It’s a big universe out there. But with astronomers churning out noteworthy cosmic discoveries and insights each and every day, you might think we’ve somehow got it all covered, with the collective might of Earth’s telescopes giving us full situational awareness of the sky.
Nothing could be further from the truth. Despite the existence of all our advanced observatories, there are still parts of the electromagnetic spectrum (and beyond) that we’re not seeing and places where we need more (or any) telescopes.
By definition, the spectrum—that is, different kinds of light—is essentially infinite in range. But even so, the visible span of the spectrum from violet to red is only about a factor of two in wavelength, while the huge range from long-wave radio to gamma rays covers more than 20 orders of magnitude. So it shouldn’t be surprising that we don’t have it all covered.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
What’s more surprising, in fact, is just how much we have managed to cover! There are thousands of visible light telescopes in operation at any given time; I have a personal one I use myself when the bugs outside aren’t too bad. Professionally speaking, there are dozens of large observatories on the ground and orbiting above it, and quite a few next-generation facilities in the pipeline—including the soon-to-be launched Nancy Grace Roman Space Telescope, which will have the Hubble Space Telescope’s sharp vision coupled with a vastly larger field of view. And archival data are important to note, too, because most things in the sky don’t meaningfully change on human timescales, making thorough surveys still relevant even if they’re years or decades in the rearview.
For example, in infrared we had the Wide-Field Infrared Survey Explorer, which scanned the whole sky to give an overview, and, of course, we still have the James Webb Space Telescope giving us the sharpest, deepest views yet in that spectral range. The Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck observatory mapped the sky in microwaves; today the Atacama Large Millimeter/Submillimeter Array (ALMA) covers smaller wavelengths. And overall there are almost as many operational radio telescopes as there are visible-light ones.
At the other end of the spectrum, the Galaxy Evolution Explorer (GALEX) surveyed the sky in ultraviolet, and Hubble has two UV cameras still in operation. Several orbiting telescopes detect x-rays, including the venerable Chandra X-ray Observatory, XMM-Newton, Neil Gehrels Swift Observatory, and more. Even gamma rays get their day in the sun (so to speak), with the Fermi Gamma-Ray Space Telescope and Swift still operating and producing amazing data.
There are some holes in our coverage, but even those have proposals to fill them. One of the most glaring gaps lies between the infrared and millimeter-wavelength radio observations, but the Probe Far-Infrared Mission for Astrophysics (PRIMA) would fill much of it. Another gap exists for radio waves with wavelengths of 10 meters or more, which are reflected by Earth’s ionosphere; to observe these, astronomers have proposed building radio telescopes on the moon’s far side. One, called the Lunar Crater Radio Telescope, would be a staggering kilometer across. Such telescopes would be sensitive to radio waves emitted by gas from the cosmic “Dark Ages,” the period a few hundred million years long after the big bang but before the first stars were born, an era we know very little about.
And even for the parts of the spectrum already thoroughly covered, it’s not necessarily greedy to still want more! Different telescopes have different functions. Some look at wide areas of the sky to do surveys, while others pinpoint specific targets; some take images, while others take spectra, dividing the incoming light into different energies (or colors, wavelengths or frequencies, all of which are different terms for essentially the same thing). Such spectroscopy is a powerful technique for in-depth studies of celestial objects, capable of revealing their rotation, motion, composition, distance, and much more. I think it self-evident that the more telescopes we have, the better we can understand the universe.
But focusing on gaps in our coverage of the spectrum can cause us to ignore other viable areas of observation.
For one, we have a bias toward studying light. But other cosmic messengers exist.
For example, accelerating masses create gravitational waves, literal ripples in the fabric of spacetime. For the vast majority of objects in the universe, these waves are too mushy to detect, but very massive objects accelerating very rapidly give off much more sharply defined waves. Black holes, in particular, are amenable to this approach, all the more so because they don’t directly emit any light at all.
The Laser Interferometer Gravitational-Wave Observatory (or LIGO) detected the first such waves in 2015, recording the otherwise invisible merger of two stellar-mass black holes. It was an extraordinary achievement; Albert Einstein predicted the existence of gravitational waves, but it took technology a century to catch up to his calculations. Several other similar observatories have come online since then to glimpse hundreds of additional events, but all this activity represents a narrow range of gravitational waves—those created when neutron stars or relatively small black holes collide.
The European Space Agency’s Laser Interferometer Space Antenna (LISA), planned for launch in 2035, will detect the much longer gravitational waves created when mammoth supermassive black holes spiral together and collide. Such collisions are thought to be the most energetic events in the known universe, but we still know very little about them. Consisting of three separate spacecraft separated by 2.5 million kilometers, LISA is too big and too sensitive for our small, noisy planet—which is why, of course, it must be put in space.
Dark matter is another problem area. We know it exists and is responsible for shaping much of the structure in the universe, but it emits no light and apparently doesn’t interact at all with normal matter except through gravity. We can detect it indirectly in the faraway universe via gravitational lensing and other methods, but we still have no way of detecting it directly right here on Earth, even though dark matter particles are presumably streaming through you and everything else on the planet as you read this! We’re still not even sure, in fact, if dark matter is a particle at all. Not one of the many experiments that have attempted to spot such particles have unequivocally found them. And, more broadly, this is all part of a rich and growing field in which our “telescopes” are detectors studying neutrinos, fragments of atomic nuclei and other nonelectromagnetic celestial emissaries.
But there’s still more we cannot see, and it may surprise you: we have vast gaps in the knowledge of our own solar system! The region out past Neptune is populated by billions of icy, rocky bodies called trans-Neptunian objects (or TNOs) left over from the solar system’s formation. Only a few thousand are known, however. They’re incredibly faint and difficult to find. The Vera C. Rubin Observatory should discover tens of thousands of them, which will hopefully allow astronomers to classify them better and get a firmer grasp on what the solar system was like in its infancy. And Rubin will discover much more than TNOs, too, by virtue of its emphasis on time-domain astronomy—the study of objects such as asteroids, novae, supernovae and active galaxies that move and vary in brightness. Although Rubin just takes visible-light images, the ability to show us the change in those images is where its real power lies.
Our more “local” limits aren’t just in the outer solar system, either; we also don’t know that much about the region near the sun. The Parker Solar Probe has been repeatedly dive-bombing the sun ever since its launch in 2018 to measure the solar environment very close to our star’s surface for the first time. Somewhere in that scarcely explored vicinity sunward of Mercury, there could be a population of small asteroids 100 meters to six kilometers in diameter; called vulcanoids, they would be too close to the sun’s mighty glare for us to easily see from Earth. If their existence is ever confirmed, they would tell us a lot about the evolution of the solar system.
We also currently can’t look for potentially hazardous asteroids coming from inside Earth’s orbit for the same reason, but NASA’s Near-Earth Object Surveyor, due to launch in 2027, will park itself in a gravitationally stable position about a million kilometers closer to the sun than Earth to look for asteroids as close as 45 degrees in the sky to our star. The plan is to catalog two thirds of the asteroids larger than 140 meters across in that volume of space.
The universe starts right over your head and continues onward for a very long way. We humans have a pretty decent view of it, one we take advantage of to learn about our origins and cosmic environment. And while there are certainly gaps in our view, we have a pretty good idea of where they are, and we should be doing our best to fill them.
