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How do we know that the CMB is from a hot, early Universe?

Towards the end of July I had an article published in The Conversation about the Cosmic Microwave Background, follow this link to read that article. After the article had been up a few days, I got this question from a Mark Robson, which I thought was an interesting one.

Mark Robson’s original question which be posed below the article I wrote for The Conversation.

I decided to blog an answer to this question, so the blogpost “What is the redshift of the Cosmic Microwave Background (CMB)?” appeared on my blog on the 30th of August, here is a link to that blogpost. However, it would seem that Mark Robson was not happy with my answer, and commented that I had not answered his actual question. So, here is his re-statement of his original question, except to my mind he has re-stated it differently (I guess to clarify what he actually meant in his first question).

I said I would answer this slightly different/clarified question soon, but unfortunately I have not got around to doing so until today due to various other, more pressing, issues (such as attending a conference last week; and also writing articles for an upcoming book 30-second Einstein, which Ivy Press will be publishing next year).

The questions and comments that Mark Robson has since posted below my article about how we know the redshift of the CMB

What is unique about the CMB data?

The very quick answer to Mark Robson’s re-stated question is that “the unique data possessed by the CMB which allow us to calculate its age or the temperature at which it was emitted” is that it is a perfect blackbody. I think I have already stated in other blogs, but let me just re-state it here again, the spectrum as measured by the COBE instrument FIRAS in 1990 of the CMB’s spectrum showed it to be the most perfect blackbody spectrum ever seen in nature. Here is the FIRAS spectrum of the CMB to re-emphasise that.

The spectrum of the CMB as measured by the FIRAS instrument on COBE in 1990. It is the most perfect blackbody spectrum in nature ever observed. The error bars are four hundred times larger than normal, just so one can see them!

So, we know, without any shadow of doubt, that this spectrum is NOT due to e.g. distant galaxies. Let me explain why we know this.

The spectra of galaxies

If we look at the spectrum of a nearby galaxy like Messier 31 (the Andromeda galaxy), we see something which is not a blackbody. Here is what the spectrum of M31 looks like.

The spectrum of our nearest large galaxy, Messier 31

The spectrum differs from a blackbody spectrum for two reasons. First of all, it is much broader than a blackbody spectrum, and this is easy to explain. When we look at the light from M31 we are seeing the integrated light from many hundreds of millions of stars, and those stars have different temperatures. So, we are seeing the superposition of many different blackbody spectra, so this broadens the observed spectrum.

Secondly, you notice that there are lots of dips in the spectrum. These are absorption lines, and are produced by the light from the surfaces of the stars in M31 passing through the thinner gases in the atmospheres of the stars. We see the same thing in the spectrum of the Sun (Josef von Fraunhofer was the first person to notice this in 1814/15). These absorption lines were actually noticed in the spectra of galaxies long before we knew they were galaxies, and were one of the indirect pieces of evidence used to argue that the “spiral nebulae” (as they were then called) were not disks of gas rotating around a newly formed star (as some argued), but were in fact galaxies outside of our own Galaxy. Spectra of gaseous regions (like the Orion nebula) were already known to be emission spectra, but the spectra of spiral nebulae were continuum spectra with absorption lines superimposed, a sure indicator that they were from stars, but stars too far away to be seen individually because they lay outside of our Galaxy.

The absorption lines, as well as giving us a hint many years ago that we were seeing the superposition of many many stars in the spectra of spiral neublae, are also very useful because they allow us to determine the redshift of galaxies. We are able to identify many of the absorption lines and hence work out by how much they are shifted – here is an example of an actual spectrum of a very distant galaxy at a redshift of $z=5.515$, and below the actual spectrum (the smear of dark light at the top) is the identification of the lines seen in that spectrum at their rest wavelengths.

