Thursday, September 12, 2013

Hubble's Law and Hubble's Legacy

In astronomy, everything in the Universe is moving relative to everything else. The Earth moves around the Sun, the Sun around the Milky Way, the Milky Way moves relative to the other Local Group galaxies, and the Local Group relative to more distant galaxies and galaxy clusters. Within the large-scale cosmic web we find bulk motions in every direction on the sky. 

Such motions can be measured using a variety of techniques depending on the objects of interest. For galaxies, this is typically achieved through the identification of known "lines" in their spectra, which shift from where they should be, where we measure them at rest in the lab. This is simply a "light" version of the known Doppler Effect for sound, where, for example, the pitch of a train goes up when approaching, then down when moving away, compared to the pitch you hear when you're on it.

In astronomy, the degree of this shift is known as either redshift (for galaxies moving away from us) or blueshift (for galaxies moving towards), as explained previously.

It was then a curious set of observations in the early 1900's that revealed that the majority of objects outside our own galaxy (then called nebulae, now known as other galaxies) were all moving away from us (i.e. redshifted), and in approximate proportion to their distance. This was explicitly seen in the pioneering work of Vesto Slipher in 1917, Knut Lundmark in 1924, and Edwin Hubble in 1929, amongst others of the time. 

In the figure below we reproduce the original "discovery" plot by Edwin Hubble, which has since come to be known as Hubble's Law, written as:

galaxy recession velocity = H0 x galaxy distance

The proportionality constant, H0, is called Hubble's constant and was determined by fitting a straight line through the data. Hubble estimated H0 = 500 km/s/Mpc at the time.

Edwin Hubble's "discovery" plot from 1929, showing that the distance to a galaxy (on the x-axis) is correlated with the speed at which it's moving away from us (called redshift, on the y-axis). Such a distance-redshift relation is strong evidence supporting the idea that the Universe is expanding.

The observation that every distant galaxy in the Universe appears to be red and not blueshifted is itself remarkable. In effect, it tells us that the motions of all galaxies beyond our local volume are in a direction away from us, and Hubble's Law tells us that the further away a galaxy is, the faster its moving away. This was, in essence, the first observational evidence of an expanding Universe!

That the Universe could be expanding was predicted by Einstein's equations of general relativity, as many of you may know. A somewhat crazy idea when first proposed, Einstein himself was unsatisfied with the concept of a dynamic space-time, which led him to update his equations with the famous cosmological constant, Lambda.

Although Hubble is solely credited with the discovery of his Law, closer examination of the literature shows a more complex history with no one single eureka moment by any individual. In fact, the redshifts that Hubble used above were entirely borrowed from Slipher's earlier work, and the distances that Hubble measured himself were unfortunately significantly flawed. The modern value of H0 is 67.3 km/s/Mpc, measured to about 2% accuracy by the Planck cosmic microwave background satellite.

As Edinburgh Royal Observatory Professor John Peacock recently argued, Hubble was perhaps somewhat fortunate to be able to demonstrate the relation given the data he had on hand at the time. However he was already an important figure in the community, very good at promoting the result, and the community of the day was equally as excited to accept it.

Regardless, suffice it to say that once the Law was established its ramifications changed our understanding of the Universe. The measurement of the Hubble expansion (and repeated confirmation over the years) heralded in the age of modern cosmology. It underpins our modern cosmological paradigm. And it is a key component to many of the observations and results that CANDELS produces using the Hubble Space Telescope.

More discussion of the Hubble constant and its use (and misuse) in astronomy data analysis can be found in my recently published paper, "Damn you little h!".

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