Dark energy is the single most important element in the universe. It influenced how the cosmos was born, how it is evolving today, and how it all will end trillions of years in the future. Right now, this energy is causing the universe to expand faster and faster; in the far future, the expansion may become so rapid that space itself will be torn apart. And yet we know next to nothing about what dark energy is. We don’t even have a proper name for it—the very term “dark energy” is little more than a scientific shrug.
Small wonder, then, that our recent DISCOVER magazine cover story about the mystery of dark energy (Confronting the Dark by Zeeya Merali) produced such an outpouring of curious reader mail. In a previous post, I addressed some of the key cosmological questions submitted by our readers. But really, that first set of responses only scratched the surface. For every letter writer who asked broadly about the nature of the Big Bang, someone else who wanted to know more about dark energy itself.
So as promised, here is a second installment addressing how scientists came to realize that energy, not matter, rules the universe.
As before, I need to put out a disclaimer up front. The answers I give here are not just my own. They are distilled from the dedicated efforts of astronomers and physicists around the world—although some of the more philosophical questions inevitably trigger more personal answers. I should also acknowledge that there is a lot we still do not know about the universe. But over the past 15 years, scientists have put together a convincing case that dark energy is real. At this point, it would require revolutionary new discoveries to disprove that.
That said, let’s move on to Cosmic Questions II: The Dark Universe.
What the scientists studying dark energy are doing is just speculation that leads nowhere. It is obvious that they have no idea what it’s all about. –Dick and Linda C.
There is a powerful question embedded in that statement. Cosmologists talk all the time about how little they understand about dark energy, about how enigmatic it is. Can they be sure that it exists at all?
To paraphrase a certain president: Yes they can.
Let me back that answer with a quick review of how dark energy was discovered in the first place. Ever since the Big Bang theory became widely accepted in the 1960s, researchers have been trying to measure a number called the “deceleration parameter.” That number describes the rate at which the universe is slowing down due to the mutual attraction of all the matter in the universe.
The rate of deceleration is significant for several reasons. It tells you the total mass of the observable universe. It tells you the fate of the universe, by showing if things are slowing down enough that they will eventually come to a halt and reverse. And it is crucial for determining the age of the universe, since it tells you how the rate of cosmic expansion has been changing since the time of the Big Bang.
To measure the deceleration, you need to compare how the universe is expanding now with the way it was expanding in the distant past. Fortunately, the finite speed of light acts like a visual time machine. A galaxy located one million light years away appears to us as it was a million years ago. A galaxy a billion light years ago appears as it was a billion years ago, and so on.
In the 1990s, a few researchers—most notably, Saul Perlmutter, Brian Schmidt, and Adam Riess, who eventually shared a Nobel prize—developed a novel way to measure the deceleration parameter by looking at very distant, extremely bright exploding stars called Type 1a supernovas. As the data rolled in, the deduced number they were getting for the deceleration parameter kept getting smaller and smaller, until it hit zero. That corresponds to a universe with no mass at all. “I guess we’re not here!” Perlmutter joked with his team. Then the number started going to less than zero. That meant the universe was speeding up, not slowing down, and some outward force, not the inward pull of gravity, was the dominant actor. Cosmologist Michael Turner proposed the name dark energy for this unknown repulsive agent.
When the first supernova papers came out in 1998, many scientists were skeptical. Since then, however, studies of cosmic acceleration have been repeated over and over, always with the same result. Dark energy is also supported by studies of the radiation left over from the Big Bang, and by studies of how clusters of galaxies have evolved over time.
Dark energy is not speculation. It is a description of the observed behavior of the universe.
Dogma be damned: I have a different idea about dark energy…
We got a lot of letters that began like that. DISCOVER readers clearly were not satisfied with the prevailing scientific view that space itself is filled with invisible energy—so much energy that its mass is far exceeds that of all the stars, planets, and gas put together. (Remember that mass and energy are equivalent: e=mc²)
It does seem counterintuitive that the universe as a whole is so different than the world we live in. On earth, everything you touch consists of atoms made of protons, neutrons, and electrons. On the cosmic scale, though, such “ordinary” matter is not ordinary at all. According to the latest cosmic measurements, it accounts for just 4.9 percent of the total. Dark matter is 26.8 percent. (For more about dark matter, see my primer on the shadow universe.) Dark energy is 68.3 percent. More than two thirds of the universe is completely immaterial.
Even cosmologists have a hard time wrapping their head around these ideas. So it is important to keep in mind that they have embraced the idea of dark energy not because it sounds cool, but because observational evidence has forced them to do it.
Scientists’ uncomfortable relationship with dark energy began with none other than Albert Einstein. In 1917 he examined the cosmic implications of his new theory of gravity (the general theory of relativity), and ran into a problem: On very large scales, gravity would tend to cause the universe to collapse. Einstein therefore invoked a possible repulsive force that could balance out gravity and allow the universe to remain static—as he and most astronomers believed it was at the time. He called it Lambda but that was, in essence, just an earlier name for dark energy.
