Testing Modified Gravity Theories with Wide Binaries from GAIA DR2"

Transcript

so good afternoon here we are with another remote seminar our guest today is Carolyn Busby tortoise from Queen Mary University of London Carolyn was had a very interesting career path so he got the PhD in 2018 working in probes of cosmology and gravitation and the supervision of will saterland Casteel collaborator of itself now his work in actually as a data scientist in one on in the second largest investment a management firm in europe handling 2% of global assets but he's still participating and working the islamic community and today is giving us a talk about testing modified gravity with binaries with Gaia data so please the stage is yours thank you very much Franco so testing at my Italian buongiorno a tutti Meccano Haralambos big tortoise and i'll be speaking to you all about testing modified gravity clearings using wide stellar binary systems from Gaia so throughout this talk I'm gonna give a brief introduction to ger versus modified gravity theories and then discuss a little bit about why binary systems themselves and as probes of gravity and then discuss about the gaia spacecraft then i'll move on to discuss a little bit about how we actually perform tests of gravity by constructing some simulations and computing observables and then the last part of the talk itself it will be using results from Geier itself so us going out and searching for these white binary systems and then ending with a summary and conclusion so as we know today the best description we have of gravity on all scales is general relativity it has been very well tested to a high accuracy on solar system scales and even lambda-cdm model that's based on gr with inclusion of dark matter and dark energy fit to cosmological observations very well and it can also explain the weak field limit so things like flat rotation curves of galaxies the CMB and so on with the inclusion of dark matter however there is a problem the problem is that we do not have a direct detection of dark matter so because of this some theories may speculate that maybe that maybe G is not the correct theory for gravity maybe it needs to be modified so their theories will go ahead and modify gravity to try and explain the Dark Sector effects of cosmology so by modifying dynamics putting in extra fields and all this stuff now no modified gravity theory has been successful in actually fitting to cosmos cause of observations but there is still a large model space for modified gravity theories which still remains partially explored so moving on to our probes of gravity which are the white Winery systems so what are these systems so these are isolated stellar binary systems with very large separations where the gravitational interaction between these two stars is comparable to what you see in the Milky Way galaxy acting on systems that are greater than eight kiloparsecs away so regions of the galaxy where you start to get a flat rotation curve of fate due to dark matter or maybe a modified gravity theory 80% of the stars within the Milky Way galaxy are stellar binary systems so even a fraction of that 80% of them being wide binary systems is sufficient amount of data for great statistics these systems they should not contain any significant amount of dark matter so meaning that it orbits should just be purely Newtonian /kup Larian and yes these systems may be tightly disrupted by other external sources and stuff like that why they are tidally disrupted and they are buoyant and they were on Bindon scales much less than the age of the galaxy Hey there should be a clear distinction between currently bound binaries entirely disrupted binary systems so user murmurs Probus gravity so why do we need another probe of gravity why use white wineries I mean we have great gravitational experiments so stuff that we've done in the solar system skill where we have constrained the parametrized post-newtonian parameters we have this amazing binary black hole mergers giving us gravitational waves and and also the recent wild waves supermassive black hole imaging so what do white wineries actually offer so what white wineries offer they actually help us constrain the Newtonian part of the gravitational field ie the time time part of the weak field matrix in gr at the very low acceleration regimes and this is complimentary to all either test of gravity on all scales and also these are purely model independent pros of gravity which I'll show you guys later on now these systems they have received little attention in the past because of their very long orbital periods and also their relative velocities by extremely slow and in the pre gaya era there wasn't any instruments that was precise enough to measure such slow relative velocities however in the pre guy catalogues they were selected quite fairly robustly using their proper motions but again because of their very slow relative velocities it wasn't precise enough for any useful dynamics and also in the pre guy era there's also a Magna tree range where there's missing Y binary systems and in this magnitude range there's a potential of thousands of white wines actually existing bright enough for any follow-up radial velocity measurements from ground blade from ground-based telescopes now moving on to discussing no bareback gaius or guys coming in to save the day so Gaia is this spacecraft that's measuring proper motion parallax ease and property of over 1 billion stars within the Milky Way galaxy and producing this 3d map of our own Milky Way galaxy and with