UR: A Crisis in Cosmology? A new perspective on the Hubble Constant Tension

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Sahil Ugale

Department of Physics, Mithibai College, The University of Mumbai

As a third-year undergraduate student majoring in physics, Sahil Ugale conducted this research along with collaborators as part of a remote research internship at National Astronomical Observatory of Japan (NAOJ) under the supervision of Dr. Maria Dainotti. This research paper was published on 29 January 2022 in the journal MDPI-galaxies.

Cosmologists agree that the universe is expanding as we observe stars at increasing distances having larger redshifts, known as Hubble’s Law. Plotting the apparent recession speed against distance, the gradient of this line gives the Hubble constant. However, if we look at a large enough distance, we can see that the expansion rate doesn’t follow a straight line, but rather a curve, suggesting the value of Hubble’s constant has changed over time. 

In the local universe, measuring the Hubble constant (H0) from observations of nearby Cepheids and SNe Ia gives 74.03 ± 1.42 kms-1Mpc-1, which does not match values calculated for the early universe using Planck data of the Cosmic Microwave Background radiation (CMB) and the best fit lambda-cold-dark-matter model ΛCDM (67.4 ± 0.5 kms-1Mpc-1). This difference of 4 to 6 σ in the Hubble constant found between these different, independent methods is significant and known as “Hubble Constant Tension”. This tension raises important questions – is there something wrong with the observations? Or is there something wrong with our best fit cosmological model ΛCDM, despite it predicting so much data correctly?

Our team focused on observations of Type Ia supernovae (SNe 1a) and Baryon Acoustic Oscillations (BAOs) in the nearby universe. We use the Pantheon sample – a compilation of 1048 spectroscopically confirmed SNe Ia from different surveys – divided into three redshift bins of increasing value. We then consider a SNe 1a + BAO set, combining the SNe data with one BAO data point in each redshift bin (so the same number of BAOs are in different bins). Using this binned redshift analysis, we investigated whether H0 really remains constant over the redshift span range of the probes (SNE + BAO) considered. 

This research extends on the previous analysis published last year in The Astrophysical Journal. This considered only the variation of H0 in the SNe Ia Pantheon sample (for both the flat ΛCDM and w0waCDM models), but not any other cosmological parameter or any other probe.

In the current analysis, we extend this to vary two cosmological parameters with H0: the total matter density parameter (Ω0m) in the ΛCDM model, and the equation of state evolution parameter (wa) in the w0waCDM model, using both the SNe and SNe + BAO data. This can be seen in Figure 1. Including BAOs in the cosmological computations allows us to check if the trend of H0 in the previous research is still evident when we include other probes. This analysis is not intended to constrain Ω0m or any other cosmological parameter, but rather to investigate the reliability of the trend of H0 with redshift.

Two graphs show the hubble constant vs redshift, one for each model. Data is split into three redshift bins with the model fit plotted also.
Figure 1: The Hubble constant vs redshift (z) for the two different models. The red color shows the SNe 1a only probe, while the blue represents SNe + 1 BAO. The left panel shows the ΛCDM model and the right panel shows the w0waCDM model.

Our new study paves the way for understanding how combined probes (SNe + BAOs) still show the evolution of the H0 by redshift and how simulations on GRB cosmology are progressing to obtain Ω0m uncertainties comparable to SNe Ia results. Using GRBs, SNe Ia, and BAOs together as cosmological probes in the near future has proven to be not only feasible, but also necessary, since the redshift range that GRBs cover is much larger than that of SNe Ia. As a result of this characteristic, GRBs will certainly be able to give new insights into the nature of the early universe and pose new constraints on future measurements of H0.

Astrobite edited by: Emma Foxell

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