by Les Johnson
The universe as we now know it is radically different from what it was known to be just one hundred years ago and it is not at all what it was perceived to be at the dawn of the space age. Many of the classic science fiction stories so many of us cherish as inspirational were written in a time when the Big Bang was a new idea, the term “black hole” hadn’t yet been coined, dark energy and dark matter would have brought blanks stares from most astronomers and physicists, and the notion that over 700 extrasolar planets had been found would have been greeted with, “are you kidding me?” Go back another half century and the simple notion that the universe contained more than just what we observed to be in our own galaxy would have been a new idea – some of that fuzzy nebulae seen in astronomers’ telescopes were not yet known to be other galaxies. And it was commonly thought that the universe was in a “steady state” (It was not thought to be expanding or contracting; it just “was.”)
The universe isn’t what it used to be.
The Expanding Universe
A timeline from the beginning of the universe (The Big Bang) until now. The timeline is continuously refined as new data pours in, such as the results from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) mission. (Image courtesy of NASA.)
We now take for granted that The Big Bang occurred. Yet, before 1929, when Edwin Hubble observed that galaxies were all moving away from each other at speeds proportional to their distance from us, the idea of The Big Bang was unknown to science. Hubble's observation that the universe was flying apart in all directions caused him to ask what the universe would have looked like in the past when the objects in the universe were closer together. In fact, he ran the clock backwards and found that there must have been a time at which all the matter in the universe originated at one point in space -- at the measured rate of expansion, that point (in both time and space) must have been about 14 billion years ago – The Big Bang.
Back to the present. Numerous observations, including data from very sensitive space satellites, have pretty much confirmed the Big Bang theory origin of the universe. Based on what we understand about physics, the expansion rate of the universe should either be constant or decreasing. If there is enough mass in the universe tugging on itself, then perhaps the rate of universal expansion will slow down and everything will crash back in on itself in a "big crunch" at some time in the future. If this is the case, then we should be able to see different expansion rates today (looking at galaxies relatively close to us) than yesterday (looking at the most distant galaxies in the universe). When we observe objects in deep space, we are looking back in time at how they were in the past -- it takes years, millions of years in the case of galaxies, for this light to reach us. So the light we see from distant galaxies left them long ago, showing us how they appeared then and not now. If the rate of expansion has not slowed, then there is not enough mass contained within the universe to slow its expansion, and then universe may be considered to be "open" and expand forever. That was the theory, anyway.
Shock of shocks, when modern instruments measured the rate of expansion here and there (in the distant galaxies), they found that the rate of universal expansion is increasing.
Authors whose major works pre-date knowledge of The Big Bang include H.G. Wells and Jules Verne. Excellent and mind-bending treatments of The Big Bang and the hypothetical “big crunch” include Poul Anderson’s Tau Zero and Stephen Baxter’s The Time Ships.
The Accelerating Expanding Universe
This diagram reveals changes in the rate of expansion since the universe’s birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pushing the galaxies apart. (Image and caption courtesy of NASA.)
Most of us are taught, and we often intuitively understand, the law of conservation of energy. Simply put, in a closed system (meaning a system that doesn’t allow energy or matter to enter or exit), the amount of energy is constant. You are free to move it around, change its form, convert between matter and energy, and generally do as you please, but when you add it all up, the amount of energy contained in the system is present and accounted for. This is fundamental to understanding virtually any physical system we’ve yet to encounter – except, perhaps, for the universe in which we live.
As discussed in above, scientists implicitly assume that the conservation of energy (and matter) applies to the universe as a whole. When you add up all the matter and energy in the universe present a few billion years ago and compare it with today, the equation balances. The disorder of the universe has increased (entropy), but no matter or energy seems to have been created or destroyed during that period. If that is the case, then how is it that the rate at which the universe is expanding is accelerating? It appears that there is some as-yet-unaccounted energy in the universe that is pumping up, accelerating, its expansion.
Various theories have been discussed to explain the phenomenon. The most widely accepted idea is there is some sort of not-yet-discovered form of energy that is applying a negative (or repulsive) pressure on a galactic scale, causing everything to move away from everything else at a faster and faster rate. Do we know anything else about this dark energy? No, that’s it – and astronomers are scrambling to gain a better understanding of what’s happening.
When did this unseen, or “dark energy” become accepted as real in physics and astronomy? Not until the 1990’s with a Nobel Prize being awarded for its discovery in 2011!
Authors whose major works pre-date “Dark Energy” include Arthur C. Clarke, Isaac Asimov, Robert Heinlein, and just about every other major science fiction writer of the 20th century!
Galaxies and Nebula
The Hubble Space Telescope took this breathtaking photograph of Galaxy M-81. We've become so accustomed to seeing other galaxies that it is difficult to imagine that their existence - as we know them today - was unknown a century ago. (Image courtesy of NASA.)
