Feeds:
Posts

## The Prediction of the Cosmic Microwave Background – the original paper

Last week I reposted my blog about the prediction of the cosmic microwave background (CMB), which I had originally written in April 2013. This month, July, marks the 50th anniversary of the first detection of the CMB, and I will blog about that historic discovery next week. But, in this blog, I wanted to show the original 1948 paper by Alpher and Hermann that predicted the CMB’s existence.

I learnt far more about the history of the CMB’s prediction whilst researching for my book on the CMB, which was published at the end of 2014 (follow this link to order a copy). In doing my research, I found out that many of the things I had been been told or had read about the prediction were wrong, so here I wanted to say a little bit more about what led up to the prediction.

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

## Gamow did not predict the CMB

Many people either do not know of the 1940s prediction of the CMB, or they attribute its prediction to George Gamow. In fact, it was his research assistants Ralph Alpher and Robert Hermann who made the prediction, but as head of the group it is often Gamow who gets the credit.

Ralph Alpher had just finished his PhD on the origin of the elements, and after the publication of the famous Alpher, Bethe, Gamow paper (see my blog here about that), Gamow started writing a series of papers on the nature of the early Universe. One of these papers was entitled “The Evolution of the Universe”, and it appeared in Nature magazine on the 30th of October 1948 (Nature 1948, volume 162, pages 680-682) – here is a link to the paper.

Gamow’s October 1948 paper in Nature was entitled “The Evolution of the Elements”.

Although a man of huge intellect and inventiveness, Gamow was often sloppy on mathematical detail. Alpher and Hermann spotted an error in some of Gamow’s calculations on the matter-density, and so wrote a short letter to Nature magazine to correct these mistakes. The letter is entitled “Evolution of the Universe”, nearly the same title as Gamow’s paper, but with no “The” at the start. The letter is dated 25 October 1948. It appeared in Nature magazine on the 13th of November 1948 (Nature 1948, volume 162, pages 774-775) – here is a link to the paper.

Here is the paper in its entirety (it is short!), and I have highlighted the part which refers to a relic radiation from the early Universe, what would become known as the cosmic microwave background.

The original paper (letter) by Alpher and Hermann which makes the first prediction of the cosmic microwave background (CMB). It was published in Nature magazine on the 13th of November 1948.

As you can see, the prediction is not the main part of the paper, it just forms two sentences!

Next week, I will blog about the accidental discovery of the CMB by Penzias and Wilson, which was published 50 years ago to this month (July).

## Five top facts about Jupiter – no. 1

Last week I said I would blog in more detail about the “five top facts” I gave to BBC Radio 5 when I was on the breakfast show back on the 3rd of February. Here is the tweet that BBC Radio 5 sent out, and below it the “five top facts” that I chose. Today I am going to blog about the first fact.

The tweet from BBC Radio 5 with the five most interesting facts about Jupiter.

The list of the five facts

## Fact 1 – Jupiter is a failed star

Jupiter is, like the Sun, mainly comprised of hydrogen and helium. In the Universe as a whole, about 75% of the Universe is hydrogen, about 24% is helium, and the remaining 1% is everything else (carbon, oxygen, nitrogen etc.). It was George Gamow and his co-workes Ralph Alpher and Robert Hermann who showed in the late 1940s that the hydrogen and helium we find in the Universe was all be created in the first few minutes after the Big Bang. In fact, the observed abundances of hydrogen and helium are one of the main pieces of evidence for the “hot big bang” theory, there is just too much helium to have all been created in stars as e.g. Fred Hoyle argued.

The other elements beyond hydrogen and helium have all been created within stars. A star like the Sun is what we call a “main sequence” star, (see my blog on the HR diagram here) and this means that it is burning hydrogen in its core. It is turning the hydrogen into helium via nuclear fusion, and the tiny mass difference between the hydrogen that goes into this reaction and the helium which comes out provides the energy of the Sun, because of Einstein’s famous equation $E=mc^{2}$. Because hydrogen is converted into helium in stars, the abundance of helium is very slowly getting greater in the Universe, and the abundance of hydrogen is very slowly going down.

