Missing Pieces of the Cosmic Puzzle
Scientists pushing into unknown territory often find themselves at a loss for words. The more mysterious the emerging landscape, the further they must reach for appropriate language to describe it.
Lately, physicists who study the big questions of the universe can be heard tossing around such terms as “quintessence,” “X Dark Matter,” “smooth stuff,” “funny energy” and “tangled strings.”
These odd verbalizations mark their first attempts to understand one of the strangest mysteries of the cosmos: Where is all the energy hiding?
It’s bad enough that cosmologists can’t find 99% of the matter that the leading theories say must lurk in the universe (the so-called dark matter). Now, they realize they’re missing as much as 70% of the energy as well.
And until they account for all the energy needed to keep the universe “in balance,” physicists can’t explain why the universe has neither exploded nor collapsed.
Solving the “missing” energy problem is all the more urgent because it lies at the pivot of major mysteries of the cosmos, including: How did the universe evolve? How will it end? What is it made of? And that always intriguing question for physicists: Was Einstein right or wrong?
Recent sightings of exploding stars, speeding away at the edges of the cosmos, may be a sign of the missing energy at work. If so, it’s unlike anything ever seen before--hence, the labored attempts at descriptives.
This unusual form of energy acts like a kind of anti-gravity, pushing distant galaxies apart. The good news is that this newly discovered “repulsive force” may solve the missing energy problem. The bad news is physicists don’t yet have a clue as to its nature, its origin or its future.
Although the evidence for a repulsive force in the universe is new, the struggle to pin down the ineffable is as old as science itself. And so is the search for an appropriate imagery to describe it.
Isaac Newton described the forces between atoms as “a most subtle spirit, which lies hid in all gross bodies.” James Clerk Maxwell, who worked out the basic equations of electromagnetism, started with “a lot of imaginary wheels and idlers in space,” according to the late Caltech physicist Richard Feynman.
Frustration Setting In
While physicists squint into the darkness for clues to the forms hidden in shadows, they understandably stumble a lot. And the search for the missing energy is so elusive that it has them flailing about with more abandon than usual.
“This is desperation,” said astrophysicist Rocky Kolb, of Fermi National Laboratory, where a group of high-powered physicists met recently to ponder the universe’s missing energy. “The pieces aren’t fitting. It’s like you understand how the heart works and how the liver works and you try to figure out how the whole thing works. For now, we have too many legs and not enough arms.”
Many physicists believe that Einstein already had the answer to the “missing energy” problem when he inserted a so-called “cosmological constant” into his equations that described the state of the universe. This “constant” was a mathematical term for the repulsive force needed to keep all the matter and energy in the universe from gravitationally pulling together, and crushing everything into oblivion.
Later, when astronomers discovered that the universe was expanding of its own accord--spreading outward from its initial spectacular entrance in the Big Bang--adding an extra outward pushing force seemed redundant. Einstein dismissed his “constant” as the biggest blunder of his professional life.
Today, cosmologists are divided over whether Einstein was right about being wrong. While some would like to resurrect the cosmological constant to explain the missing energy, others are busy creating even more exotic alternatives. However, for the present they have far too few clues to come to any sort of even tentative conclusion.
Here’s what they know:
According to a majority of the top cosmologists and astronomers who gathered at Fermilab recently, the evidence is pretty solid that distant galaxies are flying away at faster and faster speeds. The speedometer astronomers use to measure this acceleration is a type of exploding star, called a supernova Ia, that appears to be a very consistent light source.
Because all supernovas of this type emit the same amount of light, their brightness should give an accurate measure of their distance. But measuring anything at the far edge of the universe needs to be approached with a certain amount of caution.
“There’s good reason to think [the supernovas] are all the same. But extraordinary claims require extraordinary evidence,” said University of Chicago astrophysicist Michael Turner, a leading authority on the cosmological constant.
The true nature of speeding supernovas are central because they are the one piece of firm evidence that the universe is really accelerating at its outer fringes. And the acceleration, in turn, is the “smoking gun” pointing to the existence of a strange force pressing space and matter outward like a kind of negative gravity, said Turner.
“If the universe is really accelerating, then we have something out there with negative pressure, and that’s something new,” he said.
The argument that supernovas are flying off at higher and higher speeds is based on the fact that they appear dimmer than astronomers expected. If they’re dimmer, the thinking goes, they must be farther away.
But distance isn’t the only effect that makes stars look dimmer. Space dust can produce the same effect. Skeptics of the speeding supernova theory argue that intergalactic dust may be creating a cosmic-scale optical illusion. “Maybe they’re not dimmer,” suggested Bill Press of the Harvard-Smithsonian Center for Astrophysics. “Maybe dust is making them [look] dimmer.”
The observers who tracked the supernovas said they had already ruled out the presence of red dust--the most common dust in the cosmos. But Press, playing devil’s advocate, suggested they might be looking through another kind of dust.
“[Maybe] they’ve discovered a new phenomenon, all right, but it’s just gray dust.”
Hanging in the Balance
Why do astronomers believe that most of the energy in the universe is missing in the first place?
When physicists add up all the known energy and matter in the universe, they don’t get enough to explain its behavior. The energy books don’t balance.
But the universe behaves as if they do--expanding at a stately pace. That is, the gravitational attraction of all the matter and energy pulling inward seems to almost exactly balance the expansive energy pushing outward. One reason scientists believe this balance exists is that in a universe with even a little more energy and matter, gravitational attraction would have crushed us into oblivion long ago. A universe with a little less energy and matter would have blasted out in all directions after the birth of the universe, never slowing down long enough to cohere into stars, planets and people.
