The Accelerating universe is the idea that our universe is undergoing accelerated expansion: distant objects are receding from our galaxy with speeds that increase over time. The accelerating universe is related to the Hubble Law: while the Hubble parameter may be decreasing with time, it is doing so slowly enough that distant objects continue to accelerate away from us.
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In the late 1990s, observations of Type Ia supernovae suggested that the expansion of the universe is accelerating. In the past few years, these observations have been corroborated by several independent sources: the cosmic microwave background, gravitational lensing, age of the universe and large scale structure, as well as improved measurements of the supernovae.
If the acceleration continues indefinitely, the ultimate result of this trend will be that galaxies outside the local supercluster will move beyond the cosmic horizon and will no longer be visible. The unusual energy thought to be responsible for the acceleration is called dark energy. It is "dark" because it is not known what form the energy takes: it is not directly visible and has not been detected in particle physics experiments. The leading candidates are a cosmological constant and quintessence, with recent WMAP data favouring the cosmological constant. The most important property of dark energy is that it has negative pressure and can be distributed relatively homogeneously in space. An even more hypothetical form of energy, known as phantom energy, causes divergent expansion, which will tear apart the Virgo supercluster and cause a Big Rip. Measurements of acceleration are crucial to determining the ultimate fate of the universe in big bang theory.
Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter and baryons. The density of dark matter in an expanding universe disappears more quickly than dark energy (see Equation of State (Cosmology)) and, eventually, the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved but the density of dark energy is nearly unchanged (it is exactly constant for a cosmological constant).
The observation of an accelerating universe presents problems for Dyson's eternal intelligence. This theory relies on a decelerating universe which, for many years, was the dominant model in cosmology since (in the absence dark energy) the gravitational attraction of ordinary matter in the universe would act to slow the expansion. By seemingly ruling out a Big Crunch, the accelerating universe also presents great problems for Tipler's Omega Point.
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Dyson's eternal intelligence (or The Dyson Scenario) is a concept that states an intelligent being would be able to think an infinite number of thoughts in an open universe. As the universe cooled, the thoughts would be slower and slower, but there would still be an infinite number of them. The idea was published in a scientific paper by Freeman Dyson.
Two recent observations have presented problems for Dyson's scenario. The first is that the expansion of the universe appears to be accelerating rather than decelerating due to a positive cosmological constant, meaning any two regions of the universe will eventually become permanently separated from one another. The second is that there appears to be a lower bound for the temperature of a vacuum, meaning that the universe would not continue to cool indefinitely.
Also, many grand unification theories predict that protons are unstable, albeit with a very long half-life. Thus the material base for intelligence could eventually disappear (in which case another type of matter could possibly be utilized). No evidence for proton decay has yet been detected, however.
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Cosmological constant problem
Unsolved problems in physics: Why doesn't the zero-point energy of vacuum cause a large cosmological constant? What cancels it out?A major outstanding problem is that most quantum field theories predict a huge cosmological constant from the energy of the quantum vacuum. This would need to be cancelled almost, but not exactly, by an equally large term of the opposite sign. Some supersymmetric theories require a cosmological constant that is exactly zero, which further complicates things. This is the cosmological constant problem, the worst problem of fine-tuning in physics: there is no known natural way to derive the infinitesimal cosmological constant observed in cosmology from particle physics.
One possible explanation for the small but non-zero value was noted by Steven Weinberg in 1987[2]. Weinberg demonstrated that if the vacuum energy took different values in different domains of the universe, then observers would necessarily measure values similar to that which is observed: the formation of life-supporting structures would be suppressed in domains where the vacuum energy is much larger, and domains where the vacuum energy is much smaller would be comparatively rare. This argument depends crucially on the reality of a spatial distribution in the vacuum energy density. There is no evidence that the vacuum energy does vary, but it may be the case if, for example, the vacuum energy is (even in part) the potential of a scalar field such as the residual inflaton (also see quintessence). It should be noted that there are good reasons to be wary of excessive use of the anthropic principle and much current research is aimed at understanding the observed vacuum energy density by non-anthropic means.
As was only recently seen, by works of 't Hooft, Susskind[3] and others, a positive cosmological constant has surprising consequences, such as a finite maximum entropy of the observable universe. (See the holographic principle.)
More recent work has suggested the problem may be indirect evidence of a cyclic universe predicted by string theory. With every cycle of the universe (Big Bang then eventually a Big Crunch) taking about a trillion (1012) years, "the amount of matter and radiation in the universe is reset, but the cosmological constant is not. Instead, the cosmological constant gradually diminishes over many cycles to the small value observed today." [1]
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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 74% of the Universe; the rest is dark matter (22%) and normal matter (4%).
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. |
