Where did the Sun form?

2012-04-16 21:01:41
Every morning we open the curtains, the comforting glow of a yellow-burning star awaits us. All life on Earth thanks its existence to the Sun – without it, there would be no plants, no air to breathe, no animals. No us. It is almost as if the Sun has always been there. But everywhere else in the Universe, we see stars being born and passing away. Clearly, there must have been a time that the Sun came into existence too. This leads to a mind-boggling question: what did the formation of the Sun look like?


The most obvious way to answer the question is to look for stars that are presently being born. All across the sky, there are beautifully shining groups of stars, and some of them are brightly blue. Around them, often patches of turbulent gas are seen – illuminated by the stars these gas clouds can reflect an impressive wealth of colours. The gas in the surroundings of these stars is the remnant of their formation. Some few million years ago, the gas collapsed under its own weight and fragmented into seeds for stars. Later on, the seeds became so dense that they would ignite nuclear fusion and emit their energy all across the Milky Way. This is the light of the stars that we see today.

Because collapsing gas clouds fragment into several smaller cores, stars are not generally born alone. But we are not talking twins here. A full nest can contain anything between a few tens to millions of baby stars. And some time long ago, the Sun must have had a similar family.

The first indications for the age and birth environment of the Sun came from meteorites: rocks from all across the solar system that descended through the atmosphere and impacted on Earth. By analysing these meteorites in the lab, scientists discovered that many of them contained radioactive elements, meaning that after a certain amount of time, some of the rock’s atoms would decay into other elements. By measuring how many radioactive elements were still present, the scientists could derive how long this radioactive decay had been going on. This gave the stunning answer of about 4.5 billion years. It turned out the Sun is ancient.

However, the radioactive elements in the meteorites made astronomers face another problem. Radioactive elements are extremely rare in the Universe, and are only produced in supernova explosions, which mark the ends of the most massive stars’ lives. The only way the solar system could have ended up with such radioactive meteorites, is if a supernova exploded in the direct vicinity of the Sun just after it was born. This in itself is a rare event because the massive stars that end their lives as supernovae are not commonplace. The astronomers calculated that the chance of such a star being near the Sun when it formed is so low, that it would require about a thousand other stars to be sure that one of them would be massive enough to produce the needed supernova.

Again, this posed a problem. Our solar system does not only host Earth, but also seven other planets and a large belt of asteroids beyond the orbit of Neptune. The orbits of all these objects are almost perfectly circular. If the Sun would have had too many brother and sister stars, undoubtedly one of them would have passed the Sun quite closely at some point, with a devastating outcome. A close encounter with another star would gravitationally disturb the solar system, and could well put the planets on very elliptic orbits. In such a situation, the Earth would half of the year be very close to the Sun, whereas the other half of the year it would be very far away. The temperature on our planet would therefore change dramatically, and life would not have been able to exist.

Fortunately, the problem has recently been solved. If the Sun did not have more than ten thousand siblings, and if these stars drifted apart within a few tens of millions of years, then the chance of an encounter would have been so low that our solar system could have survived. It turns out that these conditions are quite common in places where we can see stars currently being born.

It’s an amazing puzzle, but if you put all the pieces together it gives a very clear picture. The Sun formed in a group of a thousand to ten thousand stars, of which at least one exploded as a supernova when the Sun was still very young. A few tens of millions of years later, the Sun’s family had drifted apart, leaving our Sun as an isolated star with a beautifully ordered system of planets around it.

Admittedly, it looks like a very specific set of circumstances that allowed the solar system to form. Have we been lucky? Time will tell – it is not clear whether life could also form in solar systems with vastly different histories. If anything, this story tells us where in the universe to look for stars that are currently forming in the same way as the Sun did. By studying such stars, astronomers will be able determine the history of the Sun with even higher accuracy. And who knows what secrets are yet to be uncovered?

DK
 
 
 

Do we await the same massacre as the dinosaurs?

2012-03-06 19:45:59
Although nothing can be said with certainty, it is very likely that the dinosaurs got extinct after an asteroid impacted on Earth. Around 65 million years ago an object with the size of about 10 kilometers crashed into the Earth near the Yucatan peninsula in Mexico. Any such event destroys most life on Earth and has a massive impact on Earth’s climate. Fortunately, the chances of such a disaster are very small, but impact chances of much smaller asteroids with sizes of 150 meters are higher. Such an asteroid could already destroy regional human settlements. Should this information worry us?

