Saturday, January 29, 2011

Our Universe Part 2: The Big Bang

We begin with an utterly unfathomable newborn universe and probe the nature of the singularity it is. How does physics itself begin?

Planck-Time

In 1929, Edwin Hubble noticed that distant stars are moving away from us in all directions and that the further away they are, the faster they are moving. He looked at the light spectra from various distant galaxies and compared them with the Sun's spectrum. He found that black lines, called absorption lines, shifted toward the red end of the spectrum in all of the distant galaxy spectra he looked at, as shown below.

(Georg Wiora; Wikipedia)

This red shift is a Doppler effect. It means that the galaxies are all moving away from us. He then compared the galaxies' brightness, an indication of its distance away, to how much its spectrum was redshifted and he discovered that the further away the galaxy was, the faster away it was moving.

This observation was the seed that brought the Big Bang theory to life. Later, using the theory of general relativity as a theoretical framework, scientists were able to extrapolate the expansion of the universe backwards to a point of infinite temperature and density about 14 billion years ago. This theory has been tested using a variety of data from both cosmology and particle physics and it has gained a great deal of scientific support. There is one catch though - the infinite values for temperature and density at the very beginning of the Big Bang. Singularities, as these are called in theoretical physics, are usually taken to be a sign that the there is something wrong with the calculations, and this point still bothers physicists. Either the theory is wrong or the laws of physics themselves break down at the exact point the universe began.  Most physicists today are willing to put their money on the latter being true, and this willingness to explore the unfathomable is bearing some very strange fruit.

The graphical timeline of the Big Bang below, part of the Wikipedia page of the same name, provides a theoretical timeline of the universe that will be very handy as we begin our exploration of its birth (the website has a clearer image).

The very beginning of the universe from zero to 10-43 seconds is called the Planck epoch. Scientists can't make any predictions about events that occurred in an interval shorter than Planck time (10-43 seconds). Planck time is the time it takes light to travel 1.6 x 10-35 meters (this length is called Planck length).

Check out this website Planck Scale to put this strange value into perspective and to find out how it is obtained. There are values for Planck energy, time, length, mass and temperature. These values are all derived from combinations of fundamental constants in physics and some of them represent the smallest (length and time) or largest (temperature) possible theoretical measurement.

Why is this Planck value so significant? When scientists explore intervals smaller than this, our current theories about gravity and space-time cease to be valid. No smaller division of length or time has any meaning according to our current theories and this is why:  Electrons and, in fact, all subatomic particles break down at this point into wave functions. A wave function is a function of a subatomic particle's spin and momentum, two characteristics that distinguish one subatomic particle from another. It is nothing more than a probability amplitude - all measurable certainty goes out the window. You can think of an electron's orbital inside an atom as an example. You cannot know its exact location; you can only know where it might be. Below is a diagram of an electron orbital in a hydrogen atom. The electron could be in any one of the three doughnut shapes. It is even accurate to say the electron is a wave function "smeared" over all three regions:


This is not an easy concept to grasp let alone accept. It feels so counter-intuitive. All quantum mechanical systems like this one break down into wave functions when we approach the Planck scale universe. That means all particles of mass and energy. It makes pinning down an exact point where the universe began impossible, and it leaves us stuck at the Planck-scale border of time and length, where the entire universe, as best as physicists can describe it, is a wave function.

You could think of Planck-scale as the ultimate limit of resolution, as in a photograph. Beyond a specific point (Planck time or length) our understanding of reality breaks down into nonsense (the picture is just a fuzzy grain).

So, we have established that we can only make sense of the universe at the baby age of 10-43 seconds and older. At zero seconds we just can't know or understand the universe. Extrapolating backward gives physicists infinite temperature and density values squished within an implied infinitely small volume: zero volume.

There is another example of a singularity in our universe - black holes. I explore them in Stellar Objects Part 5. Within a black hole, all matter and energy collapse into an infinitely dense and small space. And according to current theories, not just matter and energy disappear but information itself is obliterated. Some physicists speculate that the birth of the universe may have been a black hole operating in reverse, and they call it a white hole. This theory suggests that a big bang occurs at the core of a black hole, creating a new universe independent of its parent universe. Our universe is thought to contain countless black holes, each one potentially spawning a new universe on the other side, with its own physical laws that may be different from ours, an hypothesis called the fecund universes hypothesis.

Mother Force?

Back to our universe, what is happening at 10-43 seconds? Most physicists believe that all four of the fundamental forces are jumbled into one unified force in this ultra-energetic environment. This enormous energy, expressed as particle energy, is measured in gigaelectron volts (GeV). Researchers believe the maximum energy of the universe right after the Big Bang was about 1019 GeV. This is another example of a Planck value, a theoretical energy maximum. At this energy, all four fundamental forces are believed to unify into one "mother force."

