Department of Astronomy and Astrophysics
University of Chicago
Recession of the galaxies: distant objects
appear to be moving away from us.
Cosmic Microwave Background: we are bathed
in primordial light.
The big bang or hot expanding model of cosmology finds strong
support from the recession of the galaxies, that is, when we
look out in the night sky, distant objects appear to be
flying away from us. And also the cosmic microwave background -
the fact that we are bathed in primordial light that bears evidence
from and earlier, hotter, and denser period in the universe.
As we shall see, this evidence also supports the gravitational
instability paradigm - the picture that gravity can make wrinkles
The relation between recession and expansion is easy to
Imagine you're standing at the north pole and think
boy, I'd much rather be at the basking the sun at some equatorial
paradise. You look up the distance to it and plan your trip.
Right before you leave, you think, I'd better check the distance
again. Unbeknownst to you the radius of the earth has expanded
in the meantime. To your suprise the distance to your equatorial
paradise is now larger. You think, that's funny paradise seems
to be receding from me! This is exactly the situation we observe
with the distant galaxies. Because space itself is expanding
it looks as if the galaxies around us are all receeding into the
distance. It's easy to convince yourself that there is nothing
special about being on the north pole. Anywhere on the globe
the distance between points is increasing and hence all distant
objects appear to be receeding.
Well, maybe at least you can see paradise? Unfortunately
the wavelength of light also stretches with the expansion so that
visible light at millionths of meter wavelengths gets streched
into invisible microwaves at millimeter to centimeter wavelengths.
Given that light behaves this way, it's easy to see how the sea
of primordial light we observe in microwaves supports the hot
expanding (big bang) model. Run these pictures backwards in time.
The wavelength of light becomes shorter and shorter and matter
becomes denser and denser implying that the universe began
in a hot dense state.