The spectrum of a galaxy at a redshift of z=5.515 (top) (z=5.515 is a very distant galaxy), and the features in that spectrum at their rest wavelengths

Some galaxies show emission spectra, in particular from the light at the centre, we call these type of galaxies active galactic nucleui (AGNs), and quasars are now known to be a particular class of AGNs along with Seyfert galaxies and BL Lac galaxies. These AGNs also have spectral lines (but this time in emission) which allow us to determine the redshift of the host galaxy; this is how we are able to determine the redshifts of quasars.

Notice, there are no absorption lines or emission lines in the spectrum of the CMB. Not only is it a perfect blackbody spectrum, which shows beyond any doubt that it is produced by something at one particular temperature, but the absence of absorption or emission lines in the CMB also tells us that it does not come from galaxies.

The extra-galactic background light

We have also, over the last few decades, determined the components of what is known as the extra-galactic background light, which just means the light coming from beyond our galaxy. When I say “light”, I don’t just mean visible light, but light from across the electromagnetic spectrum from gamma rays all the way down to radio waves. Here are the actual data of the extra-galactic background light (EGBL)

Actual measurements of the extra-galactic background light

Here is a cartoon (from Andrew Jaffe) which shows the various components of the EGBL.

The components of the extra-galactic background light

I won’t go through every component of this plot, but the UV, optical and CIB (Cosmic Infrared Background) are all from stars (hot, medium and cooler stars); but notice they are not blackbody in shape, they are broadened because they are the integrated light from many billions of stars at different temperatures. The CMB is a perfect blackbody, and notice that it is the largest component in the plot (the y-axis is what is called $\nu I_{\nu}$, which means that the vertical position of any point on the plot is an indicator of the energy in the photons at that wavelength (or frequency). The energy of the photons from the CMB is greater than the energy of photons coming from all stars in all the galaxies in the Universe; even though each photon in the CMB carries very little energy (because they have such a long wavelength or low frequency).

Why are there no absorption lines in the CMB?

If the CMB comes from the early Universe, then its light has to travel through intervening material like galaxies, gas between galaxies and clusters of galaxies. You might be wondering why we don’t see any absorption lines in the CMB’s spectrum in the same way that we do in the light coming from the surfaces of stars.

The answer is simple, the photons in the CMB do not have enough energy to excite any electrons in any hydrogen or helium atoms (which is what 99% of the Universe is), and so no absorption lines are produced. However, the photons are able to excite very low energy rotational states in the Cyanogen molecule, and in fact this was first noticed in the 1940s long before it was realised what was causing it.

Also, the CMB is affected as it passes through intervening clusters of galaxies towards us. The gas between galaxies in clusters is hot, at millions of Kelvin, and hence is ionised. The free electrons actually give energy to the photons from the CMB via a process known as inverse Compton scattering, and we are able to measure this small energy boost in the photons of the CMB as they pass through clusters. The effect is known as the Sunyaev Zel’dovich effect, named after the two Russian physicists who first predicted it in the 1960s. We not only see the SZ effect where we know there are clusters, but we have also recently discovered previously unknown clusters because of the SZ effect!

I am not sure if I have answered Mark Robson’s question(s) to his satisfaction. Somehow I suspect that if I haven’t he will let me know!

Update on the GRB in M31

Well, it would seem it was all a false alarm. For one of the best and most detailed explanations of why the false detection happened, check out this link.

So, no spectacular supernova in Messier 31 yet. But we can live in hope.

Some of my favourite Messier objects – M31

In this blog I discussed the Messier catalogue, including its historical background. As I mentioned in that blog, many of the 110 objects in the Messier catalogue are amongst the most beautiful and interesting objects in the sky. In a series of blogs I am going to discuss some of my favourite objects in the Messier catalogue in more detail, including how to find them.

First off, Messier 31.