Do you remember the astronomer who discovered that the universe was expanding and gave us The Big Bang – Edwin Hubble? He’s also the man who figured out that some of those fuzzy, spiral-shaped objects, generically called “nebula”, which were so lovingly and systematically categorized by Messier (“Messier Objects”), were actually giant clusters of stars distinctly separate and very distant from our own galaxy.
Until Hubble, astronomers considered what we now know as galaxies to be “nebulae.” Exploded stars within our own galaxy were also “nebulae.” In fact, given the limitations of those early telescopes, just about anything that looked like a fuzzy cloud of dust or gas was labeled as a “nebula.” Nebulae were everywhere and there wasn’t much to distinguish between the various types.
Using the 100-inch mirror at the Mount Wilson observatory, in 1919 Hubble took the highest-resolution images to-date of the Andromeda Nebula and discovered that it contained billions of distinct stars within it. A few years later, he calculated that they must be at least ten times more distant than the most distant stars within our own Milky Way galaxy. Telescopes improved and many more nebulas were found to be, in fact, distant galaxies. Thanks to modern telescopes, including one named after Edwin Hubble flying in space 500 kilometers above the atmosphere, we now know that there are billions of galaxies in the universe.
E.E. Smith’s Gray Lensman (1939) is one of the first science fiction stories to take readers out of the Milky Way galaxy and let’s not forget the episode “By Any Other Name” from Star Trek (1968) that featured invaders from the Andromeda galaxy.
The Whirlpool Galaxy and a companion galaxy as seen by The Hubble Space Telescope. What is keeping the stars in the outermost part of the galaxy from flying off into deep space? (Image courtesy of NASA.)
Most of the matter required to keep the universe working according to the known laws of physics is, well, missing. Rather, we think it's there, we just can't see it. Way back when, about eighty years or so ago, scientists thought they had most things associated with the orbits of planets and stars worked out. Newton's laws explained the orbits of the planets fairly well and, by analogy, the orbits of stars circling the center of the galaxy.
The speed with which a planet orbits the Sun, or a moon orbits a planet, depends upon the mass of the object being orbited and the distance between them. The larger the orbital distance, the slower the orbital velocity. For example, the Earth orbits the Sun at about 30 km/sec, Jupiter goes around the Sun at 13 km/sec and Neptune at 5 km/sec.
Since the stars in a galaxy, like our Milky Way Galaxy, orbit the massive black hole at their center in a manner similar to the way the planets in our solar system orbit the Sun, one would expect that the stars nearer the center of the galaxy to orbit faster than those near the edge. Unexpectedly, when astronomers measured their orbital velocities, they weren't moving as expected. In fact, measurements indicate that all of these stars are moving at about the same angular speed. This is a real problem because, well, unless there is more mass than we can see tugging on these fast moving outer-galaxy stars, they would fly off into space -- having way too much energy to be trapped in orbit. This invisible mass must be there tugging on the stars, keeping them circling the galaxy's center and not flying off into deep intergalactic space.
There are two possible explanations for this discrepancy: 1) We don't understand orbital motion (an unlikely prospect given the recent success of the Mars Science Lander and other space probes, or 2) There is more mass (matter) in these galaxies influencing their orbital velocities than we can see -- "dark" matter. Given how well we seem to understand planetary motion, most scientists favor the second explanation. (There is, of course, the possibility that there is yet something else at work that we simply don’t understand.)
It is estimated that there must be at least 80% more mass in the universe than we can see in the stars, planets, and galaxies around us. As we've used all the tools in our observational toolboxes, scientists have determined that if this dark matter is out there, then it neither emits nor absorbs light in any part of the electromagnetic spectrum. Based on the fact that we can only detect its existence through its gravitational effects, dark matter must be fundamentally different from the matter we encounter in our everyday life. It cannot be made of protons, neutrons or electrons -- these are all easily detectable by means other than gravity.
For those that are uncomfortable inferring the existence of matter we cannot "see," I must point out that we know about many parts of our universe that we cannot see already. We cannot "see" a magnetic field, yet we can experience its effects and we know it nonetheless exists. We cannot "see" the Higgs Boson, yet experiments recently conducted in Europe are close to proving that it, too, exists.
Though first observed and discussed in 1932, Dark Matter didn’t gain popular acceptance until the late 1970’s to mid-1980’s.
Stephen Baxter’s Ring and Larry Niven’s Ringworld’s Children provide credible treatments of dark matter – without the mysticism that many other writers seem to imbue this poorly understood component of our universe.
Gamma Ray Bursts
A hydrogen bomb test in the Pacific. (Photo courtesy of National Nuclear Security Administration / Nevada Site Office)
The Cold War, nuclear weapons and astrophysics – three phrases that at first glance don’t seem directly related. Yet, without the threat of nuclear annihilation, one of the most amazing discoveries about the universe around us might not yet have occurred.
