According to Einstein's theory, when two extremely massive and dense objects orbit around each other, the space around them will get wrinkled. This can be compared to the wrinkles or waves you get when you drop a stone in the water. It is hard to grasp the idea of wrinkles in space. On Earth we do not notice these things, but under extreme conditions weird things can happen. For instance, two black holes circling around each other or two neutron stars collapsing are systems that are thought to emit gravitational waves.

Although the responsible systems are very massive and dense, the wrinkles they produce are still extremely small when they arrive here on Earth. It is therefore very difficult to detect them. To get an idea, the relative change in space due to the passage of such a wrinkle is of the order of 1 part in 1,000,000,000,000,000,000,000, a 1 with 21 zeros. This means that over the length of 1 kilometer you are looking for a wrinkle with a size less than an atom! As impossible as this may sound, nowadays detectors are being build and used that can reach such a high sensitivity.

Lasers are used to overcome the sensitivity problems, in a set-up called an interferometer. Light travels through space as a wave. Imagine you have two of the same waves moving towards each other with the same speed, the speed of light. When the two waves hit and the top of the first wave hits the top of the second wave, then these waves strengthen each other. Then these waves are ‘in phase’. However, when one wave is at his top and the other at his bottom, they dim each other. This is known as ‘out of phase’.

For lasers, the wavelength of the light is exactly known. You can use this laser light and split it in such a way that one part goes north and the other part goes east. After a few kilometers, the light hits a mirror and comes back. At some point the two laser beams will ‘hit’ each other again. If you know the exact distance and the wavelength of the laser light, then you can predict if the hitting of the two beams results in dimming or strengthening of the light.

Now, imagine that a gravitational wave comes along. The wave, a wrinkle in space, will result the stretching of the laser beam in one direction, while the other shortens a bit. If, initially, the waves were in phase, this strengthening of one arm and shortening of the other makes the waves out of phase. This means that the detector at the position where the waves ‘hit’ measures a shift in phase caused by a gravitational wave.

The detector I just described is actually functioning at the moment and is called the Laser Interferometer Gravitational Wave Observatory (LIGO). Its laser beams travel a path of about 2 to 4 km in length. The longer this distance, the weaker the wrinkles you can detect. In theory LIGO should be able to measure wrinkles as discussed above. However, the further away a source of these waves is, the weaker they are when they reach Earth. Basically, the chance that LIGO will observe a wave is still extremely small.

A few years ago NASA and ESA started developing a much larger detector known as Laser Interferometer Space Antenna (LISA). LISA is the combination of 3 satellites orbiting the Earth in a triangular pattern. The distance between the satellites must stay exactly the same and will be around 5 million kilometers. However, the principle behind LISA is the same as behind LIGO. A laser beam is sent from the central satellite to the other two, where it is being reflected and sent back to the initial sender. This sending central satellite receives the light again and measures any differences between the expected and actual arrival of the wave. The biggest difficulty is getting the satellites at exactly the right location and keeping them there. Even the smallest deviation from its position will mess up the detections.

Unfortunately, last year NASA announced to stop the LISA partnership. ESA announced to continue developing a space based gravitational wave detector, but it is not yet known if this will be comparable to LISA or less complex. Either way, LISA gives a nice idea on how difficult it is to discover the missing pieces of Einstein’s puzzle. I am convinced that at one point we will observe gravitational waves. Whether it will be from space or on the ground, Einstein’s puzzle will be solved.

NvdP

The James Webb Space Telescope (JWST) will not be an exact copy of the Hubble Space Telescope (HST). Hubble was created to view the universe at a wide variety of wavelengths. Not only was it capable of watching what is known as the visible light (the light/colours we see around us every day) but also ultraviolet and infrared light. We cannot observe these kinds of light with the naked eye, but Hubble opened up this hidden world. JWST is designed to observe the infrared part of the light spectrum. Its infrared capabilities far exceed those of the HST. Why this change of characteristics you might wonder. Why would you change something that has proven to be a great concept?

In a sense, Hubble himself is responsible for the knowledge that led to this change. Not the telescope, but the man the telescope was named after. In the early 20th century, Edwin Hubble discovered that the universe is expanding. The result of this expansion is that the light emitted by very distant galaxies changes. Light of an object moving away from you at great speeds, as the most distant galaxies do, becomes redder. At a certain point, this light has become so red that it reaches the infrared part of the light spectrum and becomes invisible to the human eye. This means that when we want to observe and study the most distant galaxies, we need a telescope that reaches even deeper into the infrared than the HST. Basically, the JWST is an extension of the HST; it looks for similar galaxies, but at distances the HST could never reach.

But if JWST is designed to look for light that we cannot see with the naked eye, does this then mean that we will never be fascinated with beautiful HST-like images? Well, yes and no. The pictures created with the JWST can be of the same astonishing beauty as HST’s, but they will be different. In order to explain why this is the case, I have to reveal a little (perhaps shocking) secret. The colours you see on the pictures made with the HST are not real. But it is not Hubble that is cheating you, it is the astronomers. Do not yet run off, suing all astronomers for lying to you. Let me explain what really happens before a Hubble picture is shown to the public.

