*The Planck epoch universe is a point of unimaginably immense energy confined in an incredibly tiny space. What space-time is made of, and how does a unified force*

*"break" into the four fundamental forces of the universe today?*

At 10

^{-43}seconds, the universe is about 10

^{-33 }centimeters across with a temperature of 10

^{32 }K. It's an unimaginably tiny bubble of indescribably immense energy. This particular size is is Planck length, the smallest theoretical size possible, and in string theory, it is the size of one "string," about 10

^{-33 }centimeters long. It is 10

^{-20}times smaller than a proton inside an atom.

How hot is HOT?

The temperature, 10

^{32 }K, is also a theoretical limit, called Planck temperature. Above this temperature, calculations break down because particle energies become so large that gravitational forces between them become as strong as the other three fundamental forces. The forces essentially melt into one unified force and predictions about everything we know about the universe, including spacetime, break down into nonsense. A recent discovery may help put this bizarre temperature into scale. Physicists recently created the highest ever measured temperature, inside the Relativistic Heavy Ion Collider, about 4 trillion degrees Celsius (about 7 x 10^{12}K). At this temperature, they observed atoms "melting" into a gluon-quark plasma soup, all within a temporary bubble. In this bubble, there was evidence that some laws of physics began to break down - the electromagnetic force and the weak force began to combine into a unified electroweak force. At 10^{32 }K, theoretically at least, quarks and gluons, and in fact all matter and energy simply melt together. You will find out what quarks and gluons are in a future article - these particles will make their first appearance in the Quark Epoch Universe. For now you can think of them as the building blocks of atomic nuclei.
Spacetime

What's really weird about this Planck universe is that there is absolutely nothing beyond it! There is not even a vacuum because even an absolute vacuum exists within the framework, or manifold as it is called, of spacetime. There is no space or time outside the tiny newborn universe, unless . . .

We start here with a Planck-time universe in which time is now beginning and the laws of physics are just shifting into place. There is a theory that the laws of physics may not be absolutely fixed. There could be many universes, each with its own physical laws, matter and force particles. Below, a ten minute interview with physicist Michio Kaku puts this multiverse concept into perspective, and he gives us an introduction to another strange concept, extra dimensions, as well:

Let's begin with what we think might be the birth of space-time, that is, the three dimensions of space and one dimension of time that exist throughout the universe today.

First we need to know what space and time are. Time seems simple until we examine it closely. We perceive time differently from spatial dimensions. Einstein's theories of relativity treat space and time as components of a four-dimensional manifold called spacetime. The quantum mechanical model treats time a little bit differently: the perception of time flowing forward in one direction is an artifact of the laws of thermodynamics. In the quantum realm there is no rule against a backward time arrow but in the macroscopic realm we live in, time reversal is forbidden. A spilled glass of milk cannot refill itself like a film being played backward. For a photon on the other hand, or any object traveling at light speed, time stops altogether. This is a consequence of relativity. It gives time a stretchiness that becomes apparent only near the speed of light or under extreme gravity. Time is not nearly as simple as it first seems and I explore the puzzle of time in the article, Time. This being said, we may be able to set aside any questions about multiple dimensions of time. There is no evidence for multiple time dimensions and adding time dimensions does not simplify the quantum mechanical equations or help marry them to general relativity. Yet there is an interesting article by physicist Itzhak Bars that explores the possibility of two time dimensions.

