January 11, 2012
Spiral galaxy NGC 3190 as seen by the European Southern Observatory's Very Large Telescope.I thought it would be fun to start the year addressing some questions that many people have about the universe. Mind you, some of these are far from simple, true to what Milan Kundera once wrote, "the only truly serious questions are the ones that even a child can formulate."
The big bang model asserts that observational evidence is consistent with a universe that originated from a dense and hot initial state some 13.7 billion years ago and that it has been expanding and cooling ever since. It's important to understand that this claim is not a "belief"; it's based on solid pieces of evidence.
The expansion is measured by studying light (visible and in other wavelengths, such as infrared and radio) emitted by distant galaxies. This light is "stretched," or Doppler shifted, into longer wavelengths like the folds of a pair of bellows. The amount of stretching indicates the recession velocity of the galaxies.
This expansion is a stretching of space itself. Galaxies are not like shrapnel from a bomb. Otherwise the universe would have a center point form where it all started. And in the universe every point is as important as any other point, the ultimate geometrical democracy: like the surface of a ball or of a very large (say infinite) table. Imagining different galaxies as coins glued to the ball, an observer in one will see all the others moving away from her. But this will be true of all other observers; no one is more central. Our universe is like that, but harder to visualize: evidence points to a flat geometry in three dimensions (the tabletop is 2-dimensional.)
Here people ask, "but if the universe is expanding what is it expanding into?" It turns out that the problem is how we picture the expansion, as a ball or a table growing into the surrounding space. Space grows as it stretches: nothing needs to be "out there." In truth, however, we are limited to only seeing so far out, due to the finiteness of the speed of light.
Our "horizon" is roughly 42 billion light-years, the distance light travelled in 13.7 billion years. (It would have been 13.7 billion light-years if there were no expansion. But the stretching of space gives light a boost and it manages to travel 3 times as far.) So, there is plenty of space "out there," beyond our horizon: we just keep on expanding and growing into the adjacent regions. (But see below.)
However, as we go back in time, the ball (or local regions of the table) becomes smaller and galaxies squeeze onto one another. Heat intensifies to such a level that all bonds that keep matter together break: no more molecules, atoms, atomic nuclei; even protons and neutrons get broken down to their constituent quarks. If we keep going back, we quickly reach a time when energies were beyond what we have tested. So, whatever happened between the "beginning" and about a trillionth of a second after the bang relies on theoretical speculation.
Here, there are two schools: one says that the beginning happened when the universe transitioned from a timeless quantum fluctuation into the expanding blob we live in. History starts at the bang, so to speak. Another says that we live in a multiverse, a possibly infinite assembly of all kinds of disconnected universes, with different properties. Ours would be but one of these. In this case, the universe, big bang and all, would be a tiny piece of a timeless cosmic entity. This is where science meets the poetic imagination. And the best part is that it may be true. If only we could test it one day.
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