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.
The spectrum—that is, different kinds of light—is essentially infinite. But the visible portion of the spectrum, from violet to red, includes a wavelength range that differs by only about a factor of two from one end to the other, whereas the huge range from long-wave radio to gamma rays spans more than 20 orders of magnitude. So it shouldn’t be surprising that we don’t have it all covered.
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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 one I use myself when the bugs outside aren’t too bad. In the professional realm, there are dozens of large observatories on the ground and orbiting Earth, as well as 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. Archival data are important to note, too, because most things in the sky don’t meaningfully change on human timescales, making thorough surveys durably 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 entire sky to give an overview, and of course, we have the James Webb Space Telescope giving us the sharpest, deepest views yet in that spectral range. The Wilkinson Microwave Anisotropy Probe and the Planck observatory mapped the sky in microwaves; today the Atacama Large Millimeter/submillimeter Array covers shorter wavelengths. And overall there are almost as many operational radio telescopes as there are visible-light ones.
One of the most glaring gaps lies between the infrared and millimeter-wavelength radio observations.
At the other end of the spectrum, the Galaxy Evolution Explorer 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, the 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 people have proposed ways 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 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 farside. 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”—a period of a few hundred million years after the big bang but before the first stars were born—which we know very little about.
And even for the parts of the spectrum already thoroughly covered, it’s not necessarily greedy to want more. Different telescopes have different functions. Some look at wide areas of the sky to do surveys; others pinpoint specific targets. Some take images; others detect 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 and distance, among many features. I think it is 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 instance, we have a bias toward studying light. But other cosmic messengers exist.
One example is gravitational waves, literal ripples in the fabric of spacetime, created by accelerating masses. For the vast majority of objects in the universe, these waves are too mushy to detect, but massive ones accelerating very rapidly give off much more sharply defined waves. Black holes are quite amenable to this approach, all the more so because they don’t directly emit any light at all.
The Laser Interferometer Gravitational-Wave Observatory detected the first such waves in 2015, recording the otherwise invisible merger of two stellar-mass black holes. It was an extraordinary achievement. Although Albert Einstein predicted the existence of gravitational waves in 1915, 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, yet we 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 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 not even sure, in fact, that dark matter consists of particles at all. Not one of the many experiments attempting to spot such particles has found them unequivocally. More broadly, all of this research is 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 huge 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 (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 astronomers hope will allow them to classify these objects better and get a firmer grasp on what the solar system was like in its infancy. And Rubin will discover much more than TNOs by virtue of its emphasis on time-domain astronomy—the study of objects such as asteroids, novae, supernovae, and/or active galaxies that move and vary in brightness. Although Rubin takes only visible-light images, its ability to show us the changes 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. Since its launch in 2018, the Parker Solar Probe has been repeatedly dive-bombing the sun to measure the environment very close to our star’s surface for the first time. Somewhere in the 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 them from Earth. If their existence were confirmed, they would tell us a lot about the evolution of the solar system.
For the same reason, we currently can’t look for potentially hazardous asteroids coming from inside Earth’s orbit. 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. Although 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.