Three researchers, Abdus Salam, Sheldon Glashow and Steven Weinberg, helped provide direct evidence that two fundamental forces, the weak force and electromagnetism, combine into one force, called the electroweak force, at energies higher than around 100 GeV, earning them a Nobel prize in 1979. The particle energy of our current universe is very low, in comparison, about 10-4 eV, or 3K above absolute zero.

The weak force, strong force, and the electromagnetic force can be theoretically unified in terms of the particles that mediate them. They are each mediated through the exchange of closely related virtual particles called gauge bosons. Quantum particles of matter, called fermions, attract or repel each other by exchanging bosons. Bosons carry energy and momentum between the fermions, changing their speed and direction. Examples of these gauge bosons are photons and gluons (remember them from the previous article?) However, no boson has yet been discovered for the fourth fundamental force, gravity, and no one has been able to fit gravity into the same theoretical framework as the other three fundamental forces.

Gravity: A Big Problem When Describing the Planck Universe

The fly in the ointment when trying to understand the Planck epoch universe is gravity. Gravity is so incredibly feeble compared to the other three forces that it is simply ignored when working on particle physics problems. Physicists don't yet have a quantum mechanical theory for gravity, as they do for the other three fundamental forces. This, at first thought, might not seem to be a huge problem, because gravity is so weak it can be ignored when describing physics at the scale of particles. Gravity is described very well, however, at the macroscopic level and especially at the cosmic scale, where its effects are significant, by using the framework of general relativity. But what about black holes and the Big Bang, where gravitational effects are enormous and take place in the super-squeezed down subatomic quantum realm? Here, both general relativity and quantum mechanics must be used to describe what is going on, and at least for now, the two theories cannot be made to work together.

What Scientists Know About Gravity

We know that gravity has some peculiar qualities:

It is the only force that acts on all particles having mass (and it has an infinite range of influence over them).
It cannot be absorbed or shielded against.
It always attracts and never repels (nothing "cancels" gravity).

Gravity was first scientifically described by Galileo Galilei and later refined by Isaac Newton. Newton's laws of gravitation work well for objects on Earth but Albert Einstein's general theory of relativity, on the other hand, is the gravitational theory we rely on today for all objects, both here on Earth and heavenly bodies. It describes gravity in terms of the geometry of space-time and it works perfectly for all macroscopic objects. Try Spacetime 101 for a very good tutorial on general relativity.

We know how gravity works on large objects but we don't know how it works at the quantum level. How do you pin down the gravitational field of a subatomic particle which is a wave function? 

Gravitons?

Unlike the gauge boson, no mediator particle has been discovered for gravity, although physicists are currently in hot pursuit of a hypothetical massless spin-2 particle called a graviton that they believe might carry out this force.  Physicists want to come up with a unified theory that resolves quantum gravity, special relativity and general relativity to explain the bizarre behavior of this very early universe, as well as black holes. The theoretical graviton could bridge the gap between quantum mechanics and general relativity. Many of the unified theories in vogue right now such as string theory, superstring theory and M-theory all depend to some extent on this theoretical graviton.

Even though there is no evidence yet for it, many physicists hold to the idea that one primordial force operated in the Planck-time universe. It is possible, according to other theorists that gravity is different from the other three fundamental forces. Einstein describes gravity as the curvature of spacetime in his theory of general relativity. It is possible that gravity, therefore, has no particle associated with it.

The recent well-publicized discovery of the Higgs boson may play a role in understanding quantum gravity. Physicists do not know why some particles have mass and others do not and they believe this massive zero-spin boson may "give" mass to those particles. If so, there could be a connection between this particle and the graviton, if it exists.

There are more questions than answers when we explore the very mysterious Planck-epoch universe. The first entity to explode from absolute nothingness is believed to be some kind of primordial force from which the four fundamental forces will evolve. This primal unified force doesn't seem likely to ever be experimentally observed because scientists would need to recreate the unimaginable energy in which it existed in order to observe it. At this point, physicists cannot even put the Planck universe into a complete theoretical framework, with gravity being the problem. Without the four fundamental forces in place yet, it is reasonable to assume that the laws of physics themselves do not exist between zero and 10-43 second. No one knows if spacetime exists right from the very beginning, or if it appears with the appearance of gravity as it breaks away from the other three (still unified) fundamental forces. This is where we are in bullet-point form:

  • There is a fuzzy primal unimaginably enormous energy from which everything in our universe has yet to evolve.
  • There is no mass and there are no objects, not even a subatomic particle, with the possible exception of some very exotic particle that has burst into existence at this point to mediate the primal unified energy, or "mother force," from which the four fundamental forces will evolve.
  • Time begins, and it has jumped mysteriously from zero to 10-43 seconds for us observers because we cannot know any smaller time unit.
  • There is as yet no framework in which energy operates.  It is about to explode into being now. Is it space-time as we know it or something stranger yet . . . ?

Stay tuned. Next: The Planck Epoch.

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