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## Looking for cosmic and gamma rays in Namibia

I am currently visiting Namibia, giving talks and meeting the astronomers here at the University of Namibia. My university, Cardiff University, has a formal university-wide collaboration with the University of Namibia through something called the Phoenix project. The level of collaboration between different departments goes from very strong to zero; for the Physics and Astronomy department it is non-zero but could be stronger. Hopefully, my visit here can help make it stronger.

Although the physics department at the University of Namibia is involved in many exciting areas of research, the one that interests me most is its involvement in a high-energy radiation system of telescopes called H.E.S.S. (High Energy Stereoscopic System). The name H.E.S.S., however, is no coincidence; it is also the name of Victor Hess, one of the founders of studies of cosmic rays. I blogged about him here.

H.E.S.S. (which I am now going to type as just HESS) is a system of telescopes which detect Cherenkov radiation produced when high energy particles (cosmic rays) or gamma rays from space strike atoms or molecules in the Earth’s atmosphere. This provides an indirect way of detecting this high energy radiation, as the radiation itself does not reach the ground; but the Cherenkov radiation it produces does reach the ground.

HESS (High Energy Stereoscopic System) is an system of telescopes which detect Cherenkov radiation, produced when cosmic rays or gamma rays from space strike atoms and molecules in the Earth’s atmosphere

But, what kind of astronomical sources does HESS detect? What is Cherenkov radiation? And, why has HESS been sited in Namibia?

## What kind of astronomical sources does HESS detect?

• non-thermal emission which is not a continuum
• line emission

I have blogged several times about blackbody radiation. Here I derived Planck’s radiation law using his original arguments of 1900, here I blogged about the fact that the Cosmic Microwave Background is a perfect blackbody (which is a very important fact in its interpretation as being due to radiation from the hot, early Universe), here I blogged about blackbody radiation and the ultraviolet catastrophe, and here I showed how we can use the fact that stars radiate as blackbodies to determine their sizes. And blackbody radiation has cropped up in several others of my blogposts.

Non-thermal continuum radiation (also known as synchrotron radiation) is something I have been planning to blog about for a while. I mentioned synchrotron radiation here when I derived the reason that accelerated electrons emit electromagnetic radiation. I will blog about the details of synchrotron radiation, as planned, in the near future; but for now I will just say that is produced when electrons spiral along magnetic field lines. As they spiral they accelerate (they are moving in a circle around the magnetic field line and moving along the field line at the same time) and, as I showed in that blogpost, accelerated electrons emit EM radiation.

Line emission is also something I have blogged about. For example, here in a blog entitled Emission Line Spectra, and here in my basic explanation of the three kinds of spectra we see in nature. It is the kind of emission given off by e.g. the Orion nebula, Messier 42.

The HESS telescope is looking for radiation which falls into the fourth category, non-thermal radiation which is not a continuum. In particular, it is looking for the high-energy end of this radiation, which is going to come from cosmic rays or gamma rays. As explained in my blogpost here, the source of cosmic rays is still hotly debated. They are not rays as such, put rather high energy charged particles. As I discussed in my blogpost here, some 89% of cosmic rays are high-energy protons (hydrogen nuclei), some 10% are high-energy helium nuclei (alpha particles), and the remaining 1% are high-energy electrons (beta particles).

We know what the cosmic rays are, but where they come from in terms of what kind of astronomical sources emit them is still a mystery. Also, we know that cosmic rays do not come from thermal sources; the energies are just too high. There is some kind of acceleration mechanism (often called cosmic accelerators) which are accelerating these charged particles to nearly the speed of light. Importantly for HESS’s work, gamma rays almost always accompany the cosmic accelerators, so we can use gamma rays to learn more about these mysterious phenomena.

Detecting gamma rays rather than cosmic rays has a distinct advantage; gamma rays travel in straight lines whereas cosmic rays, being charged particles, are bent by any magnetic fields. This has always been the main problem in determining the source of cosmic rays, as they do not travel in straight lines identifying their origin has been nigh-on impossible. I discussed that problem in more detail here.

A list of the sources detected by HESS since it saw first light in 2002 is given here. As you can see from this list, some are associated with supernovae remnants (such as the Crab nebula and the Vela nebula), some are from so-called active galactic nuclei (such as NGC 253), some are associated with Quasi-Stellar Objects (QSOs), and some have yet to be identified. The image below, taken from the HESS website, shows very high energy (VHE) gamma-ray emission from RCW 86, another supernova remnant.

A very high-energy gamma-ray image of RCW 86, a supernova remnant. To understand what 3, 5 or 7$\sigma$ significance means, read my blogpost here.

I will blog in more detail about Cherenkov radiation, showing the derivation of the formulae involved. But, for now, let me give a brief non-technical explanation.

Cherenkov radiation is produced when a high-energy charged particle (usually an electron) travels faster than the speed of light in that medium. You may think that you have heard that nothing can travel faster than the speed of light. This is true, in the sense that nothing can travel faster than the speed of light in a vacuum. But, it is possible for something to travel faster than the speed of light in a non-vacuum, where the speed of light is reduced by the medium through which the light is travelling.

