13 Things That Don't Make Sense (5 page)

Not that they were happy about it even then. The team leader, Brian Schmidt, probably put it best. His reaction, he told
Science
magazine, was “somewhere between amazement and horror.”

Nevertheless, the LBNL came out with the same conclusions shortly afterward. The results still stand. And what is pulling
the universe apart? We simply don’t know. But it is also pulling at the threads of the ultimate quest in physics.

BRIAN
Schmidt’s amazement and horror cannot have begun to plumb the depths of amazement and horror that would follow from his team’s
announcement. This is no longer just a cosmological mystery. The “nutty-sounding” observation, based on the light emitted
by a series of exploding stars, created rifts between some of the most eminent scientists on the planet. Now that the cosmological
constant is back in play, no one can agree how best to proceed. Paul Steinhardt, a theorist at Princeton University in New
Jersey, expressed his dismay that, thanks to the “cosmological constant problem,” many of our finest minds seem to have given
up on ever understanding our universe. “I’m disappointed with what most theorists are willing to accept,” he told the journal
Nature
in July 2007.

The controversy is—quite literally—much ado about nothing. The nothing in question is the universe’s “empty” space, which
is, in reality, far from empty.

The cosmos, whether it contains any matter or not, is fizzing with energy. In the 1920s, shortly after the birth of quantum
theory, which describes how nature behaves at the scale of atoms and subatomic particles, the British physicist Paul Dirac
used it to produce a quantum version of the theory behind the characteristics of electric and magnetic fields. Dirac’s
quantum field theory
eventually led to the prediction that empty space has energy. Since physicists refer to empty space as
the vacuum
, Dirac’s energy has come to be known as the
vacuum energy.

According to our best guess, this vacuum energy must be what powers the “antigravity” acceleration uncovered by the supernovae;
the vacuum energy is the cosmological constant. The trouble is, the measurements from the supernovae tell us the vacuum energy
is tiny. It is usually measured in grams. (Remember, according to Einstein’s famous equation
E
=
mc
²
, mass and energy are interconvertible.) The amount of vacuum displaced by the Earth’s volume in space would contain about
one hundredth of a gram’s worth of vacuum energy. That’s how small it is.

When, however, theorists work out the vacuum energy from quantum field theory, they get a number that is too big. Massively
too big. Their theory suggests that the vacuum energy is so big, it should have ripped the universe apart already in one massive
hyperacceleration. This is known as the cosmological constant problem and is widely accepted—even by the physicists involved—as
the
most embarrassing mismatch between theory and experiment ever. A million is a big number: a 1 followed by 6 zeroes. A trillion
has 12 zeroes. The mismatch between the measured and the theoretical value for the cosmological constant has 120 zeroes. One
hundred and twenty.

Faced with this failure, many physicists have adopted an idea first raised by the Nobel laureate Steven Weinberg in 1987.
In his book
Dreams of a Final Theory
, Weinberg suggested that a cosmological constant might exist in our universe without us ever being able to explain its value.
If ours was just one universe among many, each might have different values for its constants. Some of these universes would
no doubt be sterile, but some would lead to the production of life; there would probably be at least one where things like
humans evolved. This is the
anthropic landscape
approach to explaining the nature of the universe. (
Anthropic
means “of humans.”) The approach, when you boil it down, essentially says that our universe is the way it is because otherwise
we couldn’t be here to observe it. It doesn’t necessarily invoke a designer or any intention; it simply means if conditions
were different, no one would be around to observe them. Essentially, it says the very fact that we observe the universe limits
the range of forms it can take. The landscape bit comes from the physicists’ assertion that our universe is composed of a
hugely varied terrain, a patchwork quilt of subuniverses, each with its own unique and randomly assigned properties. There
need be no explanation for the values of the constants in each one.

As an “explanation” for the value of the cosmological constant, this is, to many physicists, abhorrent. Weinberg’s suggestion
is, says the Stanford University physicist Leonard Susskind, “unthinkable, possibly the most shocking admission that a modern
scientist could make.”

