Where does the Moon come from?

Last week it was full moon, and not just an ordinary one, but a perigee full moon, often popularly called a “Super Moon”. The orbit of the moon around the earth is elliptical, so the distance between moon and earth varies between 363.104 km (perigee) and 406.696 km (apogee). When a full moon occurs at perigee, the moon looks larger and brighter. It’s not a rare phenomenon, 9 September this year will  be the next perigee full moon, and June 2013 there was another one. It’s a bit of a media hype.

My friend Chuan took a beautiful picture of this perigee full moon, in the middle of the night, with his point and shoot camera(!), handheld, 24x zoom.

perigee full moon

The dark regions are called Mare (Sea) because in the past people believed that there was water on the moon. Actually they are basaltic plains, formed by ancient volcanic eruptions. Huge craters mark the places where meteorites have hit the moon. Here is a map of the moon with the names of craters and seas.

Names of seas and craters

We can see only one side of the moon because the moon is “tidally locked” to the earth, always showing the same face to us. This interesting phenomenon deserves a separate post..:-)  So how does the other (“dark”) side of the moon look like? It’s only after the start of the space age that we were able to explore. With a surprising result. Here is the other side of the moon

Far side of the moon

A lot of craters, but no “seas”. Why so different? Which leads to another, more basic question, where does the moon come from? Was it “born” at the same time as the sun and the other planets, ~4.5 billion years ago? Many hypotheses have been formulated, here is the theory that is generally accepted at the moment. It is called the Giant Impact Hypothesis

Not long the formation of the solar system, there was another planet, about the size of Mars, which collided with the (young) Earth. Here is an artist impression of this collision.

Theia meets Gaia

This hypothetical planet has been named Theia, after a Greek goddess, the mother of Selene, the goddess of the moon. The effect of this dramatic collision was that a large part of Theia and Gaia, as the young Earth is sometimes called, melted together, forming the present Earth, but another part of Gaia and Theia was thrown out during the collision and coalesced into the Moon.

So powerful was this collision that the new Moon and probably also part of the Earth consisted of molten magma. The Moon, being smaller, cooled faster, and because of the heat of Earth and the tidal locking, the near side of the moon got a thinner crust than the far side! According to this theory that might be the reason that the near side has had more volcanic activity than the far side. There are many more arguments in favour of this giant impact hypothesis.

Of course the next question is then, where did Theia herself come from? A very promising idea is that this planet might have been formed in  about the same orbit as Gaia. In 1772(!) the French mathematician Lagrange studied the properties of rotating systems, like the earth orbiting the sun. He discovered that there exist points in such a system, where other objects can exist in a stable way. There are five such points, nowadays called Lagrange points

lagrange points

In the Lagrange points L4 and L5 the gravitational force of Sun and Earth balance in such a way, that objects will corotate with Earth around the Sun. During the formation of the solar system, mass could have accumulated in for example L5 and formed Theia. Through the disturbance by other planets (Venus for example), this planet could, after millions of years, leave L5 and collide with Earth.




Just skip this last part if you find it too complicated…:-)

Physics Nobel Prize (2011)

The Nobel Prize for Physics in 2011?

But that is long ago, the Nobel Prize 2012 has already been awarded and in October the winners of the 2013 prize will be known!

Yes, this post is long overdue, I know 🙂 Every year I am interested, being a physicist myself, who will get the Nobel Prize for physics and for what . And nowadays often I have no idea what it is about :-(, being out of touch with the modern developments for so long already. So I was quite happy that I understood the importance of the discovery made by Perlmutter, Schmidt and Riess in 1998 that our Universe is expanding at an accelerated rate.

From left to right, Perlmutter, Schmidt and Riess


Of course you have heard about the Big Bang, the primordial explosion that created the Universe, about 14 billion year ago. As a result of this explosion the Universe is expanding and also cooling down. Proof: when we look at faraway galaxies, we observe that they are moving away from us and each other, the farther away the faster they move. And in 1965 the Cosmic Background Radiation was discovered, proof of the cooling down of the Universe.

When I was doing my PhD research, in the seventies, the Big Bang theory was widely accepted. And also that the rate of expansion should decrease with time because of the mutual gravitational attraction between all matter in the Universe. If the Universe contained enough mass, the expansion would finally stop, followed by contraction and ending in what became known as the “Big Crunch” where the whole Universe would again be concentrated in a single point. And might even start again in another Big Bang! An attractive idea in those hippie days!

