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Posts Tagged ‘Planck’

I thought it was about time I gave another update on currently the most important story in astrophysics – the BICEP2 team’s possible detection of B-mode polarisation in the cosmic microwave background. I have previously blogged about this story, for example here, here and here. But, just to quickly recap, in March the BICEP2 team announced that they had detected the B-mode polarisation in the cosmic microwave background (CMB), and argued that it was evidence of gravitational waves and cosmological inflation in the very early Universe.

Since then, controversy has been the order of the day as other astrophysicists and cosmologists have argued that the BICEP2 detection was not due to the CMB at all, but rather to emission from dust in our own Milky Way galaxy. BICEP2 on their own do not have sufficient data to rule out this possibility, something they concede in their published paper. However, it would seem that the European satellite Planck do, as it has not only observed the whole sky (including the part of the sky observed by BICEP2), but has done so at five different frequencies, compared to BICEP2’s single frequency measurement.

In the last few days, it has been announced that the BICEP2 team will formally collaborate and share data with the Planck team, which I think is good news in sorting out the controversy over the BICEP2 detection sooner rather than later.



The BICEP2 team and Planck team have announced that they will collaborate and share data to help clear up the controversy over the source of the B-mode polarisation detected by BICEP2.

The BICEP2 team and Planck team have announced that they will collaborate and share data to help clear up the controversy over the source of the B-mode polarisation detected by BICEP2.



Although the Planck measurements of the polarisation of dust in our Milky Way will presumably become public at some point (as is normal with publicly funded science projects), this would not be for many more months. By formally collaborating with Planck, the BICEP2 team will get not only earlier access to the Planck data, but just as importantly will get the experts in the Planck collaboration working with them to properly interpret the Planck measurements. It is hoped by all in the astrophysics and cosmology communities that this collaboration between BICEP2 and Planck will lead to the issue of the origin of the detected B-mode polarisation being sorted out in a timely fashion, possibly even by the end of this year.

We shall have to wait and see!

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I blogged back in March about the announcement by a team of cosmologists that they had discovered evidence for gravitational waves in the Cosmic Microwave Background (CMB). The BICEP2 experiment, which is based at the South Pole, claimed to have detected the so-called “B-mode polarisation” in the CMB, and from the strength of the signal they argued that it was the best evidence yet of both gravitational waves in the very early Universe, and of the theory of Cosmic Inflation.

However, since that announcement there has been considerable controversy in the cosmology community as to whether their result is correct or not. I have re-blogged several other people’s blogs on this controversy, for example Peter Coles’ blog here and Matt Strassler here and here. As Peter and Matt’s blogs indicated, this controversy has been swirling around in the astronomical community for the last several months; but last Thursday (the 19th of June) it made it into the New York Times.



There now seems to be considerable doubt in the cosmological community about the BICEP2 result which was announced in March.

There now seems to be considerable doubt in the cosmological community about the BICEP2 result which was announced in March.



The main concern amongst the skeptics is that the BICEP2 team did not correctly subtract the effects of dust in our own Galaxy from their signal. Our Milky Way has a lot of dust in it, it is this dust which causes the dark clouds in the band of the Milky Way which are familiar to anyone who has looked at the Milky Way in any detail, even with the naked eye. Most of the dust is in the plane of the disk, but some is above and below the plane in what we refer to as “high Galactic latitudes”. The BICEP2 team chose their patch of sky to be well below the plane of the Milky Way to try to minimise the effects of dust.

However, it may be that the amount of dust and its degree of polarisation where BICEP2 made their observations is greater than the BICEP2 team thought. If this is the case, then much of the polarised signal that BICEP2 measured may not be due to primordial gravitational waves, but instead may be dominated by this foreground contamination. As the New York Times story states, the BICEP2 team acknowledge that the foreground contamination may be greater than they assumed, but they are sticking to their claim that it is still small compared to the signal they detected.

We should know the answer to this burning issue within the next 6 to 12 months. The Planck satellite has done a detailed all-sky map of the strength of the polarised emission from dust in our Milky Way, far more detailed than any data currently available, and when these maps are released it should allow astronomers to correctly determine how much of BICEP2’s signal is due to foreground contamination. Planck will also do this at several different frequencies, and as Galactic dust is much warmer than the CMB the ratio of its signal at different frequencies will be different to that of the CMB, allowing for much better separation of the two effects.

In the meantime, we have a great insight into how science really works. Any result in science is closely scrutinised by the community, and is not accepted as being real until (a) it has been confirmed by other experiments and (b) that the community is satisfied that the interpretation of the measurement is correct, and that all other possibilities have been considered. As Carl Sagan once said

extraordinary claims require extraordinary evidence

So far, I think it is fair to say, most people in the astronomical and cosmological communities are treating the BICEP2 result with a good deal of caution, and that caution can only be allayed by further analysis and measurements.

