# What fraction of galaxies in the observable universe have we actually observed?

There are a finite number of currently observable galaxies due to the finite age of the universe and the speed of light. What fraction of these galaxies have we actually observed (by eye, telescope, etc.)?

There are around 2 trillion galaxies in the currently observable universe according to the latest estimates, obtained by integrating theoretical galaxy stellar mass functions above $10^{6} M_{odot}$ between $0 leqslant z leqslant 8$.

It's difficult to get a precise number for the total observed galaxies as the results from new surveys are being released all the time, and updated analysis of legacy data is revealing more galaxies. For example, the latest SDSS data release identified close to 200 million galaxies, whilst the ongoing Dark Energy Survey seeks to identify around 300 million galaxies. LSST, in Chile, will observe close to 20 billion galaxies, an order of magnitude greater than anything before it. However, all of these surveys operate up to relatively low redshifts. Probing to higher redshifts requires bigger, preferably space based telescopes, and much longer exposure times. As such, the high redshift universe has been poorly documented. Our understanding of galaxy formation at high redshift is not as complete as at low redshift, and so we may be under- or over-estimating the number of galaxies at this epoch.

A vague upper limit: we've observed less than 0.01% of all currently observable galaxies.

The problem is that we don't really know how many small and diffuse galaxies there are. Even in our own cosmic backyard, the local group, we are still discovering new galaxies. Since these dwarf spheroidal galaxies are by far the most common, we have essentially observed a negligible fraction of all galaxies in the observable universe, and will never be able do increase this significantly.

## How Many Galaxies Are in the Universe? A New Answer From the Darkest Sky Ever Observed

Ordinarily, we point telescopes at some object we want to see in greater detail. In the 1990s astronomers did the opposite. They pointed the most powerful telescope in history, the Hubble Space Telescope, at a dark patch of sky devoid of known stars, gas, or galaxies. But in that sliver of nothingness, Hubble revealed a breathtaking sight: The void was brimming with galaxies.

Astronomers have long wondered how many galaxies there are in the universe, but until Hubble, the galaxies we could observe were far outnumbered by fainter galaxies hidden by distance and time. The Hubble Deep Field series (scientists made two more such observations) offered a kind of core sample of the universe going back nearly to the Big Bang. This allowed astronomers to finally estimate the galactic population to be at least around 200 billion.

Why “at least”? Because even Hubble has its limits.

The further out (and back in time) you go, galaxies get harder to see. One cause of this is the pure distance the light must travel. A second reason is due to the expansion of the universe. The wavelength of the light of very distant objects is stretched (redshifted), so these objects can no longer be seen in the primarily ultraviolet and visible portions of the spectrum Hubble was designed to detect. Finally, theory suggests early galaxies were smaller and fainter to begin with and only later merged to form the colossal structures we see today. Scientists are confident these galaxies exist. We just don’t know how many there are.

In 2016, a study published in The Astrophysical Journal by a team led by the University of Nottingham’s Christopher Conselice used a mathematical model of the early universe to estimate how many of those as-yet-unseen galaxies are lurking just beyond Hubble’s sight. Added to existing Hubble observations, their results suggested such galaxies make up 90 percent of the total, leading to a new estimate—that there may be up to two trillion galaxies in the universe.

Such estimates, however, are a moving target. As more observations roll in, scientists can get a better handle on the variables at play and increase the accuracy of their estimates.

Which brings us to the most recent addition to the story.

After buzzing by Pluto and the bizarre Kuiper Belt object, Arrokoth, NASA’s New Horizons spacecraft is at the edge of the solar system cruising toward interstellar space—and recently, it pulled a Hubble. In a study presented this week at the American Astronomical Society and soon to be published in The Astrophysical Journal, a team led by astronomers Marc Postman and Tod Lauer described what they found after training the New Horizons telescope on seven slivers of empty space to try and measure the level of ambient light in the universe.

Their findings, they say, allowed them to establish an upper limit on the number of galaxies in existence and indicate space may be a little less crowded than previously thought. According to their data, the total number of galaxies is more likely in the hundreds of billions, not trillions. “We simply don’t see the light from two trillion galaxies,” Postman said in a release published earlier in the week.

How did they arrive at their conclusion?

### The Search for Perfect Darkness

There is one more constraint on Hubble’s observations. Not only can’t it directly resolve early galaxies, it can’t even detect their light due to the diffuse glow of “zodiacal light.” Caused by a halo of dust scattering light within the solar system, zodiacal light is extremely faint, but just like light pollution on Earth, it can obscure even fainter objects in the early universe.

The New Horizons spacecraft has now escaped the domain of zodiacal light and is gazing at the darkest sky yet imaged. This offers the opportunity to measure the background light from beyond our galaxy and compare it to known and expected sources.

Postman told The New York Times that going an order of magnitude further wouldn’t have offered a darker view.

“When you have a telescope on New Horizons way out at the edge of the solar system, you can ask, how dark does space get anyway,” Lauer wrote. “Use your camera just to measure the glow from the sky.”

Still, the measurement was not straightforward. In an article, astrophysicist and writer Ethan Siegel, who was not part of the study, explains how the team meticulously identified, modeled, and removed contributions from “camera noise, scattered sunlight, excess off-axis starlight, crystals from the spacecraft’s thrust, and other instrumental effects.” They also removed any images too close to the Milky Way. After all this, they were left with the faint glow of the universe, and that’s the exciting bit.

