Is the Cosmic Microwave Background a Picture of the Universe's Past?

[electerr]

Hi,
Am I right when I say that the cosmic microwave background is a picture of the radiation left from recombination but that the picture (from COBE or WMAP) that we see today is not exactly the same as it was then due to the redshifting of the radiation particles. The flucuations that we see today are also more apparent now than they have been in the past since the universe was smoother at earlier times. So the pictures we get today from COBE or WMAP are pictures of the CMB NOW not as it was 300,000yrs after the big bang. I think it is really confusing that it is sometimes said that the CMB is a representation of the universe as it was at the time of recombination. Am I on the right track or have I totally misunderstood? A big thanks for anyone who can set me straight!

 

[Jorrie]

I think that apart from the wavelength that differs due to redshift, the CMB that is observed today gives the density fluctuation map as it was at last scattering - about 1 part in 10^5. It's a bit like taking a photo of the same person in UV and in IR - it looks different, but the features are the same...

 

[electerr]

Thanks for your reply Jorrie!

Can anyone confirm this?

I thought that the universe was smoother in the past, wouldn't these changes in the density affect the way the CMB looks?

Thanks again!

 

[Chalnoth]

Can anyone confirm this?

Yes.

I thought that the universe was smoother in the past, wouldn't these changes in the density affect the way the CMB looks?

Normal matter and dark matter cluster. Light does not. When the universe became transparent after the emission of the CMB, the light basically just traveled freely.

Now, it is worth mentioning that this is only an approximation (that of light traveling freely), but it is a very good one. After all, the rest of the universe besides the lights from the CMB do interact with it to a degree. Sometimes the light goes through a galaxy cluster, for instance, and interacts with the gas cloud there (the Sunyaev Zel'dovich effect). There's also a very thin, diffuse plasma that fills the entire universe since the epoch of reionization (when the first stars turned on, they re-ionized the interstellar gas). This slightly dims the CMB over the entire sky. There's also the effect of dark energy which causes gravitational potentials to change with time from the Integrated Sachs-Wolfe Effect (when a photon enters a gravitational well it blueshifts slightly, but by the time it starts to get out of the well the well has shallowed slightly and so it redshifts a bit less than the blueshift upon entering...this causes a slight brightening of the CMB at large distance scales). There's also the fact that massive structures around the universe lens the light from the CMB, which causes light that would have arrived at one angle to arrive from another.

All of these effects are known, of course, and are small enough to be nearly undetectable at the level of the current full-sky CMB data from WMAP. But they are significant systematics that we need to take into account to understand the next-generation CMB satellite, Planck.

 

[Chronos]

As chalnoth noted, the universe [radiation wise] was clumpier in the past, as we perceive it. But, perception may be an issue.

 

[electerr]

Ok, so it is actually a picture of the past with the only difference being the wavelength. Great, now I know... Thanks for the help!