The idea is so distasteful because it turns science on its head. The philosopher Karl Popper said that science progresses
only by falsification: Someone throws up a hypothesis, and then anyone can use experimental data to attempt to shoot it down.
If the data falsify the hypothesis, you move on to the next one. Only when you have a hypothesis that has survived many shots
can you start to place some faith in what it’s saying.

With the anthropic landscape, this approach doesn’t work because the other universes are out of reach. You can’t falsify the
notion because you can never test it with experimental data. No longer do we explain why the universe is as it is; instead,
the universe is as it is because that makes it the kind of universe we can inhabit. Is this science? It might just be, Susskind
says; he thinks Weinberg is probably right. If we are to make progress toward understanding the universe, we may now have
to ditch Karl Popper and his adherents—Susskind calls them the
Popperazzi
—as the ultimate arbiters of what science is and isn’t. Perhaps we should just accept that, however much it makes the Popperazzi
fume, the laws of our universe may be as they are because of our own existence.

Difficult as this notion is to swallow, there is reason to take it seriously. Quantum field theory suggests that, if we must
use a cosmological constant to complete our description of the universe, our universe really ought to be one of very many.
It may be that, as E. E. Cummings once wrote, “there’s a hell of a good universe next door.”

At the root of this argument is the
uncertainty principle
of quantum theory, which says the fundamental properties of any system are never exactly defined but have an intrinsic fuzziness.
The uncertainty principle, when applied to quantum field theory, produces natural fluctuations in the properties of certain
regions of the universe. It is rather like having a balloon that is peppered with weak spots; as the universe inflates, these
fluctuations can grow, producing a new region of space and time. In other words, a universe containing a cosmological constant
that arises from the vacuum energy will produce new bubble universes all the time. Those bubbles will produce their own new
baby universes in turn—and so on, ad infinitum. What we think of as the universe is only one region of space-time in a frothing
sea of mini-universes.

The anthropic landscape idea has many supporters now, especially among theorists; that is why Steinhardt puts himself in the
minority. But if we can’t access these bubble universes to see whether they have different constants, aren’t we effectively
giving up on physics?

This was the root of the discussion in Brussels, the ghost of Albert Einstein looking over every shoulder. Should we be shrugging
our shoulders and putting the value of the cosmological constant down to the particular kind of universe we live in? Can we
face the idea that we may never understand what most of the universe is, that we may never get to the root of dark energy?
The answer was both yes and no: yes, it is a possibility we have to face; no, it doesn’t mean giving up hope of an explanation.
David Gross, who chaired the conference, was quick to make the point that at the first Solvay conference in 1911, the physicists
were similarly puzzled. Some materials had been shown to be emitting particles and radiation in a way that seemed to violate
the laws of conservation of mass and energy. The explanation came a few years later, when quantum theory was developed. “They
were missing something absolutely fundamental,” Gross told the 2005 Solvay assembly. “We are missing perhaps something as
profound as they were back then.”

So what is that “something fundamental”? Do we have any clues? The answer depends on whom you ask. Adam Riess, the man whose
radical, Shakespearean rhetoric pulled us into the dark energy era, offers a provocative suggestion. What if, he says, we
just don’t know enough about how gravity works? Maybe there isn’t any dark matter, and maybe there isn’t any dark energy.
Maybe for the last four centuries we’ve all been blind to tiny inaccuracies in Newton’s law of gravity, and these inaccuracies
hold the key to restoring the lost universe.

Riess isn’t the first to raise the idea, and he’s not saying it necessarily has any merit. His point is that it is a possibility,
and it has yet to be ruled out. Vera Rubin feels the same. She reckons that ninety-nine physicists out of a hundred still
believe in the existence of some dark stuff that fills the universe, its gravitational influence holding galaxies together.
But, to her eyes, changing the fundamentals of physics is starting to look like a better option.

