To visualize what Hubble's law means, take a balloon and draw some dots on it. As you inflate it, you can see that the distance between every pair of dots increases. Let out the air, catch your breath, and repeat the experiment. Imagine that you are one of the dots looking at the other dots. From the dot's point of view, the other dots all seem to be getting farther away, like ships sailing from a harbor in different directions. It doesn't matter which dot you pick; each dot sees the same thing.
Based on this simple analogy, scientists deduced that Hubble's law is exactly what you'd expect if the universe were expanding. Galaxies in the expanding universe are like dots on the inflating balloon, or ships in an immense ocean that keeps getting bigger with time.
Of course, things are a little different in the real universe. We are not on the (two-dimensional) surface of a balloon. It is our three-dimensional universe that is expanding. We cannot see it from the outside, as in the case of the balloon. This is a perfect example of how our inability to escape the universe limits our view. Sometimes it is impossible for us to visualize what is going on, and scientists are reduced to talking in purely mathematical terms.
The Hubble constant is one of most important numbers in cosmology. Going back to Hubble's law, you can see that the constant is velocity divided by distance. Since velocity itself is distance divided by time, the inverse of the Hubble constant is simply time. This time, it turns out, is the approximate age of the universe.
To understand this, imagine that you and your brother want to visit some relatives. You both leave the house at the same time, but he drives 120 miles north to your grandmother's and you drive 120 miles south to your uncle's. Both of you go an average velocity of 60 mph. When you arrive, your uncle asks how long the drive took, but let's suppose you forgot when you left. Knowing the distance and the speed, you calculate that you drove for 120 miles divided by 60 mph, or 2 hours. Your brother makes the same calculation and reaches the same conclusion. Relative to each other, you drove 240 miles at 120 mph, which again implies a 2-hour journey.
This situation is analogous to Hubble law's. Astronomers can estimate how fast our Milky Way galaxy and, say, the galaxy M100 are traveling away from each other, as well as the distance between them. By dividing distance by velocity, they deduce the time in the past when the two must have departed from the same point. You can do this for any pair of galaxies, and Hubble's law will imply that all left the same point at the same time the very essence of what we call the Big Bang.
Until a couple of years ago, the Hubble constant was only known to within a factor of two, and a fierce debate was raging about its value. But thanks in large part to Hubble Space Telescope observations, the constant is now thought to be between 60 and 70 kilometers per second per megaparsec, implying an age of about 14 billion years. The oldest known star is also about 14 billion years old.
Some other measurements of the Hubble constant have implied an age of about 10 billion years, which, if true, might call the whole Big Bang theory into question. Those measurements have caused a media hoopla these past few years, even though astronomers have known of a potential problem for decades. In fact, some earlier estimates implied an even younger universe! But the precise conversion from Hubble constant to age is not as simple as dividing distance by velocity. Other effects could cause the universe to be older than the constant implies, just as, in the previous example, the road trip could have taken more than 2 hours if you had stopped for coffee. The observations now seem to be settling on a value that is fully consistent with the Big Bang theory.
Unfortunately, this is not possible. In its formative years, the universe was too hot and dense for light to get through. It was filled with light, but also with electrons, which shoved the light particles around and kept them confined. Space was opaque. Until, that is, about 300,000 years after the Big Bang began.
At that time, the atomic nuclei grabbed the electrons and put an end to their horseplay. At last, all those photons were able to escape. They streamed off in every direction, like the flash of a bomb. The Big Bang theory says that we should see this flash in the form of a light that seems to be coming from all around us. This is the closest we can get to actually seeing the Big Bang happen. It is our cosmic horizon; we cannot see through it or beyond it.
The primordial glow. An explosion produces a bright flash, right? Well, the Big Bang produced a bright flash, too known as the "cosmic background radiation." Since we are inside the universe, we are inside the Big Bang, and so the bright flash is all around us. We can't see it with our eyes because the Big Bang occurred so long ago that the flash has dimmed. But it can still be seen by radio telescopes and, in fact, causes interference with satellite communications. The flash was produced at the time the universe became transparent, 300,000 years after the Big Bang started. For the most part, the flash is quite uniform. But when you amplify it 100,000 times, you can see bright and dark patches. These patches represent clumps of matter which later became clusters of galaxies. Image courtesy of Charles L. Bennett, NASA Goddard Space Flight Center.
Fortunately, the flash is not as intense as it was billions of years ago. The expansion of the universe has stretched the wavelength of the light and sapped its energy. In fact, the wavelength has been stretched so much that the light has turned into microwave radiation.
