Deconstructing Hubble: How a Flawed 1929 Paper Changed Cosmology
As a part of a PHYSDepot undergraduate literature group, I led a session on Hubble’s 1929 recession velocity paper. In this, I lead a “scavenger hunt” to find both the surface-level and deeper flaws in the paper, and how Hubble still ended up with a right conclusion despite these issues. The entire scavenger hunt was summarized after the fact into a PDF document provided in this post.
If you have ever spent time looking at the history of science, you quickly realize that progress is rarely a straight, clean line. Nowhere is that more apparent than in Edwin Hubble’s famous 1929 paper, “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae”.
It is widely celebrated as the observational cornerstone of our modern expanding universe. But when you look closely at the actual document, it is a fascinating case study in how brilliant scientific breakthroughs can emerge from selective data, faulty calibrations, and missed theoretical underpinnings.
To help unpack this for an undergraduate literature group on PHYSDepot, we put together a deep-dive scavenger hunt guide. Here is a look at what makes Hubble’s 1929 paper such a wild ride, from its surface-level quirks to its deeper flaws.
For this post, I will highlight the key details, but for a more in-depth view (with images and citations and a modern approach), see the PDF at the bottom of this post.
1. The Historical Stage
Back in the late 1920s, astronomy was in massive transition. Just years prior, the famous “Great Debate” questioned whether our Milky Way was the entire universe or if other “island universes” existed. Hubble settled that by identifying Cepheid variable stars in Andromeda. Meanwhile, Vesto Slipher had spent the 1910s meticulously recording the spectral redshifts of these spiral nebulae, discovering most were moving away from us.
Hubble’s 1929 paper brought these two worlds together: he took distance estimates and compared them against radial velocities, plotting a roughly linear correlation that suggested further galaxies recede faster. This launched Hubble’s Law and prompted Albert Einstein to famously abandon his “cosmological constant”.
2. The Surface-Level Scratches
While the paper’s conclusion hit the mark, reading it today reveals some humorous and questionable choices:
- The Missing Credit: Hubble evaluates 46 radial velocities but largely glosses over Vesto Slipher, who spent years doing the grueling telescope work to capture those redshifts.
- The Mysterious Groups: Hubble splits his data into nine groups based on proximity, but leaves the exact methodology or grouping criteria entirely opaque.
- The Hubble Time Paradox: Using his calculated expansion slope (\(K = 500\text{ km/s/Mpc}\)), simple inverse math (\(1/K\)) yields a universe age of roughly 2 billion years. Unfortunately, geologists already knew the Earth was older than that, creating a hilarious temporal paradox where the universe was apparently younger than its own planets!
3. The Scavenger Hunt “Gold” (Deeper Flaws)
This is where the paper truly becomes a masterclass in messy data:
- Lundmark Beat Him to It: Two years earlier, in 1924, Swedish astronomer Knut Lundmark published a very similar analysis using 44 galaxies. Because Lundmark included noisier data, his plot looked like a scattered mess. Hubble’s clean linear correlation relied heavily on his selective exclusion of messy data points.
- Lemaître Did the Theory: Georges Lemaître had already derived the expansion and calculated a similar constant in 1927 using general relativity and published data, a piece of theoretical groundwork Hubble completely missed.
- The Cepheid Calibration Catastrophe: Hubble was completely unaware that there are two distinct types of Cepheid variables (Classical Population I and older, dimmer W Virginis Population II stars). Mixing these up meant his assumed absolute magnitudes were totally wrong, causing him to underestimate galaxy distances!
- Mistaking HII Regions for Stars: For more distant galaxies, what Hubble thought were individual ultra-bright blue stars were actually entire giant glowing gas clouds (HII regions). Miraculously, this error compounded with his Cepheid error in just the right way to keep his velocity-distance slope relatively straight—albeit with a wildly incorrect value.
4. The Ultimate Takeaway
Despite being built on a house of cards, selective data filtering, mixed-up stellar populations, and swapped gas clouds, Hubble’s paper pointed humanity in the exact right direction. Later corrections by astronomers like Walter Baade and Allan Sandage eventually straightened out the distance scale, landing the universe’s age and expansion rate right where modern cosmology expects them to be.
If you want to read through the full breakdown, check out the complete scavenger hunt guide PDF:
