Every second 20,000 stars fall out of our field of view.Despite the knowledge we now have of the universe and its makeup, there is a hard limit to what we can and cannot detect. The known universe may be growing, but as it does, there is less to see.
The concept of our world and our place in it has changed a great deal over time. Before technology began to give us a picture of the dimensions of stars, galaxies and empty space itself, the belief in most early cultures was that our earth was a major component of the universe. The Greeks played a central, foundational role in describing the workings of the cosmos based on mathematical models that accurately described their observations. Ptolemy modeled the Universe with the earth fixed at the center, the sun orbiting around us, and planets also orbiting the earth but additionally fixed on invisible spheres to account for their observed paths which, unlike the sun’s, advanced but then seemed to backtrack before advancing again over successive nights.
Ptolemy’s model of the Universe was in good agreement with the best observational data available to him and was the generally accepted model of the cosmos in the western world from its introduction around the year 150 until the Copernican Revolution in 1543, that is, about 1400 years. During those many generations, most people believed their entire lives the earth to be unmoving and at the center of things. The model was not perfect, but seemed to be the best explanation of the available data, and in that sense, may be considered a scientific theory which was widely accepted during many centuries when the scientific method had yet to be well established. In fact, it is one of the most enduring scientific theories regarding the natural world that have existed.
Ptolemy’s model, with all its complicated epicycles is fascinating, and I recommend animations of the motions which can be found on YouTube. As the planets make circle on their epicycle spheres they also circle around the earth, creating pleasing geometric paths like you could create with the wonderful Spirograph design kit which has not been in toy stores for many years, but you can still see on exhibit at the Smithsonian Institute. Ptolemy’s model also made sense because it seemed obvious to any scientific observer, indeed anyone with common sense, that the earth is not moving. Everyday experience, especially in those times, made it difficult to imagine that we are on a moving body without any noticeable feeling of motion.
But Ptolemy’s long lived, meticulously crafted, aesthetically pleasing model finally ran into trouble as the Renaissance produced better observations and the bold and skeptical mindset that came to understand that not only was a new model required, the old one had no salvageable parts: it had to be torn down completely.
For the Catholic Church, which was not only a religious institution then but a reigning repository of knowledge, this was a bitter pill to swallow. In fact, formally accepting the idea that the earth was not static and central in our universe took about three hundred years after Ptolemy’s model was convincingly, at least from an observational point of view, refuted through the work of Kepler, Copernicus, Newton and Galileo.
The paradigm shift from a geocentric to a heliocentric cosmos took an incredibly long amount of time to be understood and then accepted. To this day, about one in five people polled in advanced nations believe the sun orbits the earth. Why? There are, of course, many possible explanations, a well-established, time-honored model will rarely be dismantled without resistance, especially if it means accepting the counter-intuitive proposition that the earth is actually moving at a great speed. And from the Catholic Church’s perspective, the description of God’s grand creation of the cosmos and Man only to put us on a small world that orbits the sun does take some of the glory out of it.
But the shift from the geocentric to the heliocentric model of the cosmos was only the beginning of the story of our ever diminishing position and scale in the Universe. Ptolemy put the sphere containing the stars at a distance of 20,000 times the radius of the earth. We now know with precision that the earth’s radius is 6,371 kilometers, so this would put the stars at about 127 million kilometers from us according to Ptolemy. By 1838, the Astronomer and Mathematician Friedrich Bessel determined with less than 10% error, through the method of parallax, the distance to the star 61 Cygni, which turned out to be about 10 light years, that is 100 trillion kilometers, or, about a billion times farther away than what Ptolemy had estimated. And of course, 61 Cygni is one of our “closest” neighbors. Further measurements continued to fill in the picture of a galaxy consisting of a vast number of stars (current estimates vary from 100 to 400 billion) stretching a total distance of about 200,000 light years. And then there was that fuzzy patch, barely visible to the naked eye which the French Astronomer Charles Messier catalogued as Messier 31 or M31. In 1735 the astronomer William Herschel described it as a large nebula within our galaxy. But in 1919, the 30 year old Astronomer Edwin Hubble arrived at Mount Wilson and began observations which led to a different interpretation of what these fuzzy patches are. Using Cepheid variable star luminosity as references, Hubble was able to detect two of these stars in M31, paving the way to making a concrete calculation of the distance to this object. Hubble took his sweet time before presenting these findings, but was finally convinced by his colleague Henry Russell who offered to read them in the upcoming meeting of the American Astronomical Society, since Hubble was not inclined to leave Mount Wilson.
