* Astronomy

SuperNova legacy survey first results describing the nature of dark energy

The SuperNova Legacy Survey is an international collaboration involving about 40 researchers, which aims to discover several hundred far supernovae and measure their distance. The team's first results will be published in a coming issue of Astronomy & Astrophysics.
The SuperNova Legacy Survey is the largest observational project of its kind. It started in 2003 and will last for five years.
So far, the team has measured the distance to 71 supernovae that exploded between 2 and 8 billion years ago. Many of the largest telescopes worldwide are involved in this project; the imaging part of the programme is carried out at the Canada-France-Hawaii Telescope (CFHT), in the framework of the CFHT Legacy Survey. Spectroscopic observations are obtained at the ESO/Very Large Telescope, the Gemini and Keck observatories.

Measuring the distance to faraway supernovae is a key tool for cosmologists. Supernovae are exploding stars, known to have similar brightness whatever their location in other galaxies. Observing these exploding stars can thus make it possible to measure their distances: they are known as "standard candles" for measuring long distances in the Universe.



This supernova is as bright as 100 billion Sun-like stars. It exploded 3 billion years ago.
Credit CFHTLS/SNLS/Terapix

Measurement of these distances revealed a startling phenomenon; in the late 1990s, astronomers found that the expansion of the Universe is accelerating. American astronomer Edwin Hubble first discovered this expansion in 1929. The expansion of the Universe was thought to be slowing down because of the gravitational attraction of matter. Astronomers were thus very surprised to discover this was not the case at all.
Theorists then attempted to explain the acceleration of expansion through various cosmological models. These models all involved the so-called "dark energy" concept, which is a kind of repulsive force against gravitational attraction. Nobody knows what dark energy is, but we can make an attempt to understand how it behaves.

In recent years, cosmological observations have supported that the Universe is made of about 25 % of matter and 75 % of dark energy. Unlike matter, which dilutes with expansion, dark energy appears to stay roughly constant.
The new results, to be published by the SuperNova Legacy Survey team, put strong constraints on the absence of dilution of dark energy. Einstein himself already foresaw such a kind of dark energy when he introduced the famous "cosmological constant" into his General Relativity equations. Such a constant was needed for the equations to be consistent with a static universe, as it was believed to be at that time.
When the Universe was discovered to be expanding, it seemed that the cosmological constant was no longer needed in the equations. Later, Einstein referred to it as his "greatest blunder".
The discovery of the accelerating Universe expansion suggested the need for a cosmological constant that might, among other models, explain the acceleration of the expansion. The first results of the Legacy Survey indeed show that the existence of a cosmological constant is the best way to fit their observations.
Once completed, by the end of 2008, their Survey will bring even more restrictive constraints to these cosmological models. It will help us better understand the physical nature of this cosmological constant: 80 years later, "Einstein's greatest blunder" is perhaps less of a blunder after all.

Read More

Cosmologists have long accepted that the universe is expanding.
The space between galaxies is stretching.

But why is it that normal thing like tables and chairs don’t expand?

Now, Richard Price, a physicist at the University of Texas at Brownsville, has worked out why some objects are stretched by cosmological expansion, and others are not.
Atoms are made up mostly of empty space, with electrons "orbiting" the nucleus at distances typically many hundreds of times its diameter, it seemed reasonable to ask whether the electrons would be dragged away from the nucleus by the stretching of space.
Price examined the simplest system, that of a hydrogen atom, with one negative electron orbiting a positive proton. He found if the force involved - electromagnetic in the case of atoms - binding the system together is stronger than a certain critical value, the system will be entirely unaffected by the cosmological expansion

"This means that the solar system - which is quite tightly bound by gravity - doesn't expand. Your desk doesn't expand. Your dog doesn't expand. But clusters of galaxies, which are only loosely bound by gravity, will feel this effect" - Richard Price.

In an expanding universe, what doesn't expand?
Authors: Richard H. Price

The expansion of the universe is often viewed as a uniform stretching of space that would affect compact objects, atoms and stars, as well as the separation of galaxies. One usually hears that bound systems do not take part in the general expansion, but a much more subtle question is whether bound systems expand partially. In this paper, a very definitive answer is given for a very simple system: a classical ``atom'' bound by electrical attraction. With a mathemical description appropriate for undergraduate physics majors, we show that this bound system either completely follows the cosmological expansion, or -- after initial transients -- completely ignores it. This ``all or nothing'' behaviour can be understood with techniques of junior-level mechanics. Lastly, the simple description is shown to be a justifiable approximation of the relativistically correct formulation of the problem.