Here are the possible scenarios. In the coasting scenario there is not enough mass to stop the expansion, in the middle one there is just enough mass to stop it (asymptotically), but not enough to reverse the process (as in the left scenario)


The problem was that when you counted all the visible mass in the Universe, there was just not enough to stop the expansion. It was named the “missing mass problem”.

Would it be possible to determine experimentally which scenarios was the correct one? To measure the rate of expansion, you should measure the velocity of very faraway galaxies. Measuring the velocity is not that difficult, you have to measure the Doppler shift. When an ambulance passes you, you will first hear a higher sound of the siren, and a lower sound when the ambulance is moving away from you. For light it is basically the same, here you will see a difference in colour. When a star or galaxy is approaching is, the colour is a bit bluer, when it moves away it will be redder. Measuring the “redshift” gives us the velocity.

The big problem is how to determine the distance to such a faraway galaxy! The technique used in astronomy is based on the fact that light from a light source becomes more spread out when the distance is larger. Probably every photographer is aware of this “inverse-square law”

inverse square law

So if you know how “strong” the light source itself is, you can determine the distance by measuring the amount of light at that distance. But how do we know how much light a star really produces? In general that is impossible, because you have big bright stars and small, not so bright stars.

What the Nobel Prize winners did was looking at very special events, so-called (type 1A) supernova’s. A supernova is a star that explodes at the end of its life. During a few days/weeks it can produce more light than a whole galaxy. And the intensity of this light is basically the same for each supernova explosion (of type 1A). They are extremely rare events, it is estimated that in our own Milky Way they occur only a few times in a century! But when they occur, they are so bright that they can even be observed in very distant galaxies. And there are so many galaxies.

Finally we can now explain the research done by the (competing) teams of Perlmutter and Schmidt & Riess. They looked for type 1A supernovas in distant galaxies and determined the distance and the velocity. To show you how complicated this kind of research is, here is an image of a recent supernova discovery, SN Wilson. In this image a few bright points are stars, but many are galaxies. The tiny square contains the galaxy with the supernova.


Here are three enlarged images of this tiny square. The galaxy is the round spot in the center. Left image shows the situation before the supernova exploded, in the middle one the supernova has exploded. You don’t see any difference? Let the computer “Subtract” the left image from the middle one and you get the image to the right! Voila, the supernova ..:-)!

Before and after

These images, taken by the Hubble telescope were taken by the team of Riess in 2010. The distance is 10 billion light year, which makes this galaxy the most distant one, observed until now.

As the light of this galaxy needed 10 billion years to reach us, we observe it now as it was 10 billion years ago! Looking far away means looking in the past. The scientists expected to find that in the past the expansion of the universe would be faster than it is now, as explained in the beginning of this post.

What they actually found, shocked the scientific world: the expansion of the Universe was accelerating . It was so unexpected that it was very fortunate that two research teams came to the same conclusion.

So there had to be a repulsive force, stronger than the attractive force of gravitation. This repulsive force is now named “dark energy” but we still have no clear idea what it is.

It is for this discovery that the two teams shared  the Nobel Prize.

The three scenarios, mentioned above are all wrong. It is the fourth scenario, shown below, that we now believe to be correct. There is even a possibility that this acceleration will increase so dramatically with time, that the Universe would end in a Big Rip, where finally, stars, planets, even atoms would be ripped apart.

accelerating universe

Much progress has been made since 1998, especially in the analysis of the Cosmic Background Radiation. It has confirmed that there is a repulsive force, now named “dark energy”. It has also confirmed that there is a lot of invisible matter in the Universe, now called “dark matter”. In both names “dark” describes our ignorance, at the moment we just do not know what they are. I am planning to write a separate post another time about this topic.

Let me end this post with an image that gives the distribution of “normal” matter, dark matter and dark energy in our Universe. I have seen this kind of picture numerous times, and I still find it shocking.


The stars, the planets, humans, everything is made of normal matter: protons, neutrons, electrons. We know a lot about it.  But it is only 4% of our Universe. About the other 22+74 % we know next to nothing at the moment!