On January 1st of 1925, Edwin Hubble´s communication, “Cepheids in Spiral Nebulae” was read to fellow astronomers. In it, Hubble describes that M31 is actually much larger and much further away than previously thought. It is, in fact, not a nebula in our galaxy at all, but a complete second galaxy, similar to our own Milky Way, 900,0000 light years away. Meet The Andromeda Galaxy. When Hubble’s findings were presented, the audience was thrilled, and he was rewarded with accolades and a prize. When Galileo presented his breakthrough concepts regarding the universe he was rewarded with a trial by the Inquisition.
Although there was controversy prior to Hubble’s paper, there were generally no significant objections to Hubble’s meticulous analysis. The known universe before 1925 was already mind numbingly big, but on January 1st of 1925 Hubble had, in effect expanded our estimate of its size by a factor of at least 100.
Soon after Hubble’s breakthrough, additional nebulae were studied and found to be extra-galactic as well, pushing the size of the universe still further. And in the 1960’s, a flurry of activity centered about quasars led to the understanding that these apparently faint objects, not visible to the naked eye, were actually intrinsically extremely luminous, but still further away than even the most distant galaxies that had been detected.
Our concept of the universe has taken giant steps to larger and larger sizes since the model put forth by Ptolemy was replaced. But suddenly physics seems to have exhausted the available supply of direct observational data regarding our universe’s size. Our technology has improved amazingly allowing us to explore incredibly distant features of the universe. We now know that the observable universe has a radius of 46 billion light years, which means that the total universe (including the “unobservable” part, is this size or greater. But we can´t see anything beyond this radius. And when scientist say “see” they mean we cannot detect light and therefore anything coming to us from beyond this limit. No particles of any sort, no force, no signals, no alien spaceship with news. The limit is actually posed by the age of the universe. Early on it is thought that it expanded very rapidly, faster even that the speed of light, so light has not had time to reach us from beyond the observable limit. Of course, you might say, it will reach us if we wait long enough. Unfortunately, there is no cooperation from our universe, which is still expanding, so, although we will see further in the future, objects will have dropped out of the observable sphere. In other words, our observable universe will get larger but have less objects in it. If you think about it, it seems almost malicious. The universe was formed, then ran away from us quickly, limiting what we could see, and then, by continuing to expand, slowly removes objects from our observable bubble. Dr. Don Lincoln gives a neat explanation in a Fermilab YouTube video. In it he gives the estimate that, because of this expansion, stars are dropping out of our field of vision at the rate of 20,000 per second!
Imagine an artist offers to sell you a piece of canvas on which he has painted only an eye. He tells you it is a fraction of his very unique painting, which is actually much larger. So you begin to wonder how much more the painting has. Is it actually two eyes? A face? A complete person? Or perhaps the painting has many people in it? “What is this all about?” you ask him. Don’t worry, he replies, the canvas has a very special property and will grow so you will get a larger picture if you simply wait long enough. So you take him up on the offer and soon discover that, In fact, the artist is right, and you detect that in a few days the canvas has gotten larger! Amazing, you think, the artist was telling the truth and soon I will have a complete painting! But then, you take a close look at the eye and notice that the image itself has also grown bigger, instead of the complete eye you can now only see the pupil. The longer you wait, the larger the canvas grows, and yet you get less of the entire picture. What a scam, you think, I will never know what else the painting contained, and someday I will have a huge canvas, with a few dots of ink… and then absolutely nothing on it. This is in fact the situation we find ourselves with regard to the patch of the universe we can observe. And the artist will not give a refund.
Although there is indirect evidence that may suggest how large it actually is, our knowledge of the size of the universe through direct observation now faces a hard limit. Our universe has an implacable rule regarding how fast information can travel between two points in space, which is the speed of light. But our universe also expands at a rate which makes communication between regions beyond a certain distance impossible. This distance, of course is currently huge, we can see things up to 46 billion light years away. What we see at this edge of the observable universe are things that happened a long time ago, as the light that reaches us today left billions of years ago. But, as Dr. Lincoln explains, if I sent you a message today, and you were anywhere further than 15 billion light years from me, the message would never arrive. Of course no one really has any pressing need to communicate with anyone or anything over 15 bilion light years away, but I find the fact that there is this built in limit, well.., unsettling.
