Galaxy distribution

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Mike Herman
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Galaxy distribution

Postby Mike Herman » Sun Feb 05, 2017 6:12 am

I haven't been able to find an answer, so need some guidance.

As distance from the observer increases, the surface area of a sphere around the observer increases. A smooth distribution of objects would mean that a count of those objects would increase as the square of their distance. Thus my question about galaxies:
As the distance we look into space increases, does the number of galaxies seen at a given distance go up because of that effect, or go down because we are also looking back in time and the Universe was smaller, or does the number stay the same as each effect cancels the other?
This is another way of saying that I have difficulty correlating that increasing area as the observing radius goes up with the idea of looking at a smaller and smaller Universe.

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Re: Galaxy distribution

Postby Ann » Sun Feb 05, 2017 6:52 am

Because I'm terrible at math, I'm the wrong person to answer you about math, but I should be able to say something in more general terms.

Hubble Ultra Deep Filed.
NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI),
R. Windhorst (Arizona State University), and Z. Levay (STScI)
Take a look at this picture of the Hubble Ultra Deep Field. The Hubble Telescope was made to stare at a spot in the constellation Fornax repeatedly for several months. This small field contains about 10,000 galaxies, and as you can see, the small galaxies easily outnumber that larger ones. According to the Wikipedia text, the Hubble Ultra Deep Field has a lookback time of about 13 billion years, meaning it should be able to spot some galaxies that existed some 400 to 800 million years after the Big Bang.

We clearly see more distant galaxies than nearby ones in the picture. We can also imagine that the Hubble Ultra Deep Field (HUDF) is just one tiny piece of a sphere with ourselves in the middle. According to Wikipedia, the size of the Hubble Ultra Deep Field is one thirteen-millionth of the total area of the sky. To get the total number of galaxies that might potentially be available for observation using the same method that was used for the Hubble Ultra Deep Field, you would, or so I think, need to multiply the number of galaxies in the HUDF, 10,000, by 13 million. If my calculations are correct, we should, theoretically, be able to see 130 billion galaxies in the sky.

(In reality, there are several parts of the sky where we can't search for distant galaxies at all, because our view is blocked there by thick dust.)

But the number of galaxies has not always been the same, and the distance between the galaxies most certainly hasn't been the same, either. Just after the Big Bang, galaxies must have been rare - indeed, galaxies simply can't have formed right after the Big Bang. It would seem that the first galaxies formed very soon, however, because it was announced last year that Hubble had found a galaxy that existed only 400 million years after the Big Bang.

In any case, so soon after the Big Bang galaxies must have been rare. Then, presumably, galaxy formation increased very rapidly. It seems likely that the sheer number of galaxies in the universe reached a peak very early in the history of the universe, perhaps when our universe was - I'm just guessing - 3 or 4 billion years old? The reason why galaxy formation increased so rapidly is partly that star formation kept increasing - star formation in the universe peaked when the Universe was about 3.9 billion years old, according to this article - but also because galaxy formation, which is not equivalent to star formation, increased because of of increasing structure and increasing "lumpiness" of the universe. We must assume that the incredible but not perfect smoothness of the cosmic microwave background, emitted about 400,000 years after the Big Bang, later turned into the "peaks and valleys" of galaxies and voids that we see in the universe today.

So as structure increased and star formation increased rapidly, we must assume that galaxy formation also became very vigorous. So why would the number of galaxies have reached a peak and then decreased? The reason why the number of galaxies must have gone down since its peak billions of years ago is because many galaxies have merged, so that two or more small galaxies have merged into one bigger galaxy. This is a process that must have been going on in the universe for billions of years, thereby reducing the number of independent galaxies.

The universe was also much smaller when it was young, so there must have been a time when the sky was pretty much crammed with small, often brilliantly starforming galaxies. Since then, galaxies have merged and the universe has expanded, taking the galaxies with it and making them much more widely separated.

That is about as much as I think I can say about this subject.

Last edited by Ann on Sun Feb 05, 2017 6:39 pm, edited 1 time in total.
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Re: Galaxy distribution

Postby rstevenson » Sun Feb 05, 2017 1:49 pm

To add a thought to Ann's excellent description above, for Mike's contention to be true -- that "a count of those objects would increase as the square of their distance" -- the universe would need to be both static in size and uniform in its distribution of galaxies. And as Ann has pointed out, it has been neither.


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