Space Warps - ESA Euclid

Gravitational lensing
Needles in a Haystack
About the Survey
ESA Story about the Euclid Zoo project

Gravitational Lensing

Einstein's theory of gravity, General Relativity, made a remarkable prediction. Massive objects, such as stars, would bend the space around them such that passing light rays follow curved paths. Evidence for this revolutionary theory was first obtained by Arthur Eddington in 1919, when during a solar eclipse he observed that stars near the edge of the Sun appeared to be slightly out of position (read more here). The Sun was behaving like the lens in a magnifying glass and bending the light from the background stars!

In 1937, Fritz Zwicky realized that massive galaxies (which can contain anywhere from ten million to a hundred trillion stars) or clusters of galaxies could be used to magnify distant galaxies that conventional telescopes couldn't detect. As you can see, not unlike a conventional magnifying glass, these gravitational lenses not only magnify and focus the light of the distant background galaxies but they can, and mostly do, distort them as well.

When one of these gravitational lenses happens to sit right in front of a background galaxy, the magnification factor can be up to x10 or even more, giving us a zoomed-in view of the distant universe, just at that particular point. Lenses can help us investigate young galaxies more than halfway across the universe, as they formed stars and started to take on the familiar shapes we see nearby.

Observations of the distorted background galaxy can also give us useful information about the object that is behaving as a gravitational lens. The separation and distortion of the lensed images can tell astronomers how much mass there is in the object, and how it is arranged. It is one of the few ways we have of mapping out where the dark matter in the universe is, how clumpy it is and how dense it is near the centers of galaxies. Knowing this can provide crucial information about how galaxies evolve.
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See the following lensing animations showcasing the lensed image configurations that can usually form.

Left: A distant background galaxy moves across a massive elliptical galaxy in the foreground
Center: A distant background quasar moves across a massive elliptical galaxy in the foreground
Right: A distant background galaxy moves across across a galaxy group in the foreground. The configuration of the lensed images can be very different based on how the multiple galaxies are arranged in the galaxy group.
Video credits: Alessandro Sonnenfeld.

The speeds of the background galaxies/quasars are too slow for us to see the changing image configurations in our lifetime. In reality, we only see a single static configuration.

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Needles in a Haystack

There is a lot of interesting science to be done with gravitational lenses, from precisely weighing galaxies to measuring the expansion rate of the Universe. The problem is that they are very rare. Only about one in a thousand massive galaxies is aligned with a background object well enough to cause it to appear multiply-imaged. We currently know of about 700 objects that are behaving as gravitational lenses, largely because we have become very good at observing the night sky! Modern optical surveys cover thousands of square degrees, with images sharp and deep enough to resolve about 1 lens per square degree. There should be thousands of lenses that we can detect, but we will need to look at millions of galaxy images to find them!

The ideal solution would be to get a computer to look through all of the images, but unfortunately, this is not a straightforward solution. Teaching a computer to recognize the effects of gravitational lensing is challenging, as they can be easily confused by galaxies that look very similar to a distorted background galaxy. Here we combine the outputs of such a computer program with the input from citizen scientists to get the best of both worlds: computer speed and human intuition.

Human beings have a remarkable ability to recognize patterns and detect the unusual with only minimal training. With a basic understanding of what the distorted images of galaxies that have passed through a gravitational lens look like, participants in the SpaceWarps project can help discover new examples of this amazing phenomenon, and enable our survey scientists to carry out new investigations of stars and dark matter in the Universe.

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The ESA Euclid Space Telescope

Euclid is a European Space Agency (ESA) space telescope that orbits the sun roughly 1.5 million km beyond the Earth. It has a 1.2m diameter mirror and a 600-megapixel camera. The huge camera allows it to take pictures 3 times the size of the full moon.

The Euclid mission is designed to explore the composition and evolution of the dark Universe. The space telescope will create a great map of the large-scale structure of the Universe across space and time by observing billions of galaxies out to 10 billion light-years, across more than a third of the sky. Euclid will explore how the Universe has expanded and how structure has formed over cosmic history, revealing more about the role of gravity and the nature of dark energy and dark matter.

Some of you may have participated in the Euclid Galaxy Zoo project (read about that here).

Within the Euclid final dataset, there should be hundreds of thousands gravitational lenses, but the survey data is too large for us to find them without your help! That's true for this first batch of Euclid survey data.

We will be showing images mostly centered around galaxies that are massive enough to potentially act as gravitational lenses. We've made millions of these ESA Euclid galaxy images using the ESA Datalabs digital platform. The task will then be to assess whether or not they really are lenses.

This is a tricky mission: nature is very creative, and there are galaxies of all shapes and colours out there, many of which can mimic the features of a genuine gravitational lens. You can learn more about them under Education.

The challenge is to come up with the most plausible explanation for what is going on, in collaboration with the rest of the Space Warps community. Do you think you can spot outer space being warped?

You can read more about the Euclid telescope and its exciting science here!

A head start on finding lenses from artificial intelligence

As with our previous experiments, we are combining your skills with artificial intelligence too. You will classify a combination of galaxies chosen randomly from the survey, as well as those being identified as likely candidates from an AI system. We've done this because of the impractically to screen all the data in these large datasets. While AI is giving us a head start by filtering out lots of the non-lenses, so you can focus on the more interesting objects. However, we know from our previous experiments how skilled you are at spotting lenses. So by inspecting the random sample, we hope to get an indication of what else is out there, that the AI codes may have missed.

Not only will you be able to discover new lenses, but when you mark something as 'not a lens' we'll feed that back into our AI system so it will get a little bit smarter at discriminating confusing objects. With this training, the AI will get better at choosing objects that are likely to be lenses. This way, humans and AI will discover together!

NEW (Jul 2025): Space Warps REFINE

You will also notice that we have a new work flow stream - Refine. For those of you who've been contributing to Space Warps from the beginning might recall this second from the original CFHT-LS search. This 'refine' workflow follows after the classify stage where many images are classified. In 'Refine' we are asking you to look at your top-scored lens candidates from the 'classify' stream to provide us with a grade that reflects if an image is a definite lens (A-grade), probable lens (B-grade), possible lens (C-grade) and not a lens (X-grade).