# I Number of galaxies in the Universe

1. Aug 23, 2017

### davidge

This may sound like a noob question, even more if it's asked by a undergraduate physics student, but here it's:

How do we know about the existence of 100 billion galaxies out there (even that it's an average value)? I mean, how do we know about these exact number? Why not, say, 200 billion or 50 billion?

Is there a concrete way to obtain the value out of the Relativity Theory? If not, what theory should we use to get 100 billion?

2. Aug 23, 2017

### phinds

I'm not aware of any "theory" that arrives at that number, it's just an estimate based on empirical evidence, much like how a crowd of 1,000,000+ is estimated. You cut out a small average section, count it carefully, and extrapolate to the total area (volume in the case of galaxies). I've seen recent estimates that dramatically up the size of the estimate for the observable universe (which, by the way, is what you are talking about, NOT "the universe", which may be infinite in extent and in any case is WAY bigger than the observable universe) so the estimate is apparently not very precise.

3. Aug 23, 2017

### phyzguy

As phinds said, it is just an estimate. Where did you get the idea that it was an exact number? I would characterize it as an order-of-magnitude estimate, meaning that the number of galaxies in the observable universe is probably somewhere between 10^10 and 10^12. There's also some ambiguity in just how large an assembly of stars needs to be in order to be counted as a galaxy.

4. Aug 23, 2017

### davidge

I mean a exact value on an average estimate.
How do you get that?

5. Aug 23, 2017

### phyzguy

What does an "exact value on an average estimate" mean? If it's an estimate, how can it be exact? If you asked for an estimate of the number of grains of sand on Waikiki beach, would you expect an "exact" value?
I was just explaining what an "order of magnitude estimate" of 10^11 galaxies means. As phinds explained, the actual estimate is obtained by counting galaxies in some small region of space, then extrapolating to the size of the observable universe.

6. Aug 23, 2017

### davidge

Hmm, ok.

But why is the result completely empirical? It seems a bit bizarre that we can't get the value from any of our theories. It's as if we were 10,000 years in the past, on a time when people knew everything only through observations.

7. Aug 23, 2017

### phinds

Well, YOU come up with a theory that explains it. I see no reason why there should be a theory that explains it any more than a theory that explains how many grains of sand there are on planet Earth.

8. Aug 23, 2017

### Grinkle

In some manner or maybe in various manners, physicists model the early universe and as the density decreases they can describe different epochs (eg https://en.wikipedia.org/wiki/Chronology_of_the_universe ) based on the model. It might be that predicting the number of stars / galaxies that will form on average per unit volume is within the scope of these models - does anyone know if it is?

9. Aug 23, 2017

### Staff: Mentor

Why? To run with the analoguy @phyzguy made, would you expect a theory of physics to give you the exact value for the number of grains of sand on Waikiki beach? Isn't it obvious that that number is just the accidental result of a bunch of random factors combining over time? Why would you expect the number of galaxies in the universe to be any different?

10. Aug 23, 2017

### phyzguy

Well, there are detailed simulations of the evolution of the universe from early times up to the present day. Two examples are the Illustris simulation and the Eagle simulation. These are both fascinating studies by the way. Both of these give galaxy distributions which closely match what we observe. So I guess you could consider this "theory" predicting the number of galaxies in the observable universe. But these simulations have a large number of adjustable models which are tuned to give the right answers, so it is not theory in the sense of first principles theory.

11. Aug 23, 2017

### kimbyd

As far as I know, the 100 billion number came from the Hubble Ultra Deep Field:
https://www.spacetelescope.org/images/heic0611b/

This image has nearly 10,000 galaxies in it. Divide the total area of the sky by the area of this patch, and then multiply that by the 10,000 galaxies in this patch and you get about 100 billion.

That said, there are probably many galaxies in this image that you cannot see (because they're too dim), so chances are this is a significant underestimate. By how much, I do not know. There could easily be 200 billion galaxies out there.

12. Aug 23, 2017

### davidge

13. Aug 30, 2017

### laurenrayjohn

Great replay @kimbyd. I totally agree with your point of view!

14. Aug 31, 2017

### newjerseyrunner

When making claims like that, 100 billion and 200 billion are essentially the same thing. It's understood that because of the methodology, any answer is only going to be an approximate answer, so the order of magnitude is what's interesting, not the first digits. If someone tells me that there are 100 billion galaxies without mentioning a standard deviation, I take that to mean anywhere between 50 billion and 500 billion.

15. Aug 31, 2017

### kimbyd

Nit: order of magnitude estimates are more accurately done if the cutoff is $\sqrt{10}$, which is about 3.2, so that 100 billion order of magnitude is anywhere from 32 billion to 320 billion (roughly).

That aside, this is better understood as a lower bound than an order-of-magnitude estimate. Depending upon how small a cluster of stars you want to consider to be a galaxy (e.g. would the large and small Magellanic clouds count?), the total number of galaxies might be in the trillions to tens of trillions.

16. Oct 18, 2017

### Thuring

I understand someone did a very recent estimate using some sort of new method that is apparently well accepted: 2 TRILLION galaxies within the visible universe. It used to be about 200 million. If you look it up, be sure it's a very recent report.

17. Oct 18, 2017

### Thuring

I ment "200 billion".

18. Oct 18, 2017

### kimbyd

I think my post #11 above also answers this question. If the team making the claim of more galaxies, they may just be using an observational method that goes quite a bit deeper than Hubble's Ultra Deep Field.

19. Oct 19, 2017

### davidge

@kimbyd, there's a little problem with your explanation in post #11. That's it the given photo was taken between 2003 and 2004. The value 100 billion I refered to was mentioned in the Cosmos Series in the early 80's. So there must be a source for that total number other than the Hubble's Ultra Deep Field.

20. Oct 19, 2017

### phinds

This NASA site doesn't give a number but "10 Times More Galaxies Than Previously Thought" would imply 1 to 2 trillion
https://www.nasa.gov/feature/goddar...0-times-more-galaxies-than-previously-thought