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Thursday
Oct212010

From Void, Through Inflation, To Now

Check out the picture accompanying this post (courtesy NASA). It is a graphical timeline of the history of our universe. Let's walk through it step by step - from left to right.

Quantum Fluctuations: the universe starts off as a tiny blob of quantum fuzz, perhaps 0.000...thirty more zeroes...1 centimeters in size; basically out of void and nothingness - the uncertainty inherent in the laws of quantum mechanics (see post on the quantum world for more). The physics during this period is not well understood - it lies in the realm of String Theory. The temperature of the quantum fuzz is 1000…plus twenty more zeroes degrees… at these temperatures, all the laws of physics are expected to be highly symmetric and the forces of Nature unified.

Inflation: the random quantum fluctuations get frozen out during a remarkable period known as Cosmological Inflation: a violent expansion of the universe driven by the repulsive force of dark energy (see post on Dark Energy for more). This lasts only 0.000…thirty or so zeroes…1 seconds; but it is so violent that, at the end, the universe is only about 100 times smaller in size than what we see today! The explosive expansion cools down temperatures to a comfortable 1000…fifteen more zeroes degrees.

In the next few seconds, the expansion continues but slows down dramatically (see comments about the graceful exit in another post). The laws of physics loose their symmetric form and the force laws start fragmenting into different branches, as we see them today: electromagnetism, gravity, weak force, and nuclear force. Protons and neutrons form first, then Hydrogen and Helium as the matter condenses out of the vacuum into a cooler universe. By the time we reach a few hundred thousand years since the beginning, atoms abound and the stuff in the universe goes from opaque to transparent: that's the point labeled Afterglow Light Pattern in the timeline. This is the Cosmic Microwave Background (CMB) radiation that we image today (see other post on the CMB for more). The temperature is now a chilling 3000 degrees.

The universe continues to expand and cool down at a slower rate for the next 14 billion years. We first go through the Dark Ages - when witches were burned alive and alchemy was common. Then we have the formation of the first stars about 400 million years since the beginning. Then we get galaxies, and finally here we are living our miserable lives.

This picture of the history of our universe crucially relies on that initial critical and delicate period called Inflation - when the universe underwent a violent expansion that stretched space faster than the speed of light. Without the inflationary epoch, it is effectively impossible to realize a universe that looks like ours today (see other post on the multiverse). The video accompanying this post gives an excellent and brief description of what Inflation is, including a discussion by the father of the inflationary theory, Alan Guth. Enjoy.

Wednesday
Oct202010

Made Of Star Dust

A star sustains itself through a delicate balance between two competing forces: an outward pressure created by the nuclear reactions in its core that consume its fuel - typically Hydrogen; and the gravitational inward pull due the star's weight. When the star exhausts its fuel, gravity wins out and it undergoes a catastrophic collapse. The process of the collapse may heat up the star enough to start secondary nuclear reactions that burn the by-product of the previous burning stage. But this depends on the original mass of the star. For an average star, like our Sun, the collapse ensues after the star's Hydrogen fuel is consumed and transformed into Helium, and the star never gets hot enough to start a new life. Instead, it goes through a momentary expansion in size becoming a so-called Red Giant (go Giants?). During this phase, it spews part of itself in its neighborhood - very sad but nevertheless true… it then collapses back into an uninteresting rather dim object, a white dwarf - a ball of Helium and perhaps Carbon held by the pressure of a gas of electrons. For larger stars, the core gets hot enough for new nuclear reactions to follow - lighting up the star once again as it burn other heavier elements. Eventually however, all stars will end in a total collapse. The more massive ones may form neutron stars - balls of neutron particles packed together very very (very) densely ! or more interestingly, they may end up punching a hole in space by forming a black hole (see previous post on black holes)…

For average stars, the last explosive event they undergo generates beautiful patterns of dust and gases - called planetary nebula - around the dying star. The typical size of such a pattern is about one light-year; that is it takes about a year for light to travel across the nebula. Different materials in the cloud of dust glow with different colors in the veil of the strong UV light coming from the star at the center, and they form elaborate patterns on the dark background of the cosmos. The star ends its life by painting a last spectacular masterpiece with the stuff it was made of across the sky. This material eventually seeds other stars and planets. It is said we are all made of stardust...

