How many galaxies are there in the universe? – basic sciences

By Thiago Signorini Gonçalves

The challenge of conducting a census of the universe

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The article below answers the question posed by Penelope Alves, 6, of Bahia, who wants to become an astronomer, for the “Questions for Kids, Answers from Science” series.

The question may seem trivial, but the answer is complex if not anonymous. Estimates range from 200 billion to 2 trillion, and this high figure justifies our ignorance.

To start with, how many galaxies are actually cataloged? How can a number be determined, if there are hundreds of astronomical projects, each of which maps a part of the sky? The Dark Energy Survey, one of the largest, an international collaboration with Brazilian participation, announced a total of 226 million.

However, it is often just a blur in the image. A computer algorithm suggests it’s a galaxy, but identifying stars – especially the least bright – is inaccurate. For a telescope what is a galaxy, it may be a star to another telescope.

Taking all this into account, adding the efforts of the various projects and potential iterations, we can assume that we have an index of a few billion cataloged galaxies. Far from a total of perhaps over a trillion. How, then, did we arrive at 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 intend to vote for, and opinion polls rely on a sample of a few thousand to predict how tens of millions will behave at the polls.

Identification of this sample is necessary. Voters in southeast Brazil are likely to be different from those in the northeast: you can’t do the poll in one state and show the result for the whole country. Similarly, we cannot count galaxies in a region of the sky and assume that this number applies to the entire universe.

How, then, do you make a census of the universe? One of the biggest problems is that the more numerous galaxies are the less bright, and therefore more difficult to detect. The more powerful the telescope, the better we can observe a galaxy – but how sensitive are the observatories? What are the smallest galaxies in the universe? To answer, we must know intrinsically the process of formation of these systems, which remains a mystery in many respects.

Figure 2, about 100,000 light-years away from us, shows this diversity. shines with a power of no more than eight hundred times the brightness of the sun; By comparison, the Milky Way galaxy has a brightness of 100 billion stars. However, the total mass of Section 2 is 500,000 times the mass of the Sun, which, along with its faint glow, indicates the presence of huge amounts of dark matter, which does not emit light. How was this galaxy formed? How many like him are there? These unknowns affect estimates of the total number of galaxies in the universe.

The matter becomes more complicated if we consider the distances, which are considered large even for astronomers. We can see many types of neighboring galaxies, such as Andromeda or Magellanic Clouds, but billions of light-years away we see only the brightest galaxies.

This wouldn’t be a problem if the distant galaxies were identical to our neighbors. But the speed of light is limited. If the galaxy is very far away, that means that the light took a long time, billions of years, to get here. We see the universe in its infancy.

Can we assume that galaxies in the past were the same today? No way. Continuing the polling analogy, imagine if we asked the political opinion of voters in the 1960s about the candidates for the 2022 elections! Political movements are constantly evolving, and we cannot admit that the population behaves identically at such different times.

Likewise, the distant universe reflects a certain moment in the evolution of galaxies, and the account must be calculated separately. How do we do this if we only see the tip of the iceberg, only the brightest galaxies? More powerful telescopes will be able to respond accurately – and we hope James Webb, due out later this year, will help us.

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Thiago Gonçalves is an astronomer at the Valongo Observatory/UFRJ and a science publisher.

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