The 2016 study predicted that a universe with two trillion galaxies would produce about ten times more light than the galaxies we’ve so far observed indicate. But the New Horizons team only found about twice as much light. This led them to their conclusion there are likely fewer total galaxies lurking out there than previously thought—a number closer to the original Hubble estimate.

“Take all the galaxies Hubble can see, double that number, and that’s what we see—but nothing more,” said Lauer.

### Star Gazing: The Next Generation

These observations from New Horizons aren’t the end of the story. Our ability to view the earliest universe should get a leg up this year when (fingers crossed) Hubble’s successor, the James Webb Space Telescope will launch and begin operations.

The JWST is set to observe in longer wavelengths than Hubble and is much bigger. These attributes should allow it to see even further back and image those smaller, fainter first galaxies. Like the Hubble Deep Field, if all is in working order, adding those galaxies to the census should give us an even clearer picture of the whole.

Whatever number scientists finally land on, it’s unlikely to be anything but mind-bogglingly huge. Even a few hundred billion galaxies means there’s an entire galaxy out there for every star in the Milky Way. Such research will undoubtedly cast even more light on cosmological questions about how the universe formed. But it will also beg the question: Amid the vast sea of galaxies, stars, and planets, are we really the only species to ever look out and wonder if we’re alone?

## Astroquizzical: How Can We See The Largest Amount Of The Smallest Universe?

Since light has a speed, the further out we look, the further back in time we look. But the further out we look, the greater the distance, and the "sphere" of observation is larger. But the further back in time you go, the smaller the universe as the universe is expanding. -- So, how can the farthest observational sphere be both the largest we see yet represent the smallest it was?

The Hubble Ultra Deep Field. Image credit: NASA, ESA, S. Beckwith and the HUDF Team (STScI), and B. . [+] Mobasher (STScI)

You’ve got the observational part of this spot on - at greater distances from Earth, we’re observing the universe at a point where it was physically smaller than it currently is. And, because light takes so long to get to us, the objects which we can observe back in time are the ones which are very distant from us. And, if we recall from a previous post on what the observable universe is, our most distant observable galaxies are in a shell surrounding us, which contains quite a large volume of space. So how do we manage to reconcile the fact that we’re seeing a lot of a very small universe?

If you’re familiar with redshift as a unit of distance, we can actually use that number to tell us about the size of the universe when the light from that object left its source and began its journey towards us. At a redshift of 2, we’re looking at a universe that is 1/3rd its current size. A redshift of 9 is 1/10th its current size. Effectively, add one to the redshift, and then make that into your fraction. (This math is a bit of a rough estimate, but it’s a good way to get the general scope of things in perspective).

If the universe is physically smaller, this means that the distances between galaxies are all smaller, and the entire universe is more dense than it currently is. But the critical thing to consider here is the volume of space we’re able to observe. Things that are very near us we can only see within a very small volume at greater distances we see a much larger volume of space. But if we’re headed for smaller total volumes as we go back in time, and the observed volume is going up, there’s only one way out. We are seeing a larger fraction of the Universe, as we look further back in time.

Our local environment is only a very small fraction of the current universe we expect the volume we see as our ‘nearby environment’ to be repeated many times over the course of the Universe’s total size (however large that might be), in pretty much every possible configuration of galaxies, no galaxies, and combinations of galaxies. As we look further and further back, we see a much larger fraction of the universe. Since we don’t know the total volume of the universe, we can’t really say how much that fraction changes, but it’s certainly a bigger number than for the nearby universe!

Being able to see a larger volume of space as we look further back in time is actually scientifically useful! If we look back and spot that there are a lot of galaxies which are sitting around in groups of 3 or 4 galaxies, we could reasonably conclude that those groups must be reasonably common, as we have a pretty good sample size to work with. Very nearby galaxies give us a much smaller set of galaxies to work with, since we have a smaller volume of space, so it’s harder to say how rare our local group of galaxies is, for instance.

The volume of space we’re able to see only helps us so far, though - ultimately we’re limited by the fraction of the Universe that we can see. If we weren’t limited by this, our studies of the universe would be very different!

## What is the size of the observable Universe?

If that's your conclusion, then you are moving backwards, not forwards, in your understanding.

First, the observations of the CMB are not in question. The fact that you personally have not yet looked into them does not justify you refusing to include them in what you based your belief on. If you want your beliefs to be accurate you need to look at all the available evidence. If you haven't looked at all the available evidence, the correct thing to do is not state a belief based on incomplete evidence, but to just say you haven't formed a belief yet because you haven't yet looked at all the available evidence.

Second, even ignoring the CMB and just looking at the farthest galaxies we have observed, you evidently have not looked at all the available evidence:

https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects
Third, note that that Wikipedia article says "light travel distance", which is just the light travel time multiplied by the speed of light. But as a number of posts in this thread have pointed out, that is not the same as either (a) the distance the object that emitted the light is from us now, or (b) the distance the object that emitted the light was from us when it emitted the light. So you need to make up your mind which of those distances you are interested in.

