GRAVITY+: The next leap forward for the VLTI

Over the past few decades, we’ve seen a flurry of astronomical discoveries, from the first discovered (and the first imaged) exoplanet to unveiling the supermassive black hole at the centre of our galaxy. These advancements in what we know about our Universe have been enabled by massive improvements in the technology we use to observe the cosmos, including ESO’s VLT, with its ever-changing suite of instruments and capabilities.

Despite its singular name, the VLT comprises not one but eight telescopes. Its four larger telescopes — the Unit Telescopes, or UTs — each have main mirrors 8.2 metres across. Four smaller telescopes, known as the Auxiliary Telescopes or ATs, have 1.8-metre mirrors, and, unlike the UTs, they can be moved along rails to different positions.

Light from either the ATs or the UTs can be combined through interferometry, forming the Very Large Telescope Interferometer (VLTI). It enables astronomers to observe distant objects with a resolution equivalent to a telescope of over 100 metres in diameter.

But lightwaves from a given cosmic object do not arrive exactly at the same time at each telescope, so this delay needs to be compensated with extreme precision before the light beams can be combined interferometrically. In the VLTI, lightwaves are guided through a system of underground tunnels, called delay lines, and directed with mirrors moving along rails. With this system, the incoming signals can be fine-tuned to an astonishing precision of one thousandth of a millimetre.

The VLTI instruments are where the light beams collected by each telescope finally meet and interfere. One of them is GRAVITY (now being upgraded into GRAVITY+) which looks at the near-infrared part of the spectrum. By combining the light of four VLTI telescopes and working its own extra magic to improve image resolution, GRAVITY allows astronomers to see small details on faint objects, revealing a part of the Universe previously out of reach.

The VLTI at work. The light collected by either the large Unit Telescopes or, as shown here, the smaller Auxiliary Telescopes, is redirected through underground tunnels into the VLTI lab –– the flat building at the centre of the platform. There, the light beams are combined by different instruments such as GRAVITY.

Credit: ESO/L. Calçada

Science with GRAVITY

The GRAVITY instrument –– led by Frank Eisenhauer, director at the Max Planck Institute for extraterrestrial Physics (MPE) in Garching, Germany –– has been operating since 2016. It was originally designed to precisely monitor over time the positions of stars around Sagittarius A*, the supermassive black hole at the centre of the Milky Way. In doing so, GRAVITY allowed for several groundbreaking discoveries. It tracked S2, one of the stars orbiting the black hole, whose orbit is shaped like a rosette, not an ellipse, confirming a prediction in Albert Einstein’s general theory of relativity. The light of S2 was also found to be shifted to redder wavelengths when passing close to the black hole, another key prediction of general relativity.

GRAVITY has also found applications in other fields, like exoplanets, which are challenging to look at since the stars they orbit are much brighter and very close to them. The individual UTs have directly imaged exoplanets and studied their atmospheres, but precisely tracking them very close to their host stars requires interferometry. “Prior to GRAVITY, it wasn’t possible to study exoplanets with optical interferometry,” says Laura Kreidberg, the managing director of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and co-investigator of GRAVITY+. And yet, “the VLTI turned into an exoplanet machine thanks to GRAVITY,” says Julien Woillez, GRAVITY+ project scientist based at the ESO headquarters in Garching. The instrument has enabled precise measurements of exoplanets’ masses, orbits and atmospheres, and may help us find unseen companions and moons.

These four images were obtained with GRAVITY over several years, and show several stars orbiting very close to Sgr A*, the supermassive black hole at the centre of the Milky Way. One of these stars, named S29, was observed as it was making its closest approach to the black hole at 13 billion kilometres, just 90 times the distance between the Sun and Earth. Another star, named S300, was detected for the first time in the new VLTI observations.

Credit: ESO/GRAVITY collaboration

The GRAVITY+ upgrade

While interferometry has allowed us to see things that would be otherwise impossible with a traditional telescope, it still has limitations. The individual mirrors making up the telescope array collect less light than an equivalently large single telescope, since the light that falls in the “empty space” in between these mirrors is lost.. Moreover, every time incoming light bounces off the mirrors guiding it through the delay lines, some of it is also lost. In other words, out of the two perks that large telescopes have –– detecting faint objects and small details ––, interferometry mostly gives us the latter. As a consequence, interferometry works best when observing very bright celestial objects.

To address these issues, a new upgrade to the GRAVITY instrument, called GRAVITY+, is currently being implemented. Like its predecessor, GRAVITY+ will combine the light of the VLTI’s telescopes using interferometry, but it will be assisted by updated technology, encompassing both improvements to the instrument itself and infrastructure upgrades for the VLTI.

The main problem when doing optical interferometry with ground-based telescopes is the disturbance produced by the Earth’s atmosphere. The turbulent movement of air in the atmosphere makes stars twinkle to our eyes, and it can blur the images obtained by a telescope. This is similar to how an object at the bottom of a swimming pool becomes wavy and distorted when viewed from the surface. Correcting these disturbances is essential to do optical interferometry.

We can do so with a technique known as adaptive optics, where the twinkling of a reference object is monitored in real time to measure, and correct for, atmospheric turbulence. The reference object needs to be bright, but finding a luminous star close to the actual scientific target is rare, thus limiting what can be observed.