this unprecedented precision we can get very good precise proper motion measurements and parallax is for our white binary systems however the distance precision on Gaia isn't good enough to be able to resolve the line-of-sight separation so knowing the distance between star being in forward or behind each other in the sky and also the radial velocity instrument on Kyah isn't that great but however ground-based telescopes are so by having guy are giving us the proper motions the projected velocities projected separation to luminosities and having ground-based telescopes giving us in the radial velocity measurements this should give us five out of the six face based parameters which should be enough to actually perform our tests of gravity so now moving on to how we actually perform tests or gravity using these systems so we begin by constructing simulations so we simulated around five million different unique type of orbit for each theory of gravity so so given a different shapes sizes orientations and so on so in the case from woman that for modified gravity theories however these orbits are not really defined by just a simple Sara makes s and eccentricity these orbits are defined with an effective orbit size and a quasi eccentricity and what we see is that these definitions actually come inside with the usual same H axis and eccentricity that you'll use in a standard Newtonian gravity which is great because it means that we can keep all of our initial conditions consistent across all gravity theories so if we so if we see which type of theories were actually testing we are testing theories that are dark matter emulators so things like mourned with different interpolating function tevares emergent gravity and the external field effect and the external field effect theory is a property of menthe and the way this theory works it's it's the idea of the external field from the Milky Way galaxy let's say is also being applied or acting on the internal gravitational field on the within a stellar system that resides within the Milky Way galaxy what this is happening here it's violating the weak equivalence principle in GR ie getting rid of the idea or free fall so if I show you guys an example of what these Obie's what these modified gravity theories look like within y binary orbit you get something that looks like this so by using the initial conditions of the eccentricity being naught point 7 and the same major axis being around 50,000 au you get to see that as we expect in the Newtonian theory we get a closed orbit in other modified gravity theories we see that the orbits tend to process quite a bit if you look at the external field effect theories as you can see if you increase the value of the external field effect from 0 to 1.5 a naught you see that the orbits start to become more and more Newtonian like so the orbits are great and all but as we've said before we cannot observe a full orbit solution because a full orbit solution takes 102 millions of years and we cannot live for that long so but what we can do is we can observe a snapshot of the face based parameter so ie taking each point of an orbit and computing the observables and what we do is we will study the Joint Distribution of the projected separation of the white binders themselves and also looking at the 2d velocity ratio and this ratio is the ratio between the relative velocity of the orbit itself and the circular Newtonian velocity for that current projected separation and a velocity ratio is a very convenient parameter to use because when you look at the 80th and 90th percentile values of this distribution it becomes nearly independent on an unknown eccentricity distribution for white binary systems to explain the last point in a little bit more detail so if we take the case that we have all of our parameters very well measured so ie we are we have the full 3d velocity ratio and we look at the probability distribution comparing three different types of eccentricity distribution which is in this plot here we see that when we get to larger and larger velocity ratio values we see that the 80th and 90th percentile values comparing the three eccentricity distributions starts to become indifferent which is good because it shows that becomes nearly independent in reality we only have five out of the six face based parameters so we have to make do with the 2d velocity ratio because the component that we are missing is in the line of sight separation for white binary systems but there isn't that much of a difference I mean the only difference is is that the 2d velocity ratio is basically a 3d velocity ratio just shifted to smaller values so now if we actually look at the observables of the sample orbit movie that I have shown and we look at the Joint Distribution we see we see that we get something that looks like this so this root 2 limit that you see go in horizontally is the root 2 limit for bound orbits described in Newtonian theory so in Newtonian theory nothing should ever go beyond or above this point whereas in modified gravity theories they tend to go beyond this point giving us a nice clear distinction to do some comparisons so this is just for one orbit if we do this for 5 million different types of orbits we get a scatter distribution that looks like this so here we are comparing in the Newtonian description against modified gravity theories without an external field effect and as you can see you have a very nice clear distinction with the majority of the orbits being above this rutu limit if we are comparing theories that have an external