83 votes
Accepted

Is there a better explanation of this picture showing the very distant star "Earendel"?

I did the annotation of that figure for the press release, so let me start by apologizing for the poor explanation, and then try to dig deeper into what's going on :) (although you already seem to ...
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32 votes
Accepted

If the Sun was replaced with a sun-mass black hole, would it be visually detectable?

Yes, easily with a telescope, but not with the naked eye. It is a matter of routine to detect the 1.7 arcsecond shifts caused to stellar positions when seen near to the limb of the Sun. The removal of ...
  • 120k
26 votes
Accepted

Is it possible to detect gravitational lensing behind the Moon?

The angular deflection caused by the lensing of a distant background object by the Moon is given by $$\theta \simeq 4 \frac{GM}{Rc^2},$$ where $M$ is the mass of the lensing object and $R$ is the ...
  • 120k
25 votes
Accepted

If dark matter bends light, how do we know the stuff in the sky is where we think it is?

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ ...
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23 votes
Accepted

Was the Sun's gravitational lensing observed in other solar eclipses than the one in 1919?

Yes, observations of this kind are within the technical scope of amateur astronomers. Several groups succeeded in replicating the experiment during the 2017 eclipse that crossed the USA. For example ...
  • 93.4k
16 votes

How can gravitational lensing makes a quasar appear brighter?

The quasar gives out light in all directions. The light spreads out in space. Only a very small amount of that light would be pointed exactly in the direction of your telescope. But if a large galaxy ...
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16 votes

How do we know that objects that appear in duplicate or triplicate, etc. due to strong gravitational lensing aren't actually multiple objects?

Perhaps this isn't the case for every scenario, but I can think of at least two instances where this can be determined: Stars In the case of stars, it's pretty straightforward to get the spectra of ...
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15 votes
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Are astronomers waiting to see something in an image from a gravitational lens that they've already seen in an adjacent image?

What you do is cross-correlate the observational datasets for the multiple sources and look for the "lag" that maximises the cross-correlation function. Generally speaking, the "events" are not really ...
  • 120k
11 votes
Accepted

Is it possible to observe strong gravitational lensing with amateur telescopes?

The cosmic horseshoe is beyond amateur instruments. It is a magnitude 20 object. In a large (2.5 m) professional telescope it looks like: This image taken from the SDSS III data. It is small (10'', ...
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10 votes
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What is the gravitational lensing focal distance of a white dwarf star?

The gravitational focus you are talking about is actually a minimum value, defined by parallel rays of light from a very distant star just skimming past the Sun as they are bent according to General ...
  • 120k
9 votes

Images of gravitational lensing

As mentioned in the comments, a ring will (ideally) only occur if you have perfect alignment of source, lens, and observer. This is true for point sources, for extended sources things get more ...
  • 2,235
9 votes
Accepted

Gravitational lensing in Newtonian physics

A photon is an entity defined in the context of a relativistic field theory, and so it doesn't really make sense to talk about the Newtonian bending of a photon. Necessarily, we need to substitute an ...
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8 votes
Accepted

How does gravity interact with a photon?

It is simply not true that gravity can only interact with mass. Rather, any long-range spin-2 force interacts with all energy-momentum equally, and it source is the stress-energy-momentum tensor. That ...
  • 7,684
7 votes
Accepted

How are gravitational lensing and dark matter related?

Gravitational lensing of background galaxies and quasars is used to probe the amount of gravitating matter in foreground objects such as clusters of galaxies. Here is an example, taken with the ...
  • 120k
7 votes

Is it plausible to use other stars for the proposed FOCAL mission instead of the Sun?

Gravitational lensing works from anywhere beyond the focus, so in that sense, we could use any star as a gravitational lens. The problem is that the field of view is tiny. We only get any useful ...
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7 votes
Accepted

Algorithm to fit galaxies

The two most general, publicly available packages for galaxy image fitting (other than GALFIT) are probably Imfit and ProFit. (Note that I am the developer of Imfit.) There is also Lenstronomy, which ...
  • 14.9k
7 votes

Is it possible to observe strong gravitational lensing with amateur telescopes?

If you have a good camera, a steady hand, good conditions and a lot of patience, you might. The Sunburst Arc, at a magnitude of around 17, has been observed with a 132mm refractor by Ian Woodward at ...
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7 votes

Would the stars look different if many primordial black holes were present?

The answer is no, because these effects have been looked for and not found. The microlensing surveys of the 80s and 90s specifically set out to look for the lensing signatures of compact, massive ...
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7 votes
Accepted

Is quasar 2M1310-1714 outside the observable universe?

The redshift of the quasar is 1.975, so it is nowhere near the edge of the observable universe. 17 billion light years is the comoving distance (i.e. where it is now), as you can confirm with this ...
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7 votes
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Black hole magnification factor

I'd suggest you take a look at Narayan and Bartelmann (1996). They run through all the math to answer exactly this question. I'll add here the punchline from their paper for posterity. Shown below is ...
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7 votes

Gravitational mirroring: Can we theoretically see the Milky Way using a telescope?

In theory, it's possible, but in practice it's extremely unlikely. The deflection angle is small unless the light ray passes very close to the centre of mass of a compact lensing body. From Wikipedia, ...
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6 votes
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Would gravitational waves be subject to external gravitational perturbations?

Firstly, gravitational waves (GWs) are not an echo - we measure the direct signal. The process you describe here is known as gravitational lensing, the deviation of (usually) light rays due to ...
6 votes
Accepted

What does the 'amplification factor' of gravitational microlensing mean?

Gravitational lensing is just geometry, as is optical lensing. The amplification factor describes the increase of the area of the image (at constant surface brightness). BTW, the amplification ...
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6 votes
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Do we have to take gravitational lensing into consideration when measuring the size of exoplanets?

No, there is no effect here. Why? Gravitational lensing magnification works by increasing the observed surface area of the lensed object while preserving the surface brightness. An exoplanet angular ...
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6 votes
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Is cosmic shear generally agreed to have been observed?

Yes it has been observed. There are numerous papers covering this concept. Before I get into weak-lensing (because that is what you reference in your question) I'll note that cosmic shear is simply a ...
  • 14.5k
6 votes
Accepted

With gravitational lensing, is it possible to mathematically compute the correct image of the galaxy that is being distorted?

That would be possible theoretically. However, in practice, the lensed images are highly non-linearly distorted and due to the limits in resolution, they are often limited to a thickness of a few ...
  • 1,549
6 votes
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What is the scale of things you can see with gravitational lensing?

You are right that the stars seen on the sky are within the Milky Way. Only with a large telescope is it possible to resolve individual stars in other galaxies, and only for the nearest ones. I don't ...
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6 votes
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Need help understanding the recent observation of extra-galactic planets!

Instead of reading half-comprehensible newspaper articles, I found it more helpful to go to the original source. There, the authors explain their method very clearly: There is a background galaxy, ...
6 votes
Accepted

Why is this Einstein ring pink?

OK, guessing rather wildly in the absence of solid information.... The press-release page is here; that provides a bit more information than the news article, and includes two images of (different ...
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6 votes

Is it possible to see stars that are gravitationally lensed by the sun from the ground?

The answer is no. But there's an interestign alternative. The Eddington's experiment was not about seen a gravitationally lensed star but to see a small displacement of its position (just below 2 ...
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