by Prasad Ganti

Euclid is a space based telescope launched by ESA (European Space Agency) in July 2023. Since then it has traveled to its home at the L2 Lagrangian point in space about a million miles from Earth. And sent back first images of what it has seen. Whenever a new space based observation mission is launched, I am curious to find out how it is different from other missions in the past. 

Euclid was launched on SpaceX’s workhorse rocket Falcon 9 from Kennedy Space Center in Florida (closer to my home). Why would ESA launch from the US ? ESA’s Arianne 6 rocket was not ready at that time while Arianne 5 was retired. Due to the ongoing Ukrainian war, Russian launch options were not feasible either. Since then Euclid has traveled to the L2 Lagrangian point on the Sun-Earth axis, away from Earth and Sun. It is the same parking lot where the James Webb Space Telescope is also positioned. Balanced by the gravitational pull of the Earth and the Sun, the Telescope consumes minimal energy and its fuel is expected to last for six years. The picture shown below, courtesy NASA shows the various Lagrangian points. Though not drawn to scale, it illustrates the concept well.

Euclid’s mission is to look at large swaths of sky and produce a survey of all the galaxies and their red shifts in the form of a 3D map. The distribution of the galaxies shows how much dark mass is around. While dark matter cannot be seen, its effect can be felt. While the red shift of the galaxies, which measures how fast the galaxies are receding from each other, measures the dark energy. Unlike the narrowly focused telescopes like James Webb, it will not focus on any individual galaxy or any other object. But would be observing and constructing a panoramic view of the universe dominated by dark matter and dark energy. The sharp pictures produced would be built from 600 million pixels each. 

Euclid will observe galaxies which have dark matter in front of them. Any big lump of matter sitting between us and the object, creates a distorted view of the object behind because light bends in the presence of a gravitational field. This concept is known as gravitational lensing. Regular gravitational lensing causes arcs, rings and multiple images of the objects behind. When dark matter is in front, it produces a weak gravitational lensing instead, It helps us study the dark matter by looking at the images.

Another set of observations would involve Baryonic Acoustic Oscillations (BAO) which are remnants of the Cosmic Microwave Background (CMB) radiation. The structure of the universe shows clustering of galaxies in pairs and separated by some distance. This distance is related to the sound waves in plasma in the early universe. Plasma represents the ancient cosmic soup consisting of ions. The BAO is related to these sound waves. The BAO would give some clues about the universe’s expansion and the strength of dark energy at different times during the universe’s lifetime.

Coming to the first light received by Euclid, is a galaxy nicknamed the “Hidden Galaxy” closer to our Earth. Given below is the picture courtesy ESA. It looks like an ordinary picture taken by any other telescope. The galaxy is located behind our galaxy, as viewed from edge on. We do not have a ringside view of this galaxy. Instead the galaxy is behind a mountain of dust and gas which block our view. Euclid’s piercing vision made this image possible. And this is just the beginning.

The Greek mathematician Euclid is considered as the father of geometry. He lived in Alexandria about twenty three hundred years ago. The Euclid mission will map the geometry of the universe. We will have to wait for six years to see a comprehensive 3D map of our dark universe. I am waiting to get this poster to be hung in my home.