The most important number in all of cosmology --the Hubble constant -- which describes the current rate of cosmic expansion, is causing extreme astronomical anxiety. The Hubble constant allows us to measure the age of the universe and predict its future behavior. But something is wrong and disagreement about its value is violent!

Why are the values measured by astronomers so different from those measured by cosmologists and what does it mean for our universe? There are two ways to measure the Hubble constant: one from the early universe using the stellar distance ladder and one from the Cosmic Microwave Background (CMB).

Using CMB data, the Planck team says that the universe should expand at a rate of 67.4 kilometers per second per megaparsec. Galaxies should be receding 67 kilometers per second faster for each megaparsec you look out -- just as two raisins in an expanding raisin bread separate faster the farther apart they are.

Measurements of similar features called “baryon acoustic oscillations” yield exactly the same prediction as the CMB team found: Ho = 67. But observations of the actual universe by past guest Adam Riess’s team have suggested for many years that the prediction is off.

Cosmologists calling themselves H0LiCOW published measurements of the universe’s expansion rate using six distant quasars, H0LiCOW saying H0 was 73.3 kilometers per second per megaparsec — much higher than Planck’s CMB prediction. What's crucial is how close H0LiCOW’s measurements were compared to the SH0ES collaboration— the team led by Riess. SH0ES measures cosmic expansion using a “cosmic distance ladder,” a stepwise method of gauging cosmological distances. SH0ES’ latest measurements pinpointed H0 at 74.0, well within H0LiCOW’s error bars.

How can these results be so far off? Maybe the key to resolving this tension, relaxing thousands of tense astronomers lies in something so simple you probably own dozens of them: magnets. But to solve the problem and cure the tension we need a mechanism to make magnets at very early times.
Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, have never been detected. A decade old paper reports a lower bound on the magnetic field above 3 × 10−16 gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by teraelectron volt gamma-rays in the intergalactic medium.

New constraints on the magnetization of the cosmic web using LOFAR Faraday rotation observations (available here https://arxiv.org/abs/2002.06924) show "magnetization of the large scale structure by astrophysical processes such as galactic and AGN outflows, and primordial scenarios with seed magnetic field strengths less than 0.5 nG cannot be rejected by the current data; while stronger primordial fields or models with dynamo amplification in filaments are disfavoured."

Primordial magnetism might help resolve the Hubble tension — probably the hottest topic in cosmology -- by providing a scenario for the universe to possess stress-energy at early times that is not significant at late times, measured by e.g., SHOES. Cosmologists Karsten Jedamzik & Levon Pogosian argue that weak magnetic fields in the early universe would lead to the faster cosmic expansion rate seen today. Karsten along with Tom Abel, pointed out in 2011 the effect of PMF on recombination. With Levon Pogosian, Karsten showed how PMF help recombination complete sooner, so that CMB is produced at an earlier time and with a smaller sound horizon imprinted in the peaks.

References:
NASA Fermi
Science https://www.quantamagazine.org/the-hidden-magnetic-universe-begins-to-come-into-view-20200702
A new lower bound constrains models for the origin of cosmic magnetic fields. https://www.science.org/doi/10.1126/science.1184192


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