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Bang Goes The Theory

Science: Prime-time and full-on. Bang goes the theory.

Imagine you had a big box of dark matter. What would it look like?

You’re probably imagining a big black cube, aren’t you? Actually, it would be perfectly transparent. That’s because it’s not really "dark" at all.

OK, it doesn’t emit light, but it also doesn’t absorb light either, or we’d be able to see it as shadows against the background.

You also can’t touch it. If you tried to scoop up a handful of dark matter, it would pass right through your hand. The only way we can tell dark matter is there is from the gravitational pull it has.

Right now, dark matter is streaming through your body. Our Sun and all the solar system is travelling through our galaxy, with the dark matter wafting past us and through us.

Now, we can’t see dark matter with our eyes, or touch it with our fingers, but sometimes a dark matter particle will still manage to collide with a particle of ordinary matter.

These collisions are very rare, but scientists have made dark matter detectors to look for these collisions as the dark matter streams through. It’s a bit like holding your hand out of a window of a moving car and feeling the breeze.

 A lot of the evidence for dark matter is that the visible matter seems to be moving too quickly, like the spinning of spiral galaxies, or the movement of galaxies in a galaxy cluster. There has to be some unseen matter tugging at the visible matter, to explain how quickly it’s moving. 

In fact, the discrepancy is so big that most of the matter in the Universe has to be dark matter! Everything you see around you is just a tiny fraction of what’s really there.

Not only is dark matter wafting through you right now, but you’re also warping the space around you.

 According to Einstein, every object causes some curvature in the space around it. The curvature from a person is too small to measure directly, but galaxies and clusters of galaxies cause so much warping that background things look distorted.

From that distortion we can figure out how much matter there is - which is another line of evidence for dark matter.

Some scientists have argued that Einstein got it wrong about gravity, and that what we’re calling evidence for dark matter is just a sign that we’ve got gravitational tugs from the visible matter wrong.

But an image of two galaxy clusters in a middle of a collision have made the case for dark matter very strong. The Hubble Space Telescope took a picture, and from the distorted background galaxies, the scientists figured out where the matter was. In the picture, this is shaded in blue.

[image © copyright:
X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.]

Meanwhile, the Chandra X-ray space telescope (there are lots of space telescopes!) took a picture and found out where the gas is. This is red in the picture. Now, when gas collides with gas, you get all sorts of messy turbulence and mixing and maybe shock waves. But when dark matter meets dark matter, it just passes right through.

So, even if your hand was made of dark matter, you still wouldn’t be able to scoop up a handful!

What the astronomers saw in the galaxy cluster collision is that the gas (red) got stuck in the middle, while most of the matter (blue) passed right through and out the other side. This is very hard to explain in any way, unless most of the matter is dark matter.

Find out more

Find out more about the photo above: 1E 0657-56 - NASA finds direct proof of dark matter

You can’t touch dark matter, but you can get to know it - and other fundamental parts of our universe - by studying with The Open University: How the Universe works

From LearningSpace from OpenLearn: What contributes to the spectra of galaxies?

WARNING: This blog post contains spoilers for the fourth programme in the Bang Goes The Theory series. Don't read it if you haven't seen the space challenge yet and don't want to know what happens.

While we were planning and filming an ambitious item for Bang Goes the Theory in which an 'action man' type-figure dubbed mini-Dallas is sent up to the "edge of space" by a balloon, there was a lot of discussion among the Bang gang about whether or not we could claim to be reaching 'space', and also whether Joseph Kittinger had really "parachuted from space" after his balloon ascent to 102,800 feet (31,333 metres) in 1960.

We got our mini-Dallas to pretty much the same height, but I’m afraid that the answer has to be ‘no’ in both cases, even though the sky looks gratifyingly black in our remarkable camera shots.

Mini Dallas from Bang Goes The Theory.
[image © copyright BBC]

It’s pretty obvious if you think about it. Kittinger and mini-Dallas were both carried up by a balloon, and a balloon only goes up if it (plus its 'astronaut' payload) is on average less dense than the air that it displaces. That’s how buoyancy works.

There must still be air – albeit very tenuous – at the height reached by the balloon, otherwise it could not float.

There is not a vacuum at the height reached by these extreme balloons, but the pressure is very low. In fact it is about one-hundredth of the pressure at sea-level. This means that 99% of the atmosphere’s mass is below, and only 1% of the mass of the atmosphere is above.

However, that does not mean that mini-Dallas was 99% of the way to the top of the atmosphere, because the atmosphere becomes more and more tenuous with height. If you look at this diagram that shows how atmospheric temperature varies with height, you will see that 30,000 metres is only about halfway to the top of the stratosphere, and that there are layers called the mesosphere and the thermosphere above that!

Temperature variation with height in the Earth’s atmosphere. The warming with height in the stratosphere and thermosphere are because the air molecules are warmed by absorption of ultraviolet and other radiation from the Sun

Temperature variation with height in the Earth’s atmosphere. The warming with height in the stratosphere and thermosphere are because the air molecules are warmed by absorption of ultraviolet and other radiation from the Sun.