Recall the Cold War. The United States and The Soviet Union were competing for military advantage all over the world and in the space that surrounds it. Each side was building nuclear weapons, thousands of nuclear weapons, and there was a very real fear that one side or the other would take advantage of their destructive capability to attack the other in a nuclear first strike. Missiles were built and deployed, many of which were capable of carrying multiple nuclear bombs, taking the world to a time where nuclear bombs could be exploding within minutes of them being given orders to launch. It was a scary time.
In the early part of the Cold War, both countries conducted tests of their nuclear weapons to gauge their destructiveness and to serve as a warning to the other that their weapons actually worked. After all, if someone told you they had such weapons but had never tested one, would you take the threat seriously? Robust series of nuclear testing began in the late 1950’s by both the US and the USSR.
In 1958, the United States detonated two nuclear weapons in the upper atmosphere just short of the 100-km altitude now thought of as border between our atmosphere and outer space. The first, part of Operation Hardtack, successfully exploded at an altitude of 76 kilometers. The bomb injected a significant amount of fission debris into the ionosphere and disrupted radio communication throughout the South Pacific. In addition to the disruption of the ionosphere, the bomb also produced an Electromagnetic Pulse (EMP). An EMP is a burst of electromagnetic radiation that propagates for many hundreds of kilometers, depending upon where it is detonated, causing damaging current and voltage surges in unshielded devices containing electrical circuits.
It was soon discovered that these atmospheric nuclear tests were distributing radioactive fallout across the globe and the governments of both countries soon agreed to ban this type of testing. In the future, all nuclear bomb testing would have to occur underground.
To monitor compliance with this agreement and to see if any other countries were developing and testing nuclear weapons, the US Department of Defense developed sensors to detect the characteristic radiation signature of a nuclear explosion. These sensors were built and deployed in space on a series of satellites known as Project Vela. In the late 1960’s, as the Vela satellites began flying more sensitive instruments, they reported seeing gamma ray ‘flashes’ that were similar, but not identical, to the flash produced when a nuclear bomb detonates. They detected lots of flashes – approximately one per day. The experts correctly concluded that terrestrial nuclear weapons explosions did not produce these events and they quietly classified their observations as ‘secret’ lest the information becoming public compromise the capabilities of the satellites that detected them. In 1973, the detection of these burst events was de-classified and the existence of Gamma-Ray Bursts (GRB) was announced.
GRB are short-lived bursts of gamma-ray photons. Lasting anywhere from a few thousandths of a second to several minutes, GRB’s are about a million trillion times brighter than the Sun, making them briefly the brightest source of gamma-ray photons in the universe. GRB’s occur in apparently random locations of the sky and about one burst happens each day.
NASA scientists who study GRB’s produced a computer animation showing what the event might look like from space nearby:
As recently as the early 1990s, astronomers didn't know if they originated at the edge of our solar system, in our own galaxy or very, very far away near the edge of the universe. Observations since that time, many taken from space satellites, have led astronomers to conclude that GRB’s are caused by at least two processes and that they are very, very far away. The first is the explosion of a massive star, a supernova, which may subsequently form a black hole. The second involves a class of collapsed stars where the primary material remaining are the electrically neutral neutrons – a neutron star. Fortunately, the kind of stars that produce supernovas and GRBs don’t appear to be happening anywhere close. If one were to occur within our galactic neighborhood, it could be bad news indeed.
Larry Niven’s Fate of Worlds: Return from the Ringworld and Stephen Baxter’s Moonseed are among the few SF books that even mention GRB’s.
Artist concepts of matter swirling around a black hole. (Image courtesy of NASA.)
What science fiction fan hasn’t been fascinated by the thought of a black hole? The name evokes mystery and the concept is mind blowing. But did you know that the name “Black Hole” didn’t exist before John Wheeler coined the term in 1967?
Stars are large balls of gas, mostly hydrogen gas. Though it is the lightest of all the elements, hydrogen nonetheless has mass. Mass attracts mass, a physical property which we call gravity. When huge amounts of hydrogen are attracted together to form a star, the gravity-induced pressure of the outer layers of this protostar causes the hydrogen atoms to come closer and closer together until finally they are close enough to fuse, in a process appropriately called fusion. As these elements fuse, they give off energy, which counters the gravitational pull and keeps the star from collapsing further. This rough balance between the collapsing force of gravity and the outward pressure created by fusion causes our sun to shine and allows it to warm the solar system.
