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From a Physics Student: Cosmic Conundrum Revealed by the Hubble Tension

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Do you ever wonder about the universe? While that sounds philosophical and pretentious, Beijing City International School’s physics student, Vickey, will share her thoughts on The Hubble constant.

 

The Hubble constant has been the cornerstone in understanding the expanding universe. Georges Lemaître introduces the notion of a homogeneous universe with a growing radius, establishing a relationship between the distances of galaxies and their radial velocities [1]. Here is his equation:  

 



                       (1)

 

with v/c representing the ratio of the recessional velocity (v) of an object to light speed (c), quantifying the redshift of light due to the universe’s expansion. (R2 - R1)/R1 is the change in scale factor (R) between two different epochs, R2 and R1, relative to the initial scale factor R1, calculating universe’s expansion rate. By utilizing the period-luminosity relation of Cepheid variable stars to establish a linear relationship between the distances and radial velocities of receding galaxies, Edwin Hubble's groundbreaking work provided observational evidence in favor of the expanding universe theory [2].

 

Subsequent advancements were made by cosmologists and astronomers to refining Hubble’s initial measurement. Under the direction of Wendy Freedman and her colleagues, the Hubble Space Telescope (HST) Key Project started an amazing journey to determine the Hubble parameter with never-before-seen accuracy. Numerous investigations conducted after the HST Key Project have attempted to improve measurements of the Hubble constant. But in recent years, there has been an increasing amount of disagreement between measurements made using various approaches, which has motivated researchers to look further.

 

Using the HST to examine Cepheid variables in distant galaxies, measurements in the early 1990s showed substantial inconsistencies, but by 2000 [3], a precise finding of H0 = 72 ± 8 km sec−1 Mpc−1 was reached. A later recalibration of the measurement resulted in H0 = 74 ± 2 km sec−1 Mpc−1. These results point to a young expansion age of the universe relative to some of the oldest stars, which creates a Hubble tension coupled with the theoretical expectation of a high matter density with Ωm ∼ 1.

 

By examining the temperature and polarization fluctuations in the cosmic microwave background (CMB), the lower value of the measured Hubble constant H0 = 72 ± 8 km sec−1 Mpc−1 is found, which aids the calibration of parameters in the Lambda Cold Dark Matter (ΛCDM) model. Calibrated using CMB data, the lower measurements of H0 are acquired by the “inverse distance ladder”. The higher value of H0 measured, ranging from 70 to 75 km sec−1 Mpc−1 involving significant investment of the HST, encompasses the precise (within 5%) and recent measurements in the “late” universe (seen from Fig. 1) [4, 5].


 



Fig. 1 To get data up to the 5σ confidence level, the posterior was sampled using an extended MCMC method for H0 [4].

 

Significant progress has been made in the past 24 years in the realm of cosmology. However, we are currently facing another discrepancy regarding the Hubble constant, which is smaller in magnitude compared to previous tensions but highly significant, with an 8% deviation between H0 measurements and a confidence level >∼ 5σ.

 

Resolving the Hubble tension could lead to new insights into the fundamental physics governing our universe, potentially necessitating modifications to the standard cosmological model.

 

Comment down below on what more science content you want to see from Unit-E!


{.Credits }

Writing: Vickey Q

Editing: Helen W

Formatting: Vickey Q

Posted By: Margaret Y


 

 

 

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