We believe that stars form from the collapse of huge clouds of hydrogen, things we refer to as “giant molecular clouds” (like the “pillars of creation” that I blogged about here). When these clouds are massive and cold enough they can collapse under their own gravity, and as they do so they fragment. The individual fragments are what form the stars, but a fragment has to be large enough to actually “ignite” as a star. This requires a high enough pressure and density and temperature at the core of the fragment.

Jupiter has all the ingredients to be a star, but what it lacks is the mass (or size). It is too small to create a high enough temperature and pressure at the centre to force hydrogen to fuse into helium. If it were about ten to one hundred times more massive it would have become a star, albeit a very faint one (what we call a “red dwarf”). We believe stars can range in mass from about one tenth the mass of the Sun (smaller than this and they will be “failed stars” like Jupiter) up to about 50 or maybe 100 times the mass of the Sun. Larger than this and they are beyond something called the Hyashi limit, and will just blow themselves apart before they can settle down onto the main sequence.

It was discovered in the 1960s, from observing Jupiter in the infrared, that it was hotter than it should be given its distance from the Sun. This is because the gravitational potential energy lost when the gas from which it formed collapsed was converted to heat, and this heat has been leaking out over since. It emits about twice the energy that it gets from the Sun.

## The Origin of the Elements – part 1

One of the outstanding problems in astrophysics in the 1940s was how were the elements created. In the 1920s it was realised by Cecilia Payne-Gaposchkin in her PhD work that the Sun was mainly composed of hydrogen. Then, spectral analysis of others stars and the gases of the interstellar medium led astrophycisits to realise that the Universe was composed mainly of hydrogen (about $75\%$), with the remainder being helium (about $24\%$), and only $1\%$ or so being all the other elements (oxygen, nitrogen, carbon etc.)

In the mid 1940s Russian-American physicist George Gamow started thinking about how the elements originated, and he developed a theory with his student Ralph Alpher that they were all created in the early Universe when, he argued, it would have been hotter and denser than it currently is. He published his famous paper “The Origin of Chemical Elements” in Physical Review in 1948, adding renowned physicist Hans Bethe’s name to the author list as a joke so that the paper would have the author list Alpher, Bethe, Gamow (alpha,beta, gamma, geddit? There is also a story that he tried to get his post-doctoral researcher Bob Herman to change his last name to “Delta” 😛 )

The famous Alpher, Bethe, Gamow paper, “The Origin of Chemical Elements”, which appeared in Physical Review in 1948.

In fact, Bethe played no part in writing the paper, but he was happy for Gamow to include his name for Gamow’s little joke. In this paper, Gamow and Alpher argued that all the elements were created in the early Universe. However, when others went through the details it was realised that the numbers did not add up, the Universe expanded and cooled too quickly for all the elements to be created in this way. Although it was possible for hydrogen and helium to be created in the first few minutes of the Universe, by the time the Universe was a few minutes old it had become too cool and the density too low to form the heavier elements beyond helium. Part of the reason for heavier elements not being built up in these first few minutes was due to something called the deuterium bottleneck, which I will explain in a future blog.

In the 1950s an alternative theory for the origin of the elements was put forward by Fred Hoyle and his collaborators Willy Fowler and Geoffrey and Margaret Burbidge. In a series of papers they argued that the elements had been built up in the interior of stars, the most famous of this series of papers was a 1957 paper entitled “Synthesis of the Elements in Stars” which appeared in Reviews of Modern Physics. Hoyle was the main advocate of a theory called the Steady State Theory which he had first proposed in 1948. This was a competing theory to the hot big bang theory, and so of course Hoyle did not believe any elements had been formed in a hot, dense early Universe as he did not believe such a Universe ever existed.

