Coming of Age in the Milky Way (59 page)

In a sense, the big-bang universe is a high-energy physics experiment, which can be studied at various points in history to understand how the universe evolved. Analysis of this literally universal experiment confirms that most of the stuff of the universe is invisible—the “dark matter” problem—and that much of this invisible stuff cannot be ordinary matter, like planets and stars, but must be of some exotic form, like the particles predicted by string theories. (String theory, along with M-theory, which portrays the strings as membranes, continues to be a promising method for constructing a unified, quantum theory of gravity, but it remains unfinished.)

Important clues to the nature of dark matter were found by astronomers studying supernovae. One class of these exploding stars, the type la supernovae, all seem to reach about the same maximum
brightness (once one corrects for idiosyncracies such as differing abundances of nickel and other elements.) This makes them excellent “standard candles” for measuring the distances of remote galaxies and charting the expansion rate of the universe. Professional and amateur astronomers therefore launched ambitious projects that discovered hundreds of supernovae and charted their rise to maximum brightness. When these data were analyzed, the astronomers were astonished to find that the expansion rate of the universe, rather than slowing down as had been expected, appears to be accelerating. Evidently there is something like antigravity after all—a prospect envisioned in Einstein’s general relativity theory, but not previously found in nature.

This newly discovered antigravity field, often called dark energy, could be the same force that caused cosmic inflation in the first place, having now begun to reassert itself by speeding up the expansion rate. Or it could be something entirely unknown so far, perhaps one of the scalar fields long postulated by theoretical physicists although none have yet been clearly identified in the real world. In any event, there is clearly a lot to be learned about the vacuum and the quantum fields it contains.

Given that matter and energy are equivalent (as Einstein’s e = mc
²
reveals) an antigravity field pervading space counts as matter in cosmic bookkeeping. By 2003, astronomers could estimate with some confidence that dark energy constitutes two-thirds of the mass of the universe, with dark matter (its nature also unknown) making up almost all of the other third, while planets, stars, and interstellar gas and dust—the “bright matter”—account for less than one percent of the universe by weight. When the first edition of
Coming of Age
was written, scientists were in something like the situation of accountants who could weigh a locked safe and estimate how much precious metal it contained, but didn’t know whether they were gold bars or silver coins. Now the safe has been cracked. One can see that it contains a few coins (visible matter) plus two other lockboxes, one labeled “dark matter” and the other “dark energy.” The task is to pick the locks on those two boxes.

One remarkable—if slightly unsettling—prospect presented by dark energy is that the expansion rate of the universe may not dictate its destiny. It used to be thought that if the universe today was expanding above a certain rate, it would keep expanding forever.
And perhaps it will. But the “dark energy” field that evidently is accelerating cosmic expansion might have other plans: What it giveth, it can taketh away. Theorists find that certain kinds of scalar fields can speed up expansion for a while, so that the universe looks to be eternal, then put the brakes on and induce cosmic collapse. In some of these models, today’s accelerating universe is already well into middle age, and is destined to shrink to a fiery demise in another ten billion years or so. Ten billion years is a long time—it’s twice the age of the Sun and Earth—but the notion does give one pause.

To sum up, cosmologists can with some confidence say today that the universe:

Went through an initial period of extremely rapid expansion (an “inflationary” period);


Then got hot, producing the photons seen today as the cosmic microwave background radiation, and proceeded to cool as cosmic space expanded;


Was originally made of light elements (mostly hydrogen and helium, forged in the hot big bang) from which the heavier elements subsequently were made, inside stars;


Is geometrically “flat”—that is, that cosmic space has little or no observable curvature;


Is made mostly of dark energy (two-thirds) and dark matter (one-third) plus a bit of bright matter (all the things that astronomers yet have seen); and


Is expanding at an accelerating rate, owing to the influence of the dark energy field, which (depending on what sort of field it is) may continue to act in that fashion, or might instead collapse the universe at some future time.

That is certainly not the whole picture, but it’s an impressive amount to have learned, and it provides a sturdy foundation on which future research can build.

The origin of the universe remains a great mystery, and perhaps always will, but the vacuum genesis ideas depicted in
Chapter 18
have continued to bear fruit. Several leading cosmologists, notably Andrei Linde of Stanford University, have constructed consistent and physically reasonable models in which our universe is one
among many, perhaps an infinite number of universes. In these models, “new” universes bubble up out of the vacuum of preexisting ones. Many never attain a state in which life can exist—some quickly collapse, and others keep expanding at faster-than-light velocities forever, never forming matter—but some, like ours, can harbor life. The anthropic principle begins to make more sense in such models, inasmuch as it simply describes (or attempts to describe) the cosmological conditions required for life to appear in a given universe and for its existence therefore to be registered by intelligent observers. These models may help us understand how our universe got started, but they do so at the price of removing the ultimate question of genesis to a perhaps unattainable distance. Linde, who likes to imagine bubble universes as akin to apples on a tree, has attempted to calculate how many apples there may be, and how far in spacetime the original genesis event—the taproot of the tree—may be. Such calculations are of course based on many admittedly speculative assumptions, but for what it’s worth, Linde usually gets an infinite number of apples and an infinite distance from an average universe to the taproot. If so, the meta-universe looks infinite in both space and historical time, and the question of whether it ever had an origin cannot be answered.

