WMAP results: Cosmology from the CMB

The cosmic microwave background is the radiation left over from the big bang. It’s very uniform, 2.725 Kelvin everywhere. We’re moving with respect to it, so there’s a doppler shift, and we see that as a dipole moment in the Temperature. When we subtract that out, we see variations on the order of 30 microKelvins! WMAP is a satellite (Wilkinson Microwave Anisotropy Probe) that measured these anisotropies, and they just released its year 5 data. First off, with the uniform and dipole parts subtracted out, and with the foreground from the galaxy also taken out, here’s the map of the microwave sky (a baby picture of the Universe, when it was only 380,149 years old), compared with what we used to know from the previous satellite, COBE:

As you can see, the WMAP data is far better. COBE’s angular resolution was about 7 degrees; by comparison, WMAP’s resolution is less than half of one degree. We can learn a lot about the Universe from this baby picture. Let me tell you, though, why some parts are slightly hotter and why some parts are slightly colder.

The Universe has an average density, and always did. Some places are slightly more dense; they have more energy. Other places are less dense; they have less energy. Since mass=energy, imagine it like this: to escape from the planet Earth, I need a certain amount of energy. If I move with a velocity of 11 kilometers per second, that will do it. But what if the Earth were more dense? Well, I’d need to expend more energy to escape. If it were less dense, I’d need less energy to escape. Earth is the densest planet; on the Moon, for example, escape velocity is only 2.4 kilometers per second.

Now, imagine that instead of a person, you’re a beam of light at the surface of our planet. You’re going to escape because you’re moving at the speed of light, but you have to lose energy to get out of that gravitational field.

If you’re on a denser planet, I lose more energy, and so my light gets colder than normal. If I’m on a less dense planet, I lose less energy, and the light is warmer than light that left a normal planet. Well, the light from the Cosmic Microwave Background is doing the same exact thing, except it’s leaving different regions of space instead of planets. Those blue spots are where there are overdense regions and the light we see is colder, and the red spots are underdense regions, hence that light appears warmer. And that’s why we see those patterns in the sky that we do.

But that’s not the end of the story. There’s a lot of information that we learn from looking at those patterns caused by different density regions. The whole bunch of details that we’ve learned (caution that it may get technical to those who read further) are below:

  • First off, Lambda-CDM (that the Universe is made up of a cosmological constant type of Dark Energy, Dark Matter, Baryons, and leftover radiation and neutrinos) works very well. They try lots of different models, including allowing curvature and allowing dark energy to have a different equation of state. Lambda-CDM always works, no matter what they try.
  • What are the stats of the Universe today?
    1. The present expansion rate (i.e., the Hubble constant) is 68.7 km/s/Mpc, with an error of only 3%.
    2. The age of the Universe today is 13.95 billion years, with an error of 2.4%. Comparatively, the age of the Universe when the CMB was emitted was 380,000 years, with an error of 1.5%.
    3. The composition of the Universe is 72.4% dark energy (with an uncertainty of 1.5%), 23.3% cold dark matter (with an uncertainty of 1.5%), and 4.8% normal matter (with an uncertainty of 0.3%).
    4. The size (i.e., radius) of the Universe to the CMB surface today is 14.3 Gigaparsecs, or 44.2 billion light years. The uncertainty on that number is only 1.3%.
  • The density of matter is 1/Volume, the density of a cosmological constant is constant, and what they allow is dark energy to scale as (1/volume)p, where p can be any power. They find that, -0.11 < p < 0.14, which is in pretty tight agreement with p=0, or a cosmological constant.
  • Furthermore, if there is energy in spatial curvature of the Universe, it’s less than 1.75% of the total energy. You can find more results here.
  • Also, while you’re here, check out this week’s Carnival of Space over at Bad Astronomy; you can find my post on why Mars is so dry along with a bunch of other neat stuff!


  1. #1 Dilip G. Banhatti
    March 6, 2008

    Another e-mail addresses: [banhatti@uni-muenster.de]

    A relevant paper [astro-ph/0703430v3]

    Dear Ethan Siegel, you have a good turn of the word & number. You score quite high on science numeracy!


  2. #2 ethan
    March 6, 2008


    I took a look at your paper. You’re right — Newtonian modeling of gas, dust, and stars in galactic disks do not explain flat rotation curves. You might be interested in my post on Why We Need Dark Matter in the Universe, at http://startswithabang.com/?p=109
    Glad you liked the post!

  3. #3 kim
    November 21, 2008

    Thank you!

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