If you wish to read more about the BICEP2 results and the surrounding controversy, an excellent place to start is Peter Coles’ blog here.

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Some of you may have noticed that I haven’t blogged much this last month. The reason is that I have been putting the finishing touches on a book – which has just been sent off to the publishers Springer. I am sure it will need some revision, but am also hopeful that it should be hitting the shelves / bookshops / electronic stores in the next few months.



The cover, even the title may change!

The cover, even the title may change!



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In this blog I described the first results from the Planck satellite, which is studying the Cosmic Microwave Background in greater detail than we have ever done before. But, what exactly is the Cosmic Microwave Background? Where does it come from? How was it produced?

The origin of the elements

In 1929 Edwin Hubble published evidence that the speed with which galaxies were moving away from the Milky Way was directly related to their distance from us. Although Hubble himself never explicity stated it, this is clear evidence that the Universe is expanding. If the Universe is expanding, then of course one would expect it to have been smaller in the past.

In the 1940s the Russian-American physicist George Gamow started thinking about what the early Universe would have been like. He worked on two related theories, the first that the elements would have been created in the early Universe. The second related to the fact that a smaller, denser Universe would also have been hotter in the past.

In 1948, with his PhD student Ralph Alpher, the two published a paper titled “The Origin of Chemical Elements“. As a joke, Gamow decided to add the well-known physicist Hans Bethe’s name to the paper, so that it could be called “Alpher, Bethe, Gamow” (alpha,beta, gamma – geddit? 🙂 ).


George Gamow, who worked with his PhD student Ralph Alpher on the primordial nucleosynthesis theory.

George Gamow, who worked with his PhD student Ralph Alpher on the primordial nucleosynthesis theory.


Ralph Alpher, who was George Gamow's PhD student at the time of writing the paper.

Ralph Alpher, who was George Gamow’s PhD student at the time of writing the paper.


Hans Bethe, who played no part in writing the paper.

Hans Bethe, who played no part in writing the paper.


The famous "Alpha, Bethe, Gamow" paper from Physical Review 1948

The famous “Alpha, Bethe, Gamow” paper from Physical Review 1948


Although the Alpher, Bethe, Gamow paper was groundbreaking, it was wrong in some of its details. It suggested that all the elements were created in the hot, early Universe. We now think (know?) this is not the case. Only hydrogen and helium were created in the early Universe, the other elements have all been created inside stars, something Sir Fred Hoyle worked out with co-workers in the 1950s.

Alpher and Herman’s paper on the Cosmic Microwave Background


In a related paper, Alpher and Robert Herman, who was working as a post-doctoral research assistant for Gamow, calculated that the early Universe would have been a hot opaque plasma (ionised gas), and would thus have radiated like a black body. However, this radiation would not have been able to travel through the plasma as the photons would scatter of the free electrons.


The abstract of the paper by Alpher and Hermann, which predicts a cosmic microwave background at a temperature of 5K (5 degrees above absolute zero).

The abstract of the paper by Alpher and Herman, which predicts a cosmic microwave background at a temperature of 5K (5 degrees above absolute zero).


Gamow's article in Nature, which summarises the work on the origin of the elements and of the existence of a cosmic microwave background

Gamow’s article in Nature, which summarises the work on the origin of the elements and of the existence of a cosmic microwave background


But, as the Universe expanded and cooled the plasma would become a neutral gas, in that the electrons would combine with the nuclei to produce neutral atoms, allowing the photons to travel unimpeded. They calculated that these photons, which would be able to thence travel unimpeded, would now be at a characteristic black-body temperature of 5K due to the expansion of the Universe. This in the microwave part of the spectrum, hence the name Cosmic Microwave Background.

Our current understanding is pretty much what was derived in this 1948 paper, with a few refinements. Perversely, the moment the plasma became a neutral gas, which we believe to be when the Universe was about 350,000 years old, is referred to as “re-combination”, but as I tell my students, the electrons were combing with the nuclei for the first time. This is when the fog of the early Universe lifted and is the earliest radiation we can see.

In a separate blog on the history of the Cosmic Microwave Background (CMB) I will discuss how

  1. the CMB was accidentally discovered in 1964
  2. Gamow’s work was ignored, only to be worked out again in the early 1960s


Update

You can read far more about the prediction of the CMB, and its accidental discovery, in my new book, “The Cosmic Microwave Background – How it changed our understanding of the Universe”.



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.

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.



The book can be found on the Springer website here, and on the Amazon website here.

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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.


20130322-015819.jpg


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.


20130322-015750.jpg


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.


20130322-015805.jpg

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.


20130322-053017.jpg


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

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