NASA has started a program to categorize so-called Near-Earth Objects (NEOs); objects that approach the Earth and could enter the Earth’s neighborhood.  NASA set its goal to locate 90 percent of the approximately 1000 asteroids that approach Earth and have a size of over 1 km. As of March 1, 2012, 8763 NEOs have been located of which 840 have a diameter over 1 km. Fortunately, not all of these NEOs are a potential danger to us. Most of them are never going to get close enough to Earth to even have a very small chance of impacting. However, still 1291 of the NEOs have been marked Potentially Hazardous Asteroids (PHAs).  Don’t be frightened yet, in order to be marked a PHA an asteroid has to get closer to Earth than 7.500.000 km and it should be larger than 150 m. As you can imagine the chances of a PHA to impact on Earth are still extremely small.

Recently, astronomers are keeping a closer eye on asteroid 2011 AG5. At this moment this asteroid with a size of about 140 meters has a chance of 1 in 625 that it will impact Earth on February 5, 2040. Compared to many other impact chances this chance is actually very large. However, the asteroid has only been observed for a very small time, much too small to get a reliable estimate of its impact chance. The next few years there will be several new opportunities to observe 2011 AG5 in more detail. It is very likely that these new observations will decrease the impact chance.

In the very unlikely scenario that the impact chance of 2011 AG5 increases with better observations, there are still various methods available to deflect the asteroid. A key moment in this process is the 2023 “keyhole”. A keyhole is a very small region in space, about 100 km in size for 2011 AG5. If the asteroid were to pass through this region, various gravitational interactions could perturb its orbit and cause it to end in an impact here on Earth. Therefore, countermeasures should be taken before 2011 AG5 flies through this keyhole in February 2023. Of course, at this moment it is still very unlikely that it will fly through this keyhole, but future observation will shed more light on this.

There is not yet a consensus in the international world on how to deflect potential impacting asteroids. This is also one of the major issues in this field. Not only determining which methods could be employed but also getting a consensus between the different players (US, China, Russia, Europe, etc).

Various methods can be used to avoid a collision with an asteroid. These techniques roughly divide up in two pillars: direct and indirect. In this first case, the asteroid’s path is violently disturbed. For instance, nuclear bombs or something similar can be used to completely destroy the object. Downside of this method is that the asteroid could get fragmented and pieces larger than 35 meters will still impact on Earth. Hence, if the asteroid is not fragmented enough, a larger problem arises, since now you have to destroy much more smaller pieces. Another possibility is to impact a spacecraft into the asteroid and thereby changing its direction. The advantage of these direct methods is that they are relatively cheap and can easily be conducted; the downside is that you have less control over the result. On the other hand, indirect measures like detonating nuclear bombs in the vicinity of the asteroid to change it’s path are much more difficult to perform. However, if something goes wrong you probably won’t have made things worse.

As you can see, there are various ways in which we can deflect or destroy a potentially dangerous asteroid. Currently, ESA is working on a mission to impact a spacecraft into an asteroid to test the outcome of such a crash. However so far none of the methods has been tested, so it is going to be a difficult task for the international community to decide which method to deploy. It is therefore of vital importance that we keep on working on getting a better understanding about the asteroids in our vicinity. The sooner a likely impact can be detected, the more time we have to stop it. Fortunately, the chances that an asteroid will impact Earth are small, at least the chance that something like that will happen in your lifetime. But it is certain that asteroids of all sizes keep on crashing into Earth. Should we be worried? I don’t think so. The chances of an asteroid impact are very small and there are numerous opportunities to stop an asteroid from impacting. Instead, I prefer to look on the bright side: according to some theories, life on Earth would not even have existed without asteroids... and neither could we spend a romantic night watching falling stars. I’m willing to run a small risk for that.

NvdP

 
 
 

Globular clusters survived a 13 billion year-old massacre

2012-02-14 20:07:02
 The Galactic globular cluster M80 in the constellation Scorpius contains several hundred thousand stars. Credit: HST/NASA/ESA.Our Milky Way galaxy is surrounded by some 200 compact groups of stars, containing up to a million stars each. At 13 billion years of age, these globular clusters are almost as old as the universe itself and were born when the first generations of stars and galaxies formed. Now a team of astronomers from Germany and the Netherlands have conducted a novel type of computer simulation that looked at how they were born - and they find that these giant clusters of stars are the only survivors of a 13 billion year-old massacre that destroyed many of their smaller siblings. The new work is led by Dr Diederik Kruijssen, co-founder of Project Collision and astronomer at the Max Planck Institute for Astrophysics in Garching, Germany. The results appears in a paper in the journal Monthly Notices of the Royal Astronomical Society.

