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Posts Tagged ‘dark energy’

There have been a number of rumours of late that the evidence for dark energy is suspect, and that maybe, after all, it doesn’t exist. The stories are due to a paper which was recently published in Nature Scientific Reports, for a link to the original paper follow this link. Unlike most papers published in Nature, which are behind a paywall, this paper is available in its entirety for free. The authors argue that a larger data set of Type Ia supernovae, which were used in the 1990s as evidence for an accelerating Universe, now calls into question that whole interpretation of the data.

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This paper – “Marginal evidence for cosmic acceleration from Type Ia supernovae”, published in Nature Scientific Reports on 21 October 2016, calls into question the evidence for the existence of dark energy.

The 2011 Nobel prize for physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for their original “discovery” of cosmic acceleration, so if new data now call into question that whole idea it is, clearly, big news.

However, Adam Riess has co-authored an interesting guest blogpost in Scientific American, entitled “No, Astronomers Haven’t Decided Dark Energy Is Nonexistent”, here is a link to that article. Riess and his co-author Dan Scolnic (a cosmologist based at the University of Chicago’s Kavli Institute for Cosmological Physics) point out in this blogpost that the re-analysis by Nielsen etal. reduces the confidence that the Universe is accelerating to a 3-sigma result, which is still at a confidence level of 99.7%! So, it hardly does away with the need for acceleration, at a confidence of 99.7% it is still pretty likely. True, it now falls short of the usual 5-sigma result that scientists usually require for a “definite result”; but they also take issue with the way that Nielsen etal. have analysed their data.

Also, as Scolnic and Riess point out, evidence for cosmic acceleration is not just based on the results from surveys of Type Ia supernovae. Studies of the details of the anisotropies in the cosmic microwave background also require dark energy (thought to be responsible for cosmic acceleration), and so do surveys of the large scale structure of the Universe done by surveys such as the Sloan Digital Sky Survey and the 2 Degree Field Galaxy Redshift Survey.

This model, often called the concordance model, as it is supported by these 3 separate lines of evidence, is summarised in this figure. In this diagram, “BAO” are the results from the large scale structure surveys (the acronym stands for Baryonic Acoustic Oscillations). As the figure shows, the percentage of dark energy required to explain the results of SN, CMB and BAO is about 70% (0.7 on the y-axis).

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This figure shows the so-called “concordance model”, three separate lines of evidence which support the existence of dark energy at about the 70-80% level. The figure is from Scolnic and Riess’s blogpost “No, Astronomers Haven’t Decided Dark Energy Is Nonexistent” which can be read by following this link.

You can read more about the three separate lines of evidence for dark matter in my book The Cosmic Microwave Background, How It Changed Our Understanding Of The Universe (follow this link to find out more about the book, including reviews).

My book "The Cosmic Microwave Background - how it changed our understanding of the Universe" is published by Springer

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

It seems to me that this is a lot of fuss about nothing, and that the case for cosmic acceleration is as strong as ever. What do you think?

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Yesterday (Thursday the 21st of March 2013), the European Space Agency (ESA) released its first results from the Planck satellite. The picture is shown below.


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This picture is a picture of the temperature differences in the earliest image we can obtain of the baby Universe. These temperature differences, technically called “anisotropies” are what have led to the structure we see in today’s Universe. They provide a powerful way for us to determine all kinds of things about our Universe, including its age, geometry, and what makes up our Universe.

COBE

The first satellite to provide us with a view of these anisotropies was COBE, the Cosmic Background Explorer, a NASA satellite launched in the late 1980s. In 1992 it released this image, which caused a sensation.


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The reason the image looks so “fuzzy” is because the detail with which COBE could see was limited, it only had a resolution of 7 degrees (a 7 degree patch, about 14 full Moons across, was the smallest patch it could see). The day it was released happened to be the day that Sir Arnold Wolfendale, who was then the “astronomer Royal” England, was visiting Cardiff, where I was finishing up my PhD. The press were constantly ringing the department to speak to him, and of course this was a time before mobile phones so the press kept the university’s switchboard pretty busy that day fielding calls for him.

Onwards to WMAP

Some 10 years later, a more detailed map was provided by NASA’s WMAP satellite. WMAP (Wilkinson Microwave Anisotropy Probe) had a much better resolution that COBE, as the image below shows.


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Between COBE and Planck were a number of important experiments such as BOOMERANG (a ballo-borne experiment) and DASI (based at the South Pole and led by John Carlstrom of the University of Chicago where I was based at the time) which gave some very important information, but I think it is fair to say that it was WMAP that heralded in the era of what we now call “precision observational cosmology”. Using technical analyses of the WMAP image shown above, cosmologists have been able to determine the age of the Universe (13.7 billion years), its geometry (flat), and even that only some 5% is made up of ordinary matter, with about 28% being made up of the mysterious “dark matter” and some 67% made up of the even more mysterious “dark energy”.

Why launch Planck?

Planck was launched in March 2009 by the European Space Agency. It was actually launched on the same rocket which launched the Herschel Space Observatory which I blogged about here. Planck has a number of improvements over WMAP, and over the next few years results will be released of measurements WMAP did not have the capability to make. But, its first result is its map of the anisotropies. As this fantastic article from the New York Times explains, there are a number of confirmations of our already accepted theories in this first image, but also a number of things which will require us to re-think some things we thought we knew.

For example, initial analysis of the Planck image suggests the Universe is 13.8 billion years old, not 13.7 as calculated by the WMAP data. Also, it has determined the composition of the Universe to be 4.9% normal matter, 27 dark matter and 68% dark energy, slightly different from values determined by WMAP. The value for how quickly the Universe is expanding is also found to be different, WMAP determined a value of 67 km/s/Megaparsec and Planck determines a value of 69 km/s/Megaparsec. Some of the features in the WMAP image which some argued were an artifact of the way the image was produced are still present in the Planck image, which has been produced with an entirely different satellite and processed with an entirely different method. This suggests some of these features are, in fact, real.

A lot more analysis of even this first image will be done over the next several months, and Planck will continue to make measurements over the next several years to refine the image shown above, as well as to make measurements of things like the polarisation of the radiation coming from this earliest view of the Universe.

I will leave you with this wonderful graphic from the above mentioned New York Times article.


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There has never been a more exciting time to be a cosmologist!

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