If that's your conclusion, then you are moving backwards, not forwards, in your understanding.

First, the observations of the CMB are not in question. The fact that you personally have not yet looked into them does not justify you refusing to include them in what you based your belief on. If you want your beliefs to be accurate you need to look at all the available evidence. If you haven't looked at all the available evidence, the correct thing to do is not state a belief based on incomplete evidence, but to just say you haven't formed a belief yet because you haven't yet looked at all the available evidence.

Second, even ignoring the CMB and just looking at the farthest galaxies we have observed, you evidently have not looked at all the available evidence:

https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects
Third, note that that Wikipedia article says "light travel distance", which is just the light travel time multiplied by the speed of light. But as a number of posts in this thread have pointed out, that is not the same as either (a) the distance the object that emitted the light is from us now, or (b) the distance the object that emitted the light was from us when it emitted the light. So you need to make up your mind which of those distances you are interested in.

So you insist on using distance at emission. Which will:
1) mean you will be talking about a different distance than anyone else who's talking about the size of the observable universe
2) confuse you, since the oldest observable galaxy was not the farthest at emission - every known galaxy that had emitted its currently-observed light earlier than 3 billion years ago was farther at emission than the 13.3 Gyr old one. This is another good reason to stick to distance at reception, since in that case you always get older=farther.
For example, should you include CMB (makes no sense to exclude it), your personal idea of how large the observable universe is would have to be corrected down towards 44 million light-years of radius. Even though, using the same measure of distance you adopted, it'd make nearly every other observable object, other than the nearest galaxies, to be ostensibly farther than the size of the observable universe.
Even if you'd insist on counting galaxies only, just another discovery of an even older galaxy would cause the size of the observable universe to shrink.
Does this make sense to you?

Perhaps you weren't confused about what you meant by "the size of the observable universe", but everyone else in this thread certainly was.

And if that's how you personally want to define "the size of the observable universe", that's fine as far as you personally are concerned. But as @Bandersnatch has pointed out, you are using a different definition from everyone else, which means every time you read anything at all about distances in cosmology, you are going to need to do the work of translating back and forth between your definition and everyone else's. Not to mention the other issues @Bandersnatch raised.

What you are not going to be able to do is just throw out numbers in a PF thread using your definition, without saying so, and expect everyone else to agree with your numbers, since everyone else is using a different definition.

After some help from various members on this forum I see where I went wrong on some of my assumptions. With this information and sticking within the definitions of scientific observation and observable universe I am reducing what I believe the size of the observable universe to a diameter less than 3 billion light years. My reasoning behind this conclusion is based off of the farthest galaxies we have observed. I realize cosmic background radiation should be used but unfortunately I haven't looked into how we are able to observe it.

The most distant Galaxy we have observed had a light travel distance of 13.3 billion light years. the light that we are currently observing which had that 13 billion light year travel distance was produced when this galaxy was only 2.7 billion light years away.

if you see anything wrong with my information please let me know. I've had to change my understanding many times before and obviously I'll need to change it many times in the future.

Before I go. I would really like to see what steps they took that got them to the 93 billion light year observable universe. I'm also interested if there is a map of the universe that shows where these galaxies were when the met at the light we are seeing.

Why would you be using the past size to mark the size of the observable universe?

Also, the distance you are using, which is the angular size distance, peaks at a certain point in the past, then decreases later. The peak angular size distance is just under 6 billion light years.

I think it's partially because most normal matter hasn't collapsed into galaxies. I think galaxies make up only about 10% of the normal matter.

Also, while there are some galaxies with truly massive SMBHs, most have quite small ones, such as our own Milky Way, whose SMBH has about 4 million solar masses, compared to a total galaxy mass of 600 billion solar masses.

Also relevant (suggesting 10s of thousand of black holes swarming around the SMBH at the center of the Milky Way.

Charles J. Hailey, Kaya Mori, Franz E. Bauer, Michael E. Berkowitz, Jaesub Hong, Benjamin J. Hord. A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy. Nature, 2018 556 (7699): 70 DOI: 10.1038/nature25029

I don't have a good intuition of the magnitude of these effects, only their direction. Basically, all of the estimates that I've seen are operating at the spherical cow level of approximation and I'm pretty comfortable that it would be possible do be much more accurate with only fairly modest research and calculation effort. I would think that a team of four post-docs working on the question for a year ought to be able to pin it down to perhaps +/- 10% or less with a high degree of confidence using very reliable and data rich methods.

While I don't have a good intuition for the magnitude of the potential factors, if an effect can only move in one direction as proportion of matter that is in black holes does, even very modest effects can add up over 13 billion plus years, so it makes sense to really be careful in evaluating every possibility. A very slow and infrequent process can easily make a factor of 2-4 difference over billions of years when the base number is already so small, and quantifying the frequency of very slow and infrequent processes is very difficult to do via direct measurement. That may be the difference between 0.2% and 0.6%, perhaps, but that's not nothing.

