Ask Ethan: What puzzles can the Vera Rubin Observatory help solve?

Out there in the Universe, there are both discoveries just waiting to be made and cosmic puzzles just waiting to be solved. We have an incredibly robust picture of our cosmos, at present. We know the hot Big Bang marked the beginning of our Universe as we know it some 13.8 billion years ago, set up by a preceding period of cosmic inflation that seeded the Universe with fluctuations that would eventually grow into stars, galaxies, and the cosmic web. We know that, today, our Universe is still expanding and cooling, and isn’t just full of normal matter and radiation, but large amounts of dark matter and — for the last ~6 billion years — dark energy has been causing the expansion of the Universe to accelerate.

But even with the advances brought by cutting edge observatories like Hubble, ALMA, and JWST, much remains unknown. We still haven’t found the first stars. We don’t know why there’s more matter than antimatter in the Universe. We don’t have a resolution to the Hubble tension, or why different methods of measuring the expansion rate yield different, incompatible results. But now, there’s a new telescope that just unveiled its first images (and discoveries) to the world: the NSF’s Vera C. Rubin Observatory. What potential does it have to augment our understanding of reality? That’s a question on quite a few people’s minds, such as those of John Mears and Rodney Nourse, respectively, who want to know:

“Where will Vera Rubin observations fit in with those of JWST?”
“Will the Vera C. Rubin Observatory help reconcile the Hubble tension and, if yes, how?”

Every new observatory has its own specialization, and that includes the Vera C. Rubin Observatory. Let’s go through what makes it so spectacular, and then have that lead us into what discoveries it’s likely to make.

This very deep combined image shows the interstellar object ‘Oumuamua at the center of the picture. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving interloper. This image was created by combining multiple images from ESO’s Very Large Telescope as well as the Gemini South Telescope, but would have been automatically found in just a single night with the Vera C. Rubin Observatory.

Most of us, when we think about telescopes, think of them as big light-buckets: devices that collect light from a variety of sources, at the highest resolution that the optics will allow.

That’s only partially true, however. Telescopes also:

• can be located on the ground or in space,
• have a specific field-of-view, which can be wide-field (focused on breadth) or narrow-angle (focused on depth),
• take exposures for a specific amount of time, allowing them to probe objects down to a specific faintness (or optical magnitude),
• acquire data at a specific wavelength or over a specific range of wavelengths (certain classes of phenomena only emit/absorb light over specific wavelength ranges),
• and can return to the same location they’ve observed previously a certain amount of time later, looking for changes as part of the science of time-domain astronomy.

Depending on what it is you’re trying to discover, you’ll design your telescope to optimize it in certain ways, at the expense of having it be less good in other ways. For example, JWST is a space-based, narrow-angle telescope, designed for depth and precision at infrared wavelengths.

For the Vera C. Rubin observatory, its focus is on wide-field views, taking exposures for short periods of time, designed to cover as much of the sky as possible in short periods of time, so that it can focus on time-domain astronomy: the ability to detect changes in any imaged field from one moment to the next.

The 48-inch Samuel Oschin Telescope at Mt. Palomar is where the Zwicky Transient Facility (ZTF) takes its data from. Even though it’s only a 48″ (1.3 meter) telescope, its wide field of view and rapid observing speed allows it to discover optical changes in the night sky that practically every other observatory cannot find. The Vera C. Rubin Observatory, now operational in 2025, can be viewed as its (and Pan-STARRS’s, and SDSS’s) successor.

The Vera C. Rubin observatory isn’t the first observatory to focus on these goals.

Pan-STARRS, for example, is a wide-field telescope that scans the sky repeatedly, looking for transient events, objects, and rapid changes in brightness, covering about ¾ths of the entire sky with each set of observations. Pan-STARRS has discovered more near-Earth asteroids than any other telescope and was once the world leader in terms of the number of pixels in its camera. Unlike Rubin, however, Pan-STARRS is a much smaller telescope, meaning that its views are restricted to significantly brighter objects and come in at much lower resolution than Rubin’s.

The Zwicky Transient Facility is a wide-field observatory, notable for its extraordinary camera, that images the entire northern sky every two days. It has detected an enormous number of cosmic events such as supernovae, tidal disruption events, and even kilonovae that light up the sky where no light (or much less light) was visible previously. Its wide-field, high resolution camera is also extraordinary.

There’s also the Sloan Digital Sky Survey (SDSS), which is another wide-field observatory with an extraordinary camera. What’s remarkable about SDSS compared to either Pan-STARRS or the Zwicky Transient Facility is that SDSS has all the necessary equipment to perform spectroscopy, allowing astronomers to see changes in an object’s light across a variety of different wavelengths at once.

These observatories, in many regards, can be viewed as forerunners, or predecessors, to the Vera C. Rubin Observatory.

This image from the Vera C. Rubin Observatory’s first look observations shows the galaxy pair NGC 4411, at lower right, which are far apart in 3D space and not interacting, along with the galaxies of RSCG 55, higher in the image, which are interacting.