If you're over 21, get a good glass of red wine, sit back, and watch the 3 minute slideshow I've prepared from imagery of planetary nebulae from NASA. The soundtrack is "Prayer of St. Gregory" from the Armenian-Scottish composer Alan Hovhaness. If you're under 21, use root beer instead.

Monday
Oct182010

High Fructose Physics Syrup

Mix two parts of corn starch with one part of water: rejoice! you now have a non-Newtonian fluid… 

A normal fluid is usually described by a handful of properties: density, velocity, pressure, viscosity. That's five dynamical quantities: one density, three velocities (in 3D), and one pressure - plus viscosity which is taken non-dynamical, a constant property of the fluid. Viscosity is a measure of the resistance of a fluid to flow: honey is more viscous than water since it pours more slowly; it's "stiffer" and hence has a higher constant value for its viscosity parameter. 

To predict the evolution of a flow, you need five equations from physics - for your original five properties of the fluid. Three come from Newton's second law applied to a fluid (you know, the good old force equal mass times acceleration in 3D); another comes of the fact that you are hopefully not allowed to create fluid out of nothing (mass conservation); and the last is known as an Equation of State: it tells you how the pressure and density in the fluid are related. This last equation depends on the type of fluid. It is for example different for water from that for Hydrogen gas (yes, a gas is very much a fluid: anything that flows is called a fluid). At the end of the day, you end up with a set of equations that you can try to solve: that is if you're really really (and I mean really) lucky. Most of the time, you just stare at them in despair and ponder alternate less glamorous fallback careers: the equations are just too nasty to solve by hand! So, you use a computer to determine numerical predictions… and even those blow up in your face very quickly because of a fun thing known as turbulence - the same thing that you experience in an airplane while trying to drink a cup of coffee. That's Newtonian fluid for you. You now know why I avoid fluid dynamics in my research whenever possible.

Now that you've suffered enough reading through equation counting, time to come back to non-Newtonian fluids and have some actual fun. In a Non-Newtonian fluid, the viscosity is NOT a constant… It depends on the pressure! The fluid has an additional equation relating the changing pressure to its changing viscosity - which is now dynamical… A non-Newtonian fluid becomes stiffer when you apply strong pressure: it behaves like a solid! this same stuff flows smoothly like syrup when under low pressure… As if a chameleon, the Non-Newtonian fluid changes its viscosity according to its environment! Now, I already told you that you can't solve most Newtonian fluid problems without a computer. So, forget about tackling non-Newtonian stuff even with loads of charisma and skills. This is an area of physics that is still currently under active research and investigation. In the meantime, check out the two videos attached to this post to see what we know so far about these very very interesting substances. The first video shows what happens when you bring together a group of intoxicated people and a pool of corn starch. The second video will bring out in you new emotions: anything from disgust to terror. 

Friday
Oct152010

Physicists Gone Wild

General Relativity teaches us that the fabric of space and time is malleable; like a sheet of flexible rubber. When objects with mass and energy - like stars and planets - are around, the fabric stretches in such a way that the measure of distance around the objects gets distorted. General Relativity tells us that the gravitational force is simply a result of an illusion arising from such distortions of the fabric of spacetime. Depending on what astrophysical process is at work, the distortion realizes various beautiful patterns, dynamical and complex. One of the most interesting phenomena results from violent events, such as the collision of two black holes… The event creates a ripple in the curvature of spacetime, a gravitational wave that propagates out at the speed of light nudging everything in its way…

Gravitational waves are waves that stretch and squeeze distances across space in an oscillatory pattern - as they propagate through the universe. They go through most everything and travel immense distances - carrying within them information about violent events far far away. Unlike light, they can puncture through the primordial plasma of the universe; hence, detecting them would allow us to probe the first 100,000 years since the beginning of time, past the opaque plasma that obstructs our view (see my previous post on the cosmic microwave radiation)… detection of gravitational waves would allow us to verify the theory of General Relativity beyond anything achieved so far, directly confirming the existence of exotic objects such as black hole and cosmic strings. In short, these ripples of spacetime pack in them physical information about the universe and cosmology, information that is simply unavailable from any other source we have. We desperately need to "see" them!