The annual percentage growth you need to double in mass over a billion years is really, really small (although the rule of 72 does not apply for numbers that extreme and is instead approximately linear at such a great extreme), specifically, about 0.7 parts per billion per year. Certainly, anything that causes mass growth at an average rate of one part per billion per year (about 4 * 10^21 kg per year for a small stellar black hole, which is less than five times the mass of dwarf planet Ceres) needs to be carefully considered. And, over 13 billion years, effects of less than one doubling per billion years can still be quite material, so the relevant threshold is quite a bit lower than that, more like 5* 10^17 kg per year, a bit bigger than asteroid Ida but smaller than asteroid Siwa). So it is easy to miss a potentially relevant effect.

In a spiral galaxy, stars at the core are closer to each other than those in the fringes, so an average distance between stars may not be a very accurate predictor, you'd really be more interested in knowing what percent are at a distance less than X at which the possibility of collision or formation of a gravitationally bound system is high and that percentage, even if only 1-5% say, that percentage might be quite a bit higher than what you would suspect making estimates of collision likelihood based upon mean distances even with a Gaussian distribution of distances around the mean. This would be of a piece with evidence that there are more black holes in the core of galaxies than in the fringes.

Also, the chance of stars absorbing substantial mass from other objects is probably greatest before a galaxy system is in equilibrium or when galaxies collider with each other, for example. A typical intermediate sized black hole, for example, might have a very punctuated history of mass acquisition with billions of years at a time of negligible change, in between several brief (by cosmological standards) periods of rapid mass acquisition when the gravitationally bound system that it is a part of goes out of equilibrium for some reason, leading to collisions of massive bodies.

I would think that indirect measurements - like inferring what sort of merger history you'd need to get the presently observed collection of supermassive black holes, are probably going to be more accurate than trying to estimate the numbers directly. The fact that we hadn't observed a single intermediate sized black hole ever until LIGO came on line, and now have observed not just several intermediate sized black holes, but several of them merging (which implies that this is the tip of the iceberg and that there must be far more that aren't merging and hence aren't seen by LIGO), at pretty modest distances from Earth as distances in the universe go, in just a few years, suggests to me that we may know less than we think we do about how many black holes there are out there. I'm not suggesting that intermediate size black holes make up 10% of the matter in the universe or anything like that, but intermediate sized black holes might very well be contributing more to the aggregate mass in black holes than supermassive ones do, and might be on the same order of magnitude or even a little bit more than the total population of stellar black holes. Whether that number is 0.02% or 0.1% or 0.3% of the matter in the universe has a pretty material effect on the total percentage of mass in the universe that is in black holes, given how low first order of magnitude estimates based upon direct collapse of stars into stellar black holes and supermassive black holes is to start with.

But, in principle, it doesn't seem like it would be so impossible to actually run numbers for a plausible merger history to produce the population of known supermassive black holes to get a much, much more accurate estimate of how much mass should be in intermediate sized black holes. Another useful endeavor might be to estimate how many intermediate black holes exist in the aggregate based upon the number of collisions that are observed by LIGO and other gravity wave detectors - figuring out how much total mass the observed tip of the iceberg implies does not seem like an impossibly ambitious enterprise.

I also haven't seen any really careful analyses of how much dark matter ends up in black holes, but, one often sees it described as interstellar matter streaming through many solar systems in a galaxy over time, rather than being rather static, and if this is the case, black holes should be absorbing a fairly steady flux of dark matter every single year. I don't have a good intuition of how that compares to the total volume of inferred dark matter (because a lot of dark matter is inferred to be in locations with dark matter halos that are far from any stars at all), but, in principle, it should be too hard to figure out by what percentage an average black hole grows every year by absorbing part of the passing flux of dark matter in its vicinity. In any given year, this has to be a tiny percentage of the total mass of a black hole. But, multiply that by a billion and maybe it adds up to something appreciable. I wouldn't be surprised to learn that a mature intermediate sized black hole has gained 20%-50% of its mass from absorbing dark matter (assuming that dark matter particles exist).

Similar estimates could be made for the mass-energy gain that a black hole receives from absorbing photons at various frequencies. Again, a small number no doubt, but not necessarily so small that it can be ignored until somebody has made a serious attempt to calculate it. The numbers I've seen for photon absorption by black holes suggest that even this tiny influx of mass-energy is sufficient to clearly exceed mass losses due to Hawking radiation for all stellar sized or larger black holes in the current universe.

Also, in addition to the actual percentage at any given time, I'm quite interested in the rate of change over time. On one hand, you have a percentage that is constantly increasing, but the rate of increase should be getting smaller with time as the expanding universe, in general, increases the average distance between masses and decreases the likelihood of events like colliding galaxies, and as local matter in the vicinity of existing black holes is exhausted. This is a classic asymptotic function. But, on the other hand, it can take billions of years for a star on a long term path towards a collapse into a black hole to get there, so to get the percentage change right you need to know something about how many stars of the right size were being formed X billion years earlier, so there might be pulses up and down around a purely asymptotic trend.

## “Beyond Comprehension” –‘The Observable Universe Is Only a Tiny Fraction of the Aftermath of the Big Bang’

“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what we will find when we observe these galaxies with the next generation of telescopes,” says astronomer Christopher Conselice, who led the team that discovered that there are ten times more galaxies in the universe than previously thought, and an even wider space to search for extraterrestrial life.

