java physics

Physics of the early Universe is at the boundary of astronomy and philosophy since we do not currently have a complete theory that unifies all the fundamental forces of Nature at the moment of Creation. In addition, there is no possibility of linking observation or experimentation of early Universe physics to our theories (i.e. its not possible to `build' another Universe). Our theories are rejected or accepted based on simplicity and aesthetic grounds, plus there power of prediction to later times, rather than an appeal to empirical results. This is a very difference way of doing science from previous centuries of research.

The current observations and estimates of dark matter is that 20% of dark matter is probably in the form of massive neutrinos, even though that mass is uncertain. The another 5% to 10% is in the form of stellar remnants and low mass, brown dwarfs. The rest of dark matter is called CDM (cold dark matter) of unknown origin, but probably cold and heavy. The combination of all these mixtures only makes 20 to 30% the amount mass necessary to close the Universe. Thus, the Universe appears to be open, i.e. Ω_M is 0.3.

From comparing the mass estimates to the observed amount of light from galaxies, and from the abundance of light elements, that there is a problem with the fraction of the mass of the Universe that is in normal matter or baryons. The fraction of light elements indicates that the density of the Universe in baryons is only 2 to 4% what we measure as the observed density. The rest of the mass appears to be `missing', meaning unobserved or dark.

In modern cosmology, the different classes of Universes (open, flat or closed) are known as Friedmann universes and described by a simple equation:

In this equation, `R' represents the scale factor of the Universe (think of it as the radius of the Universe in 4D spacetime), and H is Hubble's constant, how fast the Universe is expanding. Everything in this equation is a constant, i.e. to be determined from observations. These observables can be broken down into three parts gravity (matter density), curvature and pressure or negative energy given by the cosmological constant.

Historically, we assumed that gravity was the only important force in the Universe, and that the cosmological constant was zero. Thus, if we measure the density of matter, then we could extract the curvature of the Universe (and its future history) as a solution to the equation. New data has indicated that a negative pressure, or dark energy, does exist and we no longer assume that the cosmological constant is zero.

There are two possible futures for our Universe, continual expansion (open and flat), or turn-around and collapse (closed). Note that flat is the specific case of expansion to ever slowly speeds aproaching zero velocity.

One of the key factors that determines which history is correct is the amount of mass/gravity for the Universe as a whole. If there is sufficient mass, then the expansion of the Universe will be slowed to the point of stopping, then retraction to collapse. If there is not a sufficient amount of mass, then the Universe will expand forever without stopping. The flat Universe is one where there is exactly the balance of mass to slow the expansion to zero, but not for collapse.

Measuring the curvature of the Universe is doable because of ability to see great distances with our new technology. On the Earth, it is difficult to see that we live on a sphere. One stands on a tall mountain, but the world still looks flat. One can see a ship come over the horizon, but that was thought to be atmospheric refraction for a long time.

general relativity allows for spacetime to be curved, thus the whole Universe may have a non-flat geometry three possible shapes are allowed, flat, positive or negative curvature

Can the Universe be finite in size? If so, what is ``outside'' the Universe? The answer to both these questions involves a discussion of the intrinsic geometry of the Universe. There are basically three possible shapes to the Universe; a flat Universe (Euclidean or zero curvature), a spherical Universe (positive curvature) or a hyperbolic Universe (negative curvature). Note that this curvature is similar to spacetime curvature due to stellar masses except that the entire mass of the Universe determines the curvature. So a high mass Universe can have positive curvature, a low mass Universe might have negative curvature.