If I could start a new life now, I would choose astrophysics and cosmology as my field of study…:-)

Several images above have been taken from this very interesting set of lecture notes.

Largest structure in the Universe discovered

This will be a bit longish post…:-)

A few weeks ago an in international team of astronomers announced the discovery of the largest structure in the universe: a group of quasars extending over a distance of 4 billion lightyear (ly).


Quasars (Quasi-Stellar Radio Objects) were discovered about 50 years ago. They look like stars but are so distant (billions of ly away) that they can not be stars. Now we know that they are active nuclei of galaxies, surrounding a massive black hole in the center. Billions of ly away means that we observe them as they were billions of years ago  when the universe was still young.

More than 200.000 quasars are known at present. Some of them occur in (large) groups, called LQC‘s.The group that has now been discovered has 72 members (the black circles in the image). The red crosses form another, smaller group.

So, why is the discovery of this large group of quasars so exciting? To make that clear, we have to talk about the cosmological principle and the large-scale structure of the universe.

Long it has been thought that the Earth was the center of the Universe. Then it was discovered that Earth is one of several planets orbiting a star, the Sun. It is the blue marble in the image below. It is not in scale, light needs only 8 minutes to travel from the Sun to Earth, and more than four hours to reach the outermost planet Neptune.

The Solar System

Is the Sun the center of the Universe? No, the Sun is one of several hundreds of billions of stars in our galaxy, the Milky Way.  Some of you may have seen the Milky Way on a cloudless clear night far away from cities, as a white band of light across the sky. Here is an artist impression of the Milky Way as seen by an observer from outer space. The approximate location of our Sun has been indicated with a red cross. The diameter of the Milky Way is about 100.000 ly.

The Milky Way

Is then the Milky Way the center of the Universe? Again negative! Our Milky Way is just one of hundreds of billions of similar galaxies.  The scientists now think that the Universe has no center! From each location and in each direction the Universe looks the same, if you observe it on a sufficiently large scale. This is called the Cosmological Principle

So, what is a sufficiently large scale? If the galaxies would be randomly distributed in the Universe, we would not need to zoom out further. But that is not the case! Our Milky Way is a member of a group of more than 50 galaxies, bound by gravity. It is called the Local Group. Most of the galaxies in this Local Group are small ones, with the exception of our neighbour, the beautiful Andromeda galaxy.


Andromeda is bigger than the Milky Way, may contain one trillion stars and is located at a distance of 2.5 million light-years from our galaxy. Here is a “3-dimensional” sketch of the local group.

Local Group

The size of the Local Group is in the order of 10 million ly. Many more of these galaxy clusters exist, for example the Virgo Cluster, much bigger than our Local Group, consisting of more than 1000 galaxies, at a distance of 54 million ly.

Here is the Virgo Cluster. All the fuzzy blobs are galaxies, the light points are stars in our own Milky Way. Click on the image to enlarge it and take a few minutes to think about the meaning of life..:-)


We still have to zoom further out. Our Local Group, the Virgo Cluster, the Fornax cluster, the Eridanus cluster and about 100 more are part of an even larger collection, the Virgo Supercluster . Here is one more “3-dimensional” sketch of this supercluster.

Virgo Supercluster

You will see the Local Group in the center, the Fornax and Eridanus clusters and many more. We are talking now about a size of more than 100 million ly already!

Many more superclusters have been discovered. Could it be that these superclusters of galaxies are randomly distributed in the Universe. Let’s zoom out one more time! The image below shows the superclusters around us within a distance of 1 billion ly. So the width of this image is 2000 million ly.


Obviously this is not a random distribution. Clusters and superclusters are aligned along filaments filaments, with in between large portions of space almost without any galaxies. It  looks like a kind of foam-like structure and is sometimes called the Cosmic Web. In the center you notice the Virgo supercluster. Keep in mind the zooming out steps we have made to reach here! Earth → Sol → Milky Way → Local Group → Virgo Cluster → Virgo Supercluster → Cosmic Web.