Messier 31 – The Andromeda galaxy

I have already talked about this galaxy a few times before. In this blog I explained how Edwin Hubble showed in 1923 that M31 lies outside of our Milky Way galaxy. It was the first object ever shown to lie outside the Milky Way, and of course led to the realisation that many similar objects also lay beyond our Galaxy. This discovery by Hubble in 1923 totally changed our perception of our Universe and our place in it. It provided conclusive proof that our Milky Way galaxy was not the entire Universe, but just one galaxy in many. In my blog introducing the Messier catalogue it was one of the four objects I illustrated.

Messier 31 is an example of a spiral galaxy. However, in visible light its spiral arms are not very prominent. Along with our Milky Way, Messier 31 and our home Galaxy form the two largest galaxies in our Local Group. There are other galaxies in the local group, but they are all so-called dwarf galaxies and are either satellite galaxies of our Milky Way or of M31.

Messier 31 – The Andromeda galaxy.

Our current best measurements suggest that M31 is about 2 million light years away. That is, the light has taken 2 million years from when it left M31 to reach us. You are seeing the object as it was 2 million years ago! This makes it the most distant object visible to the naked eye. However, to see it with the naked eye you have to be (a) in a dark place away from light pollution and (b) know where to look. Here is a finding chart to find M31.

How to find Messier 31

Messier 31 is best seen in the late Summer and Autumn sky. The easiest way to find M31 is to first find the Square of Pegasus, an easily located asterism which forms part of the constellation Pegasus. The bright bluish-white star in the North-Eastern corner (top left as seen from the Northern Hemisphere) is called Alpheratz. If one moves to the left (East) from this star the Andromeda constellation forms a thin v-shape. The lower branch of the “v” has a bright red star called Mirach (the second star along from Alpheratz). From Mirach go up to pi-Andromodae (quite a faint star), and in a straight line an equal distance from Mirach to pi-Andromodae is where you should find the Andromeda galaxy.

However, even if you are in a dark place it is extremely difficult to see, and with your naked eye you will probably only be able to see it using your peripheral vision, the part of the eye which is more sensitive to low light levels. If you are having difficulty finding it with your eye, it is quite a good idea to find it with a low powered magnification in a telescope, and then use this to help you know what you are looking for with just your un-aided eye.

What you will see with your eye, whether un-aided or through a telescope, will probably not look much like the photograph above. Rather, you will see a fuzzy blob. Remember, the word “nebula” comes from the Latin word for cloud, and Messier 31 looks like a fuzzy cloud. Until astronomers gained a better understanding of objects in the Messier catalogue, a galaxy like Messier 31 was referred to as a “nebula” just as Messier 42, the Orion nebula, had the same term attached to it, even though we now know they are very different kinds of objects.

Messier 31 in ultra-violet light and infrared light

Although the spiral arms are not very prominent when we take an image of M31 in visible light, its appearance is quite different when we observe it in e.g. ultra-violet light and infrared light. The uv image below was taken by a mission called Galex, and shows a prominent ring of stars emitting in the uv part of the spectrum. The nucleus of the galaxy is also seen to be emitting quite strongly in uv light. It is only the hotter, younger stars which emit in the uv, cooler and older stars like our Sun are just not hot enough to emit much in ultra-violet light. The hot young stars are found in the spiral arms of spiral galaxies, in fact it is the presence of these hot young stars which give such galaxies their spiral appearance.

A prominent ring is also visible in this infrared image of M31 shown below. This image was taken with the Herschel Space Observatory, and was taken at a wavelength of 250 microns. What we are seeing in this image is not radiation from stars, but rather the emission from interstellar dust. At 250 microns we can see both warm dust heated by hot young stars and cooler dust that is heated by the more general stellar population. That is why you see you reasonable but not exact correspondence between the uv image and the infrared image.

Studying objects at different wavelengths has allowed to learn a lot more about them than if we just observe them with visible light. Nowadays, astronomers have the facilities to observe astrophysical objects from the X-ray to the radio part of the spectrum, and through doing this gain a much deeper understanding of the object than if it were just observed in visible light alone.