“The universe gives every sign of performing this balancing act,” said Kolb. “It’s as if magically the taxes you take in balance the budget to the penny.”
How do cosmologists count up all the energy and matter in the universe? By watching the motions of stars and galaxies, experimentalists calculate the total gravitational pull of matter. By cooking up equations, theorists re-create the ingredients needed to produce our universe. Both methods come up short.
“We haven’t weighed the universe with any precision,” said Turner. “But there is a good case to be made that [the total amount of matter and energy] is about 35% [of what should be there to keep the universe balanced]. There’s a very strong case for something beyond [what is already known]. . . .”
That means that 65%-70% of the universe is something else. “The evidence tells us that most of the universe is ‘funny energy,’ for lack of a more technical term, and nothing more,” Turner concluded.
Some astronomers took to calling the funny energy “smooth stuff,” to distinguish it from matter, which jells into particles and stars and galaxies, and is therefore “clumpy stuff.” Others call it “X Dark Matter” or “tangled strings,” terms which have yet to be fully understood.
The challenge, of course, is to find out what this funny energy is. For now, only its behavior gives it away. The beast leaves no obviously detectable tracks beyond its repulsive push. “We only know it behaves differently [from ordinary matter and energy],” said Kolb.
The repulsive push is actually “negative pressure”--a force that pulls in a direction opposite from gravity.
Einstein’s equations initially predicted that the universe would gravitationally collapse on itself. Since this didn’t seem to be happening, he inserted the so-called cosmological constant--his version of negative pressure.
The cosmological constant has come and gone since Einstein. “It’s the most maligned constant in the history of physics,” said Josh Frieman, one of the organizers of the Fermilab meeting.
The main current objection to the cosmological constant is that it doesn’t appear to be constant. It changes over time. “It’s messier than a constant,” said Paul Steinhardt, an astrophysicist from the University of Pennsylvania and one of the meeting organizers.
To produce the universe as we know it, the repulsive force would have had to be much weaker for most of the past 15 billion years than it is today. A constant that changes is by definition paradoxical, and therefore messy.
The idea of an inconstant constant so bothers some physicists that they proposed a new kind of funny stuff in the universe, called quintessence. The term comes from the fifth essence that ancient philosophers believed permeated the universe--in addition to the four fundamental essences of earth, air, fire and water.
Whatever it is, quintessence would be a kind of cosmological constant that changes in force as the universe evolves. “Quintessence,” said University of Pennsylvania astrophysicist Robert Caldwell, “is shorthand [for a cosmological constant that varies]. It’s dynamic, it’s real, it’s substantive. But it’s not like any other kind of matter.”
If all this sounds confusing, it is--even to (perhaps especially to) the physicists working on the problem. “No one understands it,” said Case Western Reserve University astrophysicist Lawrence Krauss. Like others, Krauss doesn’t see any need to introduce a new kind of “stuff” into the universe to account for the missing energy. Yes, the cosmological constant needs to change its value, he said. “But quintessence would have to change, too.”
Or as Turner says: “What was good enough for Einstein ought to be good enough for us.”
Curiously, the universe has been pushed apart by a repulsive force before. Called “inflation,” this very early epoch in history caused the universe to expand exponentially in the first moments of its birth. Like the current acceleration, that expansion was propelled by the vacuum of empty space.
A vacuum holds onto energy the way ice holds onto heat. When ice melts, it releases that heat, which turns into the energy of rapidly moving water molecules. In the same way, physicists believe a vacuum can melt, releasing energy.
A currently accelerating universe, said Frieman, suggests “we could be entering a new period of inflation.” If so, cosmological history is indeed repeating itself--15 billion years later. “Inflation happened, and went away,” said Steinhardt. “One question is: Will this period of inflation end?”
Making Something Out of Nothing
Whatever truth they come up with, it won’t be the first time physicists have made something out of nothing. Before Einstein, physicists had largely concluded that empty space was permeated by “luminiferous ether”--invisible, virtually undetectable material that light waves traveled through. Experiments suggested that light was a wave, and waves must wave through something.
Yet the ether had almost impossible properties. Planets and stars had to float through it with no resistance. At the same time, in order to transmit fast vibrating light, it had to be solid material.
Einstein’s theory of relativity relegated ether to a graveyard of dead ideas. His theories about space and time rendered the ether unnecessary and irrelevant. “Einstein freed us from it,” said Fermilab’s former director and Nobel laureate, Leon Lederman. “Now we need to get rid of [today’s version of ether] again.”
In the end, perhaps that will be the fate of funny energy, said Lederman. “Some kid now in junior high school will tell us [how to get rid of it].” Whether the answer to the “missing energy” turns out to be quintessence or inconstant constants or some other kind of strange stuff, the physicists will ultimately abandon their poetic words and images for the more concrete truths to be found in equations. “The imagery allows us to move forward more rapidly, but the truth is in the math,” said Caltech physicist Kip Thorne.
For the time being, the physicists will continue to speak in tongues, struggling to invent an appropriate language, sounding more like wordsmiths than scientists.
Perhaps that’s appropriate. The late Nobel laureate Niels Bohr, who first saw clearly into the fuzzy heart of atoms, said that physicists trying to describe the subatomic realm in everyday language were more poets than scientists.
“The poet, too,” he wrote, “is not nearly so concerned with describing facts as with creating images.”