In a way, the HST works similar to a regular digital camera. It has a CCD chip to record incoming photons (light particles). However, unlike your digital camera the HST does not measure the colour of the incoming light directly. Instead it has a variety of colour filters, which it uses to measure only the amount of light for a single colour. For instance, after Hubble has observed a galaxy with different filters, then astronomers combine these different images to make a colour image. In doing so, they assign red light to the image from the red filter, blue to the blue image and green to the green image. Depending on the goal of the astronomers they decide to either make an image that closely resembles the object the way you would observe it if you were in a spacecraft, or they use certain colours to emphasize their results. The latter is for instance used to study images with infrared or UV light. In order to make these results visible, astronomers often give these regions bright colours to see where strong UV light is emitted.

I hope this explanation did not take the beauty out of the Hubble images for you. Most of the images that have reached the public very closely resemble their true colours but in some cases for clarity different colours are used. This automatically answers the question I posed before, whether the JWST would still amaze us with beautiful images. Probably many beautiful images would still make it to the public, but unfortunately these images will not resemble the true colours of the object anymore. Hubble has amazed the world with beautiful images for many years and it will continue to do so for the next few years. In that view it is a loss that there will be no new Hubble. However, if you look at it from a scientific point of view, the launch of the James Webb (planned for 2018) promises to give the astronomic world so much more information about the origin of the universe and star- and galaxy formation that the loss of the images is well-compensated.

NvdP

Since the early 1990s, when the first planet around a star other than the Sun (a so-called exoplanet) was discovered and confirmed, astronomers have been keen to find more. Not only to find out how common planets actually are, but also to get an idea of their variety and properties. Is the Earth a normal planet? Or are other types more common? Are habitable planets rare? And how do they form? Motivated by these questions, the field of research has taken a massive leap during the past decade. As of this week, 700 exoplanets have been discovered. Because we are only at the beginning of all that is to come, many of these questions have not yet been answered – and it is still unknown what we will discover.

Exoplanet discoveries are strongly biased to planets with some very specific characteristics. This is mainly caused by the methods through which exoplanets are detected. Planets that orbit stars other than the Sun are generally so faint that it is impossible to see them next to the overwhelming brightness of their host star. Only a handful of exoplanets is known that can be visually discerned on an image, and even then this is achieved by blocking much of the light that is coming from the star. That way, the reflection of light on the planet’s surface or atmosphere is more easily detected. In the majority of cases, exoplanets are discovered indirectly. The most popular discovery methods utilise the influence of exoplanets on their host star to derive their presence.

The majority of exoplanets has been discovered by their gravitational pull on the star they orbit. This pull causes the star to wiggle, which can be detected by modern spectroscopic observatories. The second most popular method is based on the detection of variations in the brightnesses of stars, which are caused by planets passing in front of the star. Of course, this technique only allows the discovery of planets with such orbits that they do indeed “transit” in front of their host star when viewed from Earth. Nonetheless, it will likely become the most successful discovery method since the recently launched space observatory Kepler exploits this principle. And while only 26 of Kepler’s discoveries have currently been confirmed, a stunning 1,235 candidates are awaiting confirmation. Still, this is nothing compared to the estimated total number of at least 50 billion exoplanets in our own Milky Way galaxy. That is an awful lot of data to analyse, but fortunately you can participate – at planethunters.org you can aid the Kepler team to discover exoplanets using their observations.

First, lets have a closer look at the faster-than-light neutrinos. You have probably read all about it in the news, so I am not going to go in extreme detail on the topic, but the overall idea is the following. According to Einstein’s theory of general relativity no object with a mass can move faster than the speed of light. The less massive an object, the closer it can approach this speed, but only massless particles like a photon (a ‘light particle’) can reach it and thereby travel with this speed – hence the speed of light. Neutrinos are particles with a mass, allbeit very small. It was therefore a great shock when the scientists discovered that their neutrinos went faster than the speed of light.

If this result were to be correct, it would change the entire scientific world. Therefore scientists all over the world, including those who ran the experiment, were doing all they could to explain the results without requiring the neutrinos to have gone faster than the speed of light. A few weeks and many proposed solutions later, it appears as if very small errors led to the observed phenomenon. However, none of them have been conclusive. It was a very difficult experiment and it will be hard to rerun such an experiment again to confirm or falsify the results.

Another well-established feature of the laws of nature is that they are equal throughout the universe. It does not matter if you perform your measurements here on Earth, or somewhere in the early universe, the laws of physics should be the same everywhere. However, recently astronomers discovered a discrepancy in the theory of electromagnetism. They looked at the most distant observable universe to determine the behaviour of this force and it turned out that one of the fundamental constants in this theory varied depending on the direction in which the astronomers looked. But, as the name suggests, it is a fundamental constant, so it should not vary at all. While this result also seems to interfere with the established laws of nature, it did not become a hot topic in the news, while the faster-than-light neutrinos did. Why did the first result lead to a reaction of the whole (scientific) world, while the other only led to some excitement in a small field?

