Not long ago, and thanks to the observations of distant supernovae obtained with the Hubble Space Telescope (HST), astronomers were able to conclude that the Universe was not only expanding but also accelerating[1].
Credit: A. Riess
The idea of an expanding Universe started in the early 1920s when A. Friedmann introduced the possible fates of the Universe. It was not until 1929 that Edwin Hubble and his colleague Milton Humaason confirmed this with observations of distant galaxies, which appeared redder than those close to us, and the reddening increased with distance[2]. Dr. Hubble calculated the mathematical relationship between the galaxy's distance and the speed at which it travels away from us. The equation seems quite simple:
v = H_o * D
where H_o is a constant, D is the object's distance, and v is the speed of separation. Because of Edwin Hubble's role in confirming that the Universe was expanding, the constant H_o is now known as the Hubble constant. This equation only applies to distant galaxies, where the expansion is the only velocity contributing significantly to the measured reddening (for local galaxies, their movement through space would have to be considered also).
Why is this constant so important? By knowing the value of the Hubble constant, we can determine the age and history of the Universe; however, coming to a consensus on its value is not as simple and the source of tension among scientists due to the possible consequences of the disagreement[3]. One method used to measure this constant follows the same approach as that of Hubble; it uses independent distance indicators or standard candles, like Cepheid variable stars for the local Universe and supernovas for the more distant Universe. Another method uses models that fit the Big Bang afterglow, also known as cosmic microwave background (CMB) radiation. The CMB can be better understood as temperature variations we can see now but that trace the density of matter fluctuations in the early Universe[4]. In these models, the value of H_o is one of the parameters that need to be fixed to fit the observations.
Credit ESA/NASA/JPL-Caltech
The present measurements using supernovae as standard candles result in a value for H_o of around 73 km/ s/Mpc (link Mpc to Wikipedia). The indirect route using a Lambda-Cold-Dark-Matter (ΛCDM) model, which assumes the composition of the Universe that explains the properties of the cosmos, looks at the CMB and comes out at around 67 km/s/Mpc[3]. Both values fall within the range describing an accelerating universe; however, the expansion rate using standard candles significantly exceeds the model's prediction, giving rise to this so-called Hubble Tension. The term "tension" arises from the uneasiness astronomers feel after coming up with different values, which might mean that there is a different physics they still need to understand.
The ΛCDM model has six parameters that are estimated to match the cosmological observations and from which H_o is calculated. The method using distant indicators, like Cepheids or supernovas, is constrained by observations, and the uncertainties can be reduced by using powerful telescopes like HST. This is possible because HST can separate the stars from their neighbors in distant galaxies; which otherwise appear crowded together within a small area[1].
But now we also have the power of resolution of the James Webb Space Telescope (JWST), which the Nobel Laureate Adam Riess and a team of astronomers used to improve the precision of the Hubble constant in the local Universe. In the first year of JWST's operations, the team collected observations of the 320 Cepheids found by HST, along with four more supernova host galaxies. These observations were needed to refine previous measurements, which could be affected by the dusty environments of the host galaxies and make the stars look redder than they are. Using the sharp infrared vision of JWST and its power to look through dust, it also becomes possible to separate the light of the Cepheids from neighboring stars with little blending and reducing the noise in the measurements introduced by the presence of dust[8][9].
Credits: NASA, ESA, CSA, Adam G. Riess (JHU, STScI)
Riess states that the JWST measurements provide the strongest evidence that the errors in the measurements of HST do not play a significant role in the present Hubble Tension. This is, errors or uncertainties in the observations do not seem to explain the different values, leading astronomers to believe they are still missing something in their understanding of the cosmos. This difference might indicate that the ΛCDM model is not correct and that we need to invoke exotic dark energy, exotic dark matter, or a unique particle or field; probably we even need to revise our understanding of gravity. For sure, the study of the origin and history of our Universe will continue to be the focus of intense research in the coming years.
Credit: NASA, ESA, A. Riess (STScI), and G. Anand (STScI).
Cepheids as distance indicators
Why do astronomers use Cepheid variable stars to measure distances? Cepheids give the most precise distance measurements in the Universe closer to us -- for larger distances we need to use supernovas. Cepheids are supergiant stars, a hundred thousand times the luminosity of the Sun, that pulsate or expand and contract in size over a period of weeks. This period correlates with their relative luminosity — the longer the period, the more luminous they are -- so knowing their period, we can derive their luminosity or absolute magnitude. When we measure the observed apparent magnitude or brightness of a Cepheid star, we then can use the known luminosity to derive their distance. Also, because these stars are quite bright, we can observe them even when they are far from us, making these the gold standard tool to measure the distances of galaxies a hundred million or more light years away.
Credit: NASA, ESA, CSA, and A. Riess (STScI)
Reference:
[1] https://www.stsci.edu/~ariess/documents/Shaw%20Prize%20Lecture_web.pdf
[2] https://science.nasa.gov/people/edwin-hubble/
[3] https://news.uchicago.edu/explainer/hubble-constant-explained
[4] https://www.jpl.nasa.gov/images/pia16875-map-of-matter-in-the-universe
[5] https://lambda.gsfc.nasa.gov/education/graphic_history/hubb_const.html
[6] V.E. Kuzmichev, V.V. Kuzmichev 2022 https://arxiv.org/pdf/2211.16394.pdf
[7] https://academic.oup.com/mnras/article/507/3/3473/6366920
[8] https://webbtelescope.org/contents/early-highlights/webb-confirms-accuracy-of-universes-expansion-rate-measured-by-hubble
[9] https://blogs.nasa.gov/webb/2023/09/12/webb-confirms-accuracy-of-universes-expansion-rate-measured-by-hubble-deepens-mystery-of-hubble-constant-tension/