Globular star clusters have a remarkable characteristic: the typical number of stars they contain appears to be about the same throughout the Universe. This is in contrast to much younger stellar clusters, which can contain almost any number of stars, from fewer than 100 to many thousands. The team of scientists proposes that this difference can be explained by the conditions under which globular clusters formed early on in the evolution of their host galaxies.

The researchers ran simulations of isolated and colliding galaxies, in which they included a model for the formation and destruction of stellar clusters. When galaxies collide, they often generate spectacular bursts of star formation ("starbursts") and a wealth of bright, young stellar clusters of many different sizes. As a result it was always thought that the total number of star clusters increases during starbursts. But the Dutch-German team found the opposite result in their simulations.


While the very brightest and largest clusters were indeed capable of surviving the galaxy collision due to their own gravitational attraction, the numerous smaller clusters were effectively destroyed by the rapidly changing gravitational forces that typically occur during starbursts due to the movement of gas, dust and stars. The wave of starbursts came to an end after about 2 billion years and the researchers were surprised to see that only clusters with high numbers of stars had survived. These clusters had all the characteristics that should be expected for a young population of globular clusters as they would have looked about 11 billion years ago.

Dr Kruijssen comments: "It is ironic to see that starbursts may produce many young stellar clusters, but at the same time also destroy the majority of them. This occurs not only in galaxy collisions, but should be expected in any starburst environment. In the early Universe, starbursts were commonplace – it therefore makes perfect sense that all globular clusters have approximately the same large number of stars. Their smaller brothers and sisters that didn't contain as many stars were doomed to be destroyed."

This image of the Antennae galaxies shows a multitude of bright young star clusters, groups of stars associated with regions of intense star formation. Credit: NASA, ESA, and the Hubble Heritage Team.According to the simulations, most of the star clusters were destroyed shortly after their formation, when the galactic environment was still very hostile to the young clusters. After this episode ended, the surviving globular clusters have lived quietly until the present day.

The researchers have further suggestions to test their ideas. Dr Kruijssen continues: "In the nearby Universe, there are several examples of galaxies that have recently undergone large bursts of star formation. It should therefore be possible to see the rapid destruction of small stellar clusters in action. If this is indeed found by new observations, it will confirm our theory for the origin of globular clusters."

The simulations suggest that most of a globular cluster's traits were established when it formed. The fact that globular clusters are comparable everywhere then indicates that the environments in which they formed were very similar, regardless of the galaxy they currently reside in. In that case, Dr Kruijssen believes, they can be used as fossils to shed more light on the conditions in which the first stars and galaxies were born.
 
 
 

The quest for Einstein’s missing waves

2012-01-14 10:38:01
Ever since Einstein came up with general relativity, researchers have been looking for evidence for this theory. The problem with the theory is that it predicts some events that we cannot observe (yet). Over the years, increasing detector quality resulted in more and more evidence that Einstein's theory is at least partially correct. However, one of the theory’s predictions that so far has only been detected indirectly are the so-called gravitational waves.

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
 
 
 

Hubble is getting a successor

2011-12-12 21:30:06
Perhaps it is the best thing that ever happened to astronomy, the launch of the Hubble Space Telescope in 1990. Not only did it collect a gigantic amount of data for astronomers, it also opened up the world of astronomy to the public. What astronomers had long known, could now be seen by everybody: the immense beauty of the universe. Probably everyone has at least once been amazed by a photo made with the Hubble Space Telescope. But of course the main goal of the telescope was to help astronomers understand the secrets of the universe, and it succeeded with flying colours. Last year Hubble has regrettably received its final servicing mission and will not undergo any more assistance until the Earth’s atmosphere destroys it in a year or five from now. Fortunately, its successor has been on the drawing tables for while now. Although the astronomical world is still skeptical about the actual realization of these plans, good news came from the US senate a few weeks ago. They officially secured the founding of Hubble's successor, known as the James Webb Space Telescope.

