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Impearls: Through a Universe Darkly

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Earthdate 2004-01-16

Through a Universe Darkly

The journal Science presented its annual “Breakthrough of the Year” special issue as usual in its final installment of the year.  This last year's No. 1 Breakthrough, however, would appear to deserve honors beyond a single year's acclaim.  It seems silly to talk about ”Breakthrough of the Century” this early in the 21st century, but the scientific results this last year in astrophysics are downright breathtaking.

Here's how Science's Charles Seife describes these illuminating advances: 1

A lonely satellite spinning slowly through the void has captured the very essence of the universe.  In February, the Wilkinson Microwave Anisotropy Probe (WMAP) produced an image of the infant cosmos, of all of creation when it was less than 400,000 years old.  The brightly colored picture marks a turning point in the field of cosmology:  Along with a handful of other observations revealed this year, it ends a decades-long argument about the nature of the universe and confirms that our cosmos is much, much stranger than we ever imagined.

Five years ago, Science's cover sported the visage of Albert Einstein looking shocked by 1998's Breakthrough of the Year: the accelerating universe.  Two teams of astronomers had seen the faint imprint of a ghostly force in the death rattles of dying stars.  The apparent brightness of a certain type of supernova gave cosmologists a way to measure the expansion of the universe at different times in its history.  The scientists were surprised to find that the universe was expanding ever faster, rather than decelerating, as general relativity — and common sense — had led astrophysicists to believe.  This was the first sign of the mysterious “dark energy,” an unknown force that counteracts the effects of gravity and flings galaxies away from each other.

Although the supernova data were compelling, many cosmologists hesitated to embrace the bizarre idea of dark energy.  Teams of astronomers across the world rushed to test the existence of this irresistible force in independent ways.  That quest ended this year.  No longer are scientists trying to confirm the existence of dark energy; now they are trying to find out what it's made of, and what it tells us about the birth and evolution of the universe.

Lingering doubts about the existence of dark energy and the composition of the universe dissolved when the WMAP satellite took the most detailed picture ever of the cosmic microwave background (CMB).  The CMB is the most ancient light in the universe, the radiation that streamed from the newborn universe when it was still a glowing ball of plasma.  This faint microwave glow surrounds us like a distant wall of fire.  The writing on the wall — tiny fluctuations in the temperature (and other properties) of the ancient light — reveals what the universe is made of.

Long before there were stars and galaxies, the universe was made of a hot, glowing plasma that roiled under the competing influences of gravity and light.  The big bang had set the entire cosmos ringing like a bell, and pressure waves rattled through the plasma, compressing and expanding and compressing clouds of matter.  Hot spots in the background radiation are the images of compressed, dense plasma in the cooling universe, and cold spots are the signature of rarefied regions of gas.

Just as the tone of a bell depends on its shape and the material it's made of, so does the “sound” of the early universe — the relative abundances and sizes of the hot and cold spots in the microwave background — depend on the composition of the universe and its shape.  WMAP is the instrument that finally allowed scientists to hear the celestial music and figure out what sort of instrument our cosmos is.

The answer was disturbing and comforting at the same time.  The WMAP data confirmed the incredibly strange picture of the universe that other observations had been painting.  The universe is only 4% ordinary matter, the stuff of stars and trees and people.  Twenty-three percent is exotic matter: dark mass that astrophysicists believe is made up of an as-yet-undetected particle.  And the remainder, 73%, is dark energy.

The tone of the cosmic bell also reveals the age of the cosmos and the rate at which it is expanding, and WMAP has nearly perfect pitch.  A year ago, a cosmologist would likely have said that the universe is between 12 billion and 15 billion years old.  Now the estimate is 13.7 billion years, plus or minus a few hundred thousand.  Similar calculations based on WMAP data have also pinned down the rate of the universe's expansion — 71 kilometers per second per megaparsec, plus or minus a few hundredths — and the universe's “shape”: slate flat.  All the arguments of the last few decades about the basic properties of the universe — its age, its expansion rate, its composition, its density — have been settled in one fell swoop.