In 2016, astronomers using data from the NASA/ESA Hubble Space Telescopes and other telescopes performed an accurate census of the number of galaxies, and came to the surprising conclusion that there are at least 10 times as many galaxies in the observable universe as previously thought. The image itself was produced by the Frontier Fields Collaboration (a joint effort between NASA’s Hubble, Spitzer, and Chandra space telescopes) allowing scientists to detect galaxies that are as much as 100 times fainter than those independently captured before.

One of the most fundamental known unknowns in astronomy is just how many galaxies the universe contains. The Hubble Deep Field images, captured in the mid 1990s, revealed untold numbers of faint galaxies. It was estimated that the observable Universe contains between 100 to 200 billion galaxies.

The international team, led by Conselice from the University of Nottingham, UK, have shown that this figure is at least ten times too low.

Conselice and his team reached this conclusion using deep space images from Hubble, data from his team’s previous work, and other published data . They painstakingly converted the images into 3D, in order to make accurate measurements of the number of galaxies at different times in the Universe’s history.

In addition, they used new mathematical models which allowed them to infer the existence of galaxies which the current generation of telescopes cannot observe. This led to the surprising realization that in order for the numbers to add up, some 90% of the galaxies in the observable Universe are actually too faint and too far away to be seen — yet.

Because gravitational attraction is overwhelmed by a mysterious force latent in empty space that pushes galaxies apart from each other, all that the human species will be able to view after a hundred billion years, will be the dead and dying stars of our Local Group.

But these, say astronomer Martin Rees in On the Future, who was not part of Conselice’s team, “could continue for trillions of years—time enough, perhaps, for the long-term trend for living systems to gain complexity and ‘negative entropy’ to reach a culmination. All the atoms that were once in stars and gas could be transformed into structures as intricate as a living organism or a silicon chip—but on a cosmic scale. Against the darkening background, protons may decay, dark matter particles annihilate, occasional flashes when black holes evaporate—and then silence.”

We can only see a finite number of galaxies because there’s a horizon, a shell around us, delineating the greatest distance from which light can reach us. But that shell, observes Rees, “has no more physical significance than the circle that delineates your horizon if you’re in the middle of the ocean.”

In analyzing the data the team looked more than 13 billion years into the past. This showed them that galaxies are not evenly distributed throughout the Universe’s history. In fact, it appears that there were a factor of 10 more galaxies per unit volume when the Universe was only a few billion years old compared with today. Most of these galaxies were relatively small and faint, with masses similar to those of the satellite galaxies surrounding the Milky Way.

These results are powerful evidence that a significant evolution has taken place throughout the Universe’s history, an evolution during which galaxies merged together, dramatically reducing their total number. “This gives us a verification of the so-called top-down formation of structure in the Universe,” explains Conselice.

The decreasing number of galaxies as time progresses also contributes to the solution of Olbers’ paradox — why the sky is dark at night. The astronomer Heinrich Olbers argued that the night sky should be permanently flooded by light, because in an unchanging Universe filled with an infinite number of stars, every single part of the sky should be occupied by a bright object. However, our modern understanding of the Universe is that it is both finite and dynamic — not infinite and static.

The team came to the conclusion that there is such an abundance of galaxies that, in principle, every point in the sky contains part of a galaxy. However, most of these galaxies are invisible to the human eye and even to modern telescopes, owing to a combination of factors: redshifting of light, the Universe’s dynamic nature and the absorption of light by intergalactic dust and gas, all combine to ensure that the night sky remains mostly dark.

Astronomers are confident that the volume of space-time within range of our telescopes—‘the universe’—is only a tiny fraction of the aftermath of the big bang. “We’d expect far more galaxies located beyond the horizon, unobservable,” concludes Rees, “each of which (along with any intelligences it hosts) will evolve rather like our own.”

The Daily Galaxy via Hubble Space Telescope and Martin Rees On the Future

## Dark Energy Renders 97% Of The Galaxies In Our Observable Universe Permanently Unreachable

When you look out at a star whose light arrives after traveling towards you for 100 years, you're seeing a star that's 100 light years away, due to the fact that the speed of light is finite. But when you look out at a galaxy whose light arrives after traveling towards you for a journey of 100 million years, you're not looking at a galaxy that's 100 million light years distant. Rather, you're seeing a galaxy that's significantly farther away than that! The reason for this is that on the largest scales -- ones that aren't gravitationally bound together into galaxies, groups or clusters -- the Universe is expanding. And the longer the journey of a photon from a distant galaxy to you, the farther away a galaxy not only is, but the more distant it will actually be from the light-travel-time.

This shows up as a cosmic redshift. Since light is emitted with a particular energy, and hence a particular wavelength, we fully expect that it will arrive at its destination with a particular wavelength as well. If the fabric of the Universe were neither expanding nor contracting, but rather were constant, that wavelength would be the same. But if the Universe is expanding, the fabric of that space is stretching as shown in the video above, and hence the wavelength of that light becomes longer.

Image credit: Larry McNish of RASC Calgary Center, via http://calgary.rasc.ca/redshift.htm.

The amount that a distant cosmic source of light is redshifted gives us a window into how much the Universe has expanded since that light left its source for us. By measuring sources at a whole slew of distances, discovering their redshift and then either measuring their intrinsic vs. apparent size or their intrinsic vs. apparent brightness, we can reconstruct the entire expansion history of the Universe.