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
 
 
 

The golden era of exoplanet discoveries

2011-11-25 11:48:24
Artist's impression of the exoplanetary system HD10180 (credit: ESO/L. Calçada).Like the great explorers did on Earth five hundred years ago when they set foot on unknown continents, astronomers are currently discovering alien worlds. Contrary to the likes of Columbus and Marco Polo, astronomers are not able to set foot on these worlds in person. Instead, they use their telescopes to look for unknown planets around other stars in the vicinity of the Sun. The number of discoveries increases at a steady rate – in many ways we are living at a unique moment in time, which marks the onset of a golden era. We are witnessing how a new generation of explorers leads the way to a wealth of exoplanet discoveries.

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.

Direct image of the exoplanet Fomalhaut b, which is situated in the dust ring around the star Fomalhaut (credit: Paul Kalas et al./University of California at Berkeley/NASA).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.

The characteristics of the discovered exoplanets cover a wide range, but so far they are mainly set by whichever properties enable them to be detected. The “wiggle” and the “transit” detection methods both promote the discovery of massive planets that orbit their host stars at close distances. These “hot Jupiters” (named that way to indicate their close vicinities to their host stars and their high, Jupiter-like masses) are indeed prevalent among the several discovered exoplanets. The other side of this selection bias is that Earth-like planets are notoriously hard to detect – these orbit at large distances from their host stars, safely in the habitable zone (not too cold, not too hot), and have such low masses that they hardly pull their host stars about nor do they obscure much of the stars’ light when transiting in front of them. Despite all this, a handful of Earth-like and potentially habitable exoplanets has been found. But again, this number is dwarfed by the 500 million habitable planets that are estimated to exist in our Milky Way alone.

Artist's impression of the volcanic exoplanet Gliese 581c (credit: David A. Hardy).Perhaps the most exciting thing about the approaching era of exoplanet discoveries is that each of these alien worlds will have its unique characteristics, which undoubtedly will speak to the imagination of many. Be it the recent discovery of a planet in a binary star system, or any future detection of a twin planet to Earth, it feels tangible and real. Imagine a planet covered in oceans, or a volcanic world, encrusted by molten rock. Or what about a habitable moon that orbits a giant and barren Jupiter-like gas planet? It is only a matter of time that such worlds will all be discovered. And it is a privilege to be living at the right moment to witness it.

DK
 
 
 

Trusting the laws of physics

2011-11-10 18:03:34
A few weeks ago, the news was dominated by a discovery made by scientists at the CERN institute. They performed a series of measurements in which they wanted to calculate the velocity of neutrinos (very small, almost non-interacting particles). Just an everyday experiment, you may think, but the results were shocking. They measured that the neutrinos moved faster than the speed of light, something that is thought to be impossible. Recently, another discovery also seemed to disagree with the leading laws of physics. In this case, astronomers discovered that the electromagnetic force may not be equal throughout the universe. Again, this runs against everything that is known in physics. What do these discoveries tell us? And can we still rely on the laws of physics?

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
 
 
 

Understanding the nature of dark matter

2011-10-25 18:59:57
The large-scale structure of dark matter in the Millennium Simulation (credit: MPA Garching, V. Springel, S. White et al.)Ask an astrophysicist about the biggest challenges the field is facing and undoubtedly they will mention the elusive dark matter that takes up most of the mass in galaxies – among which is our own Milky Way. Some people consider the current mystery of dark matter an embarrassment to science. But if that were true, science would not exist. It needs mysteries to be solved. And an understanding of dark matter is coming ever closer.

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.

The Bullet Cluster(s) of galaxies, providing the strongest evidence for dark matter to date (credit: 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.)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.

A dwarf galaxy seen from the surface of a hypothetical exoplanet (credit: David A. Aguilar, CfA)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.

The second telescope is being planned at ESO and is called the European Extremely Large Telescope (E-ELT), a ground-based observatory with a mirror of 39.3 metres (see the above video for a scale model at the ESO open day) that is supposed to be ready in 2022. With the E-ELT, astronomers will look back at when the very first galaxies formed – an era during which dark matter presumably dominated galaxy formation even more so than it does today. Both new telescopes will be supported by new computer simulations, which will allow us to test new dark matter theories and interpret whatever Gaia and the E-ELT will see.

It is very hard to predict science – if not inherently impossible. But I would be surprised if we haven’t understood Walker’s and Peñarrubia’s pitless galaxies by the time the E-ELT is operational. Dark matter may be invisible... but it cannot hide.

DK

 
 
 

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What is Project Collision?

A blog on the Universe, its secrets, and our place among the stars. You can read the blog posts on new, exciting discoveries, or play the iPhone app Collision - Travel to the Moon. If you'd like to propose a blog topic, feel free to contact me using the contact form at the bottom of the screen!

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