So, for example, Cherenkov radiation is the preferred way of detecting neutrinos; the neutrinos strike a sub-atomic particle in a liquid, often heavy water (a rare event, but it does happen), and the accelerated sub-atomic particle may produce Cherenkov radiation if it is charged  (so, if it is either an electron or a proton) and if that charged particle travels faster than the speed of light in that liquid.

When high-energy cosmic or gamma rays enter the Earth’s atmosphere, Cherenkov radiation may be produced when this radiation (a gamma ray or a cosmic ray) strikes a sub-atomic particle in the atmosphere. This collision may produce an electron-positron pair, and the Cherenkov radiation occurs if this pair travel faster than the speed of light in the atmosphere.

The HESS telescopes detect this Cherenkov radiation, and so are able to pin-point the place in the atmosphere where the collision took place. Through this, we can trace back to from where in space the cosmic or gamma rays entered the Earth’s atmosphere; and hence indirectly ‘see’ the cosmic or gamma rays. In the case of gamma rays, which are not bent by magnetic fields, it allows us to construct a gamma-ray image of the astronomical source.

## Why is HESS in Namibia?

Parts of Namibia are ideal to site a telescope which is looking for Cherenkov radiation from the atmosphere. The country has some of the clearest and driest skies in the World, and the driest climate of any country in sub-Saharan Africa. The HESS telescopes are located about 100km to the south-west  of the capital Windhoek, near the Gamsberg mountain in the Khomas Highland, which is a plateau at an elevation of nearly 2km above sea level.

HESS is located some 100 km south-west of Windhoek, near the Gamsberg mountain in the Khomas Highland. This part of Namibia enjoys a warm desert climate, which with its elevation of nearly 2km above sea level makes it ideal for observing Cherenkov radiation from cosmic rays and gamma rays

The HESS telescopes saw first light in September 2002, with more telescopes being added to the system in 2004. In 2012 a much larger telescope, HESS II, went into operation which allows detection of lower energy cosmic and gamma rays.

HESS and HESS II are a collaboration between scientists from 32 scientific institutions in 12 countries, including the UK. It is an extremely exciting project, and one in which I hope my department in Cardiff can be more involved.

## Available in all good bookshops – soon!

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!

## First confirmation of cosmic inflation

To most of us, inflation is a nasty thing which sees the money in our pocket being worth less as prices go up. It’s a bad thing! But, in cosmology, a theory called cosmic inflation explains very neatly several key properties of the Universe. The theory of cosmic inflation was first suggested by Alan Guth in 1980, and yesterday (Monday the 17th of March 2014) a team led by John Kovak of Harvard University announced the first direct evidence that cosmic inflation did actually happen. There is also a Cardiff University involvement in this project.

The story on the confirmation of cosmic inflation as it appeared on the BBC science website.

## What is cosmic inflation?

In 1980, particle physicist Alan Guth was pondering some of the observed properties of the Universe, and he came up with the idea of cosmic inflation. The observed properties he was hoping to explain with his theory were

• the “Horizon problem”
• the “Flatness problem”
• the “Magnetic-monopole problem”

## The Horizon problem

When the cosmic microwave background radiation (CMBR) (the prediction of which I blogged about here) was discovered in 1964 it was recognised that it was most probably the “echo” of the Big Bang. By 1967 Bruce Partridge and David Wilkinson of Princeton University showed that the CMBR was the same from all parts of the sky down to a level of 0.1% of its 3 Kelvin temperature.

It was realised soon after this that this presented a problem, the so called “horizon problem”. It is actually perplexing that different parts of the sky should have the same CMBR temperature because when we look in different parts of the sky we are looking at parts of space which have not had the time to be in contact with each other in any way; they are simply too far apart. Therefore, a patch of sky in one direction with a particular CMBR temperature should have no knowledge of the CMBR temperature of a patch of sky in a different direction.

This is a little bit like switching on a heater in the centre of a large room. Everyone knows that it will take time for the whole room to come to the same temperature, and if the room were really really big you would not expect the corners which are far away from the heater to have the same temperature as the centre of the room next to the heater after just a few minutes. The heat just hasn’t had enough time to spread throughout the room. So, if you found that the whole room was at the same temperature, even though the heat hadn’t had enough time to spread throughout the room, it would be a bit of a puzzle. That is, in essence, the “horizon problem”.

## The flatness problem

Einstein showed in his theory of gravity, the General Theory of Relativity, that gravity causes space to bend. A Universe with lots of matter in it will have a different geometry (shape) to a Universe with less matter in it. The so-called “critical density” of the Universe would be a density that would give it a flat geometry. It was realised since the 1960s that the density of the Universe seemed to be very close to the critical density. Why should this be, when it could have any value. It could be much much more or much much less? If you do the mathematics, for the density to be within about a factor of two of the critical density today means it had to have been incredibly close to the critical density in the earliest moments of the Universe. Close to about one part in $10^{60}$!! This is the “flatness problem”.