It raises the interesting question. What does it mean for things to exist beyond the realm of what can be observed? If a tree falls in the forest and no one is around to hear it, does it make a sound? The more or less accepted answer to this famous question is that air vibrates and sound waves are created no matter whether anyone is around to hear it or not but, the experience of sound is only created in the consciousness of someone if she hears it. But what happens if the tree falls in a forest on another planet? I suppose, provided that the planet has an atmosphere, the answer would be similar, but then, you might ask, what happens if it falls beyond our observable universe? Cosmologists theorize that our universe may be merely a bubble in a much larger universe soup, or one of many universes in constant creation. The term Multiverse is so common you can find it in the title of a number of non-fiction books on the subject, as well as some good sci-fi stories.
But if theory points us in the direction of the existence of other universes, should we embrace the idea? The term falsifiability is pertinent here. If a theory can be proved wrong it is falsifiable, and that places it solidly in the realm of science. Well known, commonly accepted theories in the field of physics and other sciences are all falsifiable. If other universes can affect our own in any way, even the slightest effect at the limit of what we can observe, then the case for a second or third or nth universe can be made on solid ground and is falsifiable. But if, as it seems, our universe is a bubble into and out of which no information from other universes can flow, we are left in a strange situation. What does it mean for a universe which is undetectable, not because of limits to our technology, but because of the nature of the laws of physics themselves, to exist?
There is no clear cut answer, the very concept of what it means for something to exist comes into play, but the question is not altogether unapproachable. Think about the stars that Don Lincoln explains fall out of our observable universe. We can never observe anything about them again, but it seems reasonable to assert that they continue to shine in a region of space we do not have access to. There is also a possibility that there are tunnels in the fabric of space-time, i.e., wormholes, which could link us to places in the universe which are beyond the observable limit. This would actually place unobservable parts of the universe and perhaps other universes as well back in the category of observable. So an argument can be made that theorizing on objects and universes that are beyond observation is still in the realm of science and may actually turn out to be falsifiable.
But there is still the lingering question about existence and the unobserved tree that falls in the forest. We can go to the forest and observe the fallen tree and the scattered leaves and conclude that the air vibrated and sound waves were created. Similarly, as we have seen, careful observation, ever improving technology, mathematics and our powers of reasoning have allowed us to glean evidence leading to the understanding of the vast universe we now conceptualize with great certainty. But everything we know comes from evidence we have gathered of events that have a measurable effect. Hubble figured out that M31 was not in our galaxy but was actually a far distant galaxy in itself because he carefully observed the photons that left Andromeda 900,000 years ago and were gathered by the Hooker telescope at Mount Wilson. These photons caused minute chemical reactions on photographic plates. If Andromeda were not a faraway galaxy, the plates would show different patterns. In both the case of the tree and the galaxy, events which we can observe are directly attributable to their actions. We are, in other words, conscious of events that lead us to the conclusion that the tree produced sound waves, and Andromeda is a distant galaxy. The problem arises when consciousness gets locked out of an event and its consequences. For the moment we are, in effect, locked out of obtaining any evidence of additional universes. If they exist or not is entirely equivalent to us, that is to say nothing can be gleaned from things that occur in our realm of observation that can prove or disprove this theory. We need a bridge, perhaps a wormhole, or perhaps consciousness which can span many universes and relay it to us.
The story of how we have come to understand the size of the universe is fascinating. It is amazing that the universe turned out to have so many stars and galaxies in it and that it is so much larger than anyone imagined. But equally amazing is our accomplishment. As curious primates we asked where we were and what are home was like, and we got only the tiniest, at first invisible clues to figure it out. Then, as we slowly found these clues and began to put them together, we kept having to accept what seemed impossible. Our solid planet was actually flying through space. The pinpoints of light we see at night are actually like our sun, only much more distant, the universe is so big that even expressing its size in kilometers is impractical. Consequently our own human scale was put into context. Our curiosity, technological capability, reasoning. imagination and persistence bore fruit. But, like Galileo, maybe the reward we reaped was not as pleasant as might have been expected. Our great accomplishment resulted in understanding how tiny our world is, and how incredibly common stars and planets like our own are. It also resulted in the universe putting what looks like an end to further clues due to the observational limit our ever improving technological abilities have come up against. If the challenge of the universe was to show us how insignificant we are, the score is Universe 1 Humans 0. But if the challenge is to figure out what the universe ultimately is, our ingenuity might have the universe team starting to worry.
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