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BBC NEWS
Mysterious force's long presence
Dark energy - the mysterious force that is speeding up the expansion
of the Universe - has been a part of space for at least nine billion
years.
That is the conclusion of astronomers who presented results from a
three-year study using the Hubble Space Telescope.
The finding may rule out some competing theories that predict the
strength of dark energy changes over time.
Dark energy makes up about 70% of the Universe; the rest is dark
matter (25%) and normal matter (5%).
Understanding the nature of dark energy is arguably the biggest
problem physics is facing today
Mario Livio, Space Telescope Science Institute
"It appears this dark energy was already boosting the expansion of
the
Universe as much as nine billion years ago," said co-investigator
Adam
Riess from the Space Telescope Science Institute in Baltimore, US.
"That's out of a Universe which we think is about 13.7 billion years
old - most of the way back."
The findings are consistent with the idea of dark energy behaving
like
Albert Einstein's cosmological constant. The cosmological constant
describes the idea that there is a density and pressure associated
with "empty" space.
In this scenario, dark energy never changes; it has the same
properties across the age of the Universe.
Repulsive force
Einstein first conceived of the notion of a repulsive force in space
in his attempt to balance the Universe against the inward pull of
its
own gravity, which he thought would ultimately cause the Universe to
implode.
His cosmological constant remained a curious hypothesis until 1998,
when astronomers used observations of supernovae from ground-based
telescopes and Hubble to show that the expansion of space was
accelerating.
These findings suggested there really was a repulsive form of
gravity
in space, a force that was shortly dubbed "dark energy".
There have been many attempts to explain the nature of dark energy.
One of these is that it behaves like the cosmological constant.
Another is that dark energy behaves like a field that changes over
time. The third proposes changes to our theories of gravity to
explain
the mysterious force.
The latest data from Hubble contradict theories that dark energy
might
have behaved differently billions of years ago to how it behaves
now,
or might not even have been present. Some astronomers had thought
that
dark energy might mimic whatever was the dominant force in the
Universe at the time, such as matter for example.
Previous Hubble observations of the most distant supernovae known
revealed that the early Universe was dominated by matter whose
gravity
was slowing down the Universe's expansion rate.
The observations also confirmed that the expansion rate of the
cosmos
began speeding up about five to six billion years ago. That is when
astronomers believe that dark energy's repulsive force took over
from
that of gravity.
'Tug of war'
"Imagine that you were having a tug of war and the other end of the
rope disappears behind a curtain. Somebody else is tugging on the
other end; we'll call that dark energy," said Dr Riess.
"In 1998, we saw that the thing behind the curtain was winning, it
was
pulling harder and the Universe was accelerating.
"In 2004, we showed that was not always the case. There was a time
when you - ordinary matter - were winning. The Universe was
decelerating. Now, we have shown that, even at that time, the thing
on
the other end of the rope was beginning to pull."
The discovery comes from observations of 23 exploding stars, or
supernovae. Using Hubble to peer far across the Universe, the
astronomers were able see back to a time when the cosmos was less
than
half its present size.
"These supernovae provide cosmic mile-markers that allow us to
measure
the growth rate of the Universe about nine billion years ago," said
Adam Riess.
Mario Livio, of the Space Telescope Science Institute, added:
"Understanding the nature of dark energy is arguably the biggest
problem physics is facing today."
In October, the US space agency (Nasa) said that shuttle astronauts
would be sent to service the Hubble Space Telescope, which will fail
within two or three years without running repairs.
Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/6156110.stm
Published: 2006/11/16 22:59:09 GMT
© BBC MMVI
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