Archive for the ‘DARK ENERGY’ Category

Interacting Dark Energy — constraints and degeneracies

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Timothy Clemson, Kazuya Koyama, Gong-Bo Zhao, Roy Maartens, Jussi Väliviita

In standard cosmologies, dark energy interacts only gravitationally with dark matter. There could be a non-gravitational interaction in the dark sector, leading to changes in the effective DE equation of state, in the redshift dependence of the DM density and in structure formation. We use CMB, BAO and SNIa data to constrain a model where the energy transfer in the dark sector is proportional to the DE density. There are two subclasses, defined by the vanishing of momentum transfer either in the DM or the DE frame. We conduct a Markov-Chain Monte-Carlo analysis to obtain best-fit parameters. The background evolution allows large interaction strengths, and the constraints from CMB anisotropies are weak. The growth of DM density perturbations is much more sensitive to the interaction, and can deviate strongly from the standard case. However, the deviations are degenerate with galaxy bias and thus more difficult to constrain. Interestingly, the ISW signature is suppressed since the non-standard background evolution can compensate for high growth rates. We also discuss the partial degeneracy between interacting DE and modified gravity, and how this can be broken…..
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October 15, 2011 at 9:42 pm


Why Einstein was wrong about being wrong

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There really is a mysterious antigravity force. Einstein’s only mistake was in rejecting it.

By Michael D. Lemonick
If you want to get your mind around the research that won three astronomers the Nobel Prize in physics last week, it helps to think of the universe as a lump of dough — raisin-bread dough, to be precise — mixed, kneaded and ready to rise. Hold that thought.
Now consider Albert Einstein — not the wild-haired, elderly, absent-minded professor he became in his later years but a young, dashing scientist in his 30s. It’s 1916, and he’s just published his revolutionary general theory of relativity. It’s not necessary to understand the theory (thank goodness). You just have to accept that it gave scientists the mathematical tools they needed to forge a better understanding of the cosmos than they’d ever had.

There was just one problem. Relativity told physicists that the universe was restless. It couldn’t just sit there. It either had to be expanding or contracting. But astronomers looked, and as far as they could tell, it was doing neither. The lump of dough wasn’t rising, and it wasn’t shrinking.

The only way that was possible, Einstein realized, was if some mysterious force was propping up the universe, a sort of antigravity that pushed outward just hard enough to balance the gravity that was trying to pull it inward. Einstein hated this idea. An extra force meant he had to tinker with the equations of general relativity, but the equations seemed so perfect just as they were. Changing them in any way would tarnish their mathematical beauty…… Read the rest of this entry »

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October 14, 2011 at 11:10 pm

What do we really know about Dark Energy?

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Ruth Durrer
In this paper I discuss what we truly know about dark energy. I shall argue that up to date our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, CMB anisotropies and observations of baryon acoustic oscillations, they all simply tell us that the observed distance to a given redshift is larger than the one expected from a Friedmann Lemaitre universe with matter only and the locally measured Hubble parameter…..
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In this work I have pointed out that all present claims about the existence of dark energy have not measured ΩΛ or even less ΩDE and w directly, but just the distance redshift relation DL(z).
They then have inferred the existence of dark energy by assuming the form (3) for this relation, which holds in a FL universe. Even though many of you (especially the observers, I guess) may regard this point as trivial, I find it important to be aware of it before one is ready to postulate unobserved scalar fields with most unusual properties, or violations of General Relativity on large scales.
I have not discussed the many possible pitfalls of the observations, which weaken any one observation, but my confidence relies on the fact that independent observations with different systematics find the same result. I hope they are not too strongly influenced by ’sociology’, i.e.: if your finding disagrees with the results of others it must be wrong and therefore you do not publish it, however, if it agrees well it must be right and therefore you do not have to investigate every possible systematics which would increase your error bars and make your result less competitive”.
The beauty of research in cosmology is that data come in fast and there is justified hope that the question whether relation (3) holds for the real Universe,
will be answered in the not very far future.

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October 9, 2011 at 11:40 am


Dark Energy FAQ

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In honor of the Nobel Prize, here are some questions that are frequently asked about dark energy, or should be.

What is dark energy?

It’s what makes the universe accelerate, if indeed there is a “thing” that does that. (See below.)

So I guess I should be asking… what does it mean to say the universe is “accelerating”?

First, the universe is expanding: as shown by Hubble, distant galaxies are moving away from us with velocities that are roughly proportional to their distance. “Acceleration” means that if you measure the velocity of one such galaxy, and come back a billion years later and measure it again, the recession velocity will be larger. Galaxies are moving away from us at an accelerating rate.

But that’s so down-to-Earth and concrete. Isn’t there a more abstract and scientific-sounding way of putting it?

The relative distance between far-flung galaxies can be summed up in a single quantity called the “scale factor,” often written a(t) or R(t). The scale factor is basically the “size” of the universe, although it’s not really the size because the universe might be infinitely big — more accurately, it’s the relative size of space from moment to moment. The expansion of the universe is the fact that the scale factor is increasing with time. The acceleration of the universe is the fact that it’s increasing at an increasing rate — the second derivative is positive, in calculus-speak.

Does that mean the Hubble constant, which measures the expansion rate, is increasing?

No. The Hubble “constant” (or Hubble “parameter,” if you want to acknowledge that it changes with time) characterizes the expansion rate, but it’s not simply the derivative of the scale factor: it’s the derivative divided by the scale factor itself. Why? Because then it’s a physically measurable quantity, not something we can change by switching conventions. The Hubble constant is basically the answer to the question “how quickly does the scale factor of the universe expand by some multiplicative factor?”

