The five biggest mysteries that Euclid will help solve

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Surveying the universe beyond our galaxy, Euclid will attempt to unlock the secrets of the cosmic web and perhaps explain how invisible dark matter and exotic dark energy influence the structure and evolution of the universe. Euclid will mainly focus on two core themes of the European Space Agency’s cosmic vision programme: What are the basic physical laws of the universe? How did the universe originate and what is it made of?

1. What is the structure and history of the cosmic web?

The matter in the universe is arranged in a huge, orderly web resembling a “cosmic spider’s web”. This ‘web’ is made up of huge clusters of galaxies connected by invisible filaments of gas and dark matter. In between are giant empty regions called cosmic voids. Exploring the cosmic web is challenging because of its size, with cosmic voids spanning hundreds of millions of light-years. Euclid will survey more than a third of the sky wide to collect information about the shape, size and location of billions of galaxies.

Looking into the depths of the sky, Euclid will also look back in time and visualize ten billion years of cosmic history. That’s because the farther away a star is from us, the longer it takes for its light to reach us. By carefully mapping the shape and distribution of a large number of galaxies, Euclid will reveal the structure and history of the cosmic web. Although dark matter is not visible to us, its presence distorts the light of distant galaxies. This effect is called gravitational lensing and can be observed by Euclid, revealing how dark matter is distributed throughout the universe.

2. What is the nature of dark matter?

Despite decades of research, we still don’t know what the missing mass, the so-called dark matter, in the universe is made of. Comparison of different cosmological models and measurements so far has led to the hypothesis that most dark matter is composed of “cold” particles, meaning that they are heavy and move relatively slowly. However, it is entirely possible that some of the dark matter is composed of light particles moving at close to the speed of light, the so-called “hot” dark matter. The question of how much dark matter, if any, remains hot. Hot dark matter could consist of “ghost particles” called neutrinos, which interact very weakly with other matter. Although neutrinos were initially expected to be massless, there is now evidence that they have very little mass.

We can use Euclid’s precise measurements of cosmic structure to find out the total mass of neutrinos in our universe and the percentage of dark matter that might be made of it. Despite their gravitational field, neutrinos tend to slow down structure formation and distort them due to their fast motion. The most exciting discovery will be something we least expect. Euclid’s unparalleled observations of the universe outside the galaxy could reveal the existence of new types of fast-moving particles. Dark Universe Detective Investigates!

3. How has the expansion of the universe changed over time?

In the 1990s, cosmologists made the surprising discovery that the universe is expanding at a much faster rate than before. The universe has been expanding since its inception in the Big Bang, but until recently scientists assumed that the rate at which the universe is expanding would slow over time because the gravitational force of all matter in the universe hinders this expansion. Understanding the accelerating expansion remains one of the major challenges of cosmology and fundamental physics. Evidence for a variable rate of expansion relies on observed differences in the brightness and color of so-called “standard candles”: astronomical objects of known, constant luminosity. Distant objects will appear dimmer to us, while the stretching of space-time stretches the wavelength of light on its way to us, a blushing effect called redshift. Euclid will also measure the redshift of galaxies, which will help us determine the distance between them and us.

By scanning more than a third of the sky with a telescope sensitive enough to see the light that took 10 billion years to reach us, Euclid can tell us how the universe expanded over time. Because of its wide angle of view, Euclid can also check whether the expansion is equal in all directions. If not, it would be contrary to the so-called cosmological principle, which states that the universe on a sufficiently large scale appears symmetrical in all directions (isotropy) and from everywhere (homogeneity). This rule of thumb forms the basis of almost all models and analyzes used in cosmology.

4. What is the nature of dark energy?

Knowing in detail how (and what) the universe is expanding is one thing, but we also want to know the driving force behind it. Cosmologists have called this unknown component of the universe “dark energy.” No one knows what it is made of or if it is a form of energy! The best working hypothesis is what was proposed by Albert Einstein as far back as 1917. He included a “cosmological constant” in his calculations, which is a constant energy field that exists throughout the universe. This is an intrinsic property of the vacuum of space, so the larger the space, the greater the “vacuum energy” (dark energy) and the greater its effects.

There are other alternatives. For example, acceleration could be caused by the fifth fundamental force of nature that evolves along with the expansion of the universe. Unlike the cosmological constant, this “essence” is dynamic, time-dependent and not evenly distributed throughout the universe. Any explanation of what dark energy actually is subtly alters the way acceleration changes through cosmic time, but so far no experiment has been able to measure acceleration in sufficient detail to distinguish between possible solutions. Euclid’s careful and precise measurements will change this and hopefully reveal the true nature of dark energy.

5. Do we really fully understand gravity?

The presence of dark matter and the accelerating expansion of the universe indicate that we are missing out on something important. These two surprising discoveries have one thing in common: they’re all about gravity. Gravity holds planets, stars, solar systems, and even galaxies together. We experience it every day: it keeps us grounded and causes things to fall rather than in another direction. The best theory we have for describing gravity is Albert Einstein’s general theory of relativity. In it, he argues that gravity is not a literal attraction, but rather a consequence of massive objects bending space-time. An object with mass distorts space-time, like a heavy ball pushing down a trampoline mat. By bending the trampoline mat, the lighter balls on the trampoline mat will automatically roll to the center.

General relativity has other implications, including the existence of black holes and gravitational waves. the faster or slower passage of time for different observers, depending both on their relative speed and acceleration and on the gravitational force to which they are subjected; The paths of light are also affected by gravity. The predictions of general relativity have been proven correct time and time again. But this theory has never been tested with great precision over the vast distances and times that Euclid would cover. So Euclid will show whether this test on the largest possible scale will fail the general theory of relativity. If that’s the case, physicists will have to go back to the drawing board.

source: ESA