The key point is that Einstein invoked dark energy because it was the simplest, most consistent way to explain the observed behavior of the universe. After the 1929 discovery that the universe is expanding, Einstein realized that the outward motion of the galaxies alone could explain why things are not collapsing, and he abandoned the idea of Lambda.
[An interesting historical note. Contrary to many claims in the popular press, there’s no compelling evidence that Einstein ever called Lambda his “biggest blunder.” Astronomer Mario Livio has recently found that the “blunder” story is probably a myth. Einstein always recognized that something like dark energy was a theoretical possibility—and now it looks like that possibility is a reality.]
Could dark energy be…
- …a crystallization that produced points of compressed space?
- …a cosmic electrostatic effect?
- …a 3-dimensional bubble moving as a point on the time axis in a 5-dimensional “real” universe”?
- …a “big suck” pulling on our cosmos?
- …energy expelled by stars when they go nova?
- …mass that is too far away to ever see?
In their letters, DISCOVER’s readers offered many creative way to explain dark energy. A credible theory of dark matter requires more than creativity, however. It must adhere to the known physical laws. It must offer specific, testable descriptions. And it must match all known observations about the behavior of the universe.
One of the oddest things about the expanding universe is that it has not always been expanding the same way. Until about 5 billion years ago—roughly the time when the Earth formed, give or take—it was slowing down. Then ago things turned around and the expansion started speeding up. Because we look back in time as we look out into space, this cosmic switcherooo shows up in a strange way. The most distant (long ago) parts of the universe are decelerating, while the more nearby (recent) parts of the universe are accelerating. Astronomers can actually see that pattern and map it, again using supernovas as milemarkers in deep space.
That slowing-then-speeding universe makes sense if there is a repulsive energy embedded in the structure of space. When the universe was young, small, and dense, gravity dominated. As the universe got bigger, its density dropped and the volume of space (and energy) increased. About 5 billion years ago, dark energy finally overwhelmed gravity.
Now it’s easy to see the problems with some of the reader ideas about dark energy.
A “big suck” implies that cosmic acceleration should be most intense at the greatest distances, but in fact we see the opposite. Mass that is too far away to see likewise could affect only the farthest parts of the universe; anything that lies beyond the edge of the visible universe from our vantage point cannot physically influence us here. Likewise, dark energy cannot be the effect of mass that is too far away to see.
Some other reader ideas fail because they apply commonsense ideas to cosmic questions—and human intuition simply is not a valid guide on such unfamiliar scales. For instance, matter and energy expelled by novas or supernovas does not change the density of the universe and so cannot influence its expansion. The universe is electrically neutral overall, so it cannot be under the influence of a cosmic electrostatic effect.
Finally, some of the more exotic reader ideas are too vague to evaluate. Terms like “compressed space” or “a 5-dimensional real universe” make sense only if they are precisely defined and connected to other, well-supported physical concepts.
The important lesson here: Anyone who is serious about developing a new theory of the universe first needs to engage in a thorough study of the current theories and to talk seriously with the people who have developed them. I strongly encourage that. The more perspectives brought to bear on cosmology, the more progress it will make.
Scientists say the Big Bang should have produced as much antimatter as matter. I believe that dark matter could be antimatter. Maybe matter and antimatter repel each other, like the north poles of two magnets, and this repelling force could account for the dark energy that is mysteriously expanding our universe. – Harry P.
This letter raises a fascinating and very concrete question: Does antimatter experience antigravity?
Amazingly, nobody knows. Current theory says that antimatter and matter should react to gravity exactly the same way. But what you really want to do is run a test: Assemble a clump of antimatter, let it go, and see if it falls down or up.
A large team of physicists working on the ALPHA experiment at CERN (the European physics consortium that brought you the Large Hadron Collider) recently did just that. They created atoms of antihyrdogen, trapped them, and then let them go. The results, published last month in the journal Nature, will disappoint anyone who enjoys answers like “yes” or “no.” The experiment is so difficult that the researchers can say only that the antihydrogen atoms didn’t move much; both gravity and antigravity are compatible with the results. Still, the ALPHA results show that the measurement is possible, and a much more sensitive project called AEgISwill follow up in a couple years.
(Since matter attracts matter, it’s not clear why antimatter would repel antimatter, as the reader suggests. At any rate, running that experiment in the lab would require holding and measuring two large lumps of antimatter, something that is far beyond our current technological capabilities.)
Antimatter probably cannot solve the dark energy problem. Balloon- and space-based detectors suggest that antimatter is rare in the universe. Even if the antimatter is hidden in distant locations, its influence should decrease as the universe expands—the opposite of the observed pattern of cosmic acceleration. Still, if antimatter and matter react differently to gravity, that would require some serious tinkering with both general relativity and quantum physics.