On the face of it, the fix can be a relatively simple one. It was first suggested in 1981 by an Israeli physicist called Mordehai
Milgrom. Basically, you tweak Newton’s law of gravity so that at large distances, the kinds of distances that stretch across
galaxies and even clusters of galaxies, gravity is a little bit stronger than you’d otherwise expect. The idea is known as
Modified Newtonian Dynamics
, or
MOND,
and—despite its apparently innocuous nature—it has caused a lot of trouble.

Taking something that has worked perfectly well for four hundred years, something that was created by a man widely considered
to be the greatest scientist of all time, and suggesting it needs a little tweak is a brave move. Milgrom was not taken seriously
when he first suggested it. But he did gain a few supporters. Most notable among them was a young astronomer named Stacy McGaugh.

MCGAUGH
has taken so much flak in defense of MOND, he should be issued with a Kevlar jacket. If the way the dark matter problem was
overlooked for forty years taught Vera Rubin how dumb scientists could be, McGaugh, who used to be one of her graduate students,
taught her something else: just how resistant science is to change.

In March 1999 McGaugh gave a talk on MOND at the Max Planck Institute in Germany. No one there was willing to embrace the
idea. If you want us to take you seriously, they said, predict something; when it is borne out by experiment, we’ll listen.

A few months later McGaugh published a paper in the
Astrophysical Journal
that asked the cheeky question “What if there is no dark matter?” The result, he said, would be that a characteristic feature
in the cosmic microwave background radiation, the echo of the big bang, would be different from what the dark matter advocates
expected. The
power spectrum
, a kind of breakdown of the radiation, would show it up. Both MOND and dark matter models predicted that the power spectrum
would take the form of a series of peaks and troughs. Dark matter theory said the second peak would be slightly lower than
the first, but not significantly. Without dark matter, that second peak would be tiny, McGaugh pointed out; let’s see what
happens when the data come in.

McGaugh’s paper was published in late 1999. In the summer of 2000 Rubin was at a conference in Rome, watching him give a presentation
based on his paper to an audience of astronomers. Now there were data. And there was no second peak. None at all.

McGaugh had been granted a ten-minute slot. Rubin watched in shock as, when McGaugh ended his talk, nothing happened. “There
was not a single question afterwards,” she recalls. What’s more, she adds, the next morning some eminent cosmologist started
the discussion of the new results with not a single mention of the fact that they were different from the accepted dark matter
model.

Rubin has been impressed by MOND from that time on. Partly because she doesn’t like the idea of invoking exotic new particles
to explain a straightforward observation, and partly because mainstream astronomy has gotten too good at public relations,
and good PR, she says, suppresses proper scientific debate. Rubin has always been a fan of the underdog in science.

For a long time, MOND wasn’t even an underdog. As McGaugh will testify, it was more like a mangy dog sitting outside the conference
hall: an ad hoc idea cobbled together by an Israeli physicist with no better rationale for modifying gravity than the majority
had for invoking dark matter. But then, in 2004, Jacob Bekenstein got involved.

Bekenstein was born in Mexico City, studied physics at the Polytechnic Institute of Brooklyn and Princeton University, and
is now a professor at the Hebrew University of Jerusalem. As a young man he got up Stephen Hawking’s nose by making various
controversial proposals about black holes (which all turned out to be correct); now he is simply seen as one of our most formidable
minds. As soon as Bekenstein developed a version of Einstein’s relativity specifically tailored to show why MOND should be
taken seriously, the physics world had no choice but to sit up and listen. When Bekenstein’s relativistic MOND started fitting
rather nicely with other observations of the galaxies, what had once been a fringe idea suddenly had to be taken seriously.
And when lifelong dark matter supporters started switching sides, things started to get ugly.

SOMETIMES,
the idea that science is a neutral, careful, bias-avoiding discipline has a bad day. One such day was August 21, 2006, when
a NASA press release crowed, “NASA finds direct proof of dark matter.”