## The magnetic monopole problem

In electricity, we are all familiar with positive and negative charges. James Clerk Maxwell showed in the mid 1800s that electricity and magnetism are part of the same force, electromagnetism. And yet, you never find a magnetic monopole, you always find magnetic poles come in pairs, they always have both a north and south pole. Theoretically there is no reason why one shouldn’t find just e.g. a north pole on its own, without a south pole. This is the “magnetic monopole problem”.

## What is cosmic inflation?

Alan Guth’s idea of cosmic inflation suggested that when the Universe was incredibly young, some $10^{-36}$ seconds old, it went through a brief period of very rapid expansion. This period ended when the Universe was about $10^{-33} \text{ or } 10^{-32}$ seconds old, but in this incredibly brief period Guth argued that the Universe grew from being much smaller than a proton to something about the size of a marble. After this brief period of very rapid expansion (inflation), the expansion of the Universe settled down to the more sedate rate of expansion that we see today.

## How does cosmic inflation solve these three problems?

The horizon problem is solved by inflation because the very rapid expansion which inflation proposes would allow parts of the Universe which are now too far apart to have ever communicated with each other to have been close enough together before inflation. So, going back to my analogy with the room being heated, it is as if the room started off really small, so small that all parts of it could come to the same temperature, then it suddenly expanded so that the room we are now looking at is much much bigger.

The flatness problem is solved by cosmic inflation by drawing the analogy between the geometry of the Universe and a curved surface. If a curved surface is large enough, then on a local scale it is always going to look flat. An easy analogy to understand this is the surface of our Earth. We all know it is spherical, but on a local scale it appears flat. If the Universe underwent a period of cosmic inflation, then we are seeing such a small part of it that the small part we see is always going to appear flat, no matter what the overall geometry.

The magnetic monopole problem is solved by cosmic inflation in the following manner. The idea is that magnetic monopoles were created in large quantities before the period of cosmic inflation. They should still exist today, but because the Universe expanded so rapidly during cosmic inflation, their number density (how many there are per unit volume) is so tiny that we haven’t found any in the part of the Universe which we are able to observe.

## The discovery made by BICEP2

Until yesterday, there had been no direct evidence of anything that cosmic inflation predicted, only agreement between the theory and things which had already been observed. One prediction of the theory is that the CMBR should be polarised in a particular way with a particular amount of polarisation (you can think of polarisation as a particular twisting of radiation, instead of vibrating in all directions it only vibrates in particular directions). The BICEP2 experiment (“Background Imaging of Cosmic Extragalactic Polarization”, the “2” indicates it is the second generation of this experiment) has been using the South Pole Telescope which is, as the name implies, at the Earth’s south pole, and has been looking for a particular signature in the CMBR – the “B-mode polarisation” as it is called.

Yesterday the team announced that they had, for the first time, detected this B-mode polarisation, which is the most direct evidence yet that the theory of cosmic inflation is correct. This polarisation comes about due to gravitational waves in the very very early Universe, so the detection of the B-mode polarisation is also direct evidence of gravitational waves, which were predicted by Einstein but have never been directly detected before.

If you want to read the actual announcement paper you can find the pre-print by following this link here. Here is a screen capture of the first page of the paper.

The first page of the paper announcing the detection of evidence for cosmic inflation. Notice that Cardiff University has an involvement with Peter Ade being the first author in the alphabetical list.

Superimposed on the variations in the temperature of the cosmic microwave background (red and blue blobs) is the evidence for the B-mode polarisation (the small black swirls).

This is very exciting news for cosmology and our understanding of the earliest moments of the Universe. It suggests that our model of the early Universe, including the theory of cosmic inflation, is correct (or at least is on the right tracks). Little by little, astronomers are unfolding the mysteries of the very earliest moments of creation!

If you want to read a more technical (but still non-specialist) explanation, then this story in Sky & Telescope is pretty good. Or, you may prefer this from Sean Carroll’s blog.

## He gets my vote for the next poet laureate

Some of you may already read Professor Peter Coles’ wonderful blog. Peter was professor of theoretical astrophysics at Cardiff University until recently, but he is now head of the School of Mathematical and Physical Sciences at the University of Sussex in Brighton.

According to his blog, he is also standing for Council of the Royal Astronomical Society. Accomplished though Peter clearly is in astrophysics, I really think he has missed his true calling. As this wonderfully mature and deep composition below shows (the link for it is also here), Peter should really be a published poet. Written at the tender age of 49 and 11/12 months, I can only wait with bated breath to see how turning 50 will lead to additional complexity and insights in his poems.

When the next Poet Laureate is being determined, Peter will certainly be getting my vote (do we get to vote? Maybe one writes to the Queen, or pompous letters to “The Times” to lobby for such things?).

Enjoy!