The first page of Burbidge teal.’s famous paper “Synthesis of the Elements in Stars” published in Reviews of Modern Physics in 1957

Again, as with Alpher and Gamow’s theory, detailed calculations found flaws in the Burbidge etal. theory. Although it could explain the creation of elements beyond helium, it was not possible to create enough helium in stars to account for the approximately $25\%$ found to be present in the Universe today. In part 2 of this blog, I will explain what our current understanding is of the origin of the elements in the proportions we observe in the Universe, and what the deuterium bottleneck is and why it is important.

## The prediction of the Cosmic Microwave Background

In this blog I described the first results from the Planck satellite, which is studying the Cosmic Microwave Background in greater detail than we have ever done before. But, what exactly is the Cosmic Microwave Background? Where does it come from? How was it produced?

## The origin of the elements

In 1929 Edwin Hubble published evidence that the speed with which galaxies were moving away from the Milky Way was directly related to their distance from us. Although Hubble himself never explicity stated it, this is clear evidence that the Universe is expanding. If the Universe is expanding, then of course one would expect it to have been smaller in the past.

In the 1940s the Russian-American physicist George Gamow started thinking about what the early Universe would have been like. He worked on two related theories, the first that the elements would have been created in the early Universe. The second related to the fact that a smaller, denser Universe would also have been hotter in the past.

In 1948, with his PhD student Ralph Alpher, the two published a paper titled “The Origin of Chemical Elements“. As a joke, Gamow decided to add the well-known physicist Hans Bethe’s name to the paper, so that it could be called “Alpher, Bethe, Gamow” (alpha,beta, gamma – geddit? 🙂 ).

George Gamow, who worked with his PhD student Ralph Alpher on the primordial nucleosynthesis theory.

Ralph Alpher, who was George Gamow’s PhD student at the time of writing the paper.

Hans Bethe, who played no part in writing the paper.

The famous “Alpha, Bethe, Gamow” paper from Physical Review 1948

Although the Alpher, Bethe, Gamow paper was groundbreaking, it was wrong in some of its details. It suggested that all the elements were created in the hot, early Universe. We now think (know?) this is not the case. Only hydrogen and helium were created in the early Universe, the other elements have all been created inside stars, something Sir Fred Hoyle worked out with co-workers in the 1950s.

## Alpher and Herman’s paper on the Cosmic Microwave Background

In a related paper, Alpher and Robert Herman, who was working as a post-doctoral research assistant for Gamow, calculated that the early Universe would have been a hot opaque plasma (ionised gas), and would thus have radiated like a black body. However, this radiation would not have been able to travel through the plasma as the photons would scatter of the free electrons.

The abstract of the paper by Alpher and Herman, which predicts a cosmic microwave background at a temperature of 5K (5 degrees above absolute zero).

Gamow’s article in Nature, which summarises the work on the origin of the elements and of the existence of a cosmic microwave background

But, as the Universe expanded and cooled the plasma would become a neutral gas, in that the electrons would combine with the nuclei to produce neutral atoms, allowing the photons to travel unimpeded. They calculated that these photons, which would be able to thence travel unimpeded, would now be at a characteristic black-body temperature of 5K due to the expansion of the Universe. This in the microwave part of the spectrum, hence the name Cosmic Microwave Background.

Our current understanding is pretty much what was derived in this 1948 paper, with a few refinements. Perversely, the moment the plasma became a neutral gas, which we believe to be when the Universe was about 350,000 years old, is referred to as “re-combination”, but as I tell my students, the electrons were combing with the nuclei for the first time. This is when the fog of the early Universe lifted and is the earliest radiation we can see.

In a separate blog on the history of the Cosmic Microwave Background (CMB) I will discuss how

1. the CMB was accidentally discovered in 1964
2. Gamow’s work was ignored, only to be worked out again in the early 1960s

## Update

You can read far more about the prediction of the CMB, and its accidental discovery, in my new book, “The Cosmic Microwave Background – How it changed our understanding of the Universe”.

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

The book can be found on the Springer website here, and on the Amazon website here.