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There Are More Galaxies In The Universe Than Even Carl Sagan Ever Imagined

Even in only the fraction, we have been able to observe, the Universe is a massive realm with more galaxies than we have ever been able to count. About 40 years ago, Carl Sagan introduced the idea that there were up to 100 billion galaxies in the observable Universe and hundreds of billions of stars in the Milky Way alone. The term "billions and billions," which he never mentioned in his well-known television series Cosmos, has come to be associated with his name as well as the number of stars we typically associate with each galaxy and the total number of galaxies visible in the visible Universe.


However, we have discovered a number of significant facts that have caused us to increase that estimate of the number of galaxies by a significant amount. We estimated that there are 170 billion galaxies in the distant Universe based on the Hubble eXtreme Deep Field's most thorough observations. The estimate was far higher, at 2 trillion, according to a theoretical calculation made a few years ago, which was the first to take into consideration galaxies that are too small, faint, and far away to be seen. Even still, it's a modest estimate. If we can ever count them all, there should be at least 6 trillion galaxies and possibly as many as 20 trillion. How did we get there?

In order to estimate the number of galaxies in the universe, you must first understand that the portion of the universe that is visible to us today and in the endless future is and will always be finite. The hot Big Bang, which occurred 13.8 billion years ago, was the start of the universe as we know and understand it today. Gravitation has had plenty of time to gather the matter into clumps, collections, groups, and clusters because it contains roughly 1080 atoms, about five times as much mass in the form of dark matter, billions of times as many photons and neutrinos, and approximately five times as many photons. Due to this, stars and galaxies with various masses, sizes, brightnesses, and other characteristics have formed.

However, it's crucial to understand that there are three constraints on the quantity of "stuff" in the universe that humans can observe:


  • the limited period of time since the Big Bang
  • light's limited speed 
  • and the characteristics of the universe's expansion over time, from the Big Bang to the present.


The galaxies we see now are rich, big, massive, and evolved, with many of them serving as minor parts of far larger structures made of enormous collections of matter. The galaxies we observe from a great distance, however, are smaller, less massive, and more irregular than those from previous periods in the history of the universe. We need to comprehend how the Universe has evolved throughout the course of cosmic history if we are to estimate the number of galaxies that are visible at this time.


Galaxies comparable to the present-day Milky Way are numerous throughout cosmic time, having grown in mass and with more evolved structures 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. (Credit: NASA, ESA, P. van Dokkum (Yale U.), S. Patel (Leiden U.), and the 3-D-HST Team)

The second thing you need to understand is that no matter what kind of telescope we build, we will never be able to identify and count every single galaxy that is there throughout the entire observable Universe. While all objects are naturally brilliant, in order to see them, we must gather enough photons from them to make them visible against the cosmic background of other objects and the noise generated by our sensors. Even when they are near or in the same direct line of sight as other, larger, brighter galaxies, we still need to be able to discern them as separate galaxies with their own star populations.

From a practical standpoint, at least, this is an impossible endeavor. Building a telescope that is:

It must be indefinitely huge (to provide the required resolution), cover all visible light wavelengths simultaneously (to account for the unavoidable cosmological redshift), have an infinite field of view, and observe the entire sky for an arbitrary duration of time (to reveal the faintest objects).

All you can do is essentially make the observations your tools (and given observing time) enable you to make, and then utilize what you already know about the rules of the universe to fill in the gaps as to what must lie beyond the boundaries of existing observational capability.


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. (Credit: NASA/ESA/H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (ASU), and Z. Levay (STScI))


The Hubble eXtreme Deep Field is the most in-depth vision of the universe ever created by humans. It represents a total of 23 days of observing time, combining observations from several various wavelengths spanning the ultraviolet, optical, and near-infrared regions of the electromagnetic spectrum.

We can observe a huge number of galaxies at various distances from us inside this minuscule area of sky, which only makes up 1/32,000,000th of the entire space that is accessible to us. These consist of:

Small, medium, and large galaxies that are nearby range in brightness from quite faint to extremely bright, medium-to-large galaxies that are in between, and very distant galaxies that represent the largest galaxies that were present at those early times have the highest brightnesses of any galaxies so far away, along with a few even further-away galaxies whose light has been stretched a little.

No matter where we located them or what characteristics they have, when we add them all together, we discover that this small patch of sky contains 5,500 galaxies that can be distinguished from one another. If we extrapolate what we've observed in this small area as though it were "average," we would discover that the observable Universe should consist of 170 billion galaxies throughout the entire sky.

Each figurative slice of the sky permits us to capture objects of all different distances as long as our observations are sensitive enough to expose them, despite the fact that some parts of space are rich in nearby galaxies while others are quite deficient in them. The closest, brightest objects are the simplest to resolve, but the complete cosmic story is told throughout the entire sky, and in order to properly disclose the full breadth of what's out there, deep observations across several wavelengths are required. (Credit: OmegaCAM/ESO/INAF-VST. OmegaCen/Astro-WISE/Kapteyn Institute is acknowledged.


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