field effect and have different values of the external field effect you see that they start to become quite similar to the Newtonian description where you'll have some that will just leak over this route to limit if we look at these plots in projected separation slices and we look at the histograms we see that in this example here comparing against theories without an external field effect you see that when we get too large our projected separation values these modify gravity theories start to shift beyond this well now vertical square root to limit relative to the Newtonian theory so this should be very robustly or very easy to detect or rule out some of these theories when we look at theories with the external field effect we see that they're all bit and their distribution starts to become quite similar to the Newtonian description but however for us to actually do some good test and measure any shifts from the Newtonian theory we require a very large sample of white boundaries and we need to study the 80th and 90th percentile values and if we look at the 80th and 90th percentile values and a little bit more detail we see that yes that the shifts are quite small while they still remain non negligible so with a good sample we should be able to measure around 4 to 8 percent shifts relative to a Newtonian theory so if we start to consider some observations and what type of samples we actually expect to get so as we have said theories with an external field effect we need large sample for useful statistics so if we start putting down certain conditions in certain mass ranges using a certain luminosity function and winery separation distribution with our desired range and so on up to 200 parsecs we actually estimate that we can achieve around 10,000 wide binary systems which is actually great amount of data for great statistics because even with just a sample of 1000 well measured white binary systems we can actually detect an offset or a difference of around four to eight percent relative to a newtonian prediction and yes and this is the case of if we can actually control all systematic errors contamination things like that however there are some observational caveats and the main observational caveats are things like contamination from unbound pairs so there might be two stars that are in the sky that may that may appear to be a white binary system but they're just uncorrelated or unrelated stars that masquerade as a binary system and also one of the other main problems is that some binary systems might be a hierarchical system so one of the members of the binary system itself might be its own binary system which will affect the velocity measurements but however doing follow-up radial velocity measurements and getting precise metallicity x' and things like that we should be able to calibrate out these effects so now moving on to the last part of the talk now we've actually went to Gaia and we got ourselves some data so we went to search for some white wineries and we aim to try and produce a very clean unperturbed swamp of white binary systems so we put in conditions like magnitude less than 16 so trying to get sources that should be brighten up for for our radial velocity measurements up to around 200 parsecs and so on and in addition with our sample when will searching for wineries we cut out things like the Galactic playing clusters and we try to remove as many hierarchical systems as possible and so on to show this visually we started off with close to a million stars found in the gaya and then after searching for our white wineries and doing some cleaning we ended up with around 24,000 potential wide binary systems so as you can see here we've cut out some clusters we've cut out the collecting plane and so on to make sure that our white binary systems are isolated systems and they're not affected by any other external sources so with our data we we found where we computed the the projected separations we and the projected velocities and so on and by having the magnitudes we can use the math magnitude relationship and we can compute the circular Newtonian velocity for that current projected separation so one of the parameters used within our 2d velocity ratio so by having all of these together we can then go ahead and actually do our scatter plots and what we get is we get something that looks like this so so the red distribution the red scatter plot is the actual observed data of white binary systems the green is mother fired gravity theories without an external field effect the magenta distribution is a more realistic external field effect model and as we have shown before the black is always the Newtonian description with the root to limit for bound orbits so I'm not claiming anything here that oh there is modify gravity theory or new physics or anything like that because in this plot is only the projected velocity over the circular Newtonian velocity it's not the full 2-d velocity ratio that we need to use because we are still missing the radial velocity measurements that we need to get from ground-based telescopes but however if we look at the projected separation slices of that distribution we see that for modified gravity theories without an external food effect so the pure Dark Matter emulated theories we see that when we get to larger and larger projected seperation slices it starts to not correlate as shift away from actual from the actual observed data so I guess we can kind of safely say these theories are pretty much ruled out but however if you look closely in these plots here we can see