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%).
"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.
Source: http://news.bbc.co.uk/2/hi/science/nature/6156110.stm
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