A Universe of Two Trillion Galaxies

I just saw a press-release that describes a paper, just out, authored by Chris Conselice et al from the University of Nottingham (in the Midlands), with this here abstract:


The key conclusion of this paper is that when the universe was only a few billion years old there were about ten times as many galaxies in a given volume of space as there are within a similar volume today, but most of these galaxies were much lower mass systems than, e.g., the Milky Way. In fact their masses are similar to those of the satellite galaxies surrounding the Milky Way. These objects are numerous but so faint that even in very deep surveys with very big telescopes they are very easy to miss.

Here’s an image from a deep survey: this is from the Hubble Space Telescoper Great Observatories Deep Survey (HST-GOODS).


You can click on this to make it larger if you wish. This is typical of a “pencil beam” survey. It opens a very small window on the heavens – about a millionth of its total area of the sky – in a direction chosen to avoid having too many bright stars from our own Galaxy getting in the way. When you look at such a patch with a big telescope for a long time, what you see is basically all galaxies. The few stars in the above image can be identified by the diffraction patterns they produce, but almost every fuzzy blob in the picture is a galaxy. It looks like there are a lot of galaxies in this image, but the real number seems to be substantially higher than we thought.

When I’ve given popular talks about this kind of thing I’ve always said something like “There are at least as many galaxies in the observable Universe as there are stars in our own Galaxy”. It turns out that I was wise to include the “at least as”. There are about 100 billion (1011) stars in the Milky Way, but the latest estimate is now that there are two trillion (2 ×1012) galaxies in the observable Universe. I quote Douglas Adams:

“The Universe, as has been observed before, is an unsettlingly big place, a fact which for the sake of a quiet life most people tend to ignore. Many would happily move to somewhere rather smaller of their own devising, and this is what most beings in fact do.

I believe this explains a lot about modern politics.


8 Responses to “A Universe of Two Trillion Galaxies”

  1. Anton Garrett Says:

    Actually, we prefer to live in a place where the laws are more congenial. That’s just the anthropic principle!

  2. Actually, the universe is small. This is demonstrated in the AIP interview with Maarten Schmidt, conducted by Alan Lightman.

    Lightman: “If you could have designed the universe any way that you wanted to, how would you do it?”

    (This is a written record of an oral interview. Perhaps automatically produced. Returning to a recent theme, sometimes the interviewer is written as “Light man”. At least not “Light-man”.)

    After trying to dodge the question, a bit later Schmidt says: “Well, briefly, as one of the perpetrators of looking at the distant universe, I find it extraordinary that it is possible with human means, with pieces of glass that are no larger than this room, to see things that are interestingly far out in the universe. Sometimes it strikes me that the universe is much smaller than… All right, here we go. I would have constructed a bigger universe. I think the universe is small. There we go. If I’d had my rathers, I would do that. I find the universe too confined. I find it amazing it’s so small.” [my emphasis]

    Here’s another tidbit from Schmidt: “Anecdotally — and I’m not sure whether it was right — I heard that when it was clear that the turnover of the counts of radio sources really had been understood well by people in radio astronomy, [Martin] Ryle had expressed that [result] was almost disappointing. Because here was the end of the universe. And I share that. I mean you’d almost feel claustrophobic in a universe that is [so] small [that when] you just look at one of the first results in radio astronomy, you already see the effect of the end of the universe.”

    The entire exchange on this topic is worth reading, as is the entire interview (and for that matter all of the AIP interviews): https://www.aip.org/history-programs/niels-bohr-library/oral-histories/33967

  3. I think most of us would prefer not to rely on extrapolations to make these sorts of claims… and if you actually count what you can see (and make allowance for the fact that some of the sources are components in a single “galaxy”) – then I am not sure you need to revise the claims in your talks.

    • telescoper Says:

      Yes, this paper does extrapolate the luminosity function defined by brighter objects to “correct” the total for the ones too faint to be observed. I think that’s a reasonable approach to take. Your second point is I think very important. I don’t really see how it’s possible to decide when two or more patches of starlight are actually separate galaxies or not when they are very messy structures. For that matter, what is a galaxy anyway?

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