In the 1970s, a first indirect evidence of gravitational waves was identified. Later the results were refined and confirmed. Two massive objects known as neutron stars can form a binary system - basically orbit around each other along a circle in an exceedingly dangerous dance. As they go around, they loose energy by emitting gravitational waves and spiral inward faster and faster for an eventual collision: imagine water flushing down the toilet… Astrophysicists were able to identify such binary systems and measure the period of the spiraling motion. Even though the gravitational waves could not be detected, the spin rate of the neutron stars was increasing precisely as predicted by General Relavity...

So, now that you're energized and romanticized about detecting gravitational waves, let's bring things down to reality… Think of a gravitational wave as a ripple on the surface of the ocean. As a water wave gets past you, you rise and fall with the passing disturbance. To detect gravitational waves, you need to measure the distance between two fixed points in space. Say you hold two fingers a few centimeters  apart and let a wave from some black  hole collision light-years away pass through them; you measure the distance between your fingers and you would see this distance stretch and squeeze as the wave passes through. Here's the catch: the effect from a typical gravitational wave is expected to change the distance between two fixed points by about the radius of a proton over a distance between the Sun and the Earth… How the heck you measure that??!! Well, unachievable goals have never stopped physicists from trying; and hence the title of this post. 

There are currently two huge gravitational wave detectors in operation. They are looking for ripples in the fabric of space as they pass by our neighborhood… they're called the LIGO detectors - Laser Interferometry Gravitational wave Observatory (I guess LIGWO does not sound right...). They are capable of measuring a change in length of about 1/1000 the radius of a proton over a distance 4 kilometers… the instrument is a marvel of technological precision and physics ingenuity… check out the accompanying really good video to learn more about LIGO.  

Thursday
Oct142010

Frying Brains…  

 

This post is intended to take you all the way to the edge of sanity; so dim the lights, put on some good new age music, and brace yourself… String Theory starts with the premise that the building blocks of matter and energy are not necessarily particles - that is point-like packets of energy. The theory proposes that of the three pillars of modern physics - gravitation, quantum mechanics, and relativity - the first is really not formulated properly, but the last two are right on target. The implication is that this is the cause of the difficulty of putting gravity and quantum mechanics together: basically, blame it on gravity! The theory also proposes that all physical observables should be computable from scratch; there are no magic numbers floating around in Nature, we should be able to understand every bit of observation. The principles I just listed, while frugal and rather general, are extremely powerful. I can argue that this is all that is  needed to develop the entire field of String Theory. The idea is that anything logically consistent within this framework is fair game and is to be allowed…

After a couple of decades of hard work by a group of several hundred overworked string theorists, we now have a remarkably detailed picture of what this String Theory thing is - but the full picture is still incomplete. We are able to show that quantum mechanics can be married successfully with the gravitational force. And many outstanding puzzles of theoretical physics get very interesting resolutions, from strange black hole physics to particle physics botany. But the full narrative is still being written with research in progress, and we cannot tell yet whether this theory - in its current incarnation - is to survive the ultimate tribunal: experiments and measurements. However, given the successes of the theory, it is now highly likely that some of the new revolutionary ideas the theory has introduced into discussion are to survive within the ultimate future framework of fundamental physics.

