Thursday, January 27, 2011

Our Universe Part 4: Grand Unification Epoch

Gravity, dark matter and dark energy may be establishing themselves during this epoch. We explore them as we continue to follow the evolution of the very early universe.

Following the Planck epoch, the temperature of the universe is just cool enough to allow a phase transition in which the force of gravity separates from the other three fundamental forces. The other three forces at this point are still unified into one force called the electronuclear force. This is the first of what are called symmetry breaks. This idea was introduced in "Universe Part 3: Planck Epoch."

The universe during this epoch is between 10-43 and 10-35 seconds old as it expands from one Planck length to a size that is still magnitudes smaller than that of a quark (and a quark is a very tiny building block of a proton or neutron inside a nucleus).

Dark Matter 

The popularity of string theory has taken off in the last decade thanks to its promise in helping physicists explain what gravity is. In the previous Planck epoch article I mentioned that the force of gravity, according to string theory, is mediated by gravitons, theoretical massless spin-2 particles. These particles arise as the mathematical result of a series of theoretical equations that make up string theory. According to this theory, the graviton is not really a particle but a one-dimensional string whose vibration executes the graviton function. This theory might explain why gravity is so weak on our scale compared to the other three fundamental forces. Gravity might function in more than three spatial dimensions. The graviton string might be able to move through and across dimensions.

Some theorists have extended this rationale to dark matter. Dark matter is a relatively newly discovered but significant component of our universe. When mathematical simulations of galaxy formation are carried out, theorists find that the galaxy flies apart rather that coalescing, unless its mass is bumped up to values far beyond the values calculated for a typical galaxy based on its total atomic mass. Extra mass must be there somewhere, and dark matter, calculated to make up about 80% of the matter of the universe, was proposed to fill this void. Since then, observations such as galaxal rotational velocities and gravitational lensing have lent experimental support to the existence of dark matter.

Ordinary matter, made of atoms, interacts with electromagnetic radiation such as light, but dark matter does not. That is why dark matter is called "dark." It is totally transparent to any observational tool physicists have come up with, and that makes it very difficult to prove its existence, except indirectly.

Remember that gravity's effects are not observable on the very small subatomic quantum scale and that therefore gravity can be completely left out of quantum mechanical equations. But what if dark matter is not really matter at all but a field even more tangled up within higher dimensions (and less accessible to our three dimensions so my argument goes) than the graviton is, and so its effects are felt only on a galactic scale? In other words, in terms of dark matter, our bodies could be thought of as being analogous to subatomic particles in therms to dark matter's gravitational effects. Could there be a graviton-like particle that operates across even higher dimensions than the graviton, so it is even less tethered to our three spatial dimensions than the graviton proposed for "regular" gravity? We could think of the "dark matter" graviton string as being even more smeared across dimensions than the regular graviton. This could be a new kind of gravity that is so weak on our scale its force cannot be measured but it is critical on the scale of galaxies and larger. This is just a thought exercise on my part. 

If you are intrigued by the string theory approach, I recommend reading this interview with theoretical physicist Joe Lykken on the Nova website. It serves as an interesting and accessible introduction to the odd world of strings, branes and other dimensions and it explains how these theories might help us to understand gravity. His ideas about dark matter are based on a new concept called supersymmetry. He also offers a new way to think of those strange extra dimensions. Another theory of dark matter that intrigues me is spelled out in Rusty Rockets' article that suggests that dark matter might arise from superstrings that stretch out and decay as the universe expands, releasing, as they do so, gravitational waves.

Physicists have posed a number of dark matter theories based on gravity itself, several which suggest that even general relativity is not complete in terms of describing very large objects like galaxies. Most physicists, however, believe there is a kind of matter in the universe responsible for boosting the gravitational effects of galaxies, and we just haven't found it yet.