Read More (PDF)

Seven years ago, two research groups turned the field of cosmology on its head with their announcements that the expansion of the universe was accelerating. Their theories were based on the red-shift of light from distant supernovae, and their findings suggested that the expansion of the universe was actually accelerating, not decelerating.

The mysterious force behind this accelerated expansion was given the name "dark energy". Prior to this, conventional scientific wisdom held that the Big Bang had resulted in an expansion of the universe that would gradually be slowed down and eventually reverse, ending in a Big Crunch.
Now, cosmologists are scrambling to determine what exactly dark energy is and understand how it affects the expansion of the universe. Back in 1917, Einstein amended his General Theory of Relativity with a cosmological constant, which, if the value was right, would allow the universe to exist in a perfectly balanced, static state. Einstein later called the addition of this constant his "greatest blunder," but the notion of dark energy has revived the idea.

"The cosmological constant was a vacuum energy (the energy of empty space) that kept gravity from pulling the universe in on itself. A problem with the cosmological constant is that it is constant, with the same energy density, pressure, and equation of state over time. Dark energy, however, had to be negligible in the universe's earliest stages; otherwise the galaxies and all their stars would never have formed" - Eric Linder, Berkeley Lab.

To answer the questions posed by dark energy, Linder and Robert Caldwell of Dartmouth, have proposed several scenarios for how dark energy might interact with the universe. Their paper, to be published in Physical Review Letters, identifies models of dark energy which could be used to rule out Einstein's cosmological constant and explain the nature of dark energy. And, scientists should be able to determine which of these scenarios is correct with the experiments being planned for the Joint Dark Energy Mission (JDEM) that has been proposed by NASA.

Scientists have been arguing the question 'how precisely do we need to measure dark energy in order to know what it is?'.

JDEM could be part of the SuperNova/Acceleration Probe (SNAP), a three-mirror, 2-meter reflecting telescope in deep-space orbit that would be used to find and measure thousands of Type Ia supernovae each year. These measurements should provide enough information to clearly point towards either the thawing or freezing scenario - or to something else entirely new and unknown.

In their paper, Linder and Caldwell describe two scenarios, one they call "thawing" and one they call "freezing," which point toward distinctly different fates for our permanently expanding universe. Under the thawing scenario, the acceleration of the expansion will gradually decrease and eventually come to a stop. Expansion may continue more slowly, or the universe may even collapse.
Under the freezing scenario, acceleration continues indefinitely, like a car with the gas pedal pushed to the floor. Both of these scenarios rule out Einstein's cosmological constant. Under any scenario, however, dark energy is a force that must be reckoned with.

"Because dark energy makes up about 70 percent of the content of the universe, it dominates over the matter content. That means dark energy will govern expansion and, ultimately, determine the fate of the universe" - Eric Linder.

So where does dark energy come from? Enter the concept of quintessence. "Quintessence is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion. Whereas the cosmological constant is a very specific form of energy - vacuum energy - quintessence encompasses a wide class of possibilities" - Robert Caldwell.
Linder and Caldwell have proposed tests to see if quintessence - named after the fifth element of the ancient Greeks - is the source of dark energy.
As the basis for those tests, Linder and Caldwell used a scalar field as their model. A scalar field possesses a measure of value, but not direction, for all points in space. With this approach, they were able to show quintessence as a scalar field relaxing its potential energy down to a minimum value. An analogy might be a set of springs under tension and exerting a negative pressure that counteracts the positive pressure of gravity.

"A quintessence scalar field is like a field of springs covering every point in space, with each spring stretched to a different length. For Einstein's cosmological constant, each spring would be the same length and motionless" - Eric Linder.

Under the thawing scenario, the potential energy of the quintessence field was "frozen" in place until the decreasing material density of an expanding universe gradually released it. In the freezing scenario, the quintessence field has been rolling towards its minimum potential since the universe underwent inflation, but as it comes to dominate the universe it gradually becomes a constant value.

"If the results from measurements such as those that could be made with SNAP lie outside the thawing or freezing scenarios, then we may have to look beyond quintessence, perhaps to even more exotic physics, such as a modification of Einstein's General Theory of Relativity to explain dark energy" - Eric Linder.

Read More (PDF)