As a result of all this, String Theory requires that the world has ten space dimensions… see my previous post about compactification to see an alternative mechanism - to the one I will discuss in this post - through which this setup can still lead to the observation of only three space dimensions. Since the building blocks of the theory are not necessarily point-like, one finds that these can be in the form of tiny strings; or even in the shape of membranes… in the full 10 dimensions, you can even have a three-brane or brane for short… that is an object like a membrane but extended in three dimensions instead of two. You won't be able to visualize this (I hope), but you can view a cartoon depiction of a two dimensional membrane: it is now a good time to play the first video attached to this post to make things a bit less abstract…

And here comes the big punchline: in the context of this theory, our universe can be a three-brane… we are living in the fabric of the brane that is flopping around in a higher dimensional space. Imagine the first video of this post with a population of insects living on the membrane that is flopping around. That is us; except its a brane extended in three space dimensions instead of two, and hence we perceive the world in 3D! We are confined and welded to the three brane. In fact, we are made of the stuff of the three brane: the ripples on the brane represent nothing but the matter in our universe, including ourselves! We can show from string theory that the way these ripples behave, scatter off each other, and evolve, is indeed in tune with all the stuff around us: electrons, atoms, all the forces of Nature that we have measured… This is simply shocking; that a picture of a brane flopping around in a higher dimensional space appears from the perspective of things living on the brane as the universe we actually see today… But it gets more interesting…

If we are a three brane flying and rippling through some higher dimensional space, there may very well be other branes floating around nearby: other universes. Check out my post on the Multiverse picture for more about this topic. Let's get all the way to the edge now. Imagine a gas of universes: instead of molecules making up this gas, it's branes all over! Each brane is a universe with miserable beings living in it. As is typical in a gas, constituents of the gas will frequently collide. So, imagine another brane, our evil twins, on a collision course with our brane, our universe. Time to play the second video clip attached to this post. What would we then see from the perspective of our universe during this collision process? String Theory tells us that we would observe a violent exponential expansion of our universe… well, that's what we actually observe today (see post on inflation)… the endpoint of the collision in the second video corresponds to what I referred to as "graceful exit" in the previous post; the collision itself: the Inflationary Epoch.

A reference article: James M. Cline (2007). Braneworld Cosmology PoSstringsLHC:011,2006 arXiv: 0704.2198v1
Wednesday
Oct132010

Time To Worship The Sun

Just a few planetary steps away in our immediate neighborhood, the Sun sits as a typical average astrophysical star: a factory that burns Hydrogen into Helium through nuclear fusion. Basically, an H-bomb in empty space. Fusion is the process by which two atomic nuclei, in this case those of Hydrogen, fuse and form a new atomic nucleus, in this case that of Helium. The Helium nucleus has less energy that the initial products; the difference in energy is emitted away as radiation; we say the Sun shines... The Sun has been around for around 5 billion years. And it is expected to exhaust its Hydrogen fuel within the next 5-6 billion. So, it is middle aged and has started graying; and like most middle aged people, it goes through periods of crisis we call solar flares (this is starting to get too personal, but I'll try to carry on). We expect that at the end of its Hydrogen burning cycle, the Sun will collapse under its gravitational weight - after one last bright moment as a red giant - and fizzle away as an unremarkable dim object known as a white dwarf (now it's getting really personal). 

Think of the Sun as a hot soup of plasma, a fluid of electrically charged atoms and particles. It's turbulent and in perpetual turmoil, as the hot plasma slushes around erratically. The whole ball of fire is also spinning on an axis. When electrically charged particles move, they cast about them a magnetic field. Think of a magnetic field as a network of imaginary lines in empty space emanating from charges in motion. The meaning of such lines is simply this: if other electrically charged particles, like say electrons, wander by a region filled with these line, they will experience a magnetic force. This magnetic force is such that the wandering electrons get channeled along the lines as they spiral around them. So, the space around the Sun is threaded by a beautiful and complex entanglement of such magnetic lines around which charged particles spiral in what can only be described as an elegant and fiery cosmic dance.

But the Sun's plasma is highly turbulent. Occasionally, plasma storms on its surface generate immense eruptions of both plasma and magnetic fields. These can be so violent and energetically packed that the effect propagates all the way to the Earth and hits us in the face (well, technically hits the atmosphere in the face). We can seriously get effected by such solar weather patterns: when eruptions of ions and magnetic fields pass though electronic equipment in satellites or otherwise  (that are full of electrons) , bad things can happen. There is also a popular lore of an impending apocalypse driven by a large solar storm… Recently, the Solar Dynamics Observatory captured remarkable close-ups of solar flares. You can check some raw video by clicking here. Or view the accompanying video for more production value but really really corny background music and narrative.