Do we need to zoom out more? According to the present cosmology theories: NO. Computer simulations starting from right after the Big Bang show that this foam-like structure on a scale of hundreds of millions of light-years is to be expected. Starting point for these simulations is the measured Cosmic Background Radiation CMB), as depicted in the image below.CMB radiation

To explain the relation between this CMB image and the large-scale structure asks for another post…:-).  Basically the Standard model of Cosmology is used, including the effects of Dark Matter and Dark Energy. Here is a typical result of such a simulation. The image has a width of 1500 million ly  The bright nodes represent Superclusters. You will notice strings of galaxies and voids, quite comparable to the real Universe. At this scale, the Universe looks basically everywhere the same.

Cosmic Web

We started this post with the discovery of a group of quasars extending about 4000 million ly. Quasars are nuclei of galaxies, so in the terminology used above, they would form a “cluster”. But a cluster of this size would not fit in the above image at all!

This explains the excitement among astronomers and cosmologists. Is the Standard Model of Cosmology wrong?

Let’s wait and see!

Several images in this post come from a fascinating website: An Atlas of the Universe

God’s particle

About three months ago CERN has announced the discovery of the Higgs boson, a.k.a. the God Particle. Several of my friends have asked me if I, being a physicist, could explain what it was all about. I tried, but it was not easy.

Here is another attempt…:-)

Let me start with an overview.

In the 19th century it became increasingly clear that matter is composed of molecules, and that molecules themselves are composed of atoms. Only a limited number of different atoms exists, ninety occur in nature, quite a few more have been made in laboratories. Imagine the tremendous simplification, everything around us is composed of these building blocks!

The periodic table of elements. Uranium (92) is the heaviest element found in nature. Promethium (61) and Technetium (43) are radioactive and not found in nature, giving a total count of 90 elements occurring naturally.

A monumental breakthrough took place in the 20th century, when it was discovered that atoms themselves consisted of only three (!) elementary particles, protons, neutrons and electrons. Protons and neutrons in the nucleus of the atom with electrons orbiting around this nucleus. Just a matter of numbers. Carbon with 6 protons and 6 neutrons in its nucleus and 6 electrons around this nucleus. Add one of each, and you get Nitrogen, do this again and you get Oxygen.This amazing simplicity was one of the reasons I decided to become a physicist. Even a nuclear physicist..:-)

Actually two more particles had to be added to the list. Light also consists of particles, called photons. And some of the elements are not stable but radioactive, the nucleus can send out an electron and at the same time another particle, called neutrino. Everything controlled by four forces. The strong nuclear force, keeping the protons and neutrons in the nucleus together, the electromagnetic force, keeping the electrons in orbit, the weak nuclear force, responsible for the radioactivity and, finally, the force of gravitation.

But this is not the end of the story, soon it became more complicated again! In cosmic radiation, and also in laboratory experiments (using powerful accelerators to let elementary particles collide), new particles were discovered. Not stable, often only living for split seconds, before decaying in other elementary particles. They were named muons, pions, hyperons, a confusing multitude.

It was discovered that protons and neutrons were actually NOT elementary particles, but that they were composed of “quarks”. Not just one, but several families of quarks. Bound together by “gluons”. And the electron and the neutrino were accompanied by other particle families, the muon electron, the tau electron, with corresponding neutrino’s. The strong nuclear force is actually the force between the quarks, with three quarks forming a proton or a neutron.

It has also been discovered that the elementary forces are carried by “force particles” and that the photon is actually the force particle of the electromagnetic force. One of the big successes of the last decades, was the experimental observation of the “weak nuclear force” carriers, the W and Z bosons.

All these experiments lead to what is now called the Standard Model. Three families of quarks, three families of ‘electrons’, three elementary forces with their force particles. This leaves out until now gravitation. That is actually a big problem, but we will not discuss it here

The Standard Model. Gravity is not taken into account. There are three quark families (up-down), (charm-strange) and (top-bottom). And three “lepton” families, electron, muon and tau, with their corresponding neutrinos. Finally the force particles, photon, gluon and the (W,Z) bosons

All the particles in the picture above have been “observed”. Observed in quotes, because these particles are so short-lived that their existence must be concluded from the traces they leave behind when they die…

Much more complicated than the simple “proton-neutron-electron” model, but definitely one of the most impressive results of modern physics.

One problem remains. All these particles, the quarks, the leptons, the force particles, have mass. Some are heavy like the quarks, some are light like the leptons, the photon has no mass, the neutrinos almost nothing. Why?

