Originally published at: https://boingboing.net/2024/03/13/webb-space-telescope-confirms-hubbles-view-of-universes-rate-of-expansion.html
…
3 Likes
In other words, it wasn’t a measurement error. So new physics it is.
Well, no.
The Hubble “constant” (in this case referring directly to Edwin Hubble, who is credited with discovering it) is the large-scale average rate of expansion of the Universe, approximately 70 km/s per megaparsec, meaning that galaxies about a megaparsec, or 3.26 million light-years, distant on average appear to be moving away from us at about 70 km/sec, and with each additional megaparsec of distance, their average recession velocity increases by another 70 km/sec.
The issue is that an estimate of the Hubble constant based on observations of the cosmic microwave background (CMB) differs from the estimate obtained by telescopic observations of distant galaxies. The figure from telescopic observation has a number of sources of uncertainty that come from the various distance measurement methods involved, each of which applies to only certain distance ranges. We can quite accurately measure the recession velocity of a galaxy from Doppler shifting of light, but accurately measuring a galaxy’s distance is much harder.
The particular method that is used to measure galactic distances for relatively nearby galaxies is to observe a particular type of star known as a Cepheid variable, where there is an empirically-determined relationship between the period of variability and the total luminosity of the star (originally discovered by astronomer Henrietta Swan Leavitt). Cepheids are pretty bright so they can be observed in other galaxies, and measuring how bright they appear to us compared to the luminosity expected from their period tells us how far away they are.
Originally a number of observations of Cepheids in other galaxies were made with the Hubble Space Telescope (HST) to obtain a measurement of the Hubble constant. However, due to their distance and the difficulty of separating the Cepheids from the glow of other stars in their galaxies left some question about exactly how accurate those measurements were. The Webb Telescope, which observes in infrared and with higher resolution than HST, could be used to cross-check the HST measurement and turns out to confirm the HST figure.
The hope was that maybe the HST measurement of the Hubble constant was off due to systematic errors and Webb observations would find a value closer to that obtained from measuring the CMB. Confirming the HST measurement with Webb reduced the uncertainty in that measurement and the probability that there could be any overlap between the uncertainties of the HST and CMB measurements.
Does this really mean only new physics can explain the discrepancy? Hardly. There could still be some systematic error in either the HST/Webb or CMB Hubble constant measurements that would explain the disagreement; this is still an area of active research. Both measurements might actually be accurate but imply that the Hubble “constant” changed between the formation of the CMB in the very early Universe and the current Universe.
There are other places where cosomologists are inclined to apply speculative physics to explain observed features in Big Bang cosmology – “dark matter” to explain why galaxies and galaxy clusters have more mass than can be accounted for in their observable normal matter, “dark energy” to explain an apparent acceleration of the Hubble expansion (hence the quotes around “constant” in Hubble “constant” since it may actually be increasing over time), and “inflation” (a very brief period of extremely rapid expansion of the Universe in the first instant of the Big Bang) to explain why the large-scale mass distribution of the Universe is highly uniform. Observationally all of these properties quite clearly exist, but actual verifiable physical theories to explain dark matter, dark energy, or inflation are all still very much in question.
3 Likes
This topic was automatically closed after 5 days. New replies are no longer allowed.
