13 Things That Don't Make Sense (4 page)

The answer to the first question comes from measuring the velocities of the receding galaxies, and knowing how far away they
are. You can’t just measure how fast a galaxy is moving away from us and call that the expansion rate of the universe; the
way space expands messes with your common sense. The farther away from us a galaxy lies, the faster it is moving away from
us because the space in between Earth and the galaxy is also expanding. The result, known as
Hubble’s constant,
gives a measure of the expansion rate; currently, we think it is about seventy kilometers per second per (roughly) 3 million
light-years. The accuracy shouldn’t be taken too seriously; that value is always subject to change when a better set of measurements
come in.

Answering the second question is, in many ways, much more interesting. If the universe is still expanding after the big bang,
that expansion should be slowing down; the mutual pull of all the matter in the universe works against any further expansion.
So our cosmic future depends on how much stuff there is out there, and how it is arranged.

Cosmologists already know something about those questions from one very easy scientific observation: the fact that we exist.
For that to be the case, the universe must have expanded from its hot, dense beginnings with a particular amount of energy.
If there had been too much, any matter that was created would have been spread so thinly that gravity couldn’t have pulled
atoms together into stars, galaxies, and—eventually—humans. As the matter spread farther, its gravitational pull would have
become even weaker and the expansion energy ever more dominant. The universe would have blown itself apart before anything
interesting—humans, for example—happened.

If there had been too little expansion energy, on the other hand, gravity would have pulled all the matter together in a similar
feedback cycle: once things got closer together, their gravitational pull would have become stronger, pulling them even more.
Eventually, the fabric of the universe would have shrunk back to implode in a scenario astronomers call the
big crunch
.

Given a certain amount of expansion energy, producing a Goldilocks universe like ours—one that’s “just right”—involves a precise
distribution of matter. As a shorthand for talking about the density of gravitating matter, astronomers refer to the
Omega
value of the universe. An Omega of 1, which corresponds to a measly six hydrogen atoms per cubic meter of universe (a cubic
meter of the air around you has something like 10 million billion billion atoms), is where the matter density more or less
balances out the expansion.

According to theory, the existence of stars and galaxies relies on Omega starting out within one part in a million billion
of 1. And, because of the nature of the feedback cycle with Omega, starting out in balance means remaining in balance. Today,
if the theorists are right, Omega should still be near 1. The trouble is, we know that there’s not nearly enough matter—dark
or otherwise—to make Omega 1.

It is this problem that led to the return of Einstein’s cosmological constant, something that no one saw coming. Hubble’s
triumphant discovery of the universe’s expansion had meant the cosmological constant could be ditched. The equations of general
relativity simply didn’t need the fudge factor that produced a steady-state universe, and by 1930 Einstein’s antigravity lay
embarrassingly redundant. Who could have imagined that, nearly seventy years later, it would be back, reincarnated in the
ghostly form of dark energy?

ASTRONOMERS
first started investigating Omega in the 1930s as a means of predicting the fate of the universe. If Omega is indeed 1, the
expansion will continue at its present rate. If the theorists are wrong, and Omega is less than 1, the power behind the expansion
will increase as the matter thins out. If Omega turns out to be greater than 1, gravity will eventually win out, and our future
lies in a big crunch.

Initially, the astronomers investigated Omega by continuing Slipher and Hubble’s methods: measuring the properties of the
light from galaxies. The vast number of light sources in a galaxy meant that this never produced anything reliable, however;
it is rather like trying to measure the properties of human speech by listening to the noise of a soccer crowd. What they
needed was a single object, something whose properties you could measure and draw inferences from. In 1987 they found one.
If you want to understand the fate of the universe, it turns out you’re going to have to get to grips with exploding stars:
supernovae.

