Join the dots

Jenny greene

Aeon

0
latest

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?

Join more than 270,000 newsletter subscribers

Our content is 100 per cent free and you can unsubscribe anytime.

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 moving gas, typically seen swirling around in the gravitational potential of a supermassive black hole. So, he thought, maybe the objects were not overly massive galaxies, but a new kind of growing black hole. Did I think that was interesting?

We had never seen growing black holes quite like this before

Labbé called me because I have spent my career trying to understand how supermassive black holes first formed, and how they grow alongside galaxies. As a black-hole hunter, there are a few tools that I commonly use. All rely on seeing emission from material as it falls into a growing black hole, dissipating its own energy and angular momentum, and forming an ‘accretion disk’. Typical accretion disks around supermassive black holes make copious light at high energy, in the UV to X-ray wavelengths. This distinguishes them from typical galaxies. The only problem is that the UV is easily absorbed by even a small amount of cosmic dust, so astronomers must also look for accreting black holes with X-ray emission to get a complete census.

In my hunting, I also look for very high-speed gas, moving under a black hole’s intense gravity. In normal galaxies, gas moves around at 100-300 km/s. But if the gas sits close to a black hole, it is common to see 2,000-4,000 km/s speeds. So when Dale Kocevski and colleagues published a pre-print paper with observations of high-velocity gas around little red dots, it was a big deal. It suggested that we were seeing light from a growing black hole rather than overly massive ‘Universe breaker’ galaxies. Still, we were puzzled. We had never seen growing black holes quite like this before.

Before the JWST launch, there was a different mystery around growing black holes: we were not finding enough of them. At later times, we typically find that roughly 10 per cent of galaxies show signs of black-hole growth in their nucleus, but that percentage seemed to drop to a fraction of a per cent within the first billion years of cosmic time. This is particularly perplexing because we believe that all supermassive black holes were formed as ‘seeds’ in the first billion years of cosmic time. A big goal for JWST was to find direct evidence of growing seed black holes.

Into this confusing picture came the little red dots. Unlike growing black holes known in this epoch, the little red dots were extremely common: three to four of every 100 galaxies looks like one. We proposed that they were likely to be black holes for two reasons. One is that they are very compact. A hallmark of growing black holes is that they are extremely dense, and so emission is naturally extremely compact, compared with stars. The second reason is that fast-moving gas that Labbé had called me about.

If we are right, and the rapidly moving gas that we ubiquitously observe in the little red dots is due to motions around a massive black hole, then the implications are enormous. It seems like we go from having too few black holes compared with galaxies, to suddenly having too many black holes! If indeed we could convince ourselves that the little red dots are signposts of growing black holes, then they have something important to teach us about the birth of supermassive black holes.

This field has been moving fast. Labbé called me to show me the fast-moving gas signature in December 2022. By June 2023, we had posted a pre-print of our first paper, together claiming that little red dots were most likely growing black holes, and around the same time my colleagues Jorryt Matthee, Rohan Naidu et al coined the term ‘little red dots’. But other researchers, including Pablo Pérez-González and Guillermo Barro were less sure. They suggested that the little red dots may be powered by star formation, with the red colour due to dust preferentially removing the blue light from the young forming stars and leaving only red light behind.

How could they all disappear just 1 billion years after the Big Bang?

Our idea linking the compact sizes and fast-moving gas to growing black holes was almost instantly challenged by two observations in 2024. A group of researchers at MIT led by Minghao Yue marginally detected a very weak signal from known little red dots with extremely deep X-ray data, which are usually a telltale hallmark of growing black holes. At the same time, Christina Williams and her team searched for another unique signature: emission from very hot dust that gets that hot only when it is near an actively growing black hole. Effectively, no little red dot shows this hot dust.

Maybe, then, the red colour in a little red dot is because the stars are old – older stars are red. Around this time, Josephine Baggen and colleagues proposed that the little red dots might be very compact and early forming galaxies – in other words, they went back to the overly massive galaxy hypothesis. Maybe the compact size means that the earliest galaxies formed an incredible number of stars in a very small volume, and these are literally the densest galaxies we have ever observed. Maybe the Universe breaker story was the right one after all?

Really, it still felt like we had the story wrong. If the little red dots are powered by 10- to 100-million-sun black holes, as we and others suggested, then, because they are so common, we would have gone from too few to too many supermassive black holes at early times. Furthermore, these black holes would make up a few per cent to most of the mass in the galaxy (in contrast to 0.1 per cent today). On the other hand, if powered by galaxies, these would be the densest galaxies known. How could they all disappear just 1 billion years after the Big Bang?