In fact, one can easily argue that these earlier endeavors actually enabled the Vera C. Rubin Observatory’s design and construction: they were the proof-of-concept that such an observatory could be built: that the type of science we wanted to conduct with it was possible. The huge advantages that the Vera C. Rubin Observatory possesses over all of these earlier endeavors are as follows:

It has a much, much larger primary mirror than all the others: an 8.4-meter diameter mirror, competitive with the largest aperture optical telescopes on Earth today, allowing it to gather more light and image the Universe at higher resolution.

It has the most complex and sophisticated telescope mount ever designed, optimized for speed and to move the heavy, large optics of the observatory exactly as needed. The dome and the optical mount are separately motorized, so that even though the dome can’t move as quickly as the telescope can to the next position in the sky, the dome can begin rotating while the telescope is still completing its observations in its current position, then the telescope can begin moving and can begin taking its next observations while the dome completes its motions.

And the Vera C. Rubin Observatory also has the world’s largest, most sensitive camera to photograph the Universe with: a whopping 3200 megapixel camera, capable of imaging each field-of-view in one (or all) of six different filters, including three visible light and three infrared light filters.

This image, from Vera C. Rubin Observatory’s first release, shows the Lagoon (large, Messier 8) and Trifid (small, Messier 20) nebulae in our own Milky Way. Below, the data from three different Rubin filters of the Trifid Nebula are shown, along with a composite image derived from combining that data.

This leads to stunning, wide-field, deep, nearly all-sky images that the Vera C. Rubin Observatory can acquire. In fact, if you saw the first images released to the public by the observatory, this is likely what they focused on: known objects imaged so quickly in great detail, highlighted in a variety of filters, colors, and at depths that have only rarely been achieved in the past. But if this is what you think the Vera C. Rubin Observatory is all about, you haven’t properly understood what’s so powerful about it. And that’s okay, because I used a phrase earlier that’s of incredible importance to astronomers, but that most members of the general public haven’t even heard before: time-domain astronomy.

Put simply, many astronomical objects change with time. These changes can occur for a variety of reasons.

Objects can change because they’ve moved in their apparent position as seen from our observatories on Earth. This is most common for Solar System objects, like asteroids, comets, Kuiper belt objects, or centaurs. It can also apply to the rare interstellar interloper, such as ‘Oumuamua or Borisov. When the team of astronomers working on the initial release of the Rubin data announced that they had discovered more than 2000 additional Solar System objects, almost all of which were asteroids in our own backyard (and zero of which are potentially Earth-threatening), this was the primary reason why.

This image, one of the first released by the Vera C. Rubin Observatory, shows two galaxies within the Virgo Cluster and a series of red-green-blue streaks: asteroids in our own Solar System that photobombed this distant vista. More than 2000 new asteroids were discovered like this during Rubin’s first night observing the Universe.

Objects can change because they brighten or fainten with time, such as variable stars, both in our own Milky Way and also in other relatively nearby galaxies. The surface brightness of galaxies can also fluctuate, and the high-precision optics of the Vera C. Rubin Observatory can detect this for a variety of galaxies as well. New variable stars, including of classes such as Cepheids, Delta Scuti stars, RR Lyrae stars, Mira variables, and many others will all be discovered by this observatory.

Occasionally, stars will simply wink out of existence, leaving no trace of a cataclysm or any sort of observable remnant behind. We have observed this occurring extremely rarely with telescopes like Hubble, but with the power of the Vera C. Rubin Observatory, we should expect to find many others. This will provide us with tremendous insights into the phenomena of direct collapse: where massive stars — stars that we would have expected would be destined for a supernova explosion — don’t detonate at all, but simply collapse under their own gravity to become a black hole directly. We’ve only ever found these objects with large spans of time, like years, between observations, but with the Vera C. Rubin Observatory, we should be able to identify them as soon as they wink out of existence, allowing us to look for other signals (electromagnetic, gravitational wave, or otherwise) that might correspond with their disappearance.

The visible/near-IR photos from Hubble show a massive star, at least 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. The direct collapse of this particular object, while still under investigation, may have been triggered by a stellar companion.

Within our own galaxy, we’ll get a special window into changes in stellar properties over time. Bright stars will exhibit detectable differences at the fraction-of-a-percent level in brightness, enabling us to identify variability that would have been beneath any previous threshold for observability. We’ll be able to witness and identify stellar eruptions and flares that are far less spectacular than the great dimming event that occurred recently on Betelgeuse, or small enhancements that are much fainter than the great supernova impostor event of the massive star Eta Carinae. These sorts of stellar fluctuations are expected to be common but often go undetected; the Vera C. Rubin Observatory will change that.

But the most spectacular use of the Vera C. Rubin Observatory will be to identify extragalactic cataclysms that occur all across the Universe. This includes:

• supernovae, including in distant, gravitationally lensed galaxies that will show up with multiple images,
• kilonova events, including events that occur in galaxies that are far away and often too faint to see with all previous generations of survey telescopes,
• active black holes, both of the stellar mass and supermassive varieties, that either turn on, turn off, increase in brightness, or decrease in brightness,
• tidal disruption events that are found at greater distances and with smaller optical magnitudes than anything that Pan-STARRS, the Zwicky Transient Facility, or even SDSS could identify,
• and, most excitingly, new classes of transient events that we don’t even know to look for yet.

