The Euclid space mission is ready to launch, here’s how it will test alternative theories of gravity

The European Space Agency’s (ESA) Euclid mission will be launched into space on a Falcon9 rocket by SpaceX on or shortly after July 1. Many of us who worked on it will be in Florida to witness the nail-biting event.

The mission is specifically designed to study the dark universe, probing for both dark matter and unknown dark energy substances believed to make up 95 percent of the universe’s energy density.

But it will also be able to test some strange alternative models of gravity that could challenge Albert Einstein’s grand theory of general relativity.

Scientists have known about the existence of dark matter for almost a century now. It was proposed after astronomers noticed that galaxies in clusters had mysteriously high speeds. Such velocities would cause the clusters to evaporate unless there was extra mass holding them together. Because this matter didn’t shine in the same way as visible galaxies, it was dubbed dark matter.

Gravitational lensing is a new tool for seeing this dark material. This effect is based on our understanding of general relativity. As light travels towards us from distant galaxies, its path is bent by large clumps of matter (dark or bright) in the foreground which change its appearance (and position).

This change is easily seen near the cores of massive clusters (see image below) with galaxies stretched in arcs, appearing to be long, thin and curved. We can use this deformation to determine the amount of matter in the foreground cluster. And this once again confirms that much of the mass in these clusters is indeed dark.

Gravitational lensing in the Abell 1689 galaxy cluster.
Gravitational lensing in the Abell 1689 galaxy cluster.
NASA/CXC/MIT/E.-H Peng et al; Optician: NASA/STScI

But what could it be made of? Many physicists believe it is an unknown elementary particle. One popular candidate, which has yet to be pinpointed, is axions, originally introduced to explain why some fundamental symmetries of nature appear to be broken.

However, there are other possibilities. Rather than postulating the need for dark matter, gravity can be probed. The force of gravity could become weaker than expected on the scale of galaxies and beyond. On these scales, there are some alternative models of gravity that can explain the rotation curves of galaxies without assuming that there is dark matter. The challenge for each of these alternatives is to do it consistently across all scales.

Although there are several Earth searches for dark matter particles, so far they have not found any significant evidence. Therefore, astronomical observations of galaxy clusters remain our best option for testing the various theories that can explain dark matter. This is where Euclid will excel thanks to its exceptional resolution, providing similar sharpness to the Hubble Space Telescope (see image) across a third of the sky. By comparison, Hubble only observed 5% of the entire sky.

The number of images we will obtain of clusters will increase a hundredfold with Euclid, allowing us to study in detail the distribution of dark matter within such clusters with high precision. How dark matter is distributed could hold the key to its origin and mass, ruling out a number of possible candidate particle and gravitational theories along the way.

Dark energy and gravity

Dark matter is potentially easier to understand than dark energy, which has been proposed to explain the finding that the expansion of the universe is accelerating contrary to the prediction of Einstein’s theory of gravity. This strange substance irritates physicists and cosmologists with the simplest idea that dark energy is just the energy of empty space (vacuum energy).

Essentially, as we gain more space in an expanding universe, we gain more vacuum energy, which then drives the observed acceleration.

This simple explanation is reasonable, except for the inconvenient truth that the observed density of dark energy is many orders of magnitude lower than that predicted by quantum theory, which governs the universe on the smallest of scales. In short, this simple explanation asks more questions than it answers.

As with dark matter, an alternative explanation for dark energy is that it’s not a substance or form of energy at all, but again a sign that gravity is behaving differently on larger scales.

This has led to a flurry of new ideas that extend our theory of gravity beyond general relativity. For example, could gravity exist in more than the four dimensions (three space dimensions plus time) that the rest of the universe experiences? Are there new fundamental fields that we don’t yet know about that interact with gravity?

Or perhaps Einstein’s theory is valid for the weak gravitational fields we experience on Earth, but becomes radically different in extremely strong gravitational fields, such as those near the event horizons of black holes.

The challenge for all of these alternative gravity models is to work together, for both dark matter and dark energy. Ideally, they should work across all scales and masses, as one theory. Physicists firmly believe in Occam’s razor that the best theories have the fewest hypotheses.

Euclid will help us test these exotic gravity models by mapping the positions of millions of galaxies over vast regions of the universe. This allows us to trace the cosmic web, a spongy structure of filaments and voids in space. These appear to be deposited first in dark matter and then sprinkled with galaxies.

This cosmic web is formed from billions of years of gravitational collapse, which means its structure and statistics are sensitive to the laws of gravity at work on cosmological scales. By measuring its properties, we can determine whether a new theory of gravity would fit the data better than Einstein’s theory.

As we return to Earth, there is much excitement in the astrophysicist community about what Euclid will do. This is the first time we have a satellite dedicated to mapping dark matter and dark energy.

Euclid’s data will last a lifetime, and generations of cosmologists will spend their careers studying it. As we watch Euclid launch into the Florida sky, we’ll be one step closer to answering some of science’s most fundamental questions.

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