Dark world, here comes Euclid

By Mayank Chhaya-

The European Space Agency’s (ESA) Euclid spacecraft, which lifted off on July 1 from the Cape Canaveral Space Force Station in Florida, will attempt to answer some of the most fundamental questions about the universe. In particular, it will help infer the properties of dark energy and dark matter which have mystified astronomers for decades.

According to the ESA, “Euclid will observe billions of galaxies out to 10 billion light-years to create the largest, most accurate 3D map of the Universe, with the third dimension representing time itself. This detailed chart of the shape, position and movement of galaxies will reveal how matter is distributed across immense distances and how the expansion of the Universe has evolved over cosmic history, enabling astronomers to infer the properties of dark energy and dark matter. This will help theorists to improve our understanding of the role of gravity and pin down the nature of these enigmatic entities.”

Dark matter constitutes 27% of the universe. Contrast that with barely 5% constituted by baryonic matter or normal matter, of the kind that everything we can see and experience is made of. Add to this the scientific consensus that 68% of the universe is dark energy and one begins to get a measure of how important this dark world is.

What is even more fundamental is that there is scientific consensus that it is this yet undetected dark world which has ensured that hundreds of billions of galaxies and many trillions of stars do not eventually tear apart. Dark matter is that mysterious celestial glue that seems to hold things together.

Although the existence of dark matter has been postulated variously for four centuries since the 1600s, the way the scientific community understands and studies it was first suggested by the Swiss astronomer Fritz Zwicky in 1933. While studying the Comma galaxy cluster in 1933 and concluded that the gravitational mass of the galaxies in that particular cluster was 400 times greater than what their luminosity suggested. From that calculation Zwicky inferred the cluster had matter that could not be seen—dark.

The mass of objects such as galaxies or cluster of galaxies can be measured by how much light they produce or measuring how fast they are moving. Gravitational attraction pulls objects harder and the harder it pulls the faster things go. It was believed that those two methods would lead to the same mass. They do not. There is a giant discrepancy between the two calculations, the way Zwicky had found.

How giant is that discrepancy?  Scientists say it is roughly a factor of six that they are off by.  In other words, there is about six times more dark matter than what we can account for in what we can see. In a sense, all galaxies, stars, planets and everything else in the light world is cosmically groping about in the universe.

Early on in the history of dark matter, it was thought that a lot of the actual objects made of baryonic matter could explain the missing mass; objects such as space rocks, asteroids, rocky planets and gas which do not emit much might and hence are dark. Gas is hot and X-ray emitting but it is not in the optical range.

Over decades, astronomers using various techniques narrowed down that it cannot really be normal matter that makes up all the dark matter. As instruments get better, it is found that there are some components of matter that are made up of things that we are not used to experiencing. They are outside the Standard Model of particle physics that make up this extra mass.

What is animating the debate is a search for a new particle that could be constituting dark matter. It is an open question whether it is a particle or particles. Scientists believe it could be a single one or it could be more complicated.

In recent years, there appears to be a visible surge in the science media’s interest in dark matter because it remains one of astronomy’s big unresolved questions. Descriptions such as “the dark sector” or “the ghost world” with some loaded philosophical hints have gained ground. Could the realization that between dark matter and dark energy we barely know 5% of the universe be a source of intellectual exasperation? Euclid could help address some of those questions.

Euclid’s operational orbit will be halo around a point known as the Sun-Earth Lagrange point 2 (L2), at an average distance of 1.5 million km beyond Earth’s orbit. This special location keeps pace with Earth as it orbits the Sun. The ESA’s Gaia and NASA’s James Webb telescopes are in the same orbit. Euclid is named after the Greek mathematician Euclid of Alexandria who lived around 300 BC and founded the subject of geometry. Once the spacecraft starts collecting the data and beams back to earth, scientists will begin to get a better handle on the dark world.

It is not clear what the most striking feature of dark matter might be. Some scientists believe it could be the dark matter particle.

In the context of the pervasiveness of dark matter, Earth as everything else is surrounded by dark matter quite like neutrinos, which are much lighter particles but they are similarly hard to detect. Just as neutrinos are very hard to detect even though they are always passing through us at any given time, dark matter surrounds us.


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