Fifteen years ago, a small cabal of researchers took some of the most firmly held notions about how the universe works and turned them on their head. Until then, everyone was sure that the expanding universe was born in an explosive Big Bang and had been slowing down ever since, dragged by the gravitational pull of untold billions of galaxies. But in fact the expansion is speeding up. Everyone was sure that matter was what dominated the overall behavior of the universe. But in fact it seems that “dark energy,” not matter, is running the show. Whoops.
The May cover story in DISCOVER magazine (Confronting the Dark by Zeeya Merali) chronicles that game-changing discovery, and lays out the latest thinking about what dark energy is and how it affects the fate of the universe. As soon as the article was published, DISCOVER’s inbox began to fill with letters from curious readers wanting to know more. Here I will address sweeping, big-picture questions about cosmology. I’ll consider more specific queries about dark energy and dark matter in a following post.
Before I dive in, an important pieces of context. The answers I give here are not my own. They are distilled from the dedicated efforts of astronomers and physicists around the world, working with the greatest telescopes and instruments ever built. There is a lot we still do not know about how the universe began and how it will end. Some widely held ideas will, very likely, again be overturned. But the past century of research has yielded an amazingly detailed understanding about the overall structure and workings of the universe. OK then, on to the questions!
I have seen maps of the universe, but I never saw where it started. Is there some way we could plot the direction of all the galaxies to reverse engineer the starting point? –Roger D.
This question, and several other similar ones we received, gets at one of the most confounding yet fundamental ideas in modern cosmology. The Big Bang was not an explosion in space—it was an explosion of space. Put another way, the Big Bang took place everywhere at once because space itself emerged at the same time as matter and energy. There was no outside space that the universe expanded into (at least not in the familiar three-dimensional sense), and there is no one location we can point to that is the place where the Big Bang began.
Wherever you are sitting now, you can think of that as the center of the Big Bang. It is as accurate as picking any other location. Sorry, but that’s the real answer.
From our perspective, galaxies appear to be flying away in all directions. Observers elsewhere in the universe would see the exact same thing. There’s nothing special about our spot, because every location in a uniformly expanding universe appears to be at the center of the expansion. Plotting the direction of galaxies cannot reverse engineer the starting point; again, it will only lead right back to where you are.
You might wonder, how can galaxies all be flying through space in such neat formation? The answer again requires discarding the notion of “space” as a fixed, immutable thing. In the overall expansion of the universe, galaxies are not flying at tremendous speeds through space; space itself is expanding, increasing the total scale of the universe.
What is the shape of the universe? Is it a hollow sphere? The balloon analogy seems to suggest it is, but it can’t be that simple. –Howard L.
The balloon analogy is a visual tool that that cosmologists often use to help explain the expansion of the universe. Imagine you are sitting on the surface of an enormous balloon that is marked with dots. If the balloon is inflated, the dots appear to move away from your location in all directions. The same is true for any other observer at any other location on the balloon. Furthermore, the speed at which the dots move away is proportional to their distance. Imagine the balloon doubles in size after a minute. Dots that were an inch away are now two inches away; dots that were two inches away are now four inches away (ie, they have moved twice as far); and so on.
The problem with the balloon analogy is that it is just an analogy. On a local scale the surface of the balloon is essentially a two-dimensional membrane, but the universe is a three-dimensional space. The balloon has a geometric center in three dimensions, whereas the universe does not. “The interior of the balloon is analogous to the 4th dimension,” explains Brian Schmidt, who shared the Nobel prize for the discovery of the accelerating universe. In that sense, he argues, you really can think of the universe as a higher-dimensional sphere. I don’t know about you, but I that pretty hard to visualize.
Cosmologists do talk about the overall “shape” of space in the universe. This is a way of describing what would happen to a beam of light traveling an extremely long distance through space: Would it curve or move in a straight line? (The shape of the universe is influenced by its overall density.) A widely accepted cosmological model called “inflation,” developed in the 1980s, predicted that the universe should be almost perfectly flat. At the time, there was no way to tell, but now we know that the prediction was correct: By studying microwave radiation emitted shortly after the Big Bang, NASA’s WMAP satellite has found that the universe is flat to within a 0.4% margin of error.
How about that. The world is flat after all.
What existed 10 minutes before the Big Bang? What caused the Big Bang to occur? How many other Big Bang universes are there? – Joseph T.
The simplest and most honest answer to this question is, “nobody knows.”
