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Posts Tagged ‘CMB’

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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The European Space Agency announced last week (28th of November) that a space-based gravitational wave observatory will form one of its next two large science missions. This is exciting news for Cardiff University, as it has a very active gravitational waves research group headed up by Professor B S Sathyaprakash (known to everyone as “Sathya”). The Cardiff group, along with others in the Disunited Kingdom, played an important role in persuading ESA to make this observatory one of its next large science missions. Just over a year ago I blogged about some theoretical modelling of gravitational waves the Cardiff group had done.

The ESA plan is to launch a space-based gravitational wave observatory in 2034, which will have much more sensitivity than any current or even future ground-based gravitational wave observatory. Although NASA also had plans to launch a space-based gravitational wave observatory, there is currently none in existence, so this is really ground-breaking technology that ESA is announcing. From what I can understand, the current announcement by ESA is their commitment to the proposed LISA (Laser Interferometer Space Antenna) gravitational wave observatory. It would seem NASA has withdrawn their commitment to what was originally going to be a joint NASA/ESA mission.



ESA has chosen a gravitational wave detector to be funded.

ESA has chosen a space-based gravitational wave detector to be funded as one of its two key future missions. It should go into operation in 2034.



What are gravitational waves?

Gravitational waves are ripples in the fabric of space. They were predicted by Einstein as part of his theory of general relativity, the best theory we currently have to describe gravity. In his theory, events which involve extreme gravitational forces (such as two neutron stars orbiting each other (or merging), or the creation of a black hole) will lead to the emission of these gravitational waves. As they spread out from the source at the speed of light, they literally deform space as they pass by, just as ripples deform the surface of a pond as they spread out from a dropped stone.



An artist's impression of gravitational waves being produced around two orbiting neutron stars.

An artist’s impression of gravitational waves being produced as two black holes orbit each other.



Current gravitational wave observatories

There are several current gravitational wave observatories, all ground-based. These include VIRGO (in Italy) and LIGO (Laser Interferometer Gravitational Wave Observatory) which is in the United States. LIGO is the most sensitive of the current generation of gravitational wave detectors. LIGO actually comprises three separate detectors; one in Livingston, Louisiana and two in Hanford, Washington State. Each of the three separate detectors consists of two long arms at right angles to each other, forming a letter “L”. The idea behind these detectors is that, if a gravitational wave were to pass the detector, each of the two arms would have its length changed differently by the deformation of space as the gravitational wave passes through. Thus the detectors work on the principle of an interferometer, looking for tiny changes in the relative length of the two arms. And, when I say tiny, I mean tiny. In a 4km arm they are looking for changes of the order of 10^{-18} \text{ m}, or about one thousandth the size of a proton!



The principle of a gravitational wave detector.

The principle of a gravitational wave detector. They are essentially “interferometers”, with two arms at right angles to each other. As the gravitational waves pass the detector, space will be deformed and alter the relative lengths of the two arms.



LIGO (Laser Interferometer Gravitational Wave Detector)

Currently the most sensitive gravitational wave detector is LIGO. LIGO consists of three separate detectors, one in Livingston, Louisiana and two in Hanford, Washington State. The detector in Louisiana is shown below.



The LIGO detector in Livingstone, Louisiana. Each arm is 4km in length, and can detect changes in the relative length of the two arms of less than the size of a proton.

The LIGO detector in Livingstone, Louisiana. Each arm is 4km in length, and can detect changes in the relative length of the two arms of less than the size of a proton.



The detector in Louisiana, and one of the two detectors in Washington State, consist of two 4km long arms at right angles to each other. An event like the collapse of a 10 solar-mass star into a black hole is expected to produce a change in length in a 4km arm of about 10^{-18} \text{ m}, which is about one thousandth the size of a proton. This is just at the limit of the detection capabilities of LIGO, which is why astrophysicists are wanting more sensitive detectors to be placed into space. The other detector in Washington State has arms which are 2km in length, but just as sensitive as the detector with 4km arms at frequencies above 200 Hz, due to a different design.

LISA – Laser Interferometer Space Antenna

The ESA plans just announced will be based on the NASA/ESA plans for LISA, which have been on the drawing board for most of the last 10 years. ESA’s plan is to build two space-based interferometers, which will be in the form of equilateral triangles as this artist’s description shows.



An artist's impression of LISA, the "Laser Interferometer Space Antenna". Each interferometer will consist of an equilateral triangle, with each side 5 million km in length.

An artist’s impression of LISA, the “Laser Interferometer Space Antenna”. Each interferometer will consist of an equilateral triangle, with each side 5 million km in length.

The plan for LISA is to have arms which are 5 million km long! Compare this to the 4km long arms of LIGO. The changes in the length of a 5 million km long arm would be roughly one million times more than for LIGO, so rather than 10^{-18} \text{ m it would be } 10^{-12} \text{ m}, which should be well within the capabilities of the detectors. This means that less energetic events than the collapse of a 10-solar mass star into a black hole would be detectable by LISA. All kinds of astrophysical events which involve large changes in gravitational fields should be detectable by LISA, including the afore-mentioned creation of black holes, but also the merging of neutron stars, and even the merging of less massive stars.

But, possibly most exciting is the opportunity that gravitational waves provide to probe the very earliest moments after the Big Bang. With normal electromagnetic radiation (light, x-rays, infrared light etc.), we can only see as far back as about 300,000 years after the Big Bang. This is when the Cosmic Microwave Background Radiation was produced. Prior to this time, the Universe was opaque to EM radiation of any wavelength, because it was full of unbound electrons, and the photons would just scatter off of them and not get anywhere (see my blog here about the CMB). But, it was not opaque to gravitational waves, so they provide a way for us to see back beyond the CMB, and a unique way to learn about the conditions of the Universe in its earliest moments.

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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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