|According to Einstein’s theory of general relativity, |
gravitational attraction between masses results from
their warping of space and time.
This week I am going to deal with some popular misconceptions about space and the Universe in general. Next week I will tackle the Big Bang theory.
First off, we must realize that when we look out in space we are looking back in time. It takes a long time for the light from distant objects in the Universe to reach us. Light from the Sun takes eight minutes to reach us, and light from nearby stars takes years. Distant galaxies are seen as they looked millions or even billions of years ago. Galaxies extend out far beyond what we can see today, but how far no one knows. Because the Universe has been evolving and expanding over time, most of the light from the distant reaches of the Universe has not yet had time to reach us.
As far as we can tell, there is no edge to the Universe. Galaxies extend as far as we can detect in every direction with no sign of diminishing. Even though galaxies extend much further than we can see, we don’t know if the Universe is infinite. So when a galaxy is described as being near the edge of the Universe, what is really meant is that it is near the edge of the observable Universe.
Until Einstein showed that space has structure, astronomers thought of space as just the emptiness that contains matter. But Einstein showed, through his general theory of relativity, that space is flexible and can be warped. Now when we talk about the expansion of the Universe, we are referring to the stretching of space itself—not just galaxies moving apart through space.
The notion that space is expanding was predicted by Einstein’s theory of gravitation which describes a simple but universal relationship between matter, space and time. It was a prediction that Einstein initially couldn’t embrace. He modified his theory by adding a term to achieve a static Universe that he called a cosmological constant. Later, after observational evidence by Edwin Hubble indicated that the Universe was indeed expanding, Einstein abandoned his constant calling it his biggest blunder.
Today, we know that not only is the Universe expanding, but that it is expanding at an ever-accelerating rate. Cosmologists use the term “dark energy” to explain this mysterious energy that Einstein had embraced, then discarded. The best explanation we have today for dark energy is that it is a vacuum energy associated with virtual particles—quantum fluctuations which produce particle pairs that blink into existence and then annihilate in a times pan too short to measure. This happens everywhere, throughout the Universe. But there is a big problem with this theory—vacuum energy is far too weak to account for the acceleration of the expansion of the Universe. One thing is clear—discovering the properties of space remains one of the core problems of modern science.