In 1964 Higgs and a few others came with a theory. There might exist another force field, permeating the universe, acting as a kind of “syrup”, slowing down other elementary particles and in that way giving them inertia ( = mass!). But if that field existed, it should have its own force particle, the Higgs boson. Nicknamed the God particle, because it gave mass to all the other particles.

The theory was widely accepted, so the search for the Higgs boson was on. A fierce competition resulted between CERN and the Fermilab in USA.

It now looks like it has been found. With a mass about 130 times the mass of the proton. And so short-lived that I have not even been able to find an estimate on the Internet. So you have to look at the traces it leaves behind when it dies. Here is an artist impression.

And here is a picture of the experimental setup at CERN (the ATLAS experiment) Try to spot the human figure in the picture!

This is ATLAS, one of the four experiments at the Large Hadron Collider of CERN.

Is our understanding of the physical universe now complete? No way!

As mentioned before, there is still the problem left to combine the Standard Model with gravitation. Maybe string theory, but not everybody is convinced that this will be the solution.

There is a much bigger challenge. During the last decades it has become convincingly clear that there has to be more in our universe than quarks, electrons, photons, etc. Let’s call this “normal matter”. From what we know about the Big Bang and from the way our universe is expanding after the Big Bang, we now are sure that there are two more constituents of our Universe. There has to be “Dark Matter“, until now invisible. And there has to be “Dark Energy“, a repulsive force that actually accelerates the expansion of the Universe. That’s about all we know at the moment.

A small correction to the Standard Model? Absolutely not! Here is the present estimate. Normal matter takes only 4% (!). The rest is basically unknown at the moment!

We know a lot about the 4% normal matter and next to nothing about the rest!

A sobering thought. But also exciting. There is still a lot to discover and explore in our physical universe. And for the ambitious among you: a lot of Nobel Prizes to win!


The Great Debate: Are we alone? part 2

Two months ago I published The Great Debate: Are we alone? part 1

Here is finally part 2. My apologies for the long delay.

Quite a few of you gave their opinion about the question “Are we alone or not”.
Not surprisingly most ‘votes’ went to “We are not alone”, same as in the poll at the end of the Great Debate video.

My own opinion?
It will be wonderful and fascinating if (intelligent) life is found elsewhere in the universe, but personally I think we are alone.
Mind you, that is not arrogance, I would be more than happy if even primitive life is found elsewhere!

Let me explain why I have become (recently) more skeptical about life elsewhere in the universe.

In discussions about the probability of extraterrestrial life, you will often encounter the Drake equation.
In 1961 Frank Drake tried to make an educated guess about the number of intelligent civilisations in our own galaxy, the Milky Way.
He started with the (huge) number of stars in the Milky Way, then asked questions like: “how many stars will have planets”, “how many planets will be ‘habitable'”, “what is the chance that on such a planet (primitive) life will develop”, “what is the chance that intelligent life will evolve”, and several more of this kind of questions.
In his original estimate Drake comes to a number of about ten planets in our Galaxy at this moment, with intelligent, technologically advanced civilisations.

Many of the factors in the Drake equation are the result of guesswork.
For example, one of the points of discussion in the “Great Debate” is about the chance that intelligent life will evolve from primitive life. Drake’s estimate was 1 %. Marcy in the “Great Debate” thinks it might be close to zero. “Maybe we humans are just a freak evolutionary incident?

However, both Werthimer and Marcy agree: “primitive life will be teeming in the Universe.
Drake estimated the chance that life will develop on a habitable planet as 100%!
And Michio Kaku, an American ‘science communicator’, who always enjoys being in the limelight, goes even further: “The Laws of Probability Tell Us That the Universe Should Be Teeming With Intelligent Life Forms” Elsewhere he  writes (foolishly, IMHO) about a 100% probability!

Well, if they are right, why has until now no evidence of life been found on Mars?

A few days after my first “Are we Alone” mail, I sent you a short email about the exciting discovery of a habitable planet, orbiting Gliese 581, a red dwarf star at a distance of 20 lightyear from the sun.
Here is the picture again. Planet g is causing the excitement. The blue band is the habitable zone.

