25th-26th March 2011 - Hydrogen & Dark Matter

Well, today's topic on the Starts with a Bang blog is "How Hydrogen Teaches Us the Temperature of Dark Matter!"  I found this one a real struggle to understand but I'm very glad I took the time, with a couple of friends, to fight my way through it.  There are a few points I'm not really clear of yet, but I'm getting there.

When hydrogen's electron jumps from a high energy state to a low energy state, it emits that energy, and when they do that, they emit radiation.

• If the electron jump down to the first energy level, it emits ultraviolet light and belongs to the Lyman series.
• If it jumps down to the second energy level, it emits (mostly) visible light and belong to the Balmer series.
•  If it jumps down to the third, it gives off infrared light and belong to the Paschen series.

The jumping down to the lowest level explains the red light in pictures of forming galaxies which is the forming of new stars.

The light from distant galaxies is absorbed by hydrogen clouds it passes through, and because of red shift these absorptions are detected as different wavelengths. Light from distant sources shows many more absorption lines as it passes through more clouds. [Starts with a Bang Blog]

Hydrogen tells us dark matter is cold because if it were hot, it would be moving too quickly to 'stick together'.

"It tells us that dark matter can be WIMPs  (like from supersymmetry), because they're too massive to move quickly, or they can be particles that are born cold, like axions or (some) sterile neutrinos, because they started off moving slowly. But they can't be regular neutrinos or hot sterile neutrinos, among others, because this small-scale structure -- and hence the hydrogen lines that we see -- would get washed out at early times!"

I also watched a talk by Lawrence Krauss, which helped me to understand the above blog, as well as introducing the following ideas.

The curvature of space is measured using a very large triangle and calculating the sum of its angles.

1% of the interference on the TV after transmission ended was due to cosmic radiation.

The universe is flat, since the lumps given by cosmic radiation are 'just the right size'. This means that 70% of the energy of the universe resides in empty space and noone knows why.  This amount is consistent with the universe speeding up and it's consistent with the amount 'missing' when we 'weigh' the universe to make it flat.

In 100 billion years, there will be nothing left to look at in the universe because it will be moving away faster than the speed of light, so scientists would come to very different conclusions. What conclusions would we have come to if we'd been around a few billion years earlier?