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Hubble Tension

Why the fuss over a couple of km/s/Mpc?

You have probably heard that the universe is expanding, and perhaps even that this expansion is accelerating. A consequent observation of this is that distant objects such as galaxies appear to recede from Earth faster if they are further away.

Here is a helpful analogy: imagine a loaf of raisin bread that is rising as it is baked. A pair of raisins on opposite sides of the loaf will move away from one another at a greater rate than a pair of raisins near the center. The more dough (universe) there is between a pair of raisins (galaxies), the faster they recede from one another. See Figure 1.


This phenomenon is encapsulated in Hubble’s Law, which relates specifically to the recessional velocity due to the expansion of space.


Hubble’s Law is given by the equation v = H0 D.


Where:

v is the recessional velocity

D is the distance to the receding object

H0 is the Hubble constant


It is worth noting that distant objects will often have velocities of their own due to gravitational forces - so-called ‘peculiar velocities’.


In order to clarify the meaning of the title of this article, we must explore the unit in which the Hubble constant H0 is most often quoted: km/s/Mpc. This describes the speed (in kilometers per second) at which a distant object, such as a galaxy, is receding for every megaparsec of distance that galaxy is from Earth.


Edwin Hubble is the name most often associated with this cosmological paradigm shift; however, physicists Alexander Friedmann and Georges Lemaître worked independently on the notion of an expanding universe, deriving similar results before Hubble verified them experimentally in 1929 at the Mount Wilson Observatory, California.


What is the Hubble Tension?


Hopefully the above discussion of units and raisin bread convinced you that the Hubble constant H0 is linked to the expansion rate of the universe. The larger H0 is, the faster galaxies are receding at a given distance, thus indicating a more quickly expanding universe. Therefore, cosmologists wish to accurately measure H0 in order to draw conclusions about the age and size of the universe. The Hubble Tension arises from the contradicting measurements of H0 obtained from different experiments. See Figure 2 of Edwin Hubble.


CMB measurement


One of these experiments uses the Cosmic Microwave Background (CMB), which can be thought of as an afterglow of light from near the time of the Big Bang. The wavelength of this light has expanded with the universe ever since the period of recombination - which I mentioned in my previous article on the DESI instrument. Our current best model of the universe, called ΛCDM, can describe how the universe evolved from a hot, dense state to the universe we see today, subject to a specifically balanced energy budget between ordinary matter, dark matter, and dark energy. From fitting this ΛCDM model to CMB data from missions such as ESA’s Planck Mission, one can derive a value for the expansion rate of the universe, i.e., a value for H0. The Planck Mission measured temperature variations (anisotropies) across the CMB with unprecedented angular resolution and sensitivity.


The most recent estimate for the Hubble constant using this method gave H0 = 67.4 ± 0.5 km/s/Mpc.


Local Distance Ladder measurement


Another technique to determine the value of H0 uses the distance-redshift relation. This is a wholly observational approach. It relies on the fact that the faster an object recedes from Earth, the more the light from that object is shifted towards longer wavelengths (redshifted). Hubble’s Law relates this recessional velocity to a distance; therefore, one can expect a similar relation between distance and redshift. A ‘ladder’ is invoked since astronomers wish to use objects that are visible from a vast range of distances; the rungs of the ladder represent greater and greater distances to the astronomical light source. Each rung of the ladder contains a different kind of ‘standard candle’, which are sources with reliable, well-constrained luminosities that translate to an accurate distance from Earth. I encourage you to look into these different types; some examples are Cepheid variables, Type Ia Supernovae, and RR Lyrae variables.


When this method was employed using the Hubble Space Telescope and SH0ES (Supernova H0 for the Equation of State), a value of H0 = 73.04 ± 1.04 km/s/Mpc was obtained.


The disagreement


Clearly, these two values for the Hubble constant do not agree, nor do their uncertainty ranges overlap. Figure 3 shows some of the 21st-century measurements of H0; an excellent illustration of how the uncertainty has decreased for both methods, therefore making their disagreement more statistically significant.


Many sources of scientific engagement with the public cite this disagreement as the ‘Crisis in Cosmology!’. In the author’s opinion, this is unnecessarily hyperbolic and plays on the human instinct to pick a side between two opposing viewpoints. In fact, new methods to measure H0 have been implemented using the tip of the Red-Giant branch (TRGB) as a standard candle, which demonstrate closer agreement with the value derived from the CMB. Some cosmologists believe that eventually this Hubble Tension will dissipate as our calibration of astronomical distances improves with the next generation of telescopes.


Constraining the value of the Hubble constant is by no means low-hanging fruit for cosmologists, nor is the field in crisis. To see the progress we have made, one has to look back in time to 1929 when Edwin Hubble’s first estimate using a trend line and 46 galaxies gave H0 = 500 km/s/Mpc!


We must remain hopeful that the future holds a consistent approximation for the expansion rate and, with it, the age of our universe.



Written by Joseph Brennan

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