The universe is twice as bright as previously thought, according to the Pluto probe New Horizons. A particle that has so far only been described theoretically could explain the anomaly.

The unexpectedly high brightness of the universe could be due to the decay of a previously only theoretically described elementary particle that is a candidate for dark matter. Two researchers and one researcher from the USA are now proposing this explanation for the anomaly detected by NASA’s New Horizons probe. In several measurements, the space probe had determined values ​​for the so-called cosmic optical background radiation (COB) that are more than twice as high as previously assumed, based on data from the Hubble space telescope, among other things. The new hypothesis should be able to be checked with instruments from New Horizons.

As the group led by José Luis Bernal from Johns Hopkins University recalls, the NASA probe actually sent to explore Pluto determined in two measurements at the edge of the solar system that the universe is apparently much brighter than expected. There, the observations are not falsified by the so-called zodiacal light, i.e. reflections and scattering of sunlight on dust in the interior of the solar system. Measurements of the cosmic optical background radiation have never been carried out in a darker place.

The stars we know are not enough

The determined value of 16.37 ± 1.47 nW m ⁻² sr ⁻¹ for the COB suggested that the galaxies known to us are responsible for only half of the background light in the Universe. A possible explanation that has been suggested so far is that there are many very faint, compact galaxies just below Hubble’s limit of perception. But they should be able to be found by the James Webb space telescope, it was said at the presentation. With the theory now presented, there is another hypothesis that should be able to be checked.

In a specialist article in the science magazine Physical Review Letters, Bernal’s group explains that the elementary particle called the axion, which has so far only been described in theory, could explain the values. For that, it would first have to exist and have a mass of 8 to 20 eV. In addition, they would have to decay into photons in a very specific ratio. The hypothesis and the determined values ​​could be checked with the ultraviolet spectrometer Alice on board New Horizons, they write. Further measurements are already planned. The axion is considered a candidate for dark matter, which has not yet been experimentally confirmed.