Image credit: NASA/JPL-Caltech, of standard candles (L) and standard rulers (R) for measuring cosmic . [+] distances.

In addition, since the way the Universe expands is determined by the various types of matter and energy present within it, we can learn what our Universe is made out of:

• 68% dark energy, equivalent to a cosmological constant,
• 27% dark matter,
• 4.9% normal (protons, neutrons and electrons) matter,
• 0.1% neutrinos and antineutrinos,
• absolutely nothing else, including no curvature, no cosmic strings, no domain walls, no cosmic textures, etc.

But once we know what the Universe is made out of to this degree of precision, we can simply apply this to the laws of gravity (given by Einstein's General Relativity), and determine what the future fate of our Universe is. What we discovered, when we first applied this to the discovery of a dark energy-dominated Universe, was appalling.

Image credit: NASA & ESA, via http://www.spacetelescope.org/images/opo9919k/.

First off, it meant that all the galaxies that weren't already gravitationally bound to us would eventually disappear from view. They would speed away from us at an ever-increasing rate as the Universe continued to expand and expand and expand, unchecked by gravitation or any other force. As time went on, a galaxy would get farther away, meaning that there was an increasing amount of space between that galaxy and ourselves. But since that space continues to expand at a finite, non-decreasing rate, the galaxy appears to speed up due to the expansion of space. In reality, neither we nor that galaxy are moving very fast at all, but the space between us continues to expand, causing it to recede from our view.

But there's an inevitable conclusion that this leads to that's even more disturbing. It means that, at a particular, key distance from us, the expansion of the fabric of space itself makes it so that a photon either leaving our galaxy towards a distant one or leaving a distant galaxy headed towards ours will never reach us. The expansion rate of the Universe is so great that distant galaxies become unreachable to our own, even if we were to move at the speed of light!

Image credit: NASA, ESA, and Z. Levay (STScI). The GOODS-North survey, shown here, contains some of . [+] the most distant galaxies ever observed, a great many of which are already unreachable by us.

At present, that distance is "only" about 15 billion light years away. If you consider that our observable Universe is some 46 billion light years in radius, and that all regions of space contain (on average and on the largest scales) the same number of galaxies as one another, it means that only about 3% of the total number of galaxies in our Universe are presently reachable to us, even if we left today, and at the speed of light.

It also means that, on average, twenty thousand stars transition every second from being reachable to being unreachable. The light they emitted a second ago will someday reach us, but the light they emit this very second never will. It's a disturbing, sobering thought, but there's also a more optimistic way to view it: this is the Universe reminding us how precious every second is. It's the Universe telling us that if we ever want to travel beyond our own local group -- beyond the gravitationally bound set of objects made up of Andromeda, the Milky Way and about 50 small, satellite galaxies -- that every moment we delay is another opportunity being lost.

Image credit: E. Siegel, based on work by Wikimedia Commons users Azcolvin 429 and Frédéric . [+] MICHEL.

97% of the galaxies in our observable Universe are already unreachable to us, but the 3% that are still reachable leave us with billions of options. It's up to us to act sooner, rather than later, if we ever hope to explore them.

An observable universe is just a patch of a larger whole that any particular observer can see. There are as many observable universes as there are observers (so, infinite).
Each observable universe is finite in extent.
But if the observers are not far from one another, like two people on Earth, then the difference in what they can see is imperceptibly small. So when we talk about the observable universe, we usually mean the extent of the larger universe that people on Earth can see.

That number is the time it took its light to reach us times the speed of light. It's not really distance in any common sense of the word. If you could stop the expansion today and take a ruler to measure where it is, you'd measure a significantly higher number. Distances in cosmology are a bit wonky, is what I'm saying.

I'll tackle the second one first. Whenever an object crosses a horizon, an observer who doesn't cross that horizon never actually sees the crossing happen: instead, they see an image of the object get redshifted more and more as time goes on. This is true whether you are talking about the event horizon of a black hole or the cosmological horizon. In essence, what happens is that the finite number of photons which were emitted by the object before it crossed the horizon get spread out in time infinitely into the future. Eventually those photons will redshift so much that they can't be detected, but there is no sudden disappearance.

The first question can be complicated when you start considering theories beyond the standard model, but in the simplest models the answer is simply yes: there's plenty of stuff beyond the observable universe.

I'll tackle the second one first. Whenever an object crosses a horizon, an observer who doesn't cross that horizon never actually sees the crossing happen: instead, they see an image of the object get redshifted more and more as time goes on. This is true whether you are talking about the event horizon of a black hole or the cosmological horizon. In essence, what happens is that the finite number of photons which were emitted by the object before it crossed the horizon get spread out in time infinitely into the future. Eventually those photons will redshift so much that they can't be detected, but there is no sudden disappearance.

The first question can be complicated when you start considering theories beyond the standard model, but in the simplest models the answer is simply yes: there's plenty of stuff beyond the observable universe.

This argument doesn't imply an infinite universe. It does, however, imply that there is no boundary to the universe. This could be the case for a finite universe that wraps back on itself (e.g. spherical or toroidal shape). Or it could be infinite.