Two significant developments tipped the scales towards accreting black holes. First came the discovery of two extreme objects, the ‘Cliff’ and ‘MoM-BH*-1’. These two extreme little red dots made a very strong case that the shape of the spectrum really cannot be matched with light from any stars. Second, we went looking for evidence of emission from dust, but found none. If the red in little red dots is due to reddening, then we should be able to detect the dust in emission using telescopes like the Atacama Large Millimeter Array (ALMA) in Chile. However, when we got nice, deep ALMA observations of some of the brightest-known little red dots, we detected nothing. If there were a lot of stars there, it would require copious dust to explain the spectrum that we observe, and we rule out that dust.

But there are still problems. We still don’t have a complete explanation for the red colour if dust is not the culprit. Furthermore, there are so many little red dots that if we explain them as growing black holes with the mass of 100 million suns, then we make too much black-hole mass in the first billion years of cosmic time. Not only that, but the spatial distribution of little red dots relative to other galaxies suggests that they are really small galaxies living in small dark matter halos.

In recent months, a good fraction of the community has coalesced around the idea that there is an accreting black hole powering the little red dots, but that the black holes are growing so fast that a cocoon of dense gas forms around them. This picture can explain why we see a red colour because the gas is so thick that it absorbs the UV photons without the need for dust. A corollary of the cocoon picture is that the black holes are actually much wimpier – we had assumed a lot of power hidden by dust that just isn’t there. In that case, while we originally thought that the little red dots harboured 100-million-sun black holes, maybe they are much smaller, perhaps they even represent the end phases of seed formation.

We find ourselves at an interesting moment, after three intense years of continual surprises and right turns in our understanding. Do we have a working model? Are we transitioning from a raw discovery stage, where every new observation brings new insight, into a characterisation phase, where we fine-tune the model? This was a topic of intense debate among my collaborators as we crafted observing proposals for the next cycle with JWST. On the one hand, having a concrete set of expectations from a specific model allows us to propose sharp experiments. On the other hand, do we run the risk of missing important clues by closing our minds to a wider range of possible explanations? Also, have we brought the wider community along with us, or are many still sceptical that the little red dots are even powered by growing black holes?

It wouldn’t be the first time that such dilemmas have emerged in astronomy and physics – nor would it be the first time that we have taken wrong turns on the route to the truth. We have been reading papers from the early 1960s, when astronomers were trying to understand the nature of so-called quasi-stellar objects. We now know that they are powered by growing black holes, but in those early days many other possibilities were considered, including very compact star formation or exotic types of stars, quite similar to the possibilities that we are considering now to understand little red dots. In that instance, consensus built as more observations like variability supported the very compact nature of the power source. Ultimately, the story was fully settled in the 1990s when the Hubble Space Telescope provided compelling evidence for supermassive black holes from the motions of stars in the centres of nearby galaxies. Likewise, I suspect that our story will not be finished until we can determine the real mass of the little red dots, including the black hole if it is there.

The Universe is not broken, but it is a much more interesting place than we thought a few years ago

So, what can I tell my mother – is our understanding of the Universe broken? If the little red dots are all powered by extreme compact galaxies, then we have a problem: too many big galaxies and not enough dark matter halos to host them. If the little red dots are powered by enormous black holes, then black holes form too quickly and outpace their galaxies’ growth. My best guess today is that the little red dots are the seeds of today’s larger black holes, obscured by clouds of swirling gas that make them appear more powerful than they actually are. If they are small galaxies with small black holes, then they do not break the Universe because there are plenty of wimpy dark matter halos available to host them. We are not making too many black holes, but we are seeing the early stages of their growth.

What I tell my mother now, when she asks what is happening with the little red dots, is that the Universe is not broken, but it is a much more interesting place than we thought a few years ago. And, maybe, JWST is bringing us one step closer to taking the baby picture of a supermassive black hole that I have been dreaming about for my entire career.

That is my current best guess. But I have been wrong before.

We publish hard-won knowledge from real people who have grappled deeply with their subjects.

Your donation, whatever the size, supports our mission to ask the big questions and deliver fresh, original insights from leading thinkers.

If you value what we do, will you support us?

Your donation, whatever the size, supports our mission to ask the big questions and deliver fresh, original insights from leading thinkers.

If you value what we do, will you support us?

I would like to donate:

Select amount (US dollars):

The Universe’s biggest galaxies could hold the key to the birth of the cosmos. Why are these behemoths so hard to find?

The Big Bang’s big gaps

The current theory for the origin of the Universe is remarkably successful yet full of explanatory holes. Expect surprises

Physics as we know it is elegant and exquisitely accurate. It tells almost nothing about the deepest riddles of the Universe

essaySpace exploration

By showing us a new cosmos, the discoveries of the James Webb Space Telescope will ripple through our moral universe

Galactic position system

We can point to our home on a globe and find Earth in a model of the solar system but where are we in the Milky Way?

Our planet is a tiny porthole, looking over a cosmic sea. Can we learn what lies beyond our own horizons of perception?

Updates on everything new at Aeon.

Aeon is published by registered charity Aeon Media Group Ltd in association with Aeon America, a 501(c)(3) charity.

© Aeon

visit website

Popular

© Aeon