If we try to dissect and examine space-time, it is impossible to know exactly what the components are made of, but when physicists attempt to consider the laws of quantum mechanics and general relativity together, spacetime can be divided up into chunks as small as Planck-length. When they get down to pieces this small, spacetime loses its smooth appearance; it "boils." It becomes what is called quantum foam. To explain quantum foam, let's begin with a larger piece of space-time. This piece of space-time fabric appears completely smooth at a scale of 10

^{-12 }cm and larger; some roughness shows up at 10^{-20 }centimeters, and as we zoom in to 10^{-33 }centimeter range, the Heisenberg uncertainty principle tells us that spacetime has a certain minimum energy. This energy, called vacuum energy, means that virtual particles randomly and continuously pop into and out of existence, without violating conservation laws. With this activity, spacetime resembles a three-dimensional frothing sea.
Enter Strings

As I have mentioned in an earlier article, we have a conundrum of two well-established theories that physicists have so far been unable to mesh together into a single theory of everything. General relativity accurately describes planetary motion, the evolution of galaxies and stars and even recently observed black holes and gravitational lenses - it describes gravity perfectly on the big scale. On the other hand, quantum mechanics describes the behaviour of atoms and subatomic particles wonderfully, but it neglects gravity. For most experiments this isn't a problem because at this scale gravity is monumentally weak, but a chasm forms when physicists try to describe particle behaviour under the extreme conditions of the very early universe, where gravitational force needs to be taken into account.

String theory is a developing theory that might close this gap. It suggests that particles arise as vibrations of tiny one-dimensional Planck-length strings which themselves arise from the quantum foam I mentioned earlier. The graviton, the theoretical mediator particle of gravity, would for example be a closed string with a vibration frequency that translates into two units of spin. Likewise, electrons and quarks are one-dimensional strings with their own specific oscillations, which give them their momentums and spins. I explore string theory in the article, String Theory.

Travel to the Fourth Dimension and Beyond!

M-theory is an extension of string theory and for its equations to work, a string has to vibrate in 10 dimensions of space. Don't worry, no one can visualize what this might look like. 10 dimensions implies six extra dimensions to our four, which have not yet been experimentally verified. Also, according to the theory, these strings exist along with sheets called branes. Strings can be confined on branes like waves on the surface of a sea. Some strings may be able to move through them. According to Einstein's general relativity, the gravitational force that arises from mass tells space-time how to curve and the solutions to his equations allow for many different curvature geometries ranging from a circle, to very complex shapes. Working with a number of dimensions, the geometry will try to minimize the energy it builds up as a result of its curvature. As a result of the elegance of string theory, many physicists embrace the idea that we have three spatial dimensions with several hidden dimensions that do not change over time. Think of energy as a ball rolling down in a spiral toward the bottom of an inverted energy cone. Energy-wise, we are sitting at the bottom of that cone. More accurately we are sitting at the bottom of a three-dimensional curve, which is just one slice through a complex multidimensional mountain range. In this sense, our particular universe may be just one of many points where that ball could rest. In other words, according to string theory, there may be many stable multidimensional possibilities for a universe to adopt as it pops into being - we just happen to have three observable spatial dimensions. These dimensions determine not only which particles can exist but also which and in what form fundamental forces exist.

How do we reconcile the predicted 10 dimensions with our three observable spatial dimensions? One idea is to consider that the extra dimensions are very small. A common analogy for this multidimensional space is a garden hose. From far away it appears to have only one dimension, length. Now imagine getting closer to the hose and finding out that it contains a second dimension, its circumference. An ant crawling in one direction down the outside of it would move in two dimensions and a fly stuck inside flying around inside it would move in three dimensions. This new third dimension is only visible within very close range of the hose. If you extend this rationale to the theory of particle-wave duality, you will discover that as you experiment with particles of smaller and smaller wavelengths and you approach the radii of some of these smaller dimensions you run the chance of coming face to face with direct evidence for the existence of even more dimensions (it is hoped). In quantum mechanics, this means blasting particles with

*very high*energies, and that is one reason why people are putting so much money and effort into building better particle accelerators.
Extrapolating from this idea, we will surmise that the universe began with 10 dimensions. What caused some to contract and others to expand into our current space-time geometry?