"In a universe we can't observe" isn't a well-defined statement. The problem is that the bare term "universe" means "all that exists". Nothing that exists can be in another universe, because the universe is everything! A more precise statement is that they are outside of our observable universe, and our observable universe is only a fraction of the overall universe.

Also, for a sense of scale, it will take a couple trillion years for galaxies to redshift so much their light becomes undetectable.

"In a universe we can't observe" isn't a well-defined statement. The problem is that the bare term "universe" means "all that exists". Nothing that exists can be in another universe, because the universe is everything! A more precise statement is that they are outside of our observable universe, and our observable universe is only a fraction of the overall universe.

Also, for a sense of scale, it will take a couple trillion years for galaxies to redshift so much their light becomes undetectable.

That's what I meant. There's an overall universe or overall space with pockets of observable universes within that overall space. When you look at the 46.6 billion light year radius of the observable universe, Would an alien in MACS0647-JD galaxy see it's observable universe as 46.6 billion light years in each direction and would some of the space of it's pocket observable universe extend into a space outside of our pocket observable universe?

We know that two galaxies about 4,200 megaparsecs apart will be moving away from each other faster than the speed of light. This is about 13.7 billion light years. So there's more than enough room for objects to be more than 4,200 megaparsecs away from each other in there own observable sphere.

This is speculation but the age of the universe might point to the time we separated from another observable universe that was just like or similar to ours. I just read a really good paper:

From Planck Data to Planck Era: Observational Tests of Holographic Cosmology
Niayesh Afshordi, Claudio Corianò, Luigi Delle Rose, Elizabeth Gould, and Kostas Skenderis
Phys. Rev. Lett. 118, 041301 – Published 27 January 2017

"Imagine that everything you see, feel and hear in three dimensions, and your perception of time, in fact emanates from a flat two-dimensional field,” says Professor Kostas Skenderis from the University of Southampton.

“The idea is similar to that of ordinary holograms where a 3D image is encoded in a 2D surface, such as in the hologram on a credit card. However, this time, the entire Universe is encoded." Another way of simplifying this is through 3D films. Although not an example of a hologram, 3D films create the illusion of 3D objects from a flat 2D screen. The difference in our 3D Universe is that we can touch objects and the 'projection' is 'real', from our perspective.

Recent advances in telescopes and sensing equipment have allowed scientists to detect a vast amount of data hidden in the 'white noise' or microwaves left over from the moment the Universe was created. Using this information, the team was able to make comparisons between networks of features in the data and quantum field theory. They found some of the simplest quantum field theories could explain nearly all cosmological observations of the early Universe.

Wouldn't you expect data from the observable universe we're born from in this noise from the CMB because if this information is on the event horizon of a black hole from that universe, wouldn't it show up in the noise?

The patches depicted above grow with time, while A and B recede from each other.

You're putting too much weight on the Hubble distance - the distance where recession velocities reach the speed of light.
1. This number is only coincidentally close to the age of the universe times the speed of light
2. It doesn't mean that galaxies beyond it are unobservable

Causal patches of the universe can and do separate, but it's not equivalent to galaxies disappearing from sight, nor does it happen at one particular time (it's an ongoing process).

This post discusses how and in what sense do causal patches of observable universes separate, using light-cone graphs as a visual aid:
https://www.physicsforums.com/threa. increase-bc-of-expansion.912881/#post-5754083

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That's not true, is it? Due to the accelerated expansion of the universe, distant objects are moving outside our observable horizon all the time. Unless I am mixing up different cosmological horizons, those can get confusing. I don't think so though. In the far distant future our local group of galaxies, or at least the largest gravitationally-bound structure that we are part of, will be all that can be seen from the Milky Way. The rest of the galaxies will have accelerated away from us so much that they are beyond our observable horizon. They will have red-shifted into nothingness from our perspective.

Edit: Ok looking at the link you posted, it is the particle horizon you are talking about yes? This is a pretty loose definition of "will keep seeing it" I think. Light is quantized, so the number of photons arriving per second from these highly redshifted objects gets smaller and smaller. Eventually there will in fact be a time when no more photons arrive, and any "stragglers" will be so red-shifted that we could never detect them anyway.

The point is, even though the signals we'll be receiving will become more and more redshifted, there'll never be a time when we'll stop getting them. Crossing the cosmic event horizon now does not make the earlier light sent from the object disappear. The light from the horizon crossing event will arrive at the observer only after infinite time.
It's the same effect as with an event horizon of a black hole, that kimbyd mentioned in her post #3. There's some more about it in the post about causality and light-cones I linked above.

So the objects in question are never unobservable in principle, only in practice - due to redshift and faintness of the signal.

Yes, there's nothing controversial about that. Mostly because the other observable universes are just more of the same. Again, there's as many observable universes as there are observers.
This picture shows the situation you're describing:
View attachment 225845
The patches depicted above grow with time, while A and B recede from each other.

You're putting too much weight on the Hubble distance - the distance where recession velocities reach the speed of light.
1. This number is only coincidentally close to the age of the universe times the speed of light
2. It doesn't mean that galaxies beyond it are unobservable

Causal patches of the universe can and do separate, but it's not equivalent to galaxies disappearing from sight, nor does it happen at one particular time (it's an ongoing process).