The discovery of this radiation in 1965 was the ultimate triumph of the Big Bang theory. No other theory could account for it. Since 1989, the Cosmic Microwave Background satellite has been measuring the radiation and found that it looks almost exactly the same in all directions: It is isotropic, just as the Big Bang theory predicts.
The satellite did find that the radiation varies by a few parts in a million across the sky, which means that the universe had developed a slight unevenness by the time it set free the light. Far from disproving the Big Bang, this unevenness was a victory for models of galaxy formation based on the theory. After all, at small scales, the universe is not homogeneous. There are galaxies, planets, toadstools. At some point, the utterly smooth early universe had to start developing lumps seeds that could grow into the intricate structures we see today. The unevenness in the radiation represents those lumps. Some even speculate that the unevenness could also reflect whatever processes produced the Big Bang to begin with; one cosmologist has exclaimed we are seeing "the face of God," a fossil of creation.
At this point, the line between science and religion or philosophy becomes blurred. This isn't too surprising, since cosmology is attempting to answer some of the most fundamental questions we can ever have.
We have been able to use science to find out when time began, where we come from, and what our destiny could be. But we cannot yet say what there was before time began or what there is beyond the universe. The universe we live in is the only one we can reach by observation, and the known laws of physics cannot be extended to a hypothetical time before the Big Bang. Those are philosophical and theological issues that science cannot and does not deal with.
The Big Bang theory does say that the universe had a beginning and that it may have an end. Other theories, such as the steady-state theory, held that the universe was eternal, but this turned out to be incompatible with observations. Someday the Big Bang theory, too, may be replaced with a better, more comprehensive theory. Maybe that theory will answer the questions about "beyond" and "before." But even if the Big Bang theory is not the final answer, it is the only scientific theory that can accommodate everything we currently know about the universe.
PHILIPPE BRIEU is a research investigator at the University of Michigan in Ann Arbor. He uses supercomputers to figure out how structures in the universe formed. His email address is firstname.lastname@example.org.
GEORGE MUSSER is the editor of Mercury magazine.
The Fate of the Universe
Cosmologists aren’t just concerned with the history of the universe. They can also make educated guesses about its future.
What will happen to the universe depends on how much matter it contains. The reason is that gravity is the main force that governs the universe, and the more matter there is, the stronger that force is. Gravity (which pulls things together) thus competes with expansion (which throws everything apart).
If the total mass of the universe is greater than a certain mass, known as the critical mass, gravity will beat expansion. Eventually the expansion will slow down, stop, and reverse itself. The universe will contract until it reaches an infinitely small and dense state similar to the one at the time of the Big Bang. That point has been dubbed the Big Crunch. In theory, the cycle of expansion and contraction would then repeat. Cosmologists call this fate a closed universe.
On the other hand, if the mass of the universe is less than or equal to the critical mass, expansion will win. The universe will grow forever. After many billions of years, all stars will have died out, no new stars will be able to form, and life as we know it will no longer be able to survive. Eventually even the atoms and their constituent particles will decompose. Cosmologists call this fate an open universe or, if the mass exactly equals the critical mass, a flat universe.
In essence the fate of the universe is no different from throwing a ball up in the air. In that case, too, gravity competes with an initial force: that of your arm. Usually the ball rises, slows down, stops, and then falls back. This corresponds to a closed universe: gravity wins. But if you were strong enough you could send the ball into space and it would never come back to Earth. This corresponds to an open universe: the initial force wins.
Which fate will it be? As you might imagine, measuring the total amount of matter in the universe is not easy. Not only does this require adding up all the stars and galaxies we can see, it also requires adding up all the matter we can’t see: the dark matter whose presence is revealed only by the forces it exerts on visible matter. Right now the observations seem to indicate an open universe, while theory prefers a flat universe. Few think the universe will ever contract in a Big Crunch.
Growing pains. Like a human embryo growing in the womb, the universe started off as a tiny speck and grew to its present size and complexity. As it grew, the various things we take for granted today — space and time, physical forces, subatomic particles, atomic nuclei, atoms, stars, planets, life — were able to form in succession. In the very earliest moments, the rate of growth was much faster than today. This is why we’ve presented this timeline using a logarithmic scale. That is, the step between successive tick marks represents a factor of 10 billion in either size (vertical axis) or time (horizontal axis). The universe may continue to expand forever, or it may eventually stop expanding and contract into a Big Crunch. This diagram is based partly on a sketch by Chris D. Impey, Steward Observatory.
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