String theory tells us that, when a dimension is curled up like a circle, a closed string can wrap around it and keep it from expanding. All dimensions in the very early universe may have been wrapped up by string loops. Each string-looped dimension couldn't expand beyond one Planck-length in any direction. However, strings may wrap around a dimension in one of two directions. When two strings wrapped in opposite directions come into contact they should annihilate each other. If this process happens rapidly enough some strings should annihilate each other, allowing only some dimensions to expand, but why exactly three spatial dimensions and not four or two? Some theories suggest that this is simply because three unfurling dimensions present the right number of trajectories that are least likely to interfere with one another during a very rapid expansion. Other theories suggest that we just happen to have three expanded dimensions by chance but we could have had more (or maybe less like the 2-D world in which the Simpsons live). So, we have three dimensions growing and six dimensions settling into tiny but stable curled up shapes called Calabi-Yau shapes.

This might explain why gravity is so weak. Gravity might function in more than three spatial dimensions. The graviton string might be able to move through and across dimensions. In a three-dimensional world, the strength of gravitational attraction is squared when the distance between two masses is halved. But in four dimensions strength varies as a cube rather than a square, and in five dimensions as the fourth power and so on. It's possible that gravity isn't weak at all - it just seems that way in three spatial dimensions. The extra dimensions don't need to be large for this argument to work. This website explores how this might be so, and also how both the graviton and extra dimensions might someday be experimentally proven.

Let's go back and reconsider the beginning of spacetime fabric. We will operate on the assumption that gravitons exist and that a gravitational field is composed of an enormous number of gravitons much like an electromagnetic field is composed of an enormous number of photons, and each of these gravitons is a string executing the graviton vibration. A gravitational field is encoded in the bending of spacetime, so imagine the fabric of space-time being composed of an orderly fabric of strings all vibrating in tune. This is called a coherent state. We could ask ourselves if there is a precursor to this orderly string state, a precursor of spacetime itself. We can think of each graviton string as an indivisible unit of space-time much like an atom is an indivisible unit of an element. Now we have a problem because the whole notion of string theory presupposes strings operating

*within*a spacetime framework. And if we take this argument further, we begin to wonder if space, time and the dimensions that arise from them are not fundamental aspects of the universe but artifacts that emerge from a much more primitive state. This is a thought to consider and I have no good answer for you.
Symmetry

Let's get back to our Planck-time universe - we'll again assume that gravity is analogous to the other three fundamental forces; that it is carried out by gravitons. Physicists don't know for sure if this is a true picture of gravity. It could be just the geometry of stretchy spacetime instead, rather than a true force like the other three fundamental forces. For now, we'll treat gravity as a "normal" force, so the universe begins with one unified force, from which four different forces will arise. Think of this state as being analogous to four metals melted together to form a smooth amalgam. This amalgam is perfectly symmetrical, that is, it contains no lumps or inconsistencies. Now look at our universe today: it is full of lumps and inconsistencies - all different kinds of matter and energy clumped into gas clouds, neutron stars, galaxies and so on, separated by the vacuum of space. What happened to the symmetry?

Think of a glass of water freezing into ice. At 0

^{o}C you notice something interesting - crystals of ice are forming in random patterns. The symmetry of the "smooth" water breaks as it undergoes a phase transition into ice. This process is true of larger systems and of the universe itself. The primordial unified force in this very young universe will very soon break into the fundamental forces as it begins to cool. The image below attempts to show how the universe underwent a series of phase changes that coincide with symmetry-breaking, and the separation of the four fundamental forces from one unified force:
Because we are considering that the fundamental forces are consequences of the interaction of strings and the multidimensional branes through which they move, we can assume that the symmetry-breaking of the unified force must coincide with the expansion of four dimensions into spacetime and the settling of six additional dimensions into very small Calabi-Yau shapes.

Keeping in mind that string theory is strictly theoretical and gravity might not be a particle-mediated force, we now have at least a theoretical framework in which the universe can "become." But wait. Things are about to get even weirder! Next: The Grand Unification Epoch.

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