An article recently published in Scientific American magazine (Dark Worlds by Jonathan Feng and Mark Trodden, November 2010 issue) argues that dark matter consists of weakly interacting massive particles called WIMPs and that these theoretical particles might be part of a hidden universe interlaced with our own. Sterile neutrinos have also been recently suggested as dark matter mediator particles in Scientific American (A Whole Lot Of Nothing by Michael Mayer, January 2011). These neutrinos, unlike ordinary neutrinos, do not even interact with the weak force and are therefore virtually undetectable. Researchers have already had some luck in finding them indirectly through their proposed decay particles (they should occasionally decay from dark matter into ordinary matter), a light neutrino and an X-ray photon.

The question of when dark matter arises in the universe depends on which theory you go with, whether you consider it a part of the nature of gravity itself or composed of a type of non-atomic matter. As part of gravity itself, dark matter may be present already at 10-35 seconds, when gravity is believed to separate from the original unified force.

Many physicists think dark matter may be organized into a filamentous large-scale structure in our universe, a backbone along which galaxies form. The following seven minute video explains how they are attempting to locate and map these far-reaching dark matter filaments:

Dark Energy

Whereas dark matter makes up about 80% of the mass of our universe, dark energy truly dominates our present-day universe. It makes up about three quarters of the universe's total mass-energy (recall Einstein's E = mc2). It was first proposed when physicists noticed that the expansion of the universe is accelerating, suggesting that there must be some kind of force that opposes gravity. This energy is suspected to be present throughout the universe and does not interact with any fundamental force. It is currently believed that dark energy does not directly interact with the gravitational force but rather it works against it to influence the overall expansion of the universe. In other words, it operates despite gravitational attraction. Because energy and mass are related according to Einstein's general relativity, even in a perfect vacuum, some energy will be present and exert a very small gravitational effect. This is called vacuum energy and it is believed to arise from quantized fundamental fields such as gravity (a field being a network of strings and these strings have within them vibrational potential energy). Dark energy currently acts as negative pressure in the universe because, according to classical thermodynamics, energy must be lost in order to do work (the vibration of the strings) on a system. Energy equals pressure x volume, so within the constant and finite energy of the universe (imagine: this tiny quark epoch universe has all the energy it will ever have!), pressure must become negative as the volume of the universe expands. Some researchers think this switch-over happened about 10 billion years ago

It not only becomes negative but its value increases so it correctly predicts that the rate of expansion of the universe increases with time. A (big) problem with this theory, however, is that most quantum field theories predict a vacuum energy much larger than what dark energy-driven expansion should require. Though this theory, called quintessence, has yet to be fleshed out, it seems, at least to me, to mirror the gravitational weakness conundrum - maybe dark energy is much weaker than it is predicted to be because it too is mediated through strings moving through or smeared across several dimensions. This is also just a thought experiment on my part. A physicist  explores what we know about dark energy in the following 32-minute video:

The Universe at 10-35 Seconds

The baby universe is just beginning to expand and as a result, dark energy, gravity, and dark matter may be, for the first time, unleashing their effects. All three of these forces, if they are forces, are matters of some speculation for physicists, so our understanding of the universe at this stage is far from complete. If we accept the general idea of string theory, gravitons are believed to have made their first appearance through the first phase transition, as a result of symmetry-breaking.

The other three fundamental forces do not yet exist as we know them. They are still unified into what is called the electronuclear force. The theoretical model of this force is called the grand unified theory. Within the very high-energy environment of this grand unification epoch universe, three gauge interactions (carried out by three gauge bosons) that define the electromagnetic, weak and strong forces exist only as a single interaction. A theoretical particle (some kind of undifferentiated boson) that mediates this interaction cannot be detected in a particle accelerator because its predicted mass/energy approaches the Planck limit. Theoretically such a particle would have no defining characteristics such as spin, charge or mass as these characteristics will only result later from further symmetry-breaking of this primordial force.

The three particles, called gauge bosons, that mediate, or act as carriers of, the three fundamental forces, will begin to make their appearance when another phase transition occurs. This transition will mark the beginning of a new epoch called the inflationary epoch, to be discussed next.

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