This view of a portion of the DREaM simulated galaxy catalog provides a snippet of sky that might correspond, statistically, with what James Webb expects to see. This particular snippet showcases an incredibly rich region of relative nearby galaxies clustered together, which could provide Webb with an unprecedented view of galaxies magnified by strong and weak gravitational lensing. Even this simulated view, however, omits the majority of galaxies: too faint and distant to even be seen with JWST, the Nancy Roman Telescope, or any hitherto proposed present or future mission.
(Credit: Nicole Drakos, Bruno Villasenor, Brant Robertson, Ryan Hausen, Mark Dickinson, Henry Ferguson, Steven Furlanetto, Jenny Greene, Piero Madau, Alice Shapley, Daniel Stark, Risa Wechsler)
The Universe is a vast place, filled with more galaxies than we’ve ever been able to count, even in just the portion we’ve been able to observe. Some 40 years ago, Carl Sagan taught the world that there were hundreds of billions of stars in the Milky Way alone, and perhaps as many as 100 billion galaxies within the observable Universe. Although he never said it in his famous television series, Cosmos, the phrase “billions and billions” has become synonymous with his name, and also with the number of stars we think of as being inherent to each galaxy, as well as the number of galaxies contained within the visible Universe.
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But when it comes to the number of galaxies that are actually out there, we’ve learned a number of important facts that have led us to revise that number upwards, and not just by a little bit. Our most detailed observations of the distant Universe, from the Hubble eXtreme Deep Field, gave us an estimate of 170 billion galaxies. A theoretical calculation from a few years ago — the first to account for galaxies too small, faint, and distant to be seen — put the estimate far higher: at 2 trillion. But even that estimate is too low. There ought to be at least 6 trillion, and perhaps more like 20 trillion, galaxies, if we’re ever able to count them all. Here’s how we got there.
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role.
The first thing you have to realize about estimating the number of galaxies in the Universe is that the part of the Universe we can see — both today and ever, even into the infinite future — is and will always be finite. The Universe, as we know and perceive it, began with the hot Big Bang some 13.8 billion years ago. With some 10
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Atoms within it, about five times as much mass in the form of dark matter, as well as billions of times as many photons and neutrinos, gravitation has had plenty of time to pull the matter into clumps, collections, groups, and clusters. This has led to the formation of stars and galaxies with a variety of different properties: masses, sizes, brightnesses and more.
But what’s most important to realize is that the amount of “stuff” in the Universe that we can see is limited by three factors:
The galaxies we see today are rich, large, massive and evolved, with many being mere components in large collections of matter: groups, clusters, and even larger-scale structures. But the galaxies we see from far away — from earlier epochs in our cosmic history — are more isolated, smaller, less massive, and more irregular. If we want to estimate how many galaxies we can see today, we need to understand how the Universe has grown up over the entirety of its cosmic history.
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Galaxies comparable to the present-day Milky Way are numerous throughout cosmic time, having grown in mass and with more evolved structure at present. Younger galaxies are inherently smaller, bluer, more chaotic, richer in gas, and have lower densities of heavy elements than their modern-day counterparts, and their star-formation histories evolve over time. This was not discovered or well-known until the 1960s, when we began to see large numbers of galaxies from much earlier in our cosmic history.
The second thing you have to realize is that, no matter what sort of telescope we construct, we’re never going to be able to identify and count all of the galaxies present within the entirety of the observable Universe. All objects have an inherent brightness to them, and in order for us to be able to observe them, we need to collect enough photons from them so that they stand out from the cosmic background of other objects, as well as the noise inherent to our instruments. We also need to be able to resolve them as independent galaxies, with their own stellar populations, even when they’re close to, or along the same exact line-of-sight as, other, larger, brighter galaxies.
All you can do is practically make the observations your instruments (and allotted observing time) allow you to make, and to use what you know about the laws that govern the Universe to fill in what must lie beyond the current observational frontiers.
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Various long-exposure campaigns, like the Hubble eXtreme Deep Field (XDF) shown here, have revealed thousands of galaxies in a volume of the Universe that represents a fraction of a millionth of the sky. This image contains 5, 500 galaxies, but takes up just 1-32, 000, 000th of the total sky. But even with all the power of Hubble, and all the magnification of gravitational lensing, there are still galaxies out there beyond what we are capable of seeing.
The above image is the Hubble eXtreme Deep Field: humanity’s deepest view of the Universe, ever. Combining observations from many different wavelengths that span the ultraviolet, optical, and near-infrared portions of the electromagnetic spectrum, it represents a cumulative total of 23 days of observing time.
Within this tiny region of sky, which covers just 1/32, 000, 000th of all the space that’s accessible to us, we can observe a tremendous number of galaxies at a variety of distances from us. These include:
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When we count them all up, irrespective of where we found them or what properties they possess, we find there are 5, 500 uniquely identifiable galaxies within this tiny region of sky. If we extrapolate what we’ve seen in this tiny region as though it were “typical, ” we’d find that over the entire sky, we expect there to be 170 billion galaxies contained in the observable Universe.
Although some regions of space are rich in nearby galaxies while others are relatively poor, each proverbial slice of the sky allows us to grab objects of all different distances so long as our observations are sensitive enough to reveal them. The nearest, brightest objects are the easiest to resolve, but the entire cosmic story is told across the entire sky, and must be observed deeply and across many wavelengths in order to truly reveal the full extent of what’s out there.
Of course, we shouldn’t take that to be an estimate of the number of galaxies in the Universe; we should treat that number as a lower limit. There need to be at least as many galaxies as we can infer from what we’ve already observed, but there ought to be more. Out there, beyond the limit of what our best telescopes have been able to expose to us, should be the galaxies that are too small, too faint, too distant, or too obscured for us to see just yet.
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In the absence of the necessary data, it only makes sense to run simulations — based on the known contents of the Universe — to infer what the populations of these missing objects ought to be. By combining what we’ve already observed with the behavior of dark matter and normal matter on all scales, as well as a knowledge of galactic assembly and the history of large-scale structure formation, we should be able to make informed inferences about what else is out there.
Although the very faint end of the galactic spectrum is the most uncertain (i.e., where the smallest, lowest mass galaxies are), this technique has been leveraged over the past few years to produce a superior estimate: that there are 2 trillion galaxies out there in the observable Universe alone.
The size of our visible Universe (yellow), along with the amount we can reach (magenta) if we left, today, on a journey at the speed of light. The limit of the visible Universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. Anything that occurs, right now, within a radius of 18 billion light-years of us will eventually reach and affect us; anything beyond that point will not. Each year, another ~20 million stars cross the threshold from being reachable to being unreachable.
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