Every now and again , you ’ll read a scientific discipline story claiming the “ best picture of dark affair ” ever . If it ’s so dark , how can you see it ? In this week ’s “ Ask a Physicist ” , we ’ll bump out .
I ’ve spill a lot aboutdark matterin previous column , but did you know it’s“Dark Matter Awareness Week?”Of course you did . I ’ll celebrate by answeringa questionput to me by our own Charlie Jane Anders , who asks :
call for A Physicist : Why Believe In Dark Matter ?
I ’m funny about galaxies with coloured matter cores , and how we can “ see ” dark issue .
I ’ll do it even though she did n’t border it in the form of a question , especially since this is the sub - field that I actuallyworkin .
Dark issue is everywhere . According to our best cosmological model , it take a leak up about around 25 % of the vigor in the world , about five times as much as is found in the ordinary nuclear material that make up you , me , the sun and the earth . Dark matter is all around us , lurking stealthily .
But there ’s a longsighted way to go between suspect the existence of dark matter and actually making a single-valued function of it , and for that , we need to really understand how gravity works . I should warn you in advance that the discussion ’s going to get a small hirsute , but if you hold out , you ’ll be rewarded with pretty picture at the death .
Just enough worldwide relativity
permit me say just three things about how light and gravity oeuvre together . really , I ’m going to say just one thing , but I ’m go to say it three dissimilar ways , and with increasing complexity . finger destitute to opt the one that you ’re most comfortable with , and go with that .
somberness bend lightbeams .
Light is made of particles , and as they move past a cluster of galaxies , for object lesson , they slue slightly towards it , just as a comet would as it croak by the earth . Yes , this produces a factor of two error . For this design , I can live with that . Clusters are handsome — they can turn back hundreds or even grand of item-by-item galaxy , so the gravitative effect is pretty substantial . To my fellow physicist on site :
Massive bodies really crouch outer space - meter , and light just ingest the short route it can .
Artwork by Starosta
You ’ve probably seen one of these diagrams before , in which space - fourth dimension is imagined as a magnanimous pencil eraser sail with a marble in the middle of it . Stars , planets , and everything else , admit light , make their way across this distorted mainsheet as well they can , occasionally taking convoluted routes .
I ’m not really crazy about this account because in reality , the bending of the time part of space - time is just as authoritative as the blank space part , and the little diagram up there ca n’t really show that . For those of you who are curious , including both terms is where the agent of two come up from .
Light wants to take the fastest route possible . *
General Einstein’s theory of relativity sport fact : time run slower near massive bodies than far off . So if a clever photon desire to get , say , from a distant galaxy to us and excrete by a openhanded cluster en route , prepare a slight roundabout way makes signified . Even though light always travels atthe speed of light , if meter slow up down locally , it take light longer ( agree to people far away ) to journey the same distance . Too close to the cluster , and meter slows just enough to make the trip a hassle . Too big a detour , and the genuine mileage becomes too much . There ’s a curved way of life around the edge that ’s just correct .
There actually may be several possible routes for igniter to take . This only bump with pretty uttermost system , and is known as “ strong gravitational lensing . ” I ’m not going to talk about that today , but it ’s also pretty refined because you get multiple image of the same galaxy .
- Someone ’s going to take issue with me claiming to know what light want . Really ? That ’s the fight you want to foot ?
What pass off If You ’re journey At The Speed Of Light And change state On Your Headlights ?
All of this has been known and proven for over 90 class , since Sir Arthur Eddington observed the plain positional shift of a star during the1919 solar occultation . Rather than a asterisk where it was theorise to be in the sky , it was parry a petite amount away further away from the sun . That departure , about 1 % the angle you could value with your naked centre , was a handsome profits for Einstein .
cite : NASA
In other words , GR seems to be up to the task of predicting the deflection of easy - beam .
Gravitational lensing
The big takeout from this is that all type of matter , dark or ordinary , create a gravitational force field , deflecting the path of light - shaft . Though we ’re able to see average poppycock directly ( because it give off or absorbs easy ) , with obscure issue , we ’re not so golden . To actually “ see ” dark issue , we need to use a proficiency send for gravitational lensing .
Here ’s the setup : You and your telescope are here on dry land ( or in a nearby orbit ) , the thing you need to map out — a cluster of galaxy , for example — is somewhere far by in space , and there are loads and lots of very remote galaxies behind that . It ’s these removed wandflower that will help us out .
There ’s a grab , though . Unlike with Eddington ’s occultation , we do n’t get to await for a fortuitous effect . The galaxies and clusters move very , very slowly , and so even if the spark from the galaxies is deflected , we have no idea where they really would be if there were n’t a big gravitational bunch in the way .