The concept of a habitable zone is based on the assumption that you need liquid water for the development of life. The (surface) temperature of a planet should not be too low or too high.  For a (cool) red star like Gliese 581, this zone lies much closer to the star, than for a hotter star like our Sun.
Of course you can think about more exotic  forms of life, based on silicon, ammonia, etc. Click here for a detailed discussion.

Now, when you look at the picture above, you will notice that both Earth and Mars are orbiting in the habitable zone of the Sun.
The Viking and Phoenix missions to the Red Planet had as one of their main targets the search for life on Mars, and I am sure that many scientists were hoping, or even convinced that evidence of life would be found. So was I.
But “nothing” has been found yet. Of course more exploration is needed and quite a few new missions have been planned.
Still it is disappointing and personally I believe now that the chance that life will develop on a habitable planet, might be small, maybe even very small.
Sure, the Miller-Urey experiments have shown that it is “easy” to synthesize amino acids, the building blocks of life, when the conditions are right.
And organic compounds have been found even in interstellar clouds.

But the next step is huge. Life is characterised by two fundamental properties, replication and metabolism.
We know that this step has been made at least once, on Earth.
Even on Earth there is no evidence that this step has been made more than once! Click here for more information about what is called abiogenesis.

So, this is my position:
As soon as evidence of life will be found, on Mars or deep under the frozen oceans of Jupiter’s moon Europa , I will celebrate and be convinced that life indeed is teeming in the Universe.
Until then, I believe in the Rare Earth Hypothesis , that we might well be alone.

The Great Debate: Are we alone? part 1

On 26 April 1920, a debate took place at the Smithsonian Museum of Natural History (Washington DC), between two American astronomers, Shapley and Curtis, about the scale of the Universe. Point of contention was the distance of the “nebulae” like Andromeda.
Shapley argued that the Milky Way was the entirety of the Universe and the Andromeda nebula was inside the Milky Way.
Curtis contended that Andromeda and other nebulae were separate island galaxies.
The debate became known later as The Great Debate.
We know now that Curtis was right. The Universe is huge and our Milky Way is only one of ~ 100 billion galaxies. Each galaxy contains on average 100 billion stars.
Our Sun is one of these ~ (!) stars, and was formed relatively recently, about 4.6 billion years ago (The Universe is ~ 13.8 billion years old)
Eight planets orbit the Sun (sorry for Pluto, not a planet anymore…), and on (only?) one of them, our beautiful Earth, life developed, about 4 billion years ago.
Evolution took place. But it was only about 200.000(!) years ago that Homo Sapiens (yes, that’s us) evolved, most probably in Africa.

Was that a unique incident?

About half a year ago another astronomical debate took place, at Berkeley University, 30 April 2010, almost exactly 90 years later, and it was also called a Great Debate.
A bit preposterous, IMHO…:-)
This time the topic was: Are we alone in the Universe?
The recording of the debate can be viewed at http://seti.berkeley.edu/the-great-debate . Be warned, it takes 1.5 hours…:-), but watching it is worthwhile…

Debaters were Dan Werthimer, the chief scientist of the SETI project, and Jeff Marcy, an astronomer who has discovered more extrasolar planets than anyone else.

The SETI project is Searching for Extra Terrestrial Intelligence, so Werthimer is, not surprisingly, convinced that there are many planets with highly developed technological civilizations around us. SETI is searching for about 40 years now ‘only’, it will just take more time to find evidence for intelligent life elsewhere in the Universe, according to him.

Marcy is a skeptic. Ok, many, if not most, stars will have planets orbiting around them. And he agrees with Werthimer that primitive life will be “teeming” in the Universe.
But intelligent life? Is that an evolutionary advantage? The dinosaurs ruled the world for 200 million years and never developed intelligence with their peanut brains! Why not?
Maybe speed or a thick skull will serve you better to survive. Maybe we humans are just a freak evolutionary incident?

He has other interesting arguments. For example the water content of a planet. Quite critical.
Assuming that water has been brought to the earth by asteroids, it should be about 0.03%. Not more, nor less.
Less, and the planet will be a desert. More, and it will be just oceans. Is a technological civilisation possible under those adverse conditions?

At the end of the debate, after a Q&A session, there is a poll for the audience. Are we alone or not?

I have an opinion myself, but will only tell you in the second part of this email…:-)