there's actually a tail distribution in each one going beyond this root to limit for bound orbits so then we start to think okay what can this tail distribution actually be so we thought okay maybe it might be some chance projected stars so maybe stars that appear to be wide binary systems and so on so we've been cut so he did a couple yeah thank you so we did a couple of simulations and we saw that when we do simulations for these random chance projections we saw that the tail in the actual observed data is much more numerous more populous than the randoms so we see that okay this is clearly not due to any random chance project it starts so then we thought okay maybe there might be flyby stars so co natal flyby stars that appear from that come from the same open cluster which has the zone and these stars appear to flyby together outwards and we thought okay we take that simulation and add it to the Newtonian distribution and try to fit this to to the actual observed data and once we fitted this observed model to the actual data itself we saw that there was actually a pretty good fit with acceptable chi-squared values which you can see in points here however there is some inconsistency and inconsistency is that the fitted number of flyby systems decreases when you start to increase the projected separation slices whereas in the simulations you actually get meant to get more on flyby system at larger projected separation slices so at this moment this tail still seemed still remains as a mystery and we're still trying to figure out what's actually going on and trying to get a better understanding or flyby systems and stuff like that so just to summarize wide binaries give a great opportunity to perform tests of gravity because they reach a very low acceleration regimes and they contain no significant amount of dark matter and it's a pure modeling dependent test or gravity because you only need the velocities and the projected separation and not much else and it's also complimentary to all other tests or gravity on all scales and and what we see is that is very easy to detect and rule out some theories without an external food effects so a lot of the dark matter emulator theories and both theories with an external food effect we need a large good sample of wire binary systems to study the 80th and 90th percentile values and to conclude so we actually have a very good large sample of white binary systems and our desired range that we're actually really focusing on is projected separations of around 3 to 20,000 a you because if you go to separations much larger there are more observational caveats and other challenges at these larger separations and also modify our gravity theory effects actually start to appear at values greater than 5,000 au so in the future what we're trying to do now we're trying to improve the purity of our of our white binary sample and also trying to produce more simulations to correct for more systematic errors contamination or try to model or hierarchical velocity distribution to see how that fits in with stuff and so on and and recently we have actually submitted an observing proposal for a pilot observation of around 50 wide binary systems so around 100 stars themselves and once we get this sub sample and we have our rate of velocity measurements then we can start to see if there's if there's something else going on in the tail if there's more fly buyers hierarchical systems and so on so this should give us a lot more information to understand what's going on and in the future hopefully this sub sample expands to a larger sample and if it expands to a larger sample then we should be able to have great statistics well better statistics to perform pure mortal independent test of gravity at the very low acceleration regime thank you for listening and I hope you're all staying safe at home and I'll be happy to answer any questions that you all may have thank you thank you [Music] okay so I am far from being an expert here so I would happily leave the stage if anybody from the audience want to raise any question or otherwise I can start ok Stefano matter is asking yes no sorry the message was Stefano can you speak yes I'm really below city of readiness that can be used to test at least preliminary your results because I expect that this system were already follow so in terms of radial velocity measurements if the so I mean if you know any good instruments that are available now to do some radial velocity measurements then yeah that would be great but but yes so our sources that we're actually looking at they are so they have a magnitude range between sort of 12 and 13 so we're keeping it below 13 for our pilot study because they are bright enough for for the proposal that we put in which is for the harps for the harps telescope using the eggs mode just to get a pilot study and get a few of our radial velocity measurements but and sort of radial velocity measurements that we actually looking for is something that can precisely measure radial velocities of around 0.05 km/s so things are really extremely slow so I hope that has answered your question yes just a bit to understand when you say that in some of the model you tested the results of the inclusion of the external field model yes how does it exactly work if you are testing such a big distribution I mean I think understand the external field in this case would be the external field from the Galaxy itself or from a larger scale environment am I correct yes so it's the idea of the the external gravitational field of the Milky Way