Comment by geuis | Hacker News Reader

Comment by geuis | Hacker News Reader

How many galaxies are in the observable universe?

Quick numbers check:

Author estimates over 200k galaxies in the image. Image covers 0.004% of the sky.

200000/0.00004 = 5,000,000,000 galaxies. (edited, thanks to % help from commenters below)

Finding somewhat standardized values for this rather standard question is frustratingly vague. Estimates range from a few billion to a trillion, though the most frequently quoted number I could find was about 100 billion.

The standard for measurement in the last 10 years or so have been the Hubble Deep Field Survey images. In the last one done, there were about 10k galaxies shown. The HDFS had a square arcminute resolution of 11.5. It would require 12,913,983 HDFS-type images to cover the entire side. (ref: http://curious.astro.cornell.edu/question.php?number=720)

Using these numbers: 12913983 * 10000 = 129,139,830,000 galaxies.

So why the vast difference between the two numbers? My supposition is the new ESO image isn't as deep as the HDFS.

I would argue the larger number, ~129.134 billion galaxies is a bit more accurate.

Damn, 129 billion galaxies. Someone else has to be out there.

Most probably. THe universe is Big. But it also is Old.

Most probably someone else was or will be out there. Homo Sapiens Sapiens has a currently very short history (~200k years ?). History is 6000 years old (China?). The universe is 13 billion years Old.

Maybe we're late to the party. Or Early.

Galaxies are not evenly distributed. But for all I know this is an unusually sparsely populated part of the sky. Still, your number could shift in either direction by quite a lot.

That is an excellent point. To further refine the estimate we need measurements of the number of galactic clusters and their various densities. For example, in the Local Group (Milky Way, Andromeda, etc) there are about 50. The Virgo Supercluster contains over 2500. Getting those estimates is beyond me at the moment.

Galaxies are strung out across the universe along filaments, with vast voids in between. The voids are mostly empty and very cold, but recent observations have found hot spots in some up to millions of degrees. Overall, the universe has a soapy appearance, i.e. lots of space with the "stuff" strung together.

One thing to note is that, at least to my knowledge, there are galaxies in all directions. There may be voids along the way, but you'll always see them closer or further away.

One of the reasons that this ESO deep field view is important is that its bigger than the original Hubble Deep Field, so we can start answering questions about density and distribution.

The author's calculation seems to be off, 1.2 x 1.5 degrees is 1/72,000 of the total sky area, or roughly 0.0014% (not 0.004%). That still leaves an order of magnitude to be accounted for, but considering the ballpark nature of the 200k estimate and also this being a "dark" spot it's not very surprising.

When chucking those numbers through the drake equation it looks pretty likely. I think it comes down to a matter of time, both for searching and because I believe we have entered a very dangerous period for a species where technology can either be our savior or cause us to destroy ourselves.

I think your calculated number needs to be multiplied by 100, if I'm not mistaken.

Can you reference which sets of numbers? I made reference to a couple and want to make sure I understand.

0.004% is 0.00004, not 0.004, so the result is 50 billion galaxies.

I don't think that's right. The author clearly states its 0.004% of the sky, not 0.00004%. Simply adding a percent sign doesn't change the number.

Yes it does, actually: "per cent" means "per 100", so, divide by 100.

50% is 50/100, or 0.5 of the whole

0.004% of the sky is 0.004/100, or 0.00004 of the whole sky.

Yes, and 0.002 cents == $0.002.

It's right. 0.004% == 0.00004

Just like 100% is equal to 1. If the image were 100% of the sky, you would divide by 1, not 100.

Geuis:

.004 / 100 == .00004 / 1

And

.004% = .00004

100 / 0.004 = 25,000

So we'd need 25,000 images like that one, to fill the sky.

25,000 * 200,000 = 5,000,000,000 galaxies

Also mentioned in the article:

"we’re seeing them with light that left them shortly after the Universe itself formed."

That means, there could be a lot more, but their light has not travelled to earth yet.

What are the odds of intelligent life NOT existing in such a space?

The odds are incalculable, see previous discussion here:

http://news.ycombinator.com/item?id=3662890

and if the multiverse ends up being true that number might be slightly larger, and by slightly I mean unimaginably larger, I think it's called infinity.

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