The Search for the Edge of the Universe
by Lyon McCandless
What person has not been impressed with the night sky and its many distant points of light, constantly wheeling overhead, but never changing? The fleeting meteor or comet visitors are a hint that there is probably more happening out there than we can see, or even imagine. What is at the edge of our universe? What would we see if we could somehow be at that limit?
After thousands of years, at last astronomers have discovered that the universe outside our solar system is far from being unchanging. It is full of billions of majestic galaxies moving, colliding, radiating energy at incredible intensities. In some places new stars and planets are being created, and in others stars are disappearing forever into black holes. And we have discovered that our heritage is quite literally in the stars, because our bodies contain iron and other heavy elements that did not exist in the early universe. Billions of years ago many giant stars fused light elements into heavier elements and exploded, seeding the universe with the elements essential for our life.
About 100 years ago we found that the Sun was only a medium-sized star nowhere near the center of an inconceivably huge spiral galaxy. In 1996 the Hubble Space Telescope was pointed at a very small, apparently blank area in between the stars in an attempt to explore totally unknown regions of the cosmos. An exposure of sixteen hours astounded even veteran astronomers. There were hundreds of very distant galaxies in the tiny area! Our own Milky Way galaxy alone contains more than one hundred billion stars, and we can see billions of galaxies, almost an unbelievable number. On a very large scale, the universe is homogenous and isotropic, that is, the same in all directions. The average density of matter in the universe is fairly constant. What we can see, even with the most powerful telescopes, is probably only a small part of the whole universe.
Edwin Hubble was the first to observe that distant galaxies were receding from ours, and that the rate of recession appears to increase linearly with distance. Later astronomers refined the accuracy of the ‘Hubble constant’ relating recessional velocity to distance, and showed that that relationship holds true for much greater distances.
The fact that almost all galaxies are receding from each other immediately suggests that the universe is expanding. This idea required a minor modification to one formula of Einstein’s General Relativity theory. Projecting the expansion backward to find the common starting time is a difficult process, with some degree of uncertainty. The event was soon labeled the Big Bang. The current estimate is that the Big Bang started
approximately 15 billion years ago. Note that there is no starting place, no central point. Moments after the start of the Big Bang the whole universe was contained in one infinitesimal ball of intense energy which then expanded in four dimensions just as a balloon expands in three.
Currently there is no agreement on how or why the Big Bang started. But there is good evidence that moments after the start of the Big Bang the universe was still extremely small, was completely filled with raw energy, and was expanding very rapidly. The energy was so intense that no matter could exist. As space expanded, the energy density dropped, and elementary particles became possible. At this stage space was entirely filled with quark soup and energy. Solid! You might say wall-to-wall matter, except that there were no walls due to the curvature of the space-time continuum. In several minutes the volume of the expanding universe became much larger and the energy density dropped enough to allow densely packed, fully ionized nuclei of hydrogen, helium, deuterium and lithium to form. After 300,000 years of expansion, the energy density was low enough to allow electrons to recombine with the nuclei, thus forming ordinary matter. During the recombination process each atom radiated a quantum of light at wavelengths specific to that element.
The light of the recombination was the first radiation free to propagate through the universe. Initially in the high ultraviolet end of the spectrum, it has spread through the universe for 15 billion years. As the universe continues to expand, the wavelength of the first light stretches and its intensity diminishes, but it still carries the spectral fingerprints of the first elements. This living signature of the Big Bang is still detectable as faint microwave background very evenly distributed in all quarters of the sky. Recently sensitive satellite instruments have detected very slight variations in the radiation. These patterns are perhaps analogous to sound waves in the densely packed primordial universe. Those ‘sound’ waves eventually resulted in the clumps of matter which were destined to become giant stars and galaxies.
The first light comes to us from all directions almost uniformly in the form of microwaves. It is the oldest possible signal that we will ever receive, since electromagnetic radiation at any wavelength was not possible in free space before then. In order to get a visual image we must wait a few hundred million years more after first light for galaxies and quasars to form. Let us look into that. The light elements newly created by the Big Bang were free to drift after first light. Slight variations in the original tightly packed mass caused them to clump together; with their very weak gravity, the clumps drew in other particles. Then somewhere between 500 million and a billion years later, stars, supernovas, black holes and galaxies were formed.
We know these events took place because astronomers can see these events happening. The speed of light, 186,272 miles per second, may seem very fast, but on an inter-galactic scale it is slow. But the low speed of light means that looking through a telescope at distant objects is like looking back in time. We can’t see our own past because it is too close. When we look at the Moon we see the Moon as it was one second ago; the Sun six minutes ago; Jupiter an hour ago. We see the star Alpha Centauri as it was four years ago. Pity the paleontologists who have to study dead fossils! Astronomers can look back in time and see real stars, galaxies and clusters of galaxies full of energy, moving, exploding and interacting millions or even billions years ago.
Einstein showed that the universe is a multidimensional continuum with no boundary, just a curvature in the space-time continuum that bends three dimensional space back on itself. We don’t have to understand the mathematics if we can accept certain of its implications. One is that if you could travel long enough in any direction you would end up where you started. This is analogous to a person traveling on a globe. He thinks it is flat and unending because he cannot see the Earth as a sphere. But if he goes far enough he will be back to his starting point.