As important as WMAP is, it is not this year's only contribution to cosmologists' understanding of the history of the universe.  The Sloan Digital Sky Survey (SDSS) is mapping out a million galaxies.  By analyzing the distribution of those galaxies, the way they clump and spread out, scientists can figure out the forces that cause that clumping and spreading — be they the gravitational attraction of dark matter or the antigravity push of dark energy.  In October, the SDSS team revealed its analysis of the first quarter-million galaxies it had collected.  It came to the same conclusion that the WMAP researchers had reached:  The universe is dominated by dark energy.

This year scientists got their most direct view of dark energy in action.  In July, physicists superimposed the galaxy-clustering data of SDSS on the microwave data of WMAP and proved — beyond a reasonable doubt — that dark energy must exist.  The proof relies on a phenomenon known as the integrated Sachs-Wolfe effect.  The remnant microwave radiation acted as a backlight, shining through the gravitational dimples caused by the galaxy clusters that the SDSS spotted.  Scientists saw a gentle crushing — apparent as a slight shift toward shorter wavelengths — of the microwaves shining near those gravitational pits.  In an uncurved universe such as our own, this can happen only if there is some antigravitational force — a dark energy — stretching out the fabric of spacetime and flattening the dimples that galaxy clusters sit in.

Some of the work of cosmology can now turn to understanding the forces that shaped the universe when it was a fraction of a millisecond old.  After the universe burst forth from a cosmic singularity, the fabric of the newborn universe expanded faster than the speed of light.  This was the era of inflation, and that burst of growth — and its abrupt end after less than 10-30 seconds — shaped our present-day universe.

For decades, inflation provided few testable hypotheses.  Now the exquisite precision of the WMAP data is finally allowing scientists to test inflation directly.  Each current version of inflation proposes a slightly different scenario about the precise nature of the inflating force, and each makes a concrete prediction about the CMB, the distribution of galaxies, and even the clustering of gas clouds in the later universe.  Scientists are just beginning to winnow out a handful of theories and test some make-or-break hypotheses.  And as the SDSS data set grows — yielding information on distant quasars and gas clouds as well as the distribution of galaxies — scientists will challenge inflation theories with more boldness.

The properties of dark energy are also now coming under scrutiny.  WMAP, SDSS, and a new set of supernova observations released this year are beginning to give scientists a handle on the way dark energy reacts to being stretched or squished.  Physicists have already had to discard some of their assumptions about dark energy.  Now they have to consider a form of dark energy that might cause all the matter in the universe to die a violent and sudden death.  If the dark energy is stronger than a critical value, then it will eventually tear apart galaxies, solar systems, planets, and even atoms themselves in a “big rip.”  (Not to worry; cosmologists aren't losing sleep about the prospect.)

For the past 5 years, cosmologists have tested whether the baffling, counterintuitive model of a universe made of dark matter and blown apart by dark energy could be correct.  This year, thanks to WMAP, the SDSS data, and new supernova observations, they know the answer is yes — and they're starting to ask new questions.  It is, perhaps, a sign that scientists will finally begin to understand the beginning.

Note:  The publishers of Science, the American Association for the Advancement of Science (AAAS), have made the “Breakthrough of the Year(2003-12-19) special issue of Science available without subscription or pay-per-view.  (Normally Science costs $125 per year, publishedly weekly — and totally worth it; $56 of that is tax deductible, by the way.)  Thus, the article in question is available for free online, all one must do is register for Science Online.

The online article (linked to below) ends with numerous links to abstracts and full text of recent online research reports and review articles in this area.  (I like the title of the article from last year, by A. Gangui:  “A Preposterous Universe”!)  Interesting results, indeed.
 

References

1 Charles Seife, “Illuminating the Dark Universe,” Science, Vol. 302, Issue No. 5653 (dated 2003-12-19), pp. 2038-2039.  Does not require subscription or pay-per-view; does require registration.



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