Tuesday
Oct122010

Fish in a Pond

Things are so because things couldn't have been any different… In recent years, cosmological observations have painted a remarkably detailed picture of the history of our universe. The bad or good news (depending on your perspective) is that some of the conclusions are astounding: results suggest an extremely delicate balance all around us, so delicate that if things were to be a tiny bit different at the beginning of time, perhaps we would not be around to ask any questions…

There are several such "coincidence" and "fine tuning" issues. In the beginning, the universe was filled with dark energy (see previous post) and underwent a dramatic explosive expansion. This expansion was exponential - physicists call it the Inflationary Epoch - where the fabric of space stretched faster than the speed of light. As the universe expanded, some normal matter was generated during a period cryptically called a "graceful exit"… The expansion was so violent, that it was highly highly (and I mean highly) sensitive to the initial condition of the dark energy pervading the universe. If things were a little different, this crucial epoch of expansion of the universe may not have been realized. And this inflationary epoch is crucially needed to explain why our universe is around… Here's another perverse coincidence. As the universe continued to expand, and is now known to undergo an accelerated expansion, space stretches away from us faster than light can catch up with it… so, our horizon - farthest extent we can see into the universe - is shrinking fast… As it happens, we live around the right period that allows us to just be able to see the whole universe… Several hundred million years later, the edge of the universe would have receded away from our visual horizon… On cosmological timescales, this coincidence is quite shocking and highly unlikely.

In the context of String Theory, these issues get addressed rather explicitly. String Theory is described by a set of equations whose solution is presumably our universe. The problem is that we sort of have a situation of an embarrassment of riches: the equations admit many many solutions - amongst them potential candidates for our universe - but these realizations are often very disparate in their conclusions on how the world should look like. Too many of these solutions do look like the world we live in - but the devil is in the details. And we don't know all the solutions… You may then say that String Theory allows many universes as possibilities: call it the Multiverse picture. How can we predict anything in such a situation! Which universe are we in? Are there other ones around? Where the heck are they? Why are we in this one? There comes the anthropic principle, an old idea that has received new life in the context of these modern questions. The idea is simple: of all possible universes, only a few select ones have the right conditions to have our kind of miserable life evolve in it… we are in this universe because if things were different, we wouldn't have been around… This does have a flavor of a mentality from the Middle Ages, doesn't it? it is absurd to ask some fundamental questions because there is no answer to them beyond: "it is so, because we need it to be so to be able to ask the question"...

Here's my revised version of an argument that goes back to the Cosmologist Linde in support of the anthropic principle. Imagine a species of sophisticated fish living in a pond whose temperature is 15C. After living their lives as high quality Sushi for a while, these fish develop intelligence and become sentient. Some of the fish adopt unglamorous careers of hard work with little benefits as physicists, and start measuring the temperature of the water. Fish with even lower self esteem become theoretical physicists, and start asking: "Why is the temperature so?". Can we derive some equations that predict the temperature? You are standing outside the pond looking into it and wandering: "These must be the stupidest fish in the world: the temperature is so because if it wasn't, this species of fish wouldn't survive and be around to ask the question"…

Personally, this scenario is very troubling to me. Does this mean there are some fundamental questions in physics we can never unravel beyond a lame anthropic argument? is this the end of fundamental physics then? I don't think so. There are some recent suggestions that one may be able to get a statistical handle on such questions: we may not be able to predict which universe of the many possible ones we live in, but perhaps we can say which ones are the most likely without considering a biological factor… And that may be good enough, whether we like it or not… after all, if the fish are to evolve, they need to start looking outside the pond...

The accompanying video is slightly on the edge… but still interesting enough to lead you to ponder over some of the implications of this subject that straddles physics and philosophy.

A reference article: Leonard Susskind (2007). The Census Taker's Hat http://arxiv.org/abs/0710.1129v1 arXiv: 0710.1129v1