There is actually no definite boundary that marks the top of the atmosphere, but eventually it becomes so completely tenuous that for practical purposes it can be regarded as ‘space’. But where is this limit?

Well, I did some web searching, and I came up with this. Satellites can orbit 200 km above the Earth, free of any appreciable atmospheric drag. Clearly at 200 km, you are in ‘space’ (the International Space Station orbits at 320-347 km). Lower orbits down to about 160 km are possible, but there is too much drag for these to be stable.

The US government refuses to recognise a definition of where space begins, perhaps because it prefers to keep its option open.

However the Fédération Aéronautique Internationale recognises 100 km as the lower limit of space, whereas an encyclopedia of international law suggests 80 km as a practical limit between ‘air space’, potentially reachable by an aircraft, and ‘outer space’.

However you look at it, sadly 30,000 metres or 30 km is less than half way there, but it was a bold effort nonetheless.

Find out more

John Zarnecki looks back over fifty years of space exploration

Visit The Planets & Beyond

Consider The Open University course Planets: an Introduction

About the author

Dave Rothery is a volcanologist and planetary scientist at the Open University. His current research includes studying volcanic eruptions on the Earth and characterising planetary surfaces, especially Mercury.

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SuperWASP (the Wide Angle Search for Planets) is one of the world’s leading exoplanet detection programmes. Exoplanets are simply planets that orbit stars other than the Sun, and SuperWASP works by looking for those exoplanets that transit in front of their parent star.

Planets produce virtually no light of their own, so if they happen to pass in front of their star (from our viewpoint), then they will block out a tiny fraction of the star’s light, resulting in a slight dimming of the light that we see. Unfortunately, even planets as big as Jupiter will block out less than 1% of the light from a star like the Sun, and for a planet the size of the Earth, the fraction of starlight blocked out is a hundred times smaller still.

As a planet passes in front of its parent star, as seen from our viewpoint, so the brightness of the star is reduced slightly. (Not to scale)
[image © copyright SuperWASP]

And if this is not difficult enough, only those planetary orbits that happen to line up exactly with our line of sight will cause a transit in the first place. Orbits can be orientated at any angle, but only those within about 1 degree or so of our line of sight will cause the dip in the starlight that we can observe.

Despite all these difficulties, programmes like SuperWASP have been remarkably successful at finding exoplanets. One of the keys to SuperWASP’s success is that it can image a huge area of the sky in a single snapshot. SuperWASP in fact comprises two installations – one in the northern hemisphere on La Palma in the Canary Islands, and one in the southern hemisphere at the South African Astronomical Observatory.

Each installation consists of eight cameras on a robotic telescope mount, and each camera can take images of the sky covering an area over two hundred times that of the full Moon. This means that SuperWASP can take images of around a million stars in a single exposure.

One of the SuperWASP telescopes showing the 8 cameras on the robotic mount.
[image © copyright SuperWASP]

(For those who like the technical details, SuperWASP uses Canon 200mm focal length, f/1.8 focal ratio ‘papperazzi-style’ lenses with an aperture of 11cm each. They are backed by high quality e2v CCD detectors with 2048x2048 pixels, resulting in an image scale of 13.7 arcseconds per pixel.)

The way to find transiting exoplanets is to take many, many images of the same stars over and over again. Over the course of an observing season lasting around eight months, SuperWASP may take thousands of images of each star field, accumulating several terabytes of data in the form of images of the sky. The brightness of each star on each image is then carefully measured, resulting in a so called lightcurve of each star – its brightness variation with time.

Sophisticated computer programs then examine these millions of lightcurves looking for those that show possible repeating dips that signify the presence of a planet orbiting the star.

Not all the dips found are due to planets though. Some of the dips may just be due to random noise in the detectors or effects of the weather, and some may be due to other astronomical phenomena such as the presence of another nearby star. Therefore there is a process of carefully weeding out these so-called ‘false positives’ and then following up the remaining candidates with other, larger telescopes to verify that they are indeed transiting exoplanets.

At the time of writing (Summer 2009), the SuperWASP data archive contains 994 nights of data comprising 4,935,899 individual images. These images include 27,683,288 unique stars and give rise to lightcurves containing 165,636,715,663 separate data points. So far, the SuperWASP project has announced the discovery of 19 transiting exoplanets – about one-third of the total number of transiting exoplanets that are known – but there are many more SuperWASP planets that are at various stages of confirmation and whose discovery will be presented in the coming months.

The first 15 transiting exoplanets discovered by SuperWASP, shown to scale, compared with the Sun and Jupiter (bottom right). Each image illustrates the colour and size of the star and the relative size of the transiting planet in each case.
[image © copyright SuperWASP]

The WASP Consortium consists of astronomers primarily from the Queen’s University Belfast, Keele University, Leicester University, The Open University, St Andrews University, the Isaac Newton Group (La Palma), the Instituto de Astrofısica de Canarias (Tenerife) and the South African Astronomical Observatory. The SuperWASP-N and WASP-S Cameras were constructed and operated with funds made available from Consortium Universities and the UK’s Science and Technology Facilities Council.

Find out more

More on the background to the SuperWASP from a 2004 Open2 article

Study with The Open University: Planetary science and the search for life