Physicists call their ignorance:a ‘dark force’


Richard Moore

Original source URL:

November 17, 2006

9 Billion-Year-Old ŒDark Energy¹ Reported

A strange thing happened to the universe five billion years ago. As if God had 
turned on an antigravity machine, the expansion of the cosmos speeded up, and 
galaxies began moving away from one another at an ever faster pace.

Now a group of astronomers using the Hubble Space Telescope have discovered that
billions of years before this mysterious antigravity overcame cosmic gravity and
sent the galaxies scooting apart like muscle cars departing a tollbooth, it was 
already present in space, affecting the evolution of the cosmos.

³We see it doing its thing, starting to fight against ordinary gravity,² Adam 
Riess of the Space Telescope Science Institute said about the antigravity force,
known as dark energy. He is the leader of a team of ³dark energy prospectors,² 
as he calls them, who peered back nine billion years with the Hubble and were 
able to discern the nascent effects of antigravity. The group reported their 
observations at a news conference yesterday and in a paper to be published in 
The Astrophysical Journal.

The results, Dr. Riess and others said, provide clues and place new limits on 
the nature of dark energy, a mystery that has thrown physics and cosmology into 
turmoil over the last decade.

³It gives us the ability to look at changes in dark energy,² he said in an 
interview. ³Previously, we knew nothing about that. That¹s really exciting.²

The data suggest that, in fact, dark energy has changed little, if at all, over 
the course of cosmic history. Though hardly conclusive, that finding lends more 
support to what has become the conventional theory, that the source of cosmic 
antigravity is the cosmological constant, a sort of fudge factor that Einstein 
inserted into his cosmological equations in 1917 to represent a cosmic repulsion
embedded in space.

Although Einstein later abandoned the cosmological constant, calling it a 
blunder, it would not go away. It is the one theorized form of dark energy that 
does not change with time.

Sean Carroll, a cosmologist at the California Institute of Technology who was 
not on the team, said: ³Had they found the evolution was not constant, that 
would have been an incredibly earthshaking discovery. They looked where no one 
had been able to look before.²

The paper by Dr. Riess and his colleagues represents a sort of progress report 
from the dark side, where astrophysicists have found themselves more and more as
they try to understand what is happening to the universe.

This encounter with the invisible began eight years ago, when two competing 
teams of astronomers were using exploding stars known as Type 1a supernovas as 
cosmic distance markers to determine the fate of the universe.

Ever since the Big Bang 14 billion years ago, the galaxies and the rest of the 
universe have been flying apart like a handful of pebbles tossed in the air. 
Astronomers reasoned that gravity would be slowing the expansion, and the teams 
were trying to find out by how much and, thus, determine whether all would 
collapse one day into a ³big crunch² or expand forever.

Instead, to their surprise, the two teams, one led by Saul Perlmutter of the 
University of California, Berkeley, and the other by Brian Schmidt of the Mount 
Stromlo and Siding Spring Observatories in Australia, found that the universe 
was speeding up instead of slowing down.

But the ground-based telescopes that the two teams used could track supernovas 
to distances of just seven billion light-years, corresponding to half the age of
the universe, and the effect could have been mimicked by dust or a slight change
in the nature of the supernova explosions.

Since then, Dr. Riess, who was a member of Dr. Schmidt¹s team, and his 
colleagues have used the Hubble telescope to prospect for supernovas and dark 
energy farther out in space or back in time.

The new results are based on observations of 23 supernovas that are more than 
eight billion years in the past, before dark energy came to dominate the cosmos.
The spectra of those distant supernovas, Dr. Riess reported, appear to be 
identical to those closer and more recent examples. By combining the supernova 
results with data from other experiments like the NASA Wilkinson Microwave 
Anisotropy Probe, Dr. Riess and his colleagues could begin to address the 
evolution of dark energy.

³That¹s one of the $64,000 questions,² he said. ³Is dark energy changing?²

So far, he said, the results are consistent with the cosmological constant, but 
other answers are also possible. The possibility that it is the cosmological 
constant is a mixed blessing. Physicists concede that they do not understand it.

Dr. Carroll of Caltech said, ³Dark energy makes us nervous.²

Einstein invented his constant to explain why the universe does not collapse. 
After he abandoned it, the theory was resuscitated by quantum mechanics, which 
showed that empty space should be bubbling with staggering amounts of repulsive 
energy. The possibility that it really exists in the tiny amounts measured by 
the astronomers has flummoxed physicists and string theorists.

Because it is a property of empty space, the overall force of Einstein¹s 
constant grows in proportion as the universe expands, until it overwhelms 
everything. Other theories of dark energy like strange force fields called 
quintessence or modifications to Einstein¹s theory of gravity can change in more
complicated ways, rising, falling or reversing effects.

Astronomers characterize the versions of dark energy by their so-called equation
of state, the ratio of pressure to density, denoted by the letter w. For the 
cosmological constant, w is minus one.

Dr. Riess and his group used their data to make the first crude measurement of 
this quantity as it stood nine billion years ago. The answer, he said, was minus
one ‹ the magic number ‹ plus or minus about 50 percent. By comparison for more 
recent times, with many more supernovas observable and thus more data, the value
is minus one with an uncertainty of about 10 percent.

³If at one point in history it¹s not minus one,² Dr. Riess said, ³then we have 
killed the very best explanation.²

Getting to the precision needed to kill or confirm Einstein¹s constant, however,
will be very difficult, he conceded. One of the biggest sources of uncertainty 
is the fact that the Type 1a explosions are not completely uniform, introducing 
scatter into the observations.

The Hubble is the sole telescope that can pursue supernova explosions deeply 
enough to chart the early days of dark energy. The recent announcement that the 
National Aeronautics and Space Administration will send astronauts to maintain 
and refurbish the Hubble once again, enabling it to keep performing well into 
the next decade, is a lift for Dr. Riess¹s project. A new camera could extend 
observations to 11 billion or 12 billion years back.

Copyright 2006 The New York Times Company

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