By Thiago Signorini Gonçalves
The challenge of a cosmic count
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The following article answers the question from Penélope Alves, 6 years old, from Bahia, who wants to become an astronomer, for the series “Children’s questions, answers from science”.
The question may seem trivial, but the answer is complex, if not unknown. Estimates vary between 200 billion and 2 trillion, and this high number justifies our ignorance.
How many galaxies have already been cataloged? How can you tell how many there are when there are hundreds of astronomical projects, each of which maps part of the sky? The Dark Energy Survey, one of the largest, an international collaboration with Brazilian participation, recently reported a total of 226 million.
However, they are often just a blur in an image. A computer algorithm suggests it is a galaxy, but identifying the stars – especially the less bright ones – is imprecise. What a galaxy is to one telescope could be a star to another.
If you take all this into account, add up the effort of the various projects and the possible duplicates, you can assume that we have a catalog of several billion cataloged galaxies. Far from the sum of maybe more than a trillion. Then how did we come to this value?
At this point we are subject to statistical estimates. Imagine a presidential campaign: we can’t ask every voter in the country who they want to vote, and polls rely on a sample of a few thousand to predict how tens of millions will perform in the election.
The definition of this example is essential. Voters in southeastern Brazil are likely to be different from those in northeastern Brazil: you can’t take the poll in a single state and extrapolate the result across the country. Likewise, we cannot count the galaxies in one region of the sky and assume that this number applies to the entire universe.
Then how do you do a census of the universe? One of the big problems is that the most numerous galaxies are the least luminous and therefore difficult to spot. The more powerful the telescope, the better we can observe a galaxy – but what is the limit of sensitivity of observatories? What are the smallest galaxies in the universe? In order to answer this, we have to know intrinsically the formation process of these systems, which in many ways is still a mystery.
Segue 2, about 100,000 light years away from us, illustrates this diversity. It radiates with an intensity that is only 800 times the brightness of the sun; In comparison, the Milky Way has the brightness of 100 billion stars. However, the total mass of Segue 2 is 500,000 times the mass of the Sun, which along with its faint glow indicates the presence of large amounts of dark matter that does not emit light. How did this galaxy come about? How many of them are there? These are unknowns that affect estimates of the total number of galaxies in the cosmos.
The subject becomes more complex when we think of distances that are considered great even for astronomers. We can see many types of neighboring galaxies, like Andromeda or the Magellanic Clouds, but billions of light years away we only see the brightest.
That wouldn’t be a problem if galaxies that far away were identical to our neighbors. However, the speed of light is finite. When a galaxy is very far away, it means that it took light a long time, even billions of years, to get here. We see the universe in its infancy.
Can we assume that galaxies in the past were the same as they are today? No way. To continue with the polling analogy, imagine if we were to survey the political opinion of voters in the 1960s about candidates for the 2022 elections! Political movements are constantly evolving, and we cannot admit that populations behave the same at such different times.
Likewise, the distant universe reflects a certain moment in the evolution of galaxies, and the billing must be calculated separately. How does it work when we only see the tip of the iceberg, only the brightest galaxies? More powerful telescopes will be able to respond precisely – hopefully the James Webb, due later this year, can help us.
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Thiago Gonçalves is an astronomer at Valongo Observatory / UFRJ and a science disseminator.
We know that children can ask the best questions, and science can have great answers. The “Children’s Questions, Answers from Science” series invites a scientist each month to answer one of these fundamental questions. Do you have a suggestion for a question or story for the blog? This is how you can work together.