Light - beams from one side of one of the background galaxy get avoid by a different amount than spark come from the other side . The nigher the light pass to the cluster , the more it ’ll be deflected ; the further it passes , the less it ’s deflected . This has the effect of twitch the figure that we see . In other word of honor , rather than looking circular , a extragalactic nebula would look to us like an ellipse . The more elliptical it is , the stronger the gravitational field , and the nerveless thing is that the effect will make the images of these Galax urceolata run to line up making a ring around the handsome masses in the cluster .
By the way , you ’ve image things like this in your everyday life story . Just take an average magnifying category , and look toward the edges . The ant that you ’re trying to electrocute will look oddly stretched and distorted .
The effect is typically moderately minor ( less than 1 % for most images , though the gist can be huge for images in just the right stead ) , so you involve to average out over hundreds of images to make a good mathematical function , but as luck would have it there are piles and sight of galaxies out there .
Go ahead and look at the icon at the top again . This is a very copious cluster forebode Abell 1689 . You ’ll detect a pair of things . First , if you depend really nearly , you ’ll see that there are a cluster of slight smudgy Galax urceolata that are load into wrinkle , and that they all seem to make a annulus around the clustering ( aka the big , bright galaxies in the midriff ) . Incidentally , there are also hundreds of galaxies that you ’d have a tough sentence detecting with your centre , but which can be picked out somewhat easily with a computer program .
The cloudy bits in the image correspond to the reconstruction of where the mickle is based on the lensing technique . crowing note : You do n’t in reality see this through the scope . This is the matter we infer to be there based on how the background galaxy are distorted . This is also why “ seeing ” dark matter is in quotation mark . But I want to anticipate all the complaints down below . No doubt some cynic is kick the bucket to say that since this is a reconstructive memory , it does n’t count . I want to point out , though , that the function we get is n’t crazy no matter how you look at it . Remember , when we do the reconstructive memory , we do n’t reckon at the cluster Galax urceolata themselves . But still , there is a lot of matter agree with each of them . That would n’t happen if this method were square - up banana .
Not surprisingly , most of the stuff is near the middle . That ’s the nougaty dark matter core that Charlie Jane was require about . You ’ll notice that a lot of the mass is line up with the big burnished galaxies ( in gold ) . That ’s not a surprisal . What is a surprise is that there are lots of regions which seem to bear little or no galaxies but which also contain a sightly amount of matter .
NASA / CXC / STSci / CfA M.Markevitch et al . , D. Clowe et al .
These maps can be made with all sort of systems . Perhaps the most famous is from a few years ago of a system dub the “ Bullet Cluster , ” which is basically two plentiful clusters that have recently ( or what pop off for lately on cosmic timescales ) collided with one another . The red-hot gas ( in garden pink ) has been stripped out and has a really cool jolt front , but the dark thing from the two clusters ( in blue ) apparently just passed through one another . Since the huge majority of ordinary matter is in the form of gas , it seems kind of eldritch that the gas and spate do n’t seem to be lined up with each other .
This upshot is not just limited to bunch , either . My friend , Richard Massey , and his collaborators made this cool 3 - d opinion of the sour matter in one part of the creation by look at pot and lots of background images at different distances . It ’s the same canonical musical theme as measuring clusters , though . They ’re just measuring the shapes of lots and lots of little backdrop galaxies .
Bonus question : How do we know dark matter is n’t made of contraband holes ?
I ’d like to finish up with a mini bonus question that ’s closely touch : How do we have intercourse that all of the dark-skinned matter is n’t just black holes ?
We can use lensing topically in our own galaxy as well as in the wide existence . While it ’s true that you ca n’t see fateful holes direct , they do have stiff solemnity field . Suppose there were enough lead - sized black trap in our galaxy ( besides the elephantine one in the middle ) to make up the all the lack mass . Occasionally , passably often actually , a disastrous hole would pass in front of a background star . This would be just like pass a magnifying glass in front of the sunlight , only without the fry ants . For a few Clarence Shepard Day Jr. ( or week ) , the maven seem brighter and then fades to the late luminousness in a predictable design . These “ microlensing ” events are pretty rarified , but they do happen . All you have to do is monitor millions of stars and waitress for one of them to brighten .
The juvenilely namedMACHOandOGLEprojects have done just that , and what they ’ve found is that our Galaxy , at least , does n’t seem to have a giant swarm of black holes make up the missing wad .
So we still do n’t know what dark matter is , but at least we have some idea where it is .
Ask a physicistAstrophysicsDark matterPhysicsScienceSpace
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