With the Vera C. Rubin observatory, that means we’re going to have more opportunity for new discoveries, or what astronomers call discovery potential, because we have this telescope.

This timelapse series of radio images from the VLBA of the emissions around galaxy 1ES 1927+654 shows a significant brightening and “moving apart” of the radio features. The data indicates that these jets are outflowing at ~30% the speed of light. When observations in other wavelengths of light, such as those that are currently being acquired by the Vera C. Rubin Observatory, indicate new activity in a galaxy containing a supermassive black hole, follow-ups like this can reveal the launching of new radio jets.

It also offers a tremendous opportunity for synergy with other telescopes: both space-based and ground-based. You see, what the Vera C. Rubin Observatory will excel at is identifying those changing events — supernovae, kilonovae, black hole changes, tidal disruptions, etc. — pretty much as soon as they occur, and that will provide a “target of opportunity” for any other telescope to follow up on.

This includes a whole suite of other space-based and ground based observatories, depending on what it is that you’re seeing and what you think you can learn from it. When a supernova, kilonova, nova eruption, or tidal disruption event goes off, you can measure its brightness, the rise of its light-curve, its peak and its fall-off and decline, including by doing a pre-covery search (i.e., looking at data from before the progenitor star exploded) with Rubin alone. The same for changes in a black hole’s brightness. But with the ability to follow-up in:

• X-ray light, such as with Chandra,
• ultraviolet light, such as with Hubble’s STIS instrument,
• optical light spectroscopically, with one of any number of ground-based 10-meter-class telescopes,
• infrared light with JWST,
• or radio light with ALMA or other radio telescopes,

we can study so many of these events and phenomena in unprecedented detail, and with a rapid speed that would have been all but impossible before Rubin arrived on the scene.

This image shows a photometric JWST image of a host galaxy for a neutron star-neutron star merger, along with the location of the remnant of GRB 230307A, shown at the top left. The source is faint and barely detectable at bluer (shorter-wavelength) colors, but appears bright farther into the infrared. Many other transient events, including tidal disruption events, supernovae, and novae will be discovered by the Vera C. Rubin Observatory, enabling follow-ups with telescopes like JWST.

Finally, the Vera C. Rubin Observatory offers the possibility of providing insight into what might be today’s largest cosmic conundrum: the Hubble tension, or the debate/disagreement over how fast the Universe is expanding. It will be an outstanding way to:

• find new type Ia supernovae, the gold standard in standard candles at great cosmic distances, regardless of where in the sky they occur,
• find gravitationally lensed type Ia supernovae, enabling an independent test of the expansion rate,
• find new Cepheids and other variable stars, allowing us to calibrate the cosmic distance ladder more accurately and reduce the uncertainties in one major class of methods for measuring the expansion rate,
• to independently check many of the nearby stellar parallaxes inferred by the ESA’s Gaia mission, ensuring that there were no unaccounted for systematic errors,
• and to measure many properties of galaxies and stars over time, quantifying their brightnesses and their variabilities and any drift that occurs in those properties,

increasing statistics and reducing errors, and making us more sensitive to the rare but profound events that can help resolve one of the most controversial conundrums plaguing modern cosmologists today.

This image shows the galaxy cluster PLCK G165.7+67.0, as imaged by JWST in 2023. Contained within this image are many background galaxies that are gravitationally lensed by the foreground cluster, and a remarkable, triply-lensed supernova occurring in the large orange arc on the left side of the image. With nearly all-sky coverage and the ability to survey its entire field-of-view in just hours, the Vera C. Rubin Observatory could find many more such events.

But one other thing that I mentioned briefly deserves further emphasis: it gives us the ability to look for changes of any type in objects that we can see and identify. Will new, bright sources of light appear that represent something we’ve never seen before, like two brown dwarf stars merging to ignite into a red dwarf? Will we discover something about the flare rate of a variety of stars of different classes simply by monitoring so many of them so frequently with time? Will we discover new classes of stellar variability, because we’re looking at objects that are intrinsically fainter than we’ve seen before at high resolution and with fast return-to-the-same-object speeds? Will we discover something new and unexpected about satellites and satellite debris, even though they’re normally treated as pollution by astronomers, allowing us to better protect Earth? And will we find something, astronomically, that’s a complete surprise to us: something we don’t even know to anticipate finding at this point in time?

It’s easy to say, “based on what we know, we expect to find at least this many type Ia supernovae, this many kilonovae, this many tidal disruption events, this many new Cepheids, etc.,” because we already have good estimates (or lower bounds) on those types of event rates. We know what the Vera C. Rubin Observatory’s capabilities are, and that gives us the ability to estimate what it should be able to see based on where we’ve already looked. But the possibility of finding something novel and potentially revolutionary is part of the excitement of science: we don’t know the full extent of what we’re going to find until we look with these new tools and capabilities. If that excites you, you very likely have the spirit of curiosity needed to become a scientist yourself!

Send in your Ask Ethan questions to startswithabang at gmail dot com!