journal club

A Universe in Expansion

In this classic astronomy paper, we take a look at the discovery that introduced Hubble's Law and confirmed an expanding universe.

Classic Papers: Edwin Hubble, 1929

When Edwin Hubble published A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae, he expanded the universe on multiple levels.

He had already expanded the size of the universe when he had published four years prior, something about calculating the distance of super bright cepheid stars and placing them outside the realm of the Milky Way. No one prior to that had guessed the universe extended beyond our own galaxy.

But now, he was pushing it further. He was presenting data that would suggest that not only is the universe bigger than we imagined, but it is expanding, that expansion is accelerating, and spacetime itself is expanding.

The paper itself is short and simple. It’s based on faulty data – it took years before we sent telescopes into space to get more accurate distances. He used a small sample size, considering only 24 stars in all. He even suggested an interpretation of the data that would allow for a continuation of Einstein’s beloved but mistaken concept of the static universe.

But even if Hubble wouldn’t directly do so, the data itself stood up to Einstein, who finally had to drop his infamous cosmological constant when faced with the paper.

Albert Einstein’s Theory of Relativity had predicted that the universe was expanding a little over a decade earlier. But instead of jumping at a revolution in cosmology, the genius physicist decided to introduce a variable dubbed the Cosmological Constant to his equations. This forced a fit with the accepted model of the time – that the universe is static, unmoving. Another physicist by the name of Willem de Sitter had predicted that something like the Doppler Effect might be observed in space if gravity slowed time on the farther edges of the universe. Hubble actually gives a nod to this “de Sitter Effect” in his paper as a possible explanation for the phenomena he observed.

But in the end, Hubble’s Diagram would point us correctly to an expanding universe. This would eventually provide cosmologists with a framework for developing the Big Bang Theory. The follow-up observation? Stars are not only travelling away from us, but travelling away faster as they go. This is one of the current major questions of cosmology. The likely culprit? Dark energy, making up around 70% of the universe.

So what exactly is in this paper, published in 1929 and rocking the fields of astronomy and cosmology ever since?

Hubble’s Diagram

Let’s take a quick little guided tour through Hubble’s paper.

Hubble begins by discusses a paradox in astronomy. There have been attempts to answer this paradox, but he is unsatisfied with them. It has to do with variable K terms – a variable astronomers were using to describe velocity – of nebulae outside our galaxy.

There appeared to be different velocities of these extra-galactic objects: why? Is this a real thing?

Hubble sot to answer this question by exploring different datasets for nebulae. He looked largely at three variables: nebula distance, luminosity, and velocity.

It turns out, we can calculate (or at least, estimate) distance based off of the luminosity of a star (or in this case, nebula). So in Table 1, Hubble calculates distance based off of luminosity data, and compares it to velocity data we also had on those nebulae.

It was a small data set, but there was a definite order of luminosity.

Seeing a connection between distance and velocity, he begins to modify the velocity equation used at the time. The K term within it should be dependent on distance, since it appeared that as distance increased, the velocity increased, too.

From there, Hubble starts looking at another data set. In this case, the data doesn’t include distance. Using a few more equational tricks, he gathered estimates of the missing data and again, there was that correlation between velocity and distance.

Figure 1 is the famous diagram, the data demonstration that showcased what would come to be known as Hubble’s Law. By simply graphing his data (small and unreliable his data may be), we get a visual of this relationship he’s talking about. There is an undeniable correlation between distance and velocity.

So what?

So what’s the big deal? Why did this simple observation become so groundbreaking?

In his last paragraph, Hubble attempts to interpret some of his observations, mentioning the de Sitter effect. No where is the Doppler Effect mentioned in this paper.

The de Sitter effect would have interpreted the data to account for a static universe, but as time went on and more and more data confirmed this original observation, it became clear that something else was going on.

It seems that as we look at stars and nebulae that are farther and farther from us, they are moving faster and faster. If a car drives past you (quickly), you are accustomed to the change in frequency that results as the sound changes – high frequency as it moves towards you, then low frequency as it moves away. The same thing happens with stars – if they were moving towards us, we’d expect a blueshifting of light waves from them. Instead, we get redshifting – lower frequency – indicating a move away. Redshift is, after all, a measurement used to calculate velocity.

And with that, Hubble launched a new era of astronomy, from throwing out the cosmological constant, to Dark Energy research of today.

References:

Hubble, E. (1929). A relation between distance and radial velocity among extra-galactic nebulae. PNAS, 15, 168–173.

See also:

Hubble’s Law and the Expanding Universe

Hubble’s Diagram and Cosmic Expansion

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