But as the hydrogen fuses into helium, the supply of hydrogen begins to decline and the star collapses further. The increased pressure that results allows the helium and the other elements heavier than hydrogen to begin to fuse, again producing energy and outward pressure sufficient to prohibit the star from completely collapsing. But as heavier elements fuse, they give off less energy than their lighter counterparts. There will come a time in the life of very massive stars that their mass-induced collapse will eventually overcome the outward fusion-produced forces and the result will be a body so dense that not even light can escape – a black hole.
All the mass of the black hole is compressed into a very small volume called a singularity. Around the black hole at some distance away from the singularity is a region of space where the gravitational attraction is so strong that nothing can pull away – the event horizon. The size of black hole’s event horizon depends upon its mass. Black holes with event horizons larger than our solar system are possible. They can also be very small: if the mass of the Earth were compressed into a black hole, then its event horizon would be the size of a marble.
There are black holes throughout our galaxy formed by the collapse of stars much more massive than our Sun. (Don’t worry, our Sun won’t burn out for another few billion years and when it does, it is not massive enough to form a black hole.) At the center of our galaxy is a supermassive black hole containing the mass of at least four million stars. Current estimates place its size at about 44 million kilometers – approximately the same as the distance between our Sun and the planet Mercury. One of the most famous black holes is located 6100 light years away -- Cygnus X-1. Cygnus X-1 has an event horizon of about 26 kilometers and a mass of about fourteen times our Sun. The x-rays we see from it are the result of matter accelerating and falling into the black hole; They emanated from there at about the time the first Egyptian civilization was beginning, taking 6100 years to travel through space before being seen by our scientific instruments in 1964.
I first heard of a black hole, referred to as a “dark star,” in the Star Trek time travel episode “Tomorrow is Yesterday” (1967). 1967? Isn’t that the year the scientific term “black hole” was first used? Hmmm…
Extrasolar Planetary Systems
The Earth as seen by Apollo 8. Are there planets like Earth orbiting other stars? (Image courtesy of NASA.)
Science fiction fans have known that there are planets around other stars for a very long time -- longer, in fact, than scientists themselves. The problem with scientists is that they require proof. Science fiction readers and some scientists intuitively accepted the idea ever since it was discovered that the stars in the sky were other suns like our own – and if our star had planets, then so should other stars. (Of course!)
What fun would there be in traveling to the stars if there weren’t planets around them, harboring alien life and ancient civilizations to explore? We haven’t yet confirmed the life or alien civilizations, but we do now know that there are at least 700 planets out there, circling stars other than our own. And this number is steadily increasing. The first extrasolar planet wasn’t confirmed until 1992.
There are several ways these planets can be detected; below are some of the more common methods:
When a planet passes in front of a star, it obscures part of the light from star. Measurements of the resultant dimming can then be used to determine the size and orbital distance of the planet from that star. This is called the Transit Method.
Just as a star exerts gravitational forces on the planets that orbit it, so does a planet pull on the parentstar. Doppler measurements of the light from the star can be used to determine if a planet perturbs its orbit. This is known as the Dopper Method.
One can also use shifts in the otherwise very regular radiation emissions from a pulsar. (A pulsar is what remains from some supernova events. It is called a pulsar because it pulses radio waves in a regular pattern.) A planet orbiting a pulsar will cause small irregularities in its emissions.
The number of confirmed extrasolar planets grows yearly. It is amazing to note that almost all of the planets discovered thus far are within 300 light years of our own solar system. Based on this very limited survey, and considering that there are over 250 billion stars in the Milky Way galaxy, there may be over 150 billion planets out there within reach – if you consider traveling to another star in the galaxy to be within reach!
Unfortunately we don’t yet have the ability to image these planets directly. The light from the stars around which they orbit is so much brighter than their reflected light that they are simply lost in the noise. Several possible space missions are on the drawing board that might solve this problem, but none are yet funded.
If we do find another Earth out there, what can we do about it? The distances are so vast and our space propulsion capabilities so limited that it might be centuries or millennia before we can consider making the trip. Darn it!
The Foundation Trilogy by Isaac Asimov, the standard against which other stories of galactic civilizations are judged (and which describes an empire of countless planets throughout the galaxy) was nearly 40 years old when the first planet beyond our solar system was confirmed to exist.
Other mind-bending discoveries such as Special and General Relativity, Quantum Physics, Neutron Stars, String Theory and Brane Cosmology have changed our view of the universe -but these will be topics of a future essay.
Copyright © 2012 by Les Johnson
Baen author and anthology editor Les Johnson is also the Deputy Manager for the Advanced Concepts Office at the NASA George C. Marshall Space Flight Center in Huntsville, Alabama. Johnson and Jack McDevitt are the editors of Going Interstellar, a collection of science and science fiction stories on interstellar travel using known technology. Johnson is also the coauthor with Travis S. Taylor of SF novel Back to the Moon and of an upcoming SF collaboration with Ben Bova, Rescue Mode.