December 6, 2021

I, Science

The science magazine of Imperial College

Andrew McMahon discusses new experimental results that suggest half of the stars in the Universe may be running rogue outside of galaxies.

galaxy newResults from the Cosmic Infrared Background Experiment (CIBER), the product of an international collaboration that includes the University of California, Irvine and the California Institute of Technology, suggest that over half of the stars in the universe may not belong to galaxies. The results were recently published in the journal Science and they may mean that astronomers have to rethink some of their basic ideas about the structure of our universe.

The experiment studied fluctuations in the Extragalactic Background Light (EBL), which contains all the radiation in the universe in the infrared to ultraviolet range. This radiation comes mainly from star forming processes and is a major contribution to the energy content of the universe. The EBL is the second most energetic element of diffuse background radiation, the first being the well-known Cosmic Microwave Background Radiation (CMBR).

The CIBER experiment looked at several different regions of the sky in order to study the properties of the infrared component of the EBL, which was hoped would contain some clues as to the how some of the Universe’s earliest galaxies formed. Since the universe is expanding, any radiation that travels through it becomes effectively ‘stretched out’, making it redder as it travels farther and farther. The reddest light therefore has travelled the most distance and so comes from the earliest times in the universe. This is why the CIBER scientists wanted to study the properties of the infrared part of the spectrum.

The data collected by CIBER however found this part of the EBL to be less red than expected, suggesting that it was coming from far more modern stars. When the collected data was used to make predictions as to the amount of such light in the entire universe, the researchers stumbled across something interesting. It turns out that the amount of light present in the universe is approximately double what would be expected to be coming from the known distribution of galaxies. This means that around half of the stars in the universe may actually be sitting in intergalactic space.

It is expected that these rogue stars were mostly thrown out of their parent galaxies as the result of collisions between their parent galaxies, or from near misses where large gravitational interactions caused them to be ripped out into intergalactic space.

It remains to be seen whether these results can be verified by other experiments, but if the results are proven correct then in future we will have to think very differently about the large-scale structure of the universe.

Image reproduced from Wikimedia Commons