4. Large scale structure of the Universe. In other wavebands (Inflationary perturbations 1)

In this post, I will briefly describe what astronomers see on the sky in wavebands other than optical as well as what observations in other wavebands are especially good for.

The best instruments to observe the Universe one can have are the space based ones. Currently, the space observatories – the facet eyes of our civilization looking at the sky – include: already mentioned WMAP for observations in 1 mm – 1 cm waveband (microwave radiation), Spitzer space telescope for observations in 1 mkm – 100 mkm waveband (infrared), Solar and Heliospheric Observatory (SOHO) r observations in 50 nm – 500 nm waveband (UV), Hubble Space Telescope for observations in optical band, UV and IR, XMM Newton and Chandra Space Telescope for observations in 0.5 nm – 10 nm waveband (X rays) and INTEGRAL for observations in 100 fm – 100 pm waveband (gamma rays). Finally, we also have eyes in radio waveband (> 1cm) but these are all ground based since radio waves can easily pass through the Earth’s atmosphere.

First of all, let me assure you that the large scale structure of the Universe as seen in different wavebands is similar compared to what is seen in the optical waveband. Namely, while fluctuations of the matter density and gravitational potential are large at scales less than 1 billion ly, they become small at 1 GPc scale and larger (relative fluctuation at distances larger than 1 GPc and up to the cosmological horizon scale). So, why do we need so many costly instruments? The answer is that some objects in the Universe and physical effects can be observed only in certain wavebands.

2. Universe in radio waves

Observing the Universe in radio waves is a powerful way to get high resolution maps of very distant galaxies as well as very energetic sources of radiation such as quasars which simply cannot be detected in optical astronomy. These maps typically look as follows:

This is combined optical (black and white) and radio wave (yellow contours) map. As you can see, brightest radio wave emitting objects are not quite the same is brightest objects in optical waveband.

Another important thing is the spectral line of hydrogen is 21 cm, and the Universe is full of hydrogen. Looking at galaxies in 21 cm wave, we were able to determine the relative velocities of galactic arms and construct galactic rotation curves. The latter are considered one of the major arguments in favor of existence of dark matter as we will discuss later). In cosmology, observations at 21 cm wavelength are also of extreme importance, since they are the only probe of “dark ages” between recombination and reionization epochs.

Also, next time you consider cutting of funding of radioastronomy , please don’t forget that cosmic microwave background was first discovered on a radiotelescope

3. Universe in the infrared

Such IR telescopes as Spitzer are extremely good for looking through interstellar dust. For example, as you may know, the center of our own Galaxy is hidden due to the huge dust cloud located between us and the center of the Milky Way . That is how it looks like in the optical waveband:

The center is so powerful source of radiation that, if the dust cloud between us and the center would be absent, you could hardly notice any difference between day and night

On the other hand, that is how the center of the Milky Way Galaxy looks like in the IR:

Observations in the IR are also good for spotting very young galaxies at which the star formation is yet in the early age.

4. Universe in X rays

Clusters of galaxies are nicely probed by the observations in the X-ray waveband. Also, with the help of a X-ray telescopes such as Chandra you can very clearly see the voids in the large scale structure (Cosmic Web). The reason is that the intergalactic gas which fills the voids in the large scale structure is extremely hot (its temperature is tens of millions of K) and emits X-ray radiation.

What you see in the picture above is the galaxy cluster Abell 2029. In the optical waveband one can spot an extremely massive galaxy in the center of the cluster. This galaxy strongly attracts the interstellar gas in voids; the gas falling into the gravitational well emits huge amounts of the X-ray radiation due to the accretion (picture on the left).

Ability to see in IR, optical, X-ray and radio wavebands allows us to construct combined pictures like this one -

- Centauri A radiogalaxy, one of the most powerful sources of radio waves on the sky. As I already explained, in IR one can very clearly see through the dust hiding the center of the galaxy in optical waveband. X ray and radio pictures show two nice jets of hot gas perpendicular to the galaxy’s plain (one has no idea about them just taking the optical/IR/UV image).

5. Universe in microwaves

Let us now turn to what astronomers see in the microwave waveband. Apperently, the most important effect there is the cosmic microwave background radiation (CMB). It turns out that the Earth is bathed in the aether of microwave quanta with thermal spectrum and characteristic temperature about 2.7 K (more precisely, K according to COBE). The spatial distribution of these quanta is completely isotropic and homogeneous (in 1970s, that was the strongest indication of the homogeneity of the Universe at large scales). Let me mention that the spectrum of the CMB is the most precise thermal spectrum found in Nature. On the picture below it is shown with error bars increased 400-fold (again, taken from the COBE data):

The CMB covers lots of information about the physics of the early Universe, the large scale structure of the Universe, cosmological parameters, etc. We will discuss the features of CMB later in much more detailed manner.