Counting gamma-ray photons from extragalactic sources sets the Hubble constant

Cosmology has been recently challenged by two strongly discrepant measurements of the Hubble constant. Using alternative methods will be essential to figure out what is behind this crisis. New insights into the problem may come from counting gamma-ray photons emitted by distant sources.

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Measuring distances has been a fundamental means of exploring vast space around us for ages. Although techniques, scales and precision have changed dramatically over the last centuries, we are arguably equally driven by the notion of charting our universe as our ancient ancestors. Distance measurements have changed completely our perception of the universe and position of humanity therein. It made us realize that we live on a tiny planet revolving around its host star that rotates around the centre of our own galaxy, which in turn falls towards a massive galaxy cluster. The progression of scales is arguably still mind blowing for many of us, but this is not the end of the story.
 
Distances play a particularly important role in cosmology. Since light propagates with a constant speed in an expanding space, measurements of distances to galaxies tell us about the speed of cosmic expansion at different stages of constantly growing size of the universe. It is this kind of laborious measurements at largest scales that led to Nobel-Prize-awarded discovery of the accelerating expansion of the universe.

Although the biggest mystery of modern cosmology remains undoubtedly in the question about the physical mechanism of cosmic acceleration (what is dark energy?), recent progress in precise calibration of cosmological distances posed a new surprising puzzle. Two different methodologies of measuring absolute distance scales result in two strongly discrepant values of the Hubble constant (the current expansion speed), the problem dubbed as the Hubble constant tension. The first methodology employs a succession of techniques propagating distance determination from our local neighborhood in the Milky Way to larger scales, the so-called cosmic distance ladder. The second uses a natural yardstick whose length is known precisely from physics describing propagation of sound waves in the primordial plasma before the cosmic microwave background (CMB) radiation decoupled from matter. Here, the distance is determined by measuring the angular size of the yardstick imprinted in the map of the CMB temperature fluctuations. The resulting Hubble constant turns out to be lower (67 km/s/Mpc) than its value inferred from the cosmic distance ladder (74 km/s/Mpc).

Although enormous effort has been made to exclude many potential systematic effects behind the Hubble constant tension, it is not clear whether the problem is caused by a much more elusive errors or both measurements unveil a new feature of the cosmic reality. Since the measurement based on the CMB yardstick depends on cosmological model (describing time evolution of the expansion speed as well as physics of sound waves in the primordial plasma), the Hubble constant tension can actually indicate the first anomaly of a well-established and extensively tested standard cosmological model. Many cosmologists attempt to find possible modifications to the standard framework which could resolve the Hubble constant tension. All modifications involve quite exotic physics and none give a fully a consistent description of all cosmological observations.

A reasonable strategy to approach the Hubble constant problem is to explore alternative methods of distance calibration. Possibilities are quite limited though. A rare example of a fully independent distance calibration involves a process of interaction between gamma rays and low energy photons transforming both photons into pair of electron and positron. In cosmic space filled with an ocean of low-energy photons of extragalactic background light, the mechanism of photon-photon interaction results in a well observed attenuation of gamma-ray radiation emitted from extragalactic sources, in particular blazars (active galaxies that happen to point their jets towards Earth). The attenuation is directly proportional to distance of the source and the absolute calibration is set by the number density of extragalactic background photons (with fairly precise models based on stellar evolution computations combined with cosmological census of galaxies) and coefficients measured from particle physics experiments.

Gamma-ray attenuation was measured by FERMI-LAT (NASA's Space Gamma-ray Telescope) and ground-based Cerenkov telescopes (observing gamma rays via Cerenkov radiation) for hundreds of extragalactic sources after long observational campaigns. These exquisite data, when analyzed assuming a standard cosmological model, lead to a Hubble constant which appears to be more consistent with its low-value counterpart from the CMB-based calibration (although the preference level is quite low due to relatively large measurement error). Further work is required to improve the measurement so that it would provide more decisive conclusions. However, it is already quite remarkable at this moment that this totally independent method of distance calibration, which is rooted more in particles physics rather than in astrophysics, favors only one of the two current benchmark values of the Hubble constant.

Radek Wojtak

Assistant Professor, DARK, University of Copenhagen

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