What proportion of the stars that we can see are impacted by gravitational lensing, to the point that we see multiple versions of them. I had the thought that perhaps there are fewer stars out there than we observe, since some of them are the same star.
I believe that the number is zero. My understanding is that gravitational lensing is limited to a handful of galaxies that happen to be in the right place, and the effect is so major and obvious that they can’t be mistaken for anything else.
Stars tend to have fingerprints, of a sort. They are not just tiny bright dots, their light contains a lot of information. We would probably notice two identical stars next to each other and be able to infer lensing.
In terms of stars visible to the human eye, zero. In terms of stars visible and resolvable as stars to telescopes on Earth, so few as to be still be an infinitesimal proportion.
There have been surveys looking for gravitational lensing in an effort to estimate the proportion of dark matter that black holes make up. However they look for galaxies rather than stars. The answer is that they are there, but far too few to be the answer to dark matter.
Lensing does usually not split the image, not unless the black hole is exactly in line with the lenses object, when you may get the Einstein Cross. Usually the lenses object is just twisted in shape. So looking for galaxies that have a characteristic deformation is a good start. One that is a good subject for automated techniques. Such surveys seem to have turned up to thousands of likely lenses galaxies. Compared to other the number of visible galaxies this is still pretty much zero.
Specifically, stars have unique spectral characteristics. If you saw two stars with essentially identical spectral characteristics at a slight arcdistance away, or a star with a spectrum just slightly shifted from another one, you’d know that that you are seeing some kind of gravitational lensing. However, lensing of observable stars (microlensing) occurs very rarely and almost never strongly enough to produce separate images
As @Francis_Vaughan notes, most observed gravitational lensing is on cosmic scales with the supermassive black holes actually providing the gradient to cause lensing. This requires a very precise alignment for the mass to be inline with the target and observer, hence it is observed very infrequently.
Stranger
Thanks. On one of the science shows (can’t remember which one), a scientist described our view of the cosmos as “kaleidoscopic”, in which objects are not always where they appear and there are multiple versions of some. I guess technically he was correct, but certainly not to the extent that it sounded.
If the lensed object is a point source (i.e. quasar), you get an Einstein Cross. But if it’s an extended object (i.e. galaxy), you get an
When the line up is not quite exact, there’ll be a non-symetric set of images, still often called an Einstein Cross. If the lensed object is non-point, these images will be short arcs. BTW, Gaia has discovered a dozen Einstein Crosses:
As for microlensing, those are generally only seen for a fairly short time. A couple hours maybe. They’ll happen when a not very bright object, such as rogue planets, neutron stars, brown dwarfs, etc., happens to line up between a star and Earth. They are used to estimate the number of such objects there are in the galaxy.
Coincidentally, I was watching an episode of How the Universe Works last night, about dark matter. The physicists on the show described the observation of a supernova from the Pleiades cluster that presented itself 4 separate times (an Einstein cross?), and then again a 5th time a whole year later. The reason for this was light from the supernova bending and taking several paths of arrival around a mass of dark matter, and not a star or galaxy.
With dark matter being over 5 times more prevalent than regular matter, wouldn’t lensing due to dark matter be far more common? If so, are we certain that stars that we observe are really where we observe them to be? So while seeing multiple instances of stars and other objects due to lensing may be rare, could our view of the universe around us be distorted in any significant way; i.e. the previously mentioned “kaleidoscopic” universe?
Not for stars within our galaxy. To get gravitational lensing, you need a concentration of mass along the path of the light. The dark matter in our galaxy is spread out fairly evenly, so there’s no concentration to do the lensing. The only lensing you’ll get for stars is when some object like a white dwarf or rogue planet happens to line up exactly between a star and the Earth. Then you get what is called microlensing. That only lasts a relatively short time, on the order of a few hours.
Thanks! I appreciate the education.
NASA found the most distant known star through gravitational lensing.
Single stars which appear more than once due to gravitational lensing are extremely rare, but not completely absent from out sky.
The distant supernova AT2016jka was split by gravitational lensing into three different images with different path lengths, so the explosion appeared in each location at different times as seen from Earth. Astronomers predict that a fourth image will appear in the year 2037±2.
Kind of odd behavior for gravitationally-active matter, which has a tendency to clump. Is there a good hypothesis as to why dark matter fails to clump?
Because it doesn’t interact with anything, including itself. Gravity will get it into the same general location, but to get it to become denser, there needs to be more interaction. Even if dark matter particles (whatever they are) would just bounce off each other, the clumps would become denser. But they don’t even do that much or possibly they do just a tiny bit. But it’s not enough to do anything but make a diffuse ball that’s actually bigger than the visible galaxy.
Right. If you have “ordinary” matter, if one bit of matter falls into a spot where there are other bits of matter, it’s likely to bump into some of them, and so lose its momentum, and stay there (or at least, closer to there). But if you have a bit of dark matter falling in, there’s nothing to stop it, and so it just keeps on going, and goes back out on the other side. If it’s in a bound orbit, it’ll keep on passing through repeatedly, but still just pass through.
More importantly, when regular matter bounces off itself, it releases electromagnetic energy, sapping energy from the particles, allowing them to settle down and clump.
Dark matter doesn’t seem to interact with other dark matter, and if it does, it does so losslessly.
There is the possibility that DM will very rarely interact with regular matter. There’ve been a number of experiments trying to find this, so far without success. The typical experiment along these lines is to put ultracold matter deep in a mine and look for extra energy that would come from these collisions.
I’ve also seen proposals that if dark matter does have some small amount of interaction, either with itself or with baryonic matter, that the Galaxy’s dark matter halo would be slightly denser in the disk of the Galaxy than outside of it, together with astronomical observations that would determine whether this is so. IIRC, the proposed observation was one that some already-planned instrument or another would be making anyway, and it’d just be a matter of the correct analysis of the data. I haven’t kept up with whether the measurement in question had been done yet.