If the universe is decelerating, the Hubble constant is decreasing. If the Hubble constant is increasing, the universe is accelerating. But there’s an intermediate regime in which the universe is accelerating but the Hubble constant is decreasing — and that’s exactly where we think we are. The velocity of individual galaxies is increasing, but it takes longer and longer for the universe to double in size.

Said yet another way: Hubble’s Law relates the velocity v of a galaxy to its distance d via v = H d. The velocity can increase even if the Hubble parameter is decreasing, as long as it’s decreasing more slowly than the distance is increasing.

Did the astronomers really wait a billion years and measure the velocity of galaxies again?

No. You measure the velocity of galaxies that are very far away. Because light travels at a fixed speed (one light year per year), you are looking into the past. Reconstructing the history of how the velocities were different in the past reveals that the universe is accelerating.

How do you measure the distance to galaxies so far away?

It’s not easy. The most robust method is to use a “standard candle” — some object that is bright enough to see from great distance, and whose intrinsic brightness is known ahead of time. Then you can figure out the distance simply by measuring how bright it actually looks: dimmer = further away.

Sadly, there are no standard candles…. Read the rest of this entry »

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October 5, 2011 at 8:53 am


Europe to lead daring Sun mission

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The probe will orbit closer to the Sun than any previous spacecraft

Europe is to lead the most ambitious space mission ever undertaken to study the behaviour of the Sun.

Known as Solar Orbiter, the probe will have to operate a mere 42 million km from our star – closer than any spacecraft to date.

The mission proposal was formally adopted by European Space Agency (Esa) member states on Tuesday.

Solar Orbiter is expected to launch in 2017 and will cost close to a billion euros.

Nasa (the US space agency) will participate, providing two instruments for the probe and the rocket to send it on its way.

The Esa delegates, who were meeting in Paris, also selected a mission to investigate two of the great mysteries of modern cosmology – dark matter and dark energy….. Read the rest of this entry »

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October 5, 2011 at 8:39 am

Physics Nobel will attract controversy

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Assigning credit for a scientific discovery is never easy, especially when two rival, interacting teams of scientists are involved. That is exactly the problem that the Nobel committee must have grappled with before awarding this year’s physics prize to Saul Perlmutter, Adam Riess and Brian Schmidt.

Perlmutter led the Supernova Cosmology Project, while Schmidt and Riess were involved with the High-Z Supernovae programme. Both groups came to the surprising conclusion in 1998 that the rate of expansion of the universe is increasing, not decreasing as had been thought. So a shared prize seems fair enough.

Or is it? In 2007 Bob Crease wrote an extensive article about the same discovery that proved controversial – to say the least. Some members from both teams had been particularly worried about Crease’s article, which went through more than 20 drafts.

At issue was the fact that the teams were rivals using different techniques – as well as the question of who reported and published their work first. What Bob’s article reveals is how deeply scientific progress is indebted to ambition, desire, pride, rivalry, suspicion and other perfectly ordinary human passions.

You can read the article here.
By Hamish Johnston –

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October 4, 2011 at 1:11 pm


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Nobel Prize in Physics 2011

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From left to right, Adam Riess, Brian Schmidt and Saul Perlmutter, who have won the 2011 Nobel Prize in Physics

The Nobel Prize in Physics 2011 has been awarded to Saul Perlmutter, Brian P Schmidt and Adam G Riess for discovering the accelerating expansion of the universe

Three scientists shared the 2011 Nobel Prize for physics for the stunning discovery that the expansion of the universe is speeding up, meaning it may one day turn to ice, the prize committee said on Tuesday.

Scientists have known since the 1920s that the universe is expanding, as a result of the Big Bang some 14 billion years ago, but the discovery that this process is accelerating — and not slowing as many thought — rocked the research community.

“If the expansion will continue to speed up, the universe will end in ice,” the Nobel committee said in a statement.

Half of the 10 million Swedish crown ($1.5 million) prize money went to American Saul Perlmutter and the rest to two members of a second team which conducted similar work — U.S.-born Brian Schmidt, who is based in Australia, and American Adam Riess.

“We ended up telling the world we have this crazy result, the universe is speeding up,” Schmidt told a news conference by telephone after the award was announced in Stockholm.

“It seemed too crazy to be right and I think we were a little scared,” he added.

Nobel Committee for Physics at the Royal Swedish Academy of Sciences said in its statement that the discovery was made by looking at distant, exploding stars.

Instead of their light becoming brighter, it was fading.

“The surprising conclusion was that the expansion of the universe is not slowing down. Quite to the contrary, it is accelerating,” the committee said.

The acceleration is thought to be driven by dark energy, although cosmologists have little idea what that is.

They estimate that dark energy — a kind of inverse gravity, repelling matter that comes close to it — accounts for around three quarters of the universe. –

Yes…‘s prediction is wrong again!! (anyway… read the commnent below…)

2011 Nobel Prize in Physics will be announced within the hour! Watch the live webcast here

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October 4, 2011 at 10:02 am

Dark matter

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A review of the development of the concept of dark matter. The dark matter story passed through several stages from a minor observational puzzle to a major challenge for theory of elementary particles. Modern data suggest that dark matter is the dominant matter component in the Universe, and that it consists of some unknown non-baryonic particles. Properties of dark matter particles determine the structure of the cosmic web….
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September 27, 2011 at 8:57 am