The search for life in our universe continues, with no resolution in sight. SETI projects, operated on modest scales with private funding, have as yet detected no signal from an alien civilization, and nobody yet knows whether there is (or ever has been) life on Mars or elsewhere in the solar system.

The long-debated question of whether there are planets orbiting other stars has, however, been answered: It turns out that there are lots of them. Using a new method that involves carefully studying the motions of stars to detect planets’ gravitational pull on them—rather like a dog walker’s being tugged by the dogs on his leashes—astronomers have found more than one hundred extrasolar planets. The method works best for big dogs, so as of this writing only giant planets have yet been detected, but there is no reason to suppose that Earth-size planets aren’t abundant. Another approach involves monitoring many stars to measure slight reductions in their apparent brightness when a planet passes between them and us. These mini-eclipses should be detectable not only by professionals at mountaintop observatories, but by amateurs using digital
cameras attached to backyard telescopes as well. The stage is set for the first amateur astronomer to discover a planet since 1781, when William Herschel discovered Uranus. It’s even possible that an amateur stargazer or a high-school science student will find the first planet beyond Earth that harbors life.

We’re living in fascinating times. Stay tuned …

ABT.
Abbreviation employed in this book to mean “after the beginning of time,” which is here defined as the beginning of the expansion of the universe.

Absolute luminosity.
See
luminosity
.

Absolute magnitude.
See
magnitude
.

Absolute space.
Newtonian space, hypothesized to define a cosmic reference frame independent of its content of matter or energy. The existence of absolute space, enshrined in
aether
theory, was denied in
relativity
.

Aberration of starlight.
Displacement in the apparent location of stars in the sky, introduced by the motion of the earth.

Absorption lines.
Dark lines in a
spectrum
, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like
emission lines
, absorption lines betray the chemical composition and velocity of the material that produces them.

Acceleration.
An increase in velocity over time.

Accelerator.
A machine for speeding subatomic particles to high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction. (In the latter instance, the machine may be called a
collider
.) At velocities approaching that of light, the mass of the particles increases dramatically, adding greatly to the energy released on impact. The resulting explosion promotes the production of exotic particles, which are analyzed according to their behavior as they fly away through a particle
detector
.

Aether.
(1) In
Aristotelian physics
, the fifth element, of which the stars and planets are made. (2) In
classical physics
, an invisible medium that was thought to suffuse all space.

Alchemy.
Art of bringing parts of the universe to the perfect state toward which they were thought to aspire—e.g., gold for metals, immortality for human beings.

Andromeda galaxy.
Major spiral galaxy, 2.2 million
light-years
from Earth, Gravitationally bound to the
Milky Way galaxy
, with which it shares membership in the
Local Group
, it is currently approaching us, rather than receding as is the case for most galaxies.

Angular momentum.
The product of mass and angular velocity for an object in rotation; similar to linear momentum. In
quantum mechanics
, angular momentum is quantized, i.e., is measured in indivisible units equivalent to
Planck’s constant
divided by 2π.

Anisotropy.
The characteristic of being dependent upon direction. (Light coming with equal intensity from all directions is
isotropic;
a spotlight’s beam is anisotropic.) The
cosmic background radiation
is generally isotropic—i.e., its intensity is the same in all parts of the sky—but small anisotropics have been detected which are thought to reflect the earth’s proper motion relative to the framework of the universe as a whole.

Anthropic principle.
The doctrine that the value of certain fundamental constants of nature can be explained by demonstrating that, were they otherwise, the universe could not support life and therefore would contain nobody capable of worrying about why they are as they are. Were the
strong nuclear force
slightly different in strength, for instance, the stars could not shine and life as we know it would be impossible.

Antimatter.
Matter made of
particles
with identical
mass
and
spin
as those of ordinary matter, but with opposite charge. Antimatter has been produced experimentally, but little of it is found in nature. Why this should be so is one of the questions that must be answered by any adequate theory of the early universe.

Apparent magnitude.
See
magnitude
.

Aristotelian physics.
Physics as promulgated by Aristotle; includes the hypothesis that our world is comprised of four elements, and that the universe beyond the moon is made of a fifth element and so is fundamentally different from the mundane realm.

Asteroids.
Low-mass, solid objects that orbit the sun and shine by reflected light. Most belong to the “asteroid belt,” a zone located between the orbits of Mars and Jupiter. Though they number in the millions, their total mass is but a tiny fraction of the earth’s. Also called
minor planets
.