The Week in Space and Physics: The Search for Dark Energy

On new hints of dark energy, a camera of astronomical proportions, how NASA wants to drive on the Moon and China's latest space missions

Totality over North America. Image Credit: NASA/Keegan Barber

Until 1998 we thought the universe, on a large scale at least, was dominated by gravity. This is the force, after all, that binds planets to stars, stars to galaxies and galaxies to clusters. But gravity is also an attractive force, and so it should, over a long period of time, tend to pull things together. If the universe happened to be expanding, which we knew it was, then gravity should act to slow it down, and eventually, perhaps, to send it into reverse.

And then, after carefully studying distant supernovae, astronomers concluded this story was wrong. The universe was expanding, they said, but that expansion was not slowing down. It was instead accelerating, with the cosmos growing larger at an ever faster rate. It was a shocking result, but one that proved hard to ignore.

Clearly, physicists realised, something had to be driving the acceleration. And whatever it was, it had to be strong enough to overcome the combined gravity of the entire universe. Even worse, it had to be hidden, since centuries of measurements had spotted no sign of this powerful force acting on Earth. With no obvious answers, physicists labelled it ‘dark energy’ and started dreaming up experiments to find out more.

Today, more than twenty-five years later, we still know little about dark energy. It remains largely mysterious, even more so than dark matter, the other unknown responsible for sculpting much of the heavens. But we are starting to see the results of instruments and experiments designed to probe dark matter in more detail. And that, in turn, means we might be about to get a better fix on what it actually is.

One of these instruments is Euclid, a space-based observatory that recently began surveying millions of galaxies. Researchers hope data from the telescope will help fingerprint the presence of dark energy, and so find out how much it has pushed the expansion of the universe.

Another is DESI, an instrument based in Arizona. For the past three years DESI has been mapping out the positions and motions of galaxies billions of light years from Earth. It thus hopes to measure the expansion of the universe, and to do so in a way that allows researchers to calculate how it has changed over time.

Data from the first year of DESI’s work has now been published. In it, researchers say, are hints that dark energy has not always had a constant push. Over the last eleven billion years, it seems, the mysterious force might have grown weaker. In parallel, the expansion may be accelerating at a slower rate. One day, if that trend continues, it could even come to an end.

These are only the first results, and researchers caution the findings are not yet strong enough to be certain. With more data these signs of weakening dark energy might vanish. One thing is for certain - with DESI, Euclid and a whole set of other advanced telescopes on the way, we are about to get a lot more of that data.

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Astronomy Gets a New Camera

Engineers last week announced the completion of the largest digital camera ever built. The camera, which weighs three tonnes and has a resolution of over three thousand megapixels, will soon be taken to the deserts of northern Chile. It will then be installed on the new Simonyi Survey Telescope, part of the Vera C. Rubin observatory.

Though the specs of the camera are impressive - observatory deputy director Aaron Roodman says it can pick out a golf ball at a distance of fifteen miles - it will not make images as sharp as those from the James Webb. Indeed, no telescope on Earth can: all are limited by a blur created by the atmosphere.

That gives space-based observatories an advantage, at least when they zoom in on distant objects. But putting big telescopes in space is hard. The James Webb’s six-metre wide mirror pales compared to the eight-metre one at Simonyi, or to the forty-metre one being built for the Extremely Large Telescope. Neither would it be easy to put a camera as large or as heavy as this one into orbit.

Where this telescope will really shine, though, is in the breadth of sky it can photograph. The camera is capable of imaging the entire southern night sky in just four days, a task it will repeat almost one thousand times over the next decade. This constant survey of the night sky will create a kind of movie, allowing astronomers to precisely track changes in the heavens above us.

That will prove useful for all sorts of things. The telescope should help find thousands of new asteroids and comets scattered around the solar system. It will let astronomers map out the stars in our galaxy more precisely, and to pick up on small shifts in the light coming from them.

Its biggest goal, however, is to search the sky for signs of distortion created by dark matter. Tracking this allows researchers to map out how dark matter is spread across the cosmos, and, hopefully, will shed some light on how this mysterious substance is shaping our universe.

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NASA Looks for a Moon Buggy

NASA last week selected three companies to start designing lunar rovers for the Artemis program. The vehicles will replicate many of the features of the Apollo rover, which allowed astronauts to travel up to five miles from their lander. But NASA also wants the new rovers to work without humans onboard, so that they can steer autonomously across the lunar surface.

The project starts with a year-long feasibility study. During this time the companies will flesh out their designs and prepare them for a preliminary review with NASA. Afterwards one of the three companies will be awarded a larger contract to demonstrate the rover. That will mean building it, delivering it to the Moon and then proving it can actually work.

Ideally this would all happen before Artemis V, which is currently envisioned as the third crewed mission of the Artemis series. NASA is hoping the Artemis V crew will then be able to use the rover to travel short distances around the Moon. But since the rovers will be unpressurised, and therefore unable to offer much in the way of life support, they are unlikely to use them for trips of more than a few miles.

After the astronauts leave, however, NASA could then use the rovers to transport things between sites on the Moon. That would allow future missions to make use of instruments and equipment left behind by earlier missions, and so begin building out the infrastructure needed to support a long term presence on the Moon.

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China Shoots for the Moon

America is not the only nation trying to build up infrastructure for the Moon. In recent weeks China has launched a set of satellites designed to support future missions to the surface. The largest of these was Queqiao-2, a relay satellite that will provide communications for the Chang’e-6 lander. That mission - which aims to return a sample from the far side of the Moon - will lift-off in May.

Two smaller satellites - Tiandu-1 and 2 - also made it to lunar orbit. Both aim to demonstrate technologies for navigation and communications on the Moon. If they are successful they could pave the way for a future constellation of satellites around the Moon offering services similar to GPS for future Chinese missions.

One other Chinese effort to reach the Moon seems to have ended in failure. A pair of satellites - DRO-A and B, launched on March 13, though Chinese media reported they had failed to reach the intended orbit. Observers then saw the satellites raise their altitude, as though heading towards the Moon, but neither seems to have made it. What went wrong is unclear, but it seems the two satellites were supposed to test navigation systems around the Moon.