Universe

In the beginning, the universe was created.
This has made a lot of people very angry and has been widely regarded as a bad move.
- The Hitchhiker's guide to the Galaxy

Since 1928 when Edwin Hubble discovered the expansion of our universe, we had a more or less clear picture as to what is going on. However in 1998 new astronomical observations shattered this picture: the universe is expanding in an accelerating manner, and scientists invented the concept of dark energy to explain this strange behavior. What follows is a quick guide of what the picture of the cosmos is today.

From his observatory in Mountain Wilson near Los Angeles, California Hubble studied the redshifts of various galaxies. The redshift is the amount that the frequency of the light coming from a galaxy is shifted towards red when the galaxy moves away from us (this is the optical analog of the acoustic Doppler effect). The faster a galaxy moves away, the highest the redshift. To his amazement, Hubble discovered that ALL galaxies are moving away from us (and from each other at the same time), and their speed depends on their distance from us. So galaxies further away are moving faster (even faster than the speed of light, more on that in a moment) than the nearby galaxies.

How fast do they move? Hubble's constant states this fact, and the current best guess (as of May 2003) is 71 Km/sec/Mpc. A Mpc (Mega parsec) is about 3.3 million light years, so Hubble's constant is more convenient units is 0.007% per million years. This simply means that every one million years, all the distances in the universe expand by 0.007%. Note that the galaxies themselves do not move; the space itself expands at that rate, and new space is created all the time. The expansion is not caused by the motions of the galaxies but because of the uniform expansion of space.

The walls of the room you are in right now have expanded about 0.1nm since last year because of that effect.

Since the universe is expanding right now, we can trace back the motion to the past and therefore deduce that there must have been a moment when it all started, a moment named the Big Bang. Neither space nor time existed then; they were both born and they keep expanding since that point (this is why questions of the type "What was before the Big Bang don't make much sense). Today Hubble's expansion is the result of that moment of absolute creation. (Actually we don't know what happened at t=0; we only know the history of the universe from the moment t=10^-43 seconds after the Big Bang: the laws of physics break down if we try to go before that time).

Since the most distant appear to be moving faster, can they move faster than the speed of light? Sure! Einstein's theory of relativity limits motion up to the speed c, but in this case we don't have to deal with motion: this is the rate at which space moves and is created; there is no restriction into that matter. The most distant known object today (a quasar) appears to be 13 billion light years away. The age of the universe is approximately 14 billion years. However this is how the object looked like at the age of 1 billion years (since the light we receive was emitted at that moment), and this is almost the furthest we can "see" (since light cannot go faster). However, this is not where the object is now; if we magically freeze the universe right now and use a meterstick to find the distance from the earth to that distant quasar, we will find it to be about 27 billion light years, taking into consideration the expansion of the universe. This happens because the 0.1nm that is how much space was newly inserted in one year in your room adds up to millions of light years over a period of billion of years for that particular quasar. The quasar right now (together with trillions of other stellar objects) is moving faster than the speed of light away from us, due to the stretching of space between us and the quasar.

One would expect that due to the gravitational attraction, the expansion of the universe would slow down gradually because of the attractive forces. Therefore the expansion should be less now that what it was in the past. However in 1998 researchers studied the light intensity coming from distant supernovae, and found that they were dimmer than expected. This could only be attributed to the fact that the supernovae where further away, further than the Hubble expansion predicted. This discovery has devastating results: it means that the galaxies are moving apart faster that we thought, that the expansion does not have a constant rate but instead it is accelerating! The expansion is in fact speeding up instead of slowing down...

Theorists have set forth since then to explain this strange behavior. It seems as if gravity is not attractive but repulsive over large distances. The cause of this behavior was attributed to a presence called dark energy. To be more specific, the general theory of relativity predicts that the strength of gravity does not depend only on the mass of an object (as in Newton's theory) but also on its internal pressure. For example the sun, a hot gas, has an internal pressure that causes a slightly bigger gravitational pull than the pull of an object of an equivalent mass (and no pressure). The dark energy is characterized by negative pressure: it may have a mass (energy density) that results in an attractive force, but the internal pressure is so negative that it overcomes the attractiveness and therefore the final result is a repulsive force. Note that some elastic objects such as a rubber sheet also have negative pressure, in the sense that the pressure is inward (an elastic object tends to stretch back). However in the case of dark energy we need an entity that exists everywhere without being noticeable; its detection is one of the most important tasks that science has to deal with today.

A second plausible(?) explanation that can cause the repulsiveness of gravity comes from string theory. According to string theory there are more dimensions in our world apart from the 4 ones we can sense easily. The basic idea here is that the gravitons (particles of gravity) are leaking into other dimensions as we proceed into larger scales and therefore lessen the effect of gravity. it's as if we have a bucket full of water, but the bucket has holes in its surface and these holes are small on the top side of the bucket but get bigger as we go more down the surface. The water leaks very slightly near the top side but largely as we proceed downwards. In the same way gravitons stay into 3 spatial dimensions into close distances but have the tendency to leak more as we go over large distances.

This leaking phenomenon is analogous to the polarization of dielectrics: by applying an electric field into certain materials (thus by shooting photons at them) we can set a preferred direction of propagation: then photons move easier in a certain direction that in an other one. In the same way, the gravitons gravitationally polarize our world in such a way that at certain large distances they cannot propagate freely and they are forced to leak elsewhere.

Physicists and cosmologists are confused as they are faced with one of the greatest mysteries of our time, trying to explain the strange behavior of our universe. It's as if we live again into medieval times, where people are uncertain of what is going on and resolve to "magic" to find a way out. Because it seems to me that dark energy and extra dimensions are no less ad-hoc than the magic of that times: strange, unknown entities that seem to defy logic. The only difference is that our era is so scientific that they are not regarded as insane ideas by most people.

The University of Southern California does not screen or control the content on this website and thus does not guarantee the accuracy, integrity, or quality of such content. All content on this website is provided by and is the sole responsibility of the person from which such content originated, and such content does not necessarily reflect the opinions of the University administration or the Board of Trustees