Peering into the origins of our Universe, astronomers found something that shouldn’t be there: what are those little red dots?

by Jenny Greene  BIO

Photo courtesy Jenny Greene/NASA/James Webb Space Telescope

is a researcher and professor of astrophysics. Currently the Eugene Higgins Professor of Astrophysical Science at Princeton University in New Jersey, she and her collaborators publish numerous articles about the evolution of galaxies and supermassive black holes with cosmic time. She also teaches algebra and astronomy in New Jersey prisons.

Edited byRichard Fisher

If you’re interested in the themes of this Essay, come along to our event in London on 21 April where we’ll explore the twisted hearts of black holes and the torrid birth of the early Universe.

A few years ago, my mother called me up to ask whether the Universe was broken. She had read an article about some puzzling observations of some very massive galaxies, shortly after the Big Bang.

My mother, a retired PhD biologist, keeps tabs on my public talks about my work searching for supermassive black holes. However, usually when she sends me articles, they are about political events or children’s book authors. So I knew that these findings had broken through in a different way.

The James Webb Space Telescope (JWST) had observed the early Universe, and taken baby pictures we didn’t expect to see. Only a few hundred million years after the Big Bang, astronomers had found a shockingly high number of massive galaxies – with a similar number of stars as the Milky Way has today. They would come to be nicknamed ‘Universe breakers’, because our models didn’t anticipate so many, so early. Galaxies need dark matter to hold them together, but if dark matter behaves as we think, then there would not have been enough dark matter halos for all the massive galaxies to live in. Only a few months into observations with our brand-new telescope, and it seemed everything we thought we knew about the nature of the Universe was (maybe) called into question.

I laughed. Our understanding of the Universe is fine, I told my mother, and this is just astronomers getting ahead of themselves. The JWST is a new telescope, and something is making those galaxies look more massive than they are. Before we go breaking the Universe, we should look for a simpler explanation. When I had first heard about the galaxies, I assumed they were some kind of telescope calibration error. That turned out to be utterly wrong, like most of my guesses so far.

The truth is, we’re still not 100 per cent sure what’s going on. Since then, my colleagues and I have named these puzzling early galaxies ‘little red dots’, so-called because they’re very compact and luminous sources – and most of their light arrives at wavelengths that our eye perceives as red. They are also unlike any galaxy that we have seen before. Every time we think we understand what we’re observing, a new insight creates additional confusion. So, do the little red dots really break the Universe, or is there another explanation?

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The first billion years of cosmic history was a very active time in the growth of galaxies. Astronomers have found galaxies as early as 300 million years after the Big Bang, and in this epoch we are actively searching for signs of pristine metal-free stars, and signs of the formation of the first seed black holes. However, because such limited time had passed since the Big Bang, most of the galaxies seen to date were pretty small, with many fewer stars than the Milky Way. So when the astrophysicist Ivo Labbé and colleagues found that the young Universe may actually have been populated by red massive galaxies, it created quite a stir in both scientific circles and the media.

Not long after my mother read about these mysterious massive galaxies, Labbé also called me. He now had more information about one of the objects; he had learnt that they feature very rapidly........

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