We’ve been seeing supernovae in the skies for centuries; the Danish astronomer Tycho Brahe reported seeing one in 1572, more
than thirty years before the invention of the telescope. They occur when a star gets too big and collapses under its own gravity.
During the few weeks or months over which this collapse takes place, transforming the star into a neutron star or even a black
hole, it shines with the power of 10 billion suns. On Monday, February 23, 1987, we saw such a sight. The explosion of Sanduleak-69
202, a blue giant star in the Large Magellanic Cloud galaxy, was notable for two reasons. First, because it turned the star
into the brightest supernova seen since 1604. Second, because its light was the first to give a standard for measuring cosmic
distances.

The way certain supernovae—they are known as Type 1a Supernovae—emit their light has a peculiar characteristic that makes
them supremely appealing to astronomers. Type 1a explode because they have sucked too much material from a nearby star. Analyze
the spectrum of the light from this kind of explosion, and how fast its brightness fades away, and it will tell you how far
the light traveled to Earth, and how the expansion of space stretched the light on its journey.

The only drawback is that you have a limited window of opportunity. With supernovae, timing is everything. If you want to
get useful information, you have to find it within a couple of weeks of the light first reaching Earth. Since an explosion
happens about once per century in each galaxy, that means scanning a lot of galaxies with your telescope.

This kind of drudge work is a long-standing problem for astronomers. Inside Flagstaff’s Lowell Observatory, for example,
you can experience the agonizing nature of astronomy in Slipher’s day. When he led the search for Pluto, the technique used
was a celestial Spot-the-Difference. Put two photographic plates of the same region of the sky, taken on different nights,
into a machine called a blink comparator, and you can shuttle between the two almost entirely similar views. The winner is
the first to spot the one white dot—in the mess of white dots—that has moved. That shifting white dot is the planet you are
looking for.

Fortunately, in the Lowell exhibition, someone marked the displaced dot with a big white arrow. Modern image-reading technology
has made spotting the appearance of a supernova even easier; today, we have computers to provide the big white arrow. They
can compare two different photographs of the sky, then highlight the differences. Some of those will be asteroids; some will
be the varying brightness associated with the black holes at the center of galaxies; some will be false signals—bright flashes
from subatomic particles hitting Earth’s atmosphere. And, just occasionally, one will be a distant supernova.

The first strong interpretations of supernova data came in June 1996 from a team based at California’s Lawrence Berkeley National
Laboratory (LBNL). This announcement was made at a cosmology meeting convened to celebrate the 250th birthday of Princeton
University, the adopted intellectual home of Albert Einstein. A perfect place to begin the resurrection of his cosmological
constant, as it turned out.

When astronomers first got close to using supernovae to chart the universe’s expansion, they were convinced they were going
to find a deceleration. After all, the power of the big bang should be running out; gravity had taken over, and the brakes
were firmly on. It turns out, though, that the universe is not so simple.

At first glance, the LBNL results confirmed suspicions. The supernova light suggested that the universe’s expansion was slowing
down: the gravitational pull of the universe’s contents was decelerating the cosmos and setting Omega to somewhere around
1.

But it was a controversial finding. All the known gravitating matter in the universe—including the dark matter—gave an Omega
of only 0.3. Had everyone underestimated the amount of dark matter? It seemed unlikely; by this time various different methods
for determining the mass of galaxies were in use, and each showed there was significantly more gravitating matter than we
could see, and each gave approximately the same numbers.

If dark matter was on a fairly solid footing, what was going on? The cosmologists Michael Turner and Lawrence Krauss were
at the Princeton meeting, and they had an answer ready. Why not keep the dark matter at 0.3 but let something else make up
the missing 0.7? Instead of looking for some extra matter, why not assume it is actually extra energy? Bring back Einstein’s
cosmological constant, they said.

As is proper, experiment won out over the theorists’ speculations. When Saul Perlmutter published his LBNL group’s results,
the supernova data indicated that gravitating matter could account for pretty much all of Omega. No one needed to bring back
the cosmological constant; someone just needed to sort out the dark matter discrepancy. There must be more out there.

