Dark Energy and the Nobel in Physics

The Nobel Prize for physics was announced and it goes to Saul Permutter of Berkeley, Brian Schmidt from the Australian National Observatory, and Adam Reiss from some school in Baltimore with a lacrosse team. The prize was given for the discovery that the rate of expansion of the universe is increasing.

We've known for almost a century that the universe is expanding. When something moves relative to us, there is a Doppler shift. Think of the sound of a fire engine siren going past you. As it approaches, the tone goes up and as it recedes, the tone goes down. The same thing happens with light. So, when we look around us at stars which give off light in the same way and so should have roughly the same color of light emitted from them, we notice some of them have light that is shifted towards the red end of spectrum and others that have their light shifted towards the blue. The father away the star is, the more likely it is to be moving away and moving away faster (which we know from the amount of the shift). This means that the universe is expanding. Think of raisin bread dough rising. As the dough expands, the raisins separate, but the raisins that are farthest apart at the start of the rise are farther apart than those that started off closer together. A longer distance over a fixed time means a greater relative velocity. This is what we see when we look out.

Of the four forces in nature, the only one that acts in a non-trivial fashion over any significant distance is gravitation which is an attractive force. As such, if the universe is expanding and full of objects pulling on each other in the opposite direction to the expansion, we expect that gravity is putting on the brakes. We weren't sure by how much, but the universe had to be at most constant in its expansion.

What the two teams led by these three physicists did was to look at objects very far away that are very, very old. Assuming the universe's expansion to be constant or slowing due to gravity, they expected to measure certain quantities in a certain range. But that's not what they found -- and they we looking independently. The only way these observations make sense is if the universal expansion is not slowing, but accelerating. But the only force out there that is operative is gravity which can only slow it down. What is causing this acceleration? That answer remains to be found. Some contend that it's a new force caused by something unknown to us but given the very cool name "dark energy." Others think that a fifth force is unlikely, but are at a loss to explain it. What ever it ends up being, it's just darn good stuff.