galaxy is also acting or is adding on to the internal gravitational field between two stars within a stellar system that's within the Milky Way galaxy itself and the new idea is that if you've got this external field effect acting on systems within an internal system within the Milky Way galaxy itself or in our case binary systems have a gravitational an internal gravitational field between the two stars and they you have the additional external gravitational field from the galaxy as adding on to that what would the what would the the orbit actually look like and what we see is that we actually get something that's very similar to Newtonian however this theory is also a little bit dependent - or it has an anklet dependency and a radial dependency from the galactic center so when you are applying these to the observed sample you need to guess the distance of every difference Tariq has to impose the potential at that location yes okay yes so in trying to we try to determine the orientation the phase space how is projected in the sky every way possible in actually getting velocity measurements for these systems okay thanks good answer then we have other pair of questions so if what intensity I can speak I would just let her ask the question otherwise I can read okay so I have two small questions thank you so the first one is in the scatter plot the 2d velocity ratio is plotted against the distance of the white banner is from us in astronomical units I looking at this scatter plot yeah and they simulated this one so this one it's so we don't have the full 2d velocity ratio because at the moment here we have the velocities from proper motions from Geier itself only divided by the circular Newtonian velocity for that current projected separation so we still don't have the full 3d relative velocity to scale it with this circular Newtonian velocity for this current projected separation so this is why we need to do the follow-up radial velocity measurements so we can get the full 3d velocities and see what these orbits actually look like or maybe this gaturro here they might be hierarchical systems or something else going on but once we actually do the radial velocity follow-up measurements and we find out okay this is all contaminant then we just when we can clean this away from our sample okay so yes it was because on the x-axis I see they and they saw another question and so according to all these tests of the best theories of modified gravity that you have tested are the one that included the external field F that's right yes at the moment the ones with the external field effect are the ones that are well then well it's not as good as the Newtonian theory at this point here but it is it is putting up a good fight in actually doing some fits here to the observed data yeah so theories without the mud without the external field effects are things like month and it's interpolating functions emergent gravity and theories like that we can probably we can safely say they'll roll out and I guess we can say they are fully ruled out once we have this follow-up radial velocity measurements okay so I'm not trying to make anything concrete or claim anything at this moment because there's still more information that needs to be collected okay thank you very much you're welcome okay any more question from the audience anywhere from Italy I don't see any there is a quick one arid here have you tried some of the Gaia rotational velocity readable I guess it's really real so we with a try however it I just tried it for fun to see what type of things that we get but however for the radial velocity measurements from choir the instrument is not great it's it's not that accurate at all hence why we need to do follow-up radial velocity measurements so we need something with an accuracy that can reach naught point naught 5 km/s but what we can do we can use the radial velocity from Gaia it just it wouldn't be accurate and they'll be very large errors okay sorry it just curious so I noticed that some of actually all of your targets I see bright stars and some of for those bright stars radio velocity can reach 0.1 something so how difference would this introduced to the to your results if the accuracy of the radio velocity like from your requirement 0.05 if it is 0.1 how large it will be the uncertainty if you use this not good enough data so with the uncertainty so if I try and remember my notes for the so I think we need so we need pretty good uncertainties what is 0.1 if it's so if if it's no point 1 kilometers per second in Gaia well no point 1 kilometers per second error I think from what I remember we needed something that was much less than that because I think around no point 1 km/s it still becomes quite difficult for the to get the absolute velocity from from car itself and also it becomes quite difficult to determine or distinguish whether if there's some contamination or anything like that so when all five is the requirement to distinguish different models it at once so I I can't remember I apologize life I do remember I'll send you an email and give you details but yeah so okay so uh so yeah I yeah I apologize I can't I'll say Judy bowl ater on in terms of accuracy and more and what type of errors that we are actually looking for so we're trying to be very very precise with these measurements to make sure that there is nothing else going on okay thank you or you can send you can send VP later to me Anakin forwarding case yes no problem thank you very much okay I think this is it thanks a lot I'm going to stop the recording now