The trouble was, Perlmutter’s results raised problems of their own. If you know the matter density in the universe, the current
expansion rate (Hubble’s constant), and how much the universe’s expansion is slowing down, you can work out how long it is
since the expansion started; the age of the universe, in other words. With an Omega of 1 that is entirely due to matter, the
deceleration from the Lawrence Berkeley data put the universe’s age at not more than 8 billion years old. Unfortunately, astronomers
who had analyzed the light from the universe’s oldest stars set
their
age at around 15 billion years old. It doesn’t take a Harvard-trained mind to work out that the universe simply can’t be 8
billion years old if the stars are nearly twice that age. If there was a problem with the cosmological constant making up
Omega, there was also a problem with having a matter-induced Omega of 1. The only reliable fact, it seemed, was that dark
matter made up 0.3 of Omega; everything else was up for grabs.

Not everyone was disappointed by this impasse; Robert Kirshner, for one, was rather pleased. The Harvard astronomer was worried
that his own supernova results were coming too slowly to compete with the LBNL team; that his team had been beaten to the
punch. But it seemed the race to understand the fate of the universe was still wide-open.

In his book
The Extravagant Universe
Kirshner tells the story behind the supernova searches and the reinstatement of Einstein’s cosmological constant with great
clarity and wit. In the end, he turned the tables and came out first with the result that defined a new era in cosmology.
But only after he had defeated his own prejudices.

Kirshner’s team, composed of a handful of researchers from all over the world, was using supernova observations from telescopes
on mountaintops in Chile, Arizona, and Hawaii. Like the LBNL group, they would look out for new supernovae, month after month,
then follow up any really promising candidates by taking over the Hubble Space Telescope for some detailed observations. Hubble
could tease out information on a supernova’s distance from Earth and how the spectrum of its light varied as the explosion
ran its course.

Eventually, they had what they needed. And they didn’t like it one bit.

The distant explosions were fainter than they should have been: the light was having to travel farther than it should have.
It was Adam Riess, a Berkeley-based astronomer on Kirshner’s team, who first said it out loud: the data pointed to an acceleration.
The expansion of the universe was speeding up.

It was impossible. But try telling that to the supernovae. Every time Riess used the supernova data—the luminosity, the redshift,
and the fade over time—to work out a value for Omega, his calculations told him the universe contained a negative amount of
mass. The only way to make sense of it was to assume that mass wasn’t the only force at work in the universe’s expansion.
Add in a cosmological constant, and it all made sense. Given the choice between invoking negative mass and resurrecting Einstein’s
long-abandoned cosmological constant, the constant won out. But only just.

By January 1998, it was clear from conference presentations that the LBNL team’s data were now also pointing in the same direction;
they had refined their analysis, sorting out problems like how to account for the way interstellar dust would affect the observations.
The thing was, no one wanted to get it wrong. Announcing the return of Einstein’s cosmological constant became a battle of
nerves, a test of each team’s faith in their experimental abilities. To make the claim, or to wait a little, do a few more
tests, look again for the mistakes in handling the data? The prize was to be first to produce the scientific result of the
decade. The risk was sharing Einstein’s egg on the face.

Kirshner didn’t like the result, and he certainly didn’t want to taste any egg. He admits to doing everything he could to
make this go away. On January 12, 1998, he e-mailed Riess with some advice. “In your heart, you know that this is wrong,”
he wrote.

Riess replied that evening in a long e-mail to the team. His reply sounds almost Shakespearean, like something Henry V might
have said if he was an astrophysicist. “Approach these results not with your heart or head but with your eyes,” he wrote.
“We are observers after all!”

At the end of February, they came out with the results. A media storm followed. Riess eloquently told the CNN audience that
the universe’s expansion was accelerating, the cosmos was literally blowing itself apart—and Einstein’s cosmological constant
was back, pushing on the fabric of the universe. Kirshner came out with a rather un-Shakespearean sound bite, reported on
February 27 in the
Washington Post
. “This is nutty-sounding,” he admitted. “But it’s the simplest explanation.”