Question: How can you use the Big Bang to test the laws of physics when it happened so long ago?

Mark Jackson answered on 11 Mar 2014:

Because the Big Bang has a fossil!

For the first few hundred thousand years after the Big Bang, the Universe was like a thick soup full of elementary particles (electrons, quarks, neutrinos, photons, etc.). These particles had so much energy that they didn’t want to settle down and make quiet little atoms, they preferred to just fly around by themselves. Since many of the particles were electrically charged (electrons, positrons, quarks, etc.) they kept absorbing or emitting photons. This means that photons won’t get very far before disappearing. Since we only see something by having photons bounce off it and into our eye, there was no way to see anything! It took about 380,000 years before the Universe cooled down enough to atoms to form. The photons that happened to exist at that moment could then fly through space without fear of hitting any charged particles. If this seems confusing, imagine a cloud: you can see the surface of the cloud because the photons bounce of it and then into our eye, but you can’t see inside the cloud because the photons just bounce off the water vapor. Just replace “water vapor” with “charged particles” and it’s the exact same idea for the early Universe. This earliest light is called the Cosmic Microwave Background.

Once the photons were free to travel, they continued to do so for billions of years and are still out there now! It’s like seeing a fossil of the Big Bang: the very first light in the Universe is still around us but just very faint. It was only recently that technology got good enough that we could see the light at all (Penzias and Wilson did this in the 1960s), and only very recently that we could measure slight differences in the light’s pattern across the sky (the COBE experiment in the 1990s). Modern satellites such as the Planck satellite can take pictures with breathtaking precision such as at http://goo.gl/lwp1nh.