Luckily, we can create artificial stars using lasers. These powerful beams of light excite sodium atoms 90 kilometres high in the atmosphere, creating a glowing artificial “star”. The VLTI currently relies on natural stars (while you may have seen that UT4 is equipped with lasers, they are used for that telescope only). As part of the GRAVITY+ upgrades, one laser will be installed in each of the other UTs. Not having to rely on bright natural stars to perform adaptive optics corrections will allow the VLTI to observe essentially every region of the southern sky.

Even after the blurring caused by the atmosphere is corrected by adaptive optics at each telescope, air turbulence still makes the light from an astronomical object arrive at each telescope at slightly different times. To fix this we need an extra step called fringe tracking, which synchronises these signals. But this again requires observing a bright reference object. A GRAVITY+ upgrade that has already been implemented allows fringe tracking with a reference object located far from the science target.

Without taming the atmosphere, the interference signal that results from combining the light beams captured by each telescope vanishes in a fraction of a second. The combination of improved adaptive optics and fringe tracking will allow the VLTI to look at cosmic objects for longer periods at a time, thus making it possible to observe fainter objects than ever before.

The GRAVITY instrument at the VLTI lab. In normal operations the instrument is completely closed, but here we see it partially open when it was being tested back in 2015.

Credit: MPE/GRAVITY team

New science with GRAVITY+

The GRAVITY+ upgrades will open up previously inaccessible lines of research. Astronomers will be able to use the VLTI to observe fainter objects, which were either too far from Earth or emitted too little light to be studied before with interferometry. “GRAVITY was first made for the [Milky Way’s] galactic centre, and GRAVITY+ will help us go fainter into the galactic centre,” says Fréderic Gonté, GRAVITY+ project manager at ESO. “But we are touching many more science subjects than planned at the beginning, opening several new windows.”

Antonia Drescher, a PhD student at MPE and part of the GRAVITY+ team, is excited about the implications for the Milky Way’s Sagittarius A* black hole. “Stars and hot gas orbit Sgr A*, thus providing the unique opportunity to test general relativity in the vicinity of a massive black hole,” she says. One example is measuring how fast the black hole spins. According to general relativity, spacetime around a spinning black hole spins too, altering the orbits of very close-by stars in a predictable way called Lense-Therring precession. A new GRAVITY+ observing mode with improved sensitivity is already helping to find stars close to the black hole to measure this effect. “It is super exciting to see how the instrumental upgrades improve the data we obtain,” Drescher says.

While the black hole at the centre of our galaxy still holds plenty of secrets, others farther away from Earth will also be perfect targets for GRAVITY+. To directly measure the mass of a black hole, astronomers track the movement of gas and stars around it — the faster these move, the more massive the central black hole is. Recently, a team led by Taro Shimizu, a senior scientist in the Infrared Astronomy group at MPE involved in some of the GRAVITY+ updates since 2019, used the upgraded fringe tracking of GRAVITY to measure the mass of a black hole so far away that light from its surroundings took 11 billion years to reach us. Just over a year ago, such a record-breaking measurement is what Shimizu was most excited about, “but now we have already achieved that!” he marvels.

The upcoming upgrades are set to keep breaking records. “Next, with the new adaptive optics and especially the laser guide stars on all UTs,” Shimizu says, “we will be able to observe many more [distant supermassive black holes] due to the significant increase in sensitivity and even potentially push to [larger distances] if we can find the right target. No other instrument or facility has or will have this ability.”

GRAVITY+’s improved sensitivity will also let it look at other elusive objects, like exoplanets. For Kreidberg, who studies exoplanets’ atmospheres and ultimate habitability, GRAVITY+ will enable “directly imaging planets inaccessible to any other facility. We will learn a lot about planet formation with these detections!”

What’s next for GRAVITY and the VLTI

The GRAVITY+ upgrades are being rolled out incrementally, so as not to disrupt the VLTI’s operations. This adds an extra challenge, making the upgrade a feat of project management as well as engineering: “[You have to plan] everything nine months in advance, because you’re requesting operational time” says Gonté. “[And] after you implement everything [you must ensure] you still have the same performance [as before] or better.”

The GRAVITY+ project began in 2022 and Gonté estimates it is already about two-thirds complete, with the upgrades expected to be finished by early 2026. The project is a cooperation between ESO and a consortium led by MPE. For Gonté, this collaboration is the best thing about working on GRAVITY+: “The spirit inside the consortium, the willingness to perform well, the motivation, it’s really exceptional,” he says. Woillez agrees: “The people working on the project [all] dream of the same thing. That direction and that common shared purpose, that is it for me.”

The next big astronomical discovery might come from somewhere unexpected, and there are ongoing efforts to make the VLTI and optical interferometry more accessible for the broader astronomical community. “It’s the revolution we’ve been waiting for quite some time and it’s a big pleasure to see it happening now,” Woillez says. “The good thing when you start having a [broader] community, they are more aware of the science opportunities and (...) bring different visions on where the science should go. And that’s when they [can] meet people who build instruments and the spark happens.”