In order to answer this question it is good to be aware of the different consequences. Almost the whole of physics is in someway build on the fact that nothing moves faster than the speed of light. It comes back in every area of physics, from Einstein’s famous equation, which relates mass to energy (E=Mc²) to Maxwell’s equations that describe electromagnetism. It all depends on the speed of light and it is hard to imagine what the consequences would be if something would move faster than the speed of light, but you can probably imagine it would be big. Although such a world changing observation is every scientists’ dream, I think that such a result could scare the scientists not responsible for the result. On the other hand, the varying behaviour of the electromagnetic force does not have a great impact on the description of nature here on Earth. Perhaps its behaviour is different in another part of the universe, but it still behaves in the same way here. Also, several scientists are working on a ‘Theory of everything’ in which forces differ throughout the (history of the) universe.

So what is there for us to learn from all this? I can understand the skepticism of the scientific world towards the observation of faster-than-light neutrinos. Skepticism is an important characteristic of a scientist, especially when it comes to results disagreeing with well-established theories. It is important to take a critical look before throwing away a theory that has been shown to work for many years. However, at some point a scientist has to accept the fact that a different theory explains a phenomenon better even if it implies a massive change in physics. Better experiments lead to a changing view on the world. So we can always rely on the laws of nature, but we can never keep them as the ‘truth’. It is exactly this mechanism that brings science forward. Without it we would still only use Newtonian physics and there would not even be something like general relativity to disagree with.

NvdP

Nobody knows what dark matter is – if it even is matter at all – but we can derive some of its properties. The first indirect evidence for its existence was found in 1933, when the astronomer Fritz Zwicky realised that the galaxies in the Coma Cluster are orbiting each other at velocities that are too high for them to be kept together by the gravity of their visible stars. In other words, if it were just the stars that gravitationally bind the galaxies, the Coma Cluster should have flown apart long ago. And it hasn’t. Zwicky concluded that there has to be a form of unseen mass that completely dominates the gravity in the Coma Cluster.

Since the 1930s, further evidence for dark matter has been steadily accumulating. The motions of stars in our own Milky Way galaxy, gravitational lensing by clusters of galaxies, the cosmic microwave background, and the formation of cosmic structure on the very largest scales have all made clear that it isn’t the gravity of stars that govern the evolution of the universe. It is something else.

There are several theories about what that “something” could be, such as a new kind of exotic particle, but none of them have been conclusive. Regardless of what it exactly is, the large-scale behaviour of dark matter has now been thoroughly modelled in computer simulations. Astrophysicists have used these to obtain a clear picture of what the properties of dark matter should be in order to be consistent with the universe around us. The field of research is unceasingly active. In the press and on the web, there are regular news releases that present spectacular results from observations or theoretical models that either confirm or argue against what we know of dark matter. They sometimes give the impression that even scientists are at a loss as to what it all means. But while some discoveries may contradict known results, the truth is that eventually they all help us to figure out what dark matter is.

Last week, Matthew Walker (Harvard) and Jorge Peñarrubia (Cambridge, UK) presented their recent analysis of dwarf galaxies. It was the type of news that goes against what we know. From computer simulations, it was always thought that dark matter would pile up in the centres of galaxies – kind of like a pit in a fruit. For a long time, this has been one of the hallmark characteristics of dark matter theory. Walker and Peñarrubia measured the motions of stars in two nearby dwarf galaxies and discovered that the pit was absent. Whatever dark matter is, it doesn’t always follow the rules we think it should. The big question is now whether both galaxies are exceptions, or that our models for dark matter need to be adjusted.

Among the astrophysicists at the institute where I conduct my research, there are several tens of people working every day to answer this question and to understand the nature of dark matter. Across the globe, they are joined by thousands more. Dark matter research is unique in that it needs input from several branches of science to succeed. Astronomers and astrophysicists provide observations and models of outer space, particle physicists contribute measurements from the largest particle accelerators in the world, and theoretical physicists make a big effort to combine the results in a fundamental explanation of dark matter.

At the European Space Agency (ESA) and the European Southern Observatory (ESO), which is stationed in Germany but has several gigantic telescopes on the southern hemisphere, scientists and engineers are preparing two spectacular new observatories that will provide crucial new insight in the large-scale behaviour of dark matter. The first of these will be a space telescope, like Hubble. ESA’s Gaia is scheduled for launch in March 2013, and will provide unprecedented measurements of the motions of stars in the Milky Way. Its observations will be so precise, that it will be able to see a hair from a thousand kilometres away. Such precision will enable astronomers to derive in groundbreaking detail how each star is affected by gravity – and where the dark matter is that causes the attraction.