Why Do the Galaxies Flee Us?

Why Do the Galaxies Flee Us?

We know that the light from a galaxy is the sum of the light emitted by the billions of stars within it. As the light leaves theses stars, certain frequencies or colors are absorbed by the atoms in the stars’ outermost layers. The resulting lines permit us to tell that stars millions of light-years away contain the same chemical elements as our Sun and the nearby stars. The spectra of all the distant galaxies are red-shifted and, still more startling, the more distant the galaxy is, the more red-shifted is its spectral lines.

The most obvious explanation of the red shift is in terms of the Doppler effect: the galaxies are receding from us; the more distant the galaxy the greater its speed of recession.

But why should the galaxies be fleeing us? Could there be something special about our location in the universe, as if the Milky Way had performed some inadvertent but offensive act in the social life of galaxies? It seems much more likely that the universe itself is expanding, carrying the galaxies with it. Astronomers Humason and Hubbly discovered the Big Bang – if not the origin of the universe then at least its most recent incarnation.

Why do the galaxies flee us? Don’t they like us, our behaviour and how we smell? (Quotations from Meg Jorgensen). Image: Nebulae © Elena

Thus, almost all of modern cosmology – and especially the idea of an expanding universe and a Big Bang – is based on the idea that the red shift of distant galaxies is a Doppler effect and arises from their speed or recession.

But there are other kinds of red shift in nature. There is, for example, the gravitational red shift, in which the light leaving an intense gravitational field has to do so much work to escape that it loses energy during the journey, the process perceived by a distant observer as a shift of the escaping light to longer wavelengths and redder colors. Since we think there may be massive black holes as the centers of some galaxies, this is a conceivable explanation of their red shifts.

However, the particular spectral lines observed are often characteristic of very thin, diffuse gas, and not the astonishingly high density that must prevail near black holes. Or the red shift might be a Doppler effect due not to the general expansion of the universe but rather to a more modest and local galactic explosion. But then we should expect as many explosion fragments traveling toward us away from us, as many blue shifts as red shifts. What we actually see, however, is almost exclusively red shifts no matter what distant objects beyond the Local Group we point our telescopes to.

There is nevertheless a nagging suspicion among some astronomers that all may not be right with the deduction, from the red shifts of galaxies via the Dopller effect, that the universe is expanding.