Your “Guide to the Dark Side” didn’t account for light energy. Surely, all that electromagnetic radiation, that would fry us to a crisp if it were not for Earth’s atmosphere, must count for something. — Gregg D
Now here’s an intriguing thought. Light and other forms of electromagnetic radiation contain mass in the form of energy. Perhaps that mass helps pull things together, or perhaps the pressure of radiation helps push things apart. It almost sounds like a zen koan: Could light energy be dark energy?
Measuring the total amount of radiant energy in the universe is quite easy, actually, because this is the stuff we can see. It turns out that radiation accounts for less than 0.01% of the mass of the universe (once again using special relativity to convert energy into its equivalent mass). That is too small to have a substantial effect on cosmic expansion, and the net effect is to slow things down, not speed them up.
There is that humbling lesson again: The things we see in the night sky—both the stars and the light by which they shine—make up a very small portion of what is out there.
The article made no mention of the Higgs boson. If I understand it correctly the Higgs creates a field; does that energy affect gravitational pull? –Roger
Let’s break this question into two parts. Does the Higgs field affect gravity? Most definitely, since the Higgs field is what gives other particles their mass, and without mass they would have no gravitational attraction. (At least, that’s what last year’s discovery seems to confirm.) So in that sense, the Higgs field is a central player in our whole cosmic drama.
Now the second part. Does the Higgs field have anything to do with dark energy? Very different question. Dark energy may be related to vacuum energy, the residual energy of all the fields that exist in empty space. Vacuum energy includes familiar things like the electromagnetic field (the one that allows light to exist) along with all the other known fields…including the Higgs field. So the Higgs could be one component of the energy that makes up dark energy. But we are deep into speculative territory here, since nobody yet has a well-established model of where dark energy comes from.
The cosmic expansion, we are told in this DISCOVER article, is happening only at large scales and is having no impact on smaller things like the Milky Way, planets, and atoms. Yet, one of the projected results of the expansion is the ripping apart of planets, atoms, and other small-scale things. How are the two reconciled? – D. V. Thompsop
It all comes down to timescales. Right now, dark energy makes itself felt only at the largest cosmic scales. Wait a while—and by a while I mean anywhere from tens to hundreds of billions of years—and the story may change. Recall that the balance between the inward pull of gravity and the outward push of dark energy seems to be steadily shifting in dark energy’s direction as the universe keeps expanding, creating more space and diluting the existing matter.
If the essential nature of dark energy stays constant, space will keep expanding faster and faster. As that happens, it will be harder and harder for gravity to hold things together. At the same time, the visible edge of the universe—the distance at which space moves away from us at the speed of light—will keep migrating closer and closer. Eventually the expansion of space will overwhelm the gravity that loosely holds clusters of galaxies together. In about a trillion years (about 100 times the current age of the universe) all other galaxies will disappear from view and we will live in our own pocket of isolation. “It’s going out in the bleakest fashion I can think of,” says Brian Schmidt. “It’s eternity, but it’s nothingness at the same time.”
Unless it isn’t. Nobody knows if dark energy stays the same over time. It could get stronger, in which case things get even worse. In this scenario, the expansion of space will become so rapid that it will start to disassemble our own galaxy, the Milky Way. Then things will keep going, until the accelerating expansion of space starts pulling apart stars, planets, people, atoms–everything–in a final Big Rip. The Big Rip might come as soon as 100 billion years from now (still long after the sun will have burned out, in case you are keen on long-range planning). Then again, dark energy might decay or even flip around, causing the universe to stop expanding and fall back in on itself. Some cosmological theories even suggest that a new Big Bang will happen after the current universe empties out.
The moral: Don’t count on a gloomy fate. There are so many theories on the table right now that you can pretty much pick the way you want the story to end…for now.
If space exploded out of nothingness to create the universe we inhabit now, this begs the question–Did the universe create God? –Gail S.
This is a completely unanswerable question, so forgive me for attempting anyway.
If you believe in a personal God—the kind of God who answers prayers, performs miracles, and speaks directly to people—then science has nothing to say on the matter. Such a God exists out of space and time and does not live within the laws of physics, so cosmological discoveries are irrelevant.
Einstein had a different view of God, one that is widely shared in one form or another by many other scientists who study the universe. “I believe in Spinoza’s God who reveals himself in the orderly harmony of what exists, not in a God who concerns himself with fates and actions of human beings,” Einstein told Rabbi Herbert Goldstein in 1921. When George Smoot called his map of the cosmic microwave background “the face of God,” or when Leon Lederman refers to the “God particle,” they are speaking roughly in this context. Insofar as the laws of physics emerged at the time of the Big Bang, you could say that the God we know appeared then too. Then the question of “what came before the Big Bang?” becomes equivalent to the question of whether there is a deeper, more timeless form of God.
Recent theories about multiple universes that exist in infinite time (also described in my previous post) provide a place for God to exist before our universe—and after, if there is an after–if you choose to interpret them that way. Those theories could also explain how our universe began, and what came before the Big Bang. As yet these theories are untestable, though, so they still live in the realm of metaphysics as much as physics.
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