Oh, but plenty of people are wiling to theorize. There are many ideas out there in the scientific literature. In the 1920s and 1930s a number of scientists, including Albert Einstein, considered the possibility of an eternal, cyclic universe that expands, contracts, and rebounds over and over. Those original models failed because they violated the second law of thermodynamics; essentially, the universe would keep running down instead of resetting. But the idea of endless rebirth is so appealing that it keeps coming back.
One form is the ekpyrotic cosmology co-developed by Paul Steinhardt at Princeton University. In this model, the Big Bang was sparked by the collision of two “branes”—three dimensional worlds moving through higher-dimensional space. Picture two crinkled pieces of paper banging into each other and you have the right idea, within the limits of visualization. When the branes hit, our universe was born and the two branes moved apart. After a trillion years or so they will collide again, triggering a new Big Bang and a new universe, and then again and again. According to Steinhardt’s calculations, the cycle could keep going essentially forever without violating thermodynamics.
Another type of eternal cosmological model emerges from the theory of inflation—the same one that predicted that the universe is flat. Cosmologists Andre Linde and Alan Guth, two of the creators of inflation theory, realized that this model could allow not just a single Big Bang but endless Big Bangs, each giving rise to new universes. In this model of eternal inflation, our universe is just one of a multitude—a multiverse—which could be infinite in extent and duration. Each universe is born from a quantum fluctuation in an energy field, which rapidly buds off and expands into a new universe. The inflation field can be thought of as the trigger that made the Big Bang go bang. Guth once called this “the ultimate free lunch.”
And things get weirder. Each universe could have its own laws of physics, meaning that some would be almost exactly like ours and some would be completely different. String theory (which attempts to build a single set of rules to explain all particles and forces) predicts there could be 10500 different types of universes. For now this is pure speculation, however.
The underlying theory of inflation, on the other hand, accurately matches many of the observed properties of the universe, and it has received impressive empirical support. Inflation predicts a specific pattern in the cosmic microwave background, the radiation left over from the Big Bang. The WMAP and Planck satellites have observed just such a pattern. That does not prove that inflation is correct, but it sure does make the theory look more credible.
If the Big Bang initially expanded the universe faster light, doesn’t that violate Einstein’s belief that nothing can exceed the speed of light? –Rick B.
If inflationary model of cosmology is correct, the universe expanded faster than light—much, much faster than light—in the first 10-30 second of existence. At first blush that sure seems like a violation of Einstein’s special theory of relativity, which states that nothing can go faster than light. More specifically, though, special relativity states that no object with mass can match (or exceed) the speed of light. In the early universe, objects were not moving through space faster than light; space itself was moving faster than light, which does not violate Einstein at all.
Sounds like cheating, doesn’t it? But this concept is completely true both to the letter and the spirit of Einstein’s theory. Special relativity explains the behavior of light and moving objects, and accounts for why the laws of physics look the same to all observers. The hyper-expansion of space would not affect the local laws of physics, and any objects receding faster than light would be fundamentally unobservable and hence irrelevant.
Once again, the key is dispensing with the idea of objects moving through space and getting used to the idea that space itself can stretch. That is also essential to understanding the current thinking about dark energy and the accelerating expansion of the universe.
The author refers to the redshift related stretching of light as arising from the Doppler Effect, but this is not true. It is from space stretching which is distinctly different from the elongation of wavelength from the Doppler Effect. –Tom M.
The writer is correct. As distant galaxies move away from Earth, their light gets stretched and reddened. The resulting “redshift” is how Edwin Hubble (drawing on data from unsung astronomer Vesto Slipher) deduced the apparent expansion of the universe in 1929. Many scientists—including Hubble himself—have attributed that reddening to the Doppler effect, even though that explanation is not technically accurate.
The Doppler effect causes waves to pile up if they are moving toward you and to stretch out if they are moving away. The classic example is the siren of a fire engine, which shifts to a higher note as the engine approaches you and suddenly shifts to a lower note as it passes by and begins to recede. Astronomers observe Doppler shifts all the time, measuring how various objects are moving toward or away from their telescopes. This is one of the primary ways that scientists have identified planets around other stars.
But as I keep saying (and please bear with me), the expansion of the universe is due to an expansion of space itself, not to the motion of galaxies through space. As light waves move through expanding space, they themselves get expanded and shifted to the red. (The balloon analogy is useful again: Think what would happen if you drew a wave on the balloon and then blew it up.) The result is essentially equivalent to a Doppler shift, but the root cause is very different. For this reason, the redshifts of distant galaxies are properly known as cosmological redshifts. A tip of the hat to Tom M. for catching a subtle but important error.
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