This post discusses how and in what sense do causal patches of observable universes separate, using light-cone graphs as a visual aid:
https://www.physicsforums.com/threa. increase-bc-of-expansion.912881/#post-5754083

Thanks for the response and thanks for the visual. That's exactly what I'm saying with Barbara and Adam.

I do disagree on your second part though and I agree with kurros.

That's not true, is it? Due to the accelerated expansion of the universe, distant objects are moving outside our observable horizon all the time. Unless I am mixing up different cosmological horizons, those can get confusing. I don't think so though. In the far distant future our local group of galaxies, or at least the largest gravitationally-bound structure that we are part of, will be all that can be seen from the Milky Way. The rest of the galaxies will have accelerated away from us so much that they are beyond our observable horizon. They will have red-shifted into nothingness from our perspective.

Edit: Ok looking at the link you posted, it is the particle horizon you are talking about yes? This is a pretty loose definition of "will keep seeing it" I think. Light is quantized, so the number of photons arriving per second from these highly redshifted objects gets smaller and smaller. Eventually there will in fact be a time when no more photons arrive, and any "stragglers" will be so red-shifted that we could never detect them anyway.

The point is, even though the signals we'll be receiving will become more and more redshifted, there'll never be a time when we'll stop getting them. Crossing the cosmic event horizon now does not make the earlier light sent from the object disappear. The light from the horizon crossing event will arrive at the observer only after infinite time.
It's the same effect as with an event horizon of a black hole, that kimbyd mentioned in her post #3. There's some more about it in the post about causality and light-cones I linked above.

So the objects in question are never unobservable in principle, only in practice - due to redshift and faintness of the signal.

Again, that's the 'in practice' part. Even classically, given enough time any light wave will become so redshifted, that you'd need an impractically large detector to see it. But it doesn't mean the signal isn't there.

I'm talking about the meaning of the event horizon. If the worldline of an object ever was within the event horizon of an observer, the event of the object's crossing of the horizon will be observed only after infinite time.
Particle horizon is more in line with what the OP is talking about when he asks about observable universes.

It does in the quantum case though. In the example I gave, there is probably no signal at all, not even one photon. That is literally zero signal, unless you happen to get extremely lucky. So "probably zero signal" is philosophically quite different to "an extremely weak signal" I would say.

Though I am not sure if there is some super bizarre quantum argument to save things here, something about how the wavefunction is still here even though it has an extremely low amplitude.

What @Bandersnatch is true (modulo redshift).

Here's an interesting article I just read that touches on this.

I think it stands to reason that there's this overall space filled with observable pocket universes that share the same physics. Hawking's latest paper tries to reduce the physics of these pockets and I think that's on the right track. I think if you want universes with all of these different physical laws you need to look for evidence that supports the string theory landscape and 10^500 false vacua. It goes onto say:

Wrong. Astronomy news articles almost universally report cosmological distances using light travel time, the amount of time that the light with which we’re seeing an object took to travel from the object to us. For relatively nearby galaxy, say 20-30 million light years away, that’s fine. In those cases, the light travel time is close enough to the co-moving or “proper” distance, the distance between us and the remote galaxy “right now”, that it doesn’t make a real difference. But when we look at objects that are billions of light years away, there starts to be an increasingly significant difference between the proper distance and the light travel time.

Those farthest viewable galaxies that are 13.2 billion light years away in light travel time are over 30 billion light years away in proper distance. The cosmic microwave background, the most distant thing we can see, is 46 billion light years away. So, in “proper” distances, the radius of the observable universe is 46 billion light years.

That's crazy that we can only reach 3% of the galaxies in our observable universe today. That doesn't bode well for the fate of everything in our universe but then again maybe it does. Maybe the expansion of space eventually leads to the birth of new universes.

This is why I do think it's more than a coincidence that the radius of the Hubble's sphere is close to the age of the universe in light of the recent published paper I linked to earlier called From Planck Data to Planck Era. Maybe the age of our universe is when we were born from another universe that was like ours. Maybe a black hole formed in that universe and out of the other end we were born and this is why there's this data in the noise of the CMB. Maybe that information came from a parent universe.

That would mean you could get infinities within infinities of the same or similar things occurring if at the end of every universe or if every black hole can give birth to other universes.

## But How Much Galaxy is Two Trillion Galaxies?

Up to now, astronomers usually said we know of about 200 billion galaxies in the observable universe (meaning out to our event horizon, a look-back time of 13.8 billion years). Now the number can be said to be about 2 trillion, with the caveat that this estimate doesn't go back a full 13.8 billion years, it's 600 million years short. (Not many galaxies could have formed before then.) The only reason the number is 10 times bigger now is that you can legitimately include more of those littlest early building blocks they're no longer so theoretical. The total amount of stuff — stars and gas — hasn't changed.

So no, we do not "also have to update the number of stars in the observable universe, which now numbers around 700 sextillion," as some uninformed science writers are saying. That's what they get for taking press-release hype literally.

The Lambda-CDM model predicts that the earliest clumps that formed in the smooth material after the Big Bang should have averaged about a million solar masses each (dark matter and normal matter combined). That's about the mass of a typical globular cluster today, and a millionth the total mass of the Milky Way. That's the mass down to which Conselice's team ran their extrapolations to come up with their count.