Many of Einstein’s predictions have been validated by experiment, and most astronomers accept his concept of a curved space-time continuum. The exact shape is the subject of much debate, and depends on how much matter is really present, which is another open question. What does this mean regarding the ultimate “Edge”? It means that space never reaches an edge or a boundary. Now thanks to Edwin Hubble, we know that there is an absolute maximum distance that we will ever be able to see with any possible type of instrument. It’s an interesting edge, one that is easier to understand: the edge of the observable universe. If we had a perfect telescope, how far would we be able to see, and what is happening at this limit, the edge of the observable universe?
Many clues to the past of the cosmos lie in the distant, dim regions that are a challenge to the ingenuity of our astronomers. The best instruments are still not enough. But several natural phenomena have added significantly to our ability to detect and analyze distant objects. The first is the discovery of a class of very active galaxies called Quasars, these ‘quasi-stellar objects.’ At first, quasars were interesting only because they didn’t fall into categories established for other galaxies. But when their extreme distances were determined, it was calculated that each quasar had a brightness exceeding one trillion times that of our Sun. Thus, quasars turned out to be near the limit of lookback time.
The second phenomenon that helped extend our view into the farthest reaches is based on a principle predicted by Einstein. Einstein suggested that the gravity of a large mass in space could bend light from a distant galaxy and magnify it much the way a lens magnifies
light. This truly amazing process increases the intensity of light received by the Hubble Space Telescope by a factor of approximately 30. The distance and characteristics of the remote galaxy may then be determined by spectral analysis of the received light. The most distant galaxy observed to date using this technique is receding at 95% of the speed of light, and is at a distance of eighty billion trillion miles. It takes light 13.5 billion years (13.5 bly) to travel this distance. The universe was approximately one billion years old when light left that galaxy.
How far would we be able to see with a perfect telescope? By “perfect telescope” we mean a device that derives astronomical information from received electromagnetic waves of any wavelength. We will assume that the instrument is large enough and sensitive enough to collect the needed electromagnetic radiation if it exists (visible, X-ray, gamma ray, infra-red, radio, microwave). Assuming the perfect telescope, there are three other factors that determine the absolute limit to observations made from the Earth: the speed of electromagnetic radiation in space, the rate of expansion of the universe, and the age of the universe. Fortunately, many experiments have verified Einstein’s premise that electromagnetic radiation propagates in vacuum at a speed that is constant (186,272 miles per second, or 3 x 108 meters per second) for any inertial reference system, for any direction, and for all wavelengths.
What happens when an object under observation is receding at a high velocity? The velocity of the object has no effect on the speed of the light received by the observer. However, the frequency of the received light is shifted towards the red end of the spectrum due to the expansion of the universe. The most distant galaxy observed to date has a red shift factor of approximately five. This means that the wavelength of received light is five times as long as it would be for a stationary source. What would happen if the velocity of a receding object approached the speed of light? As the speed of the object approaches light speed the wavelength of the light received on Earth would approach infinity, and the received energy, which is inversely proportional to the wavelength, would approach zero. In other words, the object would be undetectable. Using the Hubble formula, we can calculate that an object at a distance of approximately 14.5 billion light years (bly) would be receding at the velocity of light. Any object more distant than 14.5 bly is receding from earth too rapidly, and is not viewable from earth by any means.
In summary, we can say that even a perfect telescope would be unable to observe objects beyond approximately 14 billion light years distance due to the redshift cutoff. This is very close to the time when stars and quasars were first formed after the Big Bang. Going back farther we would be in the dark period with no stars, only atoms of hydrogen, deuterium and lithium.
The question, “What could we see if we were near the edge of our observable universe?” leads to an area of inquiry that is generally skipped over lightly by most cosmologists. Suppose you were an observer in a galaxy 10 billion light years distant from Earth. The accepted model says that the universe is homogenous and isotropic on a large scale. This means that there are no special locations such as a center or a corner. The universe would appear much the same to an observer at any location in the entire space-time continuum. The lumpiness of black holes and clusters of galaxies averages out on a large scale. At a distance of ten billion light years from Earth you would see the same sort of universe that we see here, and you would also be limited to an observable distance of approximately 14 billion light years. You would see different constellations, but in general your night sky is much like ours. This is a somewhat dull answer to the question, “What could we see if we could be near that limit?”, but let us look into the implications.
Observations of distant objects from Earth are impossible when the velocity of recession approaches the speed of light. Yet if you were at the edge you would see galaxies more distant than the edge. We would like to say that the distant galaxies are receding at speeds faster than light, but the very term “faster than light” gets us into the Relativity trap because Relativity has grabbed the “light” yardstick as its prime directive. The mathematics of Relativity does not deal with speeds greater than the speed of light because Special Relativity is based on observation. “Observation” implies measurable relative parameters. We lack the mathematical tools to describe in detail what goes on in the region outside our observable universe because we have had no need for such mathematics. We must clear this semantics hurdle before much progress can be made. The idea that light is